tag:blogger.com,1999:blog-37414104180967168272019-08-25T01:41:28.337-07:00Unizor - Creative Mind through Art of MathematicsUnizor is a site where students can learn the high school math (and in the future some other subjects) in a thorough and rigorous way. It allows parents to enroll their children in educational programs and to control the learning process.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.comBlogger376125tag:blogger.com,1999:blog-3741410418096716827.post-54619554922461790012019-08-18T06:19:00.001-07:002019-08-18T06:19:56.956-07:00Unizor - Physics4Teens - Energy - Energy of Nucleus - Fusion<br /><br /><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Nucleus Fission</u><br /><br /><br /><br /><i>Fusion</i> is a <i>nuclear reaction</i>, when light nuclei are brought together and combined into a heavier ones.<br /><br />The reason for this reaction to release the energy is the difference <br />between amount of energy needed to overcome the repulsion between nuclei<br /> because they have the same positive electric charge (this energy is <br />consumed by <i>fusion</i>) and the potential energy released by <i>strong forces</i>, when the formation of a combined nucleus occurs (this energy is released by <i>fusion</i>).<br /><br /><u>The former is less than the latter</u>.<br /><br /><br /><br />When the light nuclei are fused into a heavier one, the excess of potential energy of <i>strong forces</i>, released in the process of <i>fusion</i>,<br /> over the energy needed to squeeze together protons against their <br />repulsion is converted into thermal and electromagnetic field energy.<br /><br /><br /><br />Analogy to this process can be two magnets separated by a spring.<br /><img src="http://www.unizor.com/Pictures/FusionAnalogy.png" style="height: 400px; width: 200px;" /><br /><br />The magnets represent two separate protons, the magnetic force of attraction between them represents the <i>strong force</i><br /> that is supposed to hold the nucleus together, when these particles are<br /> close to each other, the spring represents the electrical repulsive <br />force between them, acting on a larger distance, as both are positively <br />charged.<br /><br />It's known that magnetic force is inversely proportional to a square of a<br /> distance between objects, while the resistance of a spring against <br />contraction obeys the Hooke's Law and is proportional to the length of <br />contraction.<br /><br />On the picture magnets are separated. To bring them together, we have to<br /> spend certain amount of energy to move against a spring that resists <br />contraction. But the magnetic attraction grows faster then the <br />resistance of the spring, so, at some moment this attraction will be <br />greater than the resistance of a spring. At this moment nothing would <br />prevent magnets to fuse.<br /><br /><br /><br />As is in the above analogy, if we want to fuse two protons, we have to bring them together sufficiently close for <i>strong forces</i> to overtake the repulsion of their positive charges.<br /><br /><br /><br />Consider the following nuclear reaction of <i>fusion</i>.<br /><br />One nucleus of hydrogen isotope <i>deuterium</i> <b><sup>1</sup>H<sup>2</sup></b> with atomic mass 2 contains one proton and one neutron.<br /><br />One nucleus of hydrogen isotope <i>tritium</i> <b><sup>1</sup>H<sup>3</sup></b> with atomic mass 3 contains one proton and two neutrons.<br /><br />If we force these two nuclei to fuse, they will form a nucleus of <i>helium</i> <b><sup>2</sup>He<sup>4</sup></b> and releasing certain amount of energy:<br /><br /><b><sup>1</sup>H<sup>2</sup> + <sup>1</sup>H<sup>3</sup> = <sup>2</sup>He<sup>4</sup> + <sup>0</sup>n<sup>1</sup></b><br /><br /><br /><br />It's not easy to overcome the repulsion of protons. High temperature and<br /> pressure, like in the core of our Sun, are conditions where it happens.<br /> On Earth these conditions are created in the nuclear bomb, using the <br />atomic bomd to achieve proper amount of heat and pressure, thus creating<br /> an uncontrlled <i>fusion</i>.<br /><br />Controlled nuclear reaction of <i>fusion</i> is what scientists are working on right now. So far, it's still in the experimental stage.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-43415492098950650422019-08-12T18:50:00.001-07:002019-08-12T18:50:19.438-07:00Unizor - Physics4Teens - Energy - Energy of a Nucleus - Fission<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/6WdOzCFKxxw" width="480"></iframe><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br/><br/><u>Nucleus Fission</u><br /><br /><i>Fission</i>, first of all, is a <i>nuclear reaction</i>, when heavier nuclei are split into lighter ones.<br/>The reason for this reaction to release the energy is the difference between amount of energy needed to break <i>strong forces</i> that hold the nucleus together (this energy is consumed by <i>fission</i>) and amount of potential energy in positively charged and repelling protons inside nucleus (this energy is released by <i>fission</i>).<br/><u>The former is less than the latter</u>.<br/><br/>When the heavy nucleus is broken into parts, the excess of potential energy of squeezed together protons against their repelling force over the energy of strong forces that keep nucleus together is converted into thermal and electromagnetic field energy.<br/><br/>Analogy of this is a spring squeezed tightly and held in this position by a thread. A thread plays the role of <i>strong forces</i>, while a potential energy of a squeezed spring plays the role of protons kept close to each other by a this force. When you cut a thread, the spring will release the potential energy, similarly to protons repelling from each other.<br/><br/>Electrically positively charged protons repel each other and, at the same time, are bonded together by <i>strong forces</i> inside a nucleus. At the same time neutrons are also bonded by <i>strong forces</i> among themselves and with protons without any repulsion.<br/>So, the more neutrons the nucleus has - the stronger it is. Neutrons only add "bonding material" to a nucleus without adding any repelling forces that work against the nucleus' stability.<br/><br/>Uranium-238 with 92 protons and 146 neutrons (<sup>92</sup>U<sup>238</sup>) naturally occurs on Earth and is relatively stable.<br/>Uranium-235 with the same 92 protons and 143 neutrons (<sup>92</sup>U<sup>235</sup>) has less "bonding material" (less neutrons) and is more susceptible to fission.<br/><br/>All it takes to break the nucleus of <sup>92</sup>U<sup>235</sup> is a little "push" from outside, which can be accomplished by bombarding it with neutrons. In the process of <i>fission</i>, caused by hitting a nucleus of <sup>92</sup>U<sup>235</sup> with a neutron, it can transforms into Barium-141 with 56 protons and 85 neutrons <sup>56</sup>Ba<sup>141</sup>, Krypton-92 with 36 protons and 56 neutrons <sup>36</sup>Kr<sup>92</sup> and 3 free neutrons.<br/>As we see, the numbers of protons is balanced (input: 92, output: 56 and 36), as well as a number of neutrons (input: 1 free hitting neutron and 143 in a nucleus of <sup>92</sup>U<sup>235</sup> total 144, output: 85 in a nucleus <sup>56</sup>Ba<sup>141</sup>, 56 in a nucleus of <sup>36</sup>Kr<sup>92</sup> and 3 new free neutrons total 144).<br/><br/>Let's express this reaction in a formula (letter <i><b>n</b></i> denotes a neutron):<br/><i><b><sup>0</sup>n<sup>1</sup> + <sup>92</sup>U<sup>235</sup> =<br/><sup>56</sup>Ba<sup>141</sup> + <sup>36</sup>Kr<sup>92</sup> + 3·<sup>0</sup>n<sup>1</sup></b></i><br/><br/>What's interesting in this reaction is that it not only produces energy because we break a heavy nucleus into lighter ones, but also that it produces 2 new neutrons that can bombard other atoms, causing a <b>chain reaction</b> and, potentially, an explosion (atomic bomb). However, if we absorb extra neutrons, it will allow to slowly release of nuclear energy (nuclear power stations).<br/><br/>Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-69734146366792166512019-08-05T18:46:00.001-07:002019-08-05T18:46:22.756-07:00Unizor - Physics4Teens - Energy - Energy of a Nucleus<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/zi6gTkhDIds" width="480"></iframe><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Energy of Nucleus</u><br /><br /><br /><br />In this lecture we will analyze the energy aspect of nucleus - the central part of an atom.<br /><br /><br /><br />By now we have built a pyramid of energy types, related to the depth of our view inside the matter.<br /><br /><br /><br />First, we analyzed the <i>mechanical energy</i> - the energy of moving macro-objects.<br /><br /><br /><br />Our next view deep into the world of macro-objects uncovered the <i>molecules</i> - the smallest parts of macro-objects that retain their characteristics. The movement of these molecules was the source of <i>thermal energy</i>, which we often call the <i>heat</i>.<br /><br /><br /><br />Next step inside the molecules uncovered <i>atoms</i>, as the molecules'<br /> components. There are about 100 types of atoms and their composition <br />inside the molecules creates all the thousands of different molecules. <i>Chemical reactions</i><br /> change the composition of atoms in molecules, thereby creating new <br />molecules from the atoms of old molecules. This process broke some <br />inter-atomic bonds and created the new ones and is the source of <i>chemical energy</i>.<br /><br /><br /><br />Now we look deep inside the atoms and find there 3 major <b>elementary particles</b> - electrically positively charged <i>protons</i> and electrically neutral <i>neutrons</i> inside a small but heavy <i>nucleus</i> and electrically negatively charged <i>electrons</i>,<br /> circulating around nucleus on different orbits. For electrically <br />neutral atoms the numbers of protons and electrons are equal. <i>Nuclear energy</i> is hidden inside the nucleus and is the subject of this lecture.<br /><br /><br /><br />The first question we would like to answer is "What holds nucleus, its <br />protons and neutrons, together, considering protons, as electrically <br />positively charged particles must repel each other?"<br /><br /><br /><br />The answer is simple. There are other forces in the Universe, not only <br />electrostatic ones, that act in this case. These intra-nucleus forces <br />that hold the nucleus together are called <i><b>strong forces</b></i>. They are <b>strong</b><br /> because they are the source of attraction between the protons that is <br />stronger than electrostatic repelling. However, these strong forces act <br />only on a very small distance, comparable to the size of a nucleus <br />inside an atom. For example, at a distance <i>10<sup>−15</sup>m</i> the strong force is more than 100 times stronger than electrostatic one.<br /><br /><br /><br />If, by regrouping <i>protons</i> and <i>neutrons</i>, we will be able to create different <i>atoms</i> (inasmuch as regrouping <i>atoms</i> in <b>chemical reaction</b> we create new <i>molecules</i>), a new source of energy, based on <b>strong forces</b>, the <i>nuclear energy</i>, can be uncovered in the course of <b>nuclear reaction</b>.<br /><br /><br /><br />There is another form of <i>nuclear reaction</i> related to <br />transformation of elementary particles. Under certain circumstance a <br />neutron inside a nucleus can transform into proton and, to keep the <br />total electrical charge in balance, it emits an electron. This reaction <br />is called <i>beta-decay</i> and it also produces energy in the form of electromagnetic waves of very high frequency (<i>gamma-rays</i>).<br /><br /><br /><br />Nuclear reactions are a very powerful source of nuclear energy, which is<br /> so much more powerful than other types of energy, that, if misused, it <br />might represent a danger for life on our planet.<br /><br /><br /><br />There is a clear analogy between nuclear and chemical reactions.<br /><br />What happens with atoms in the chemical reaction, happens with protons <br />and neutrons in nuclear reaction. Some atomic bonds break in a chemical <br />reaction, some are created. Some nuclear bonds between protons and <br />neutrons break in a nuclear reaction, some are created.<br /><br /><br /><br />Sometimes the chemical reaction happens by itself, as long as <br />participating substances are close together, but sometimes we have to <br />initiate it, like lighting methane gas with a spark or a flame of a <br />match to initiate continuous burning.<br /><br />Similar approach is valid for nuclear reaction. Sometimes it happens by <br />itself, but sometimes it should be started, like bombarding the nucleus <br />with neutrons, after which it continues by itself.<br /><br /><br /><br />Here is an interesting fact.<br /><br />Physicists have measured the masses of protons, neutrons and many <br />different nuclei that contain these protons and neutrons and have <br />discovered that the sum of masses of individual protons and neutrons is <br />greater than the mass of a nucleus that contain these exact particles.<br /><br />For example,<br /><br />mass of proton is 1.0072766 atomic mass units or 1.6726·10<sup>-27</sup>kg,<br /><br />mass of neutron is 1.0086654 atomic mass units or 1.6749·10<sup>-27</sup>kg.<br /><br />At the same time, mass of deuterium nucleus, that contains 1 proton and 1<br /> neutron is 2.0135532 atomic mass units, which is smaller than the sum <br />of masses of proton and neutron (1.0072766 + 1.0086654 = 2.015942).<br /><br />This so-called "mass defect" is directly related to nuclear energy - the energy of <i>strong forces</i> that hold the nucleus together.<br /><br /><br /><br />A simplified explanation of this effect is based on the law of energy <br />conservation. Consider the force of gravity between a planet and an <br />object above its surface. The object has certain potential energy and, <br />if dropped to the ground, this potential energy transforms into other <br />forms, like kinetic, thermal etc.<br /><br /><br /><br />Similarly, if we consider two independent neutrons (or neutron and <br />proton, or two protons) on a very small distance from each other, but <br />not forming a nucleus, there is a potential energy of the <i>strong forces</i><br /> acting between them. If we let these two particles to form a nucleus, <br />analogously to an object falling towards the surface of a planet, this <br />potential energy should be transformed into other forms, like thermal.<br /><br /><br /><br />Now the Theory of Relativity comes to play, that has established the equivalence of <b>mass</b> and <b>energy</b> by a famous formula <i><b>E=m·c²</b></i>.<br /> According to this equivalence, if some energy is released during the <br />formation of a nucleus from individual protons and neutrons, there must <br />be certain amount of mass released associated with this energy. That is <br />the explanation of "mass defect".<br /><br /><br /><br />It should be noted that to form a nucleus of deuterium from 1 proton and<br /> 1 neutron is easier than to form a nucleus that contains more than one <br />proton, because electrostatic repulsion between positively charged <br />protons prevents their bonding. So, to bring protons sufficiently close <br />to each other for <i>strong forces</i> to overcome the electrostatic <br />repulsion, we have to spend some energy. The net energy released by <br />forming a nucleus from protons and neutrons is the difference between <br />the energy released from <i>strong forces</i> taking hold of these particles inside a nucleus and the energy consumed to overcome repulsion of protons.<br /><br /><br /><br />Actually, as we attempt to form bigger nuclei, the energy we have to <br />spend to overcome electrostatic repulsion forces become greater than <br />amount of energy released by forming a nucleus. This border line is <br />approximately around the nucleus of iron <i><b>Fe</b></i>. Forming iron <br />and heavier elements from protons and electrons is a process that <br />consumes more energy than releases. These heavier nuclei will produce <br />energy, if we reverse the procedure, breaking them into individual <br />protons and neutrons.<br /><br /><br /><br />The mechanisms described above are used in nuclear reactors and atomic <br />bomb, where heavier elements are broken into lighter ones (fission), <br />releasing energy, and in hydrogen bomb, where lighter elements are <br />bonded together to release the energy (fusion).Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-20467534891672103982019-07-29T14:09:00.001-07:002019-07-29T14:09:21.377-07:00Unizor - Physics4Teens - Energy - Atoms and Chemical Reactions - Interat...<br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Interatomic Bonds</u><br /><br /><br /><br />Atoms in a molecule are bonded together to form a stable chemical substance or compound.<br /><br />The mechanism of bonding is quite complex and different for different <br />molecules. In fact, the complexity of these bonds is outside of the <br />scope of this course. However, certain basic knowledge about molecular <br />bonding and molecular structure is necessary to understand the following<br /> lecture, where we will make certain calculations related to energy <br />produced or consumed in chemical reactions.<br /><br /><br /><br />The key to a mechanism of bonding atoms into molecules lies in an internal structure of atoms.<br /><br />For our purposes we can consider the orbital model of atom as consisting<br /> of electrically positive nucleus and electrically negative electrons <br />circulating on different orbits around a nucleus. <b>This is only a model</b>,<br /> not an exact representation of what's really happening inside the atom,<br /> but this model gives relatively good results that correspond to some <br />simple experiments.<br /><br /><br /><br />Two different particles can be found in a nucleus - positively charged <br />protons and electrically neutral neutrons. The number of protons inside a<br /> nucleus and electrons circulating on different orbits around a nucleus <br />should be the same for electrically neutral atoms in their most common <br />state.<br /><br /><br /><br />For reasons not well understood by many physicists, each orbit can have <br />certain maximum number of electrons that can circulate on it without <br />"bumping" into each other. The higher the orbit - the more electrons it <br />can hold. The lowest orbit can hold no more than 2 electrons, the next -<br /> no more than 8, the next - no more than 14 etc.<br /><br /><br /><br />Consider a few examples.<br /><br /><br /><br />1. Let's consider the structure of a simplest molecule - the molecule of<br /> hydrogen, formed by two atoms of hydrogen. Each hydrogen atom has one <br />electron on the lowest orbit around a nucleus. The maximum number of <br />electrons on this orbit is two, in which case the compound becomes much <br />more stable. So, two atoms of hydrogen grab each other and the two <br />electrons, each from its own atom, are shared by a couple of atoms, thus<br /> creating a stable molecule of hydrogen with symbol <i><b>H<sub>2</sub></b></i>. The bond between two atoms of hydrogen is formed by one pair of shared electrons, so structurally the molecule of hydrogen <i><b>H<sub>2</sub></b></i> can be pictured as<br /><br /><i><b>H−H</b></i>.<br /><br /><br /><br />2. Atom of oxygen has 8 electrons - 2 on the lowest orbit and 6 on the <br />next higher one. The next higher orbit is stable when it has 8 <br />electrons. So, two atoms of oxygen are grabbing each other and share 2 <br />out of 6 electrons on the outer orbit with another atom. So, each atom <br />has 4 "personal" electrons, 2 electrons that it shares with another atom<br /> and 2 electrons that the other atom shares with it. Thus, the orbit <br />becomes full, all 8 spots are filled. The bond between two atoms of <br />oxygen is formed by two pairs of shared electrons, so structurally the <br />molecule of oxygen <i><b>O<sub>2</sub></b></i> can be pictured as<br /><br /><i><b>O=O</b></i><br /><br />(notice double link between the atoms).<br /><br /><br /><br />3. Our next example is gas methane. Its molecule consists of one atom of<br /> carbon (6 electrons, 2 of them on the lowest orbit, 4 - on the next <br />one) and 4 atoms of hydrogen (1 electron on the lowest orbit of each <br />atom). Obviously, having only 4 electrons on the second orbit, carbon is<br /> actively looking for electrons to fill the orbit. It needs 4 of them to<br /> complete an orbit of 8 electrons. Exactly this it finds in 4 atoms of <br />hydrogen that need to complete their own lowest orbit. Sharing <br />electrons, one atom of carbon and 4 atoms of hydrogen fill their <br />corresponding orbits, thus creating a molecule of methane <i><b>CH<sub>4</sub></b></i> with can be pictured as<br /><br /><i><b> H<br /><br /> |<br /><br /> H−C−H<br /><br /> |<br /><br /> H</b></i><br /><br /><br /><br />4. Carbon dioxide molecule contains 1 atom of carbon, that needs 4 <br />electrons to complete its orbit, and 2 atoms of oxygen, each needs 2 <br />electrons to complete its orbit: <i><b>CO<sub>2</sub></b></i>. By <br />sharing 2 electrons from each atom of oxygen with 4 electrons from atom <br />of carbon they all fill up their outer orbit of electrons and become a <br />stable molecule, pictured as<br /><br /><i><b>O=C=O</b></i><br /><br />(notice double link between the atoms).<br /><br /><br /><br />5. Ethanol molecule contains 2 atoms of carbon, 1 atom of oxygen and 6 atoms of hydrogen connected as follows<br /><br /><i><b> H H<br /><br /> | |<br /><br /> H−C−C−O−H<br /><br /> | |<br /><br /> H H</b></i><br /><br />(notice single bond between atoms of carbon and oxygen in ethanol, while<br /> the bond between them in carbon dioxide has double link)<br /><br /><br /><br />6. Hydrogen peroxide molecule contains 2 atoms of hydrogen and 2 atoms of oxygen connected as follows<br /><br /><i><b>H−O−O−H</b></i><br /><br />(notice single bond between atoms of oxygen, not like in a molecule of oxygen)<br /><br /><br /><br />Numerous examples above illustrate that bonds between atoms can be <br />different, even between the same atoms in different molecules. That's <br />why it is important to understand the structure of molecules, how <br />exactly the atoms are linked and what kind of links exist between them. <br />This is the basis for calculation of the amount of energy produced or <br />consumed by chemical reactions that rearrange the atoms from one set of <br />molecules to another.<br /><br /><br /><br />Obviously, bonds <i><b>O−O</b></i> and <i><b>O=O</b></i> are different. <br />The first one is facilitated by one shared electron, the second one - by<br /> two. The amounts of energy, needed to break these bonds, are different <br />too. Therefore, when calculating the energy of chemical reaction, it's <br />important to understand the kind of bond between atoms in each separate <br />case.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-66298229959949839832019-07-23T12:21:00.001-07:002019-07-23T12:21:16.716-07:00Unizor - Physics4Teens - Energy - Chemical Energy of Atomic Bonds<br /><br /><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Energy of Atomic Bonds<br /><br />in Molecules</u><br /><br /><br /><br />In this lecture we will analyze the energy aspect of chemical reactions.<br /><br />Consider the reaction of burning of methane. This gas is used in regular<br /> gas stoves, so the reaction happens every time we cook something.<br /><br />A molecule of methane consists of one atom of carbon <i><b>C</b></i> and four atoms of hydrogen <i><b>H</b></i>, the chemical formula of methane is <i><b>CH<sub>4</sub></b></i>.<br /><br />You can imagine a molecule of methane as a tetrahedron, in its center is<br /> an atom of carbon and on each of its four vertices is an atom of <br />hydrogen.<br /><br />A molecule of oxygen, as we know, consists of two atoms of oxygen and has a chemical formula <i><b>O<sub>2</sub></b></i>.<br /><br /><br /><br />As a result of the reaction of burning of methane, water and carbon dioxide are produced, according to the following equation:<br /><br /><i><b>CH<sub>4</sub> + 2O<sub>2</sub> = 2H<sub>2</sub>O + CO<sub>2</sub></b></i><br /><br />So, during this reaction<br /><br />(a) four atomic bonds between carbon and hydrogen in one molecule of methane are broken,<br /><br />(b) one atomic bond in each molecule of oxygen (out of two) are broken,<br /><br />(c) two atomic bonds between hydrogen and oxygen in each molecule of water (out of two) are created,<br /><br />(d) two atomic bonds between carbon and oxygen in a molecule of carbon dioxide are created.<br /><br /><br /><br />Amounts of potential energy of the different atomic bonds are <br />experimentally determined, which would lead to calculation of the amount<br /> of chemical energy released (for exothermic) or consumed (by <br />endothermic) reaction.<br /><br /><br /><br />To make experiments to determine potential energy of the bonds inside a <br />molecule, we have to make experiments with known amounts of components <br />in chemical reaction. The reaction above includes one molecule of <br />methane and two molecules of oxygen. Obviously, we cannot experiment <br />with one or two molecules. The solution is to experiment with <b>proportional</b> amounts of components, say, 1 million of molecules of methane and 2 million of molecules of oxygen.<br /><br /><br /><br />To explain how to do this, we have to get deeper into atoms. Physics <br />models atoms as consisting of three kinds of elementary particles - <br />protons (electrically positively charged), neutrons (electrically <br />neutral) and electrons (electrically negatively charged). This is a <br />relatively simple model, that corresponds to most of experiments, though<br /> the reality is more complex than this. For our purposes we can view <br />this model of atom as a nucleus, that contains certain number of protons<br /> and neutrons, and a number of electrons circulating the nucleus on <br />different orbits.<br /><br /><br /><br />Electrons are very light relatively to protons and neutrons, so the mass<br /> of an atom is concentrated, mostly, in its nucleus. Protons and neutron<br /> have approximately the same mass, which is called <i>atomic mass unit</i>. So, the mass of an atom in <i>atomic mass units</i><br /> ("atomic weight") is equal to the number of protons and neutrons in its<br /> nucleus. This mass is known for each element of the Periodic Table of <br />Mendeleev, that is for each known atom.<br /><br />For example, it is determined that atom of hydrogen <i><b>H</b></i> has atomic weight of 1 atomic unit, atom of carbon <i><b>C</b></i> has atomic weight of 12, atom of oxygen <i><b>O</b></i> has atomic weight 16.<br /><br /><br /><br />Knowing atomic weights of atoms, we can calculate atomic weight of molecules. Thus, the atomic weight of a molecule of methane <i><b>CH<sub>4</sub></b></i> is 12+4=16. Atomic weight of a molecule of oxygen <i><b>O<sub>2</sub></b></i> is 16+16=32. Atomic weight of water <i><b>H<sub>2</sub>O</b></i> is 2+16=18.<br /><br /><br /><br />Now we can take components of any chemical reaction proportional to the <br />atomic weight of corresponding molecules, which will result in <br />proportional number of molecules. For example, not being able to <br />experiment with one molecule of methane <i><b>CH<sub>4</sub></b></i> and two molecules of oxygen <i><b>O<sub>2</sub></b></i>, we can experiment with <i><b>16 gram</b></i> of methane and <i><b>64 gram</b></i><br /> of oxygen, and the proportionality of the number of molecules will be <br />preserved - for each molecule of methane there will be two molecules of <br />oxygen.<br /><br /><br /><br />As you see, taking amount of any mono-molecular substance in grams <br />equaled to the atomic weight of the molecules of this substance (called a<br /> <i>mole</i>) assures taking the same number of molecules, regardless of the substance. This number is the Avogadro Number and is equal to <i><b>N=6.02214076·10<sup>23</sup></b></i>.<br /><br />Thus, one <i>mole</i> of methane <i><b>CH<sub>4</sub></b></i> (atomic weight of <i><b>C</b></i> is 12, atomic weight of <i><b>H</b></i> is 1) weighs 16g, one <i>mole</i> of silicon <i><b>Si<sub>2</sub></b></i> (atomic weight of <i><b>Si</b></i> is 14) weighs 28g, one <i>mole</i> of copper oxide <i><b>CuO</b></i> (atomic weight of <i><b>Cu</b></i> is 64, atomic weight of <i><b>O</b></i><br /> is 16) weighs 80g etc. And all those amounts of different substances <br />have the same number of molecules - the Avogadro number (approximately, <br />of course).<br /><br /><br /><br />The theory behind the atomic bonds inside a molecule is quite complex <br />and is beyond the scope of this course. Based on this theory and <br />experimental data, for many kinds of atomic bonds there had been <br />obtained an amount of energy needed to break these bonds, that is its <br />inner chemical energy.<br /><br />Thus, chemical energy of atomic bonds inside a mole of methane <i><b>CH<sub>4</sub></b></i><br /> is 1640 kilo-joules (because a molecule of methane has 4 bonds between <br />carbon and each atom of hydrogen, each bond at 410KJ), inside a molecule<br /> of oxygen <i><b>O<sub>2</sub></b></i> - 494 kilo-joules (1 bond between 2 atoms oxygen at 494KJ), inside a molecule of carbon dioxide <i><b>CO<sub>2</sub></b></i> is 1598 kilo-joules (2 bonds between carbon and each atom of oxygen, each 799KJ), inside a molecule of water <i><b>H<sub>2</sub>O</b></i> is 920 kilo-joules (2 bonds between oxygen and each atom of hydrogen, each 460KJ).<br /><br /><br /><br />Let's go back to methane burning:<br /><br /><i><b>CH<sub>4</sub> + 2O<sub>2</sub> = 2H<sub>2</sub>O + CO<sub>2</sub></b></i><br /><br />This chemical reaction converts 1 mole of methane (16g) and 2 moles of <br />oxygen (64g) into 1 mole of carbon dioxide (44g) and 2 moles of water <br />(36g).<br /><br />The energy we have to spend to break the atomic bonds of 1 mole of methane and 2 moles of oxygen, according to above data, is<br /><br /><i><b>E<sub>in</sub> = 1640 + 2·494 = 2628 KJ</b></i><br /><br />The energy we have to spend to break atomic bonds of 2 moles of water and 1 mole of carbon dioxide, according to above data, is<br /><br /><i><b>E<sub>out</sub> = 2·920 + 1598 = 3438 KJ</b></i><br /><br />The net energy is<br /><br /><i><b>E<sub>net</sub> = 2628 − 3438 = −810 KJ</b></i><br /><br />This net energy is the amount of thermal energy released by burning 16g <br />of methane, using 64g of oxygen, obtaining as a result 44g of carbon <br />dioxide and 36g of water.<br /><br />Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-52607433522978610102019-07-18T14:41:00.001-07:002019-07-18T14:41:54.070-07:00Unizor - Physics4Teens - Energy - Atoms<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Atoms and Chemical Reaction</u><br /><br /><br /><br />Discussing <i>mechanical energy</i>, we analyzed the movement of objects.<br /><br />When talking about <i>thermal energy (heat)</i>, we had to go deeper inside the objects and analyzed the movement of <i>molecules</i>, the smallest parts of objects that retained the properties of objects themselves.<br /><br />Now we go even deeper, inside the molecules, in search of new kinds of energy.<br /><br /><br /><br />The components of <i>molecules</i> are called <i>atoms</i>. Currently there are more than 100 kinds of <i>atoms</i>, classified in the Mendeleev's Periodic Table.<br /><br /><br /><br />Different combinations of these <i>atoms</i> in different quantities make up all kinds of <i>molecules</i>, each with its own properties.<br /><br /><br /><br />In some cases a single <i>atom</i> makes up a <i>molecule</i>. For example, a single atom of iron (denoted by symbol <b>Fe</b>) makes up a molecule of iron.<br /><br />In some other cases a pair of <i>atoms</i> of the same type makes up a <i>molecule</i>. For example, two atoms of oxygen (denoted by symbol <b>O</b>) make up a molecule of oxygen (denoted by symbol <b>O<sub>2</sub></b>).<br /><br />In more complicated cases a few <i>atoms</i> of different types make up a <i>molecule</i>. For example, two atoms of hydrogen (denoted by symbol <b>H</b>) and one atom of oxygen (<b>O</b>) make up a molecule of water (<b>H<sub>2</sub>O</b>).<br /><br /><br /><br />One of the most complicated molecules that contains many elements in <br />different quantities is a molecule of protein that has about half a <br />million of atoms.<br /><br /><br /><br /><b>Chemical energy</b> is a <i>potential energy</i> of bonds between <i>atoms</i> that hold them together in a <i>molecule</i>.<br /><br /><b>Chemical reaction</b> is a process of re-arranging of <i>atoms</i> in a group of <i>molecules</i>, getting, as a result, a group of other <i>molecules</i>.<br /><br />During <b>chemical reactions</b> some bonds between atoms are broken and some are created. Therefore, the <u>energy might be either released or consumed</u> in the process of <b>chemical reaction</b>.<br /> This energy, stored in the molecules as potential energy of atomic <br />bonds and released or consumed during chemical reaction, is classified <br />as <i>chemical energy</i>.<br /><br /><br /><br />Let's consider a few examples of chemical energy.<br /><br /><br /><br />1. <i>Coal burning</i><br /><br />One molecule of carbon, that consists of one carbon atom <i><b>C</b></i>, and one molecule of oxygen, that consists of two oxygen atoms <i><b>O<sub>2</sub></b></i>, when brought together and lit up, will join into one molecule of carbon dioxide <i><b>CO<sub>2</sub></b></i><br /> in the process of burning. After the chemical reaction of burning is <br />initiated, it will maintain itself, as the process of burning produces a<br /> flame that lights up new molecules of carbon, joining them with oxygen.<br /><br />The chemical reaction<br /><br /><i><b>C + O<sub>2</sub> = CO<sub>2</sub></b></i><br /><br />is endothermic (consumes heat energy) in the very beginning, when we <br />have to light up the carbon, but, as soon as the reaction started, it <br />becomes exothermic, that is it produces heat energy, because the <br />potential energy of atoms inside molecules of carbon and oxygen together<br /> is greater than potential energy of atoms inside a molecule of carbon <br />dioxide.<br /><br /><br /><br />2. <i>Making water from hydrogen and oxygen</i><br /><br />Two molecules of hydrogen, each consisting of two hydrogen atom <i><b>H<sub>2</sub></b></i>, and one molecule of oxygen, that consists of two oxygen atoms <i><b>O<sub>2</sub></b></i>, when brought together and lit up, will join into two molecules of water <i><b>H<sub>2</sub>O</b></i><br /> in the process of hydrogen burning. After the chemical reaction of <br />burning is initiated, it will maintain itself, as the process of burning<br /> produces a flame that lights up new molecules of hydrogen, joining them<br /> with oxygen.<br /><br />The chemical reaction<br /><br /><i><b>2H<sub>2</sub> + O<sub>2</sub> = 2H<sub>2</sub>O</b></i><br /><br />is endothermic (consumes heat energy) in the very beginning, when we <br />have to light up the hydrogen, but, as soon as the reaction started, it <br />becomes exothermic, that is it produces heat energy, because the <br />potential energy of atoms inside two molecules of hydrogen and one <br />molecule of oxygen together is greater than potential energy of atoms <br />inside two molecules of water.<br /><br /><br /><br />3. <i>Photosynthesis</i><br /><br />This is a complicated process, during which the light from sun, air <br />components (such as carbon dioxide, nitrogen and oxygen), water and <br />whatever is in the soil are converted by the plants into chemical energy<br /> that maintains their life. This is an endothermic process, and, as its <br />result, plants grow. In most cases they consume carbon dioxide from the <br />air, break it into carbon and oxygen, consume the water from the soil, <br />break it into hydrogen and oxygen (they need sun's radiation energy to <br />break the molecules of <i><b>CO<sub>2</sub></b></i> and <i><b>H<sub>2</sub>O</b></i>),<br /> use the carbon, hydrogen and part of oxygen to produce new organic <br />molecules they consist of and release the unused oxygen back into <br />atmosphere.<br /><br /><br /><br />4. <i>Battery</i><br /><br />Battery consists of three major components: <b>anode</b>, <b>cathode</b> and <b>electrolyte</b><br /> in-between anode and cathode. As a result of a chemical and <br />electro-magnetic reaction between the molecules of anode and <br />electrolyte, some electrons are transferred from anode to electrolyte. <br />Then, as a result of a chemical and electro-magnetic reaction between <br />the molecules of cathode and electrolyte, some electrons are transferred<br /> from electrolyte to cathode. As the result, there are extra electrons <br />on the cathode, which were taken from the anode, thus creating <br />electrical potential.<br /><br /><br /><br />These simple examples explain the general mechanism of chemical energy, <br />released or consumed in the course of chemical reaction, that transforms<br /> molecules by rearranging their atoms' composition. As a result of a <br />chemical reaction and change in the atomic composition of molecules, <br />potential energy of bonds between atoms in molecules is changing. If the<br /> total potential energy of the resulting molecules is greater than the <br />potential energy of the bonds inside original molecules, the process is <br />endothermic, it consumes energy. In an opposite case the process is <br />exothermic, it produces energy.<br /><br /><br /><br />The exothermic process of extracting chemical energy using chemical <br />reaction is the key to getting energy from gasoline in the car engine, <br />producing heat and light in the fireplace by burning wood, it's the <br />source of energy in all living organisms, including humans. We exist <br />because our body knows how to extract chemical energy from the food.<br /><br /><br /><br />Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-28046097549836138762019-07-08T12:35:00.001-07:002019-07-08T12:35:28.868-07:00Unizor - Physics4Teens - Energy - Heat Transfer Problems<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Heat Transfer - Problems</u><br /><br /><br /><br /><i>Problem 1</i><br /><br /><br /><br />Determine the power of the heat source inside the room required to <br />maintain a certain difference between inside and outside air <br />temperature, given the following:<br /><br />(a) the difference between inside and outside temperature <i><b>δ=T<sub>out</sub>−T<sub>home</sub></b></i><br /><br />(b) the room has only one wall facing the outside air, and the area of this wall is <i><b>A</b></i><br /><br />(c) the thickness of the wall is <i><b>L</b></i><br /><br />(d) the wall is made of solid material with a coefficient of <i>thermal conductivity</i> <i><b>k</b></i>.<br /><br /><br /><br /><i>Solution</i><br /><br /><br /><br />Let <i><b>x</b></i> be the distance from a point inside the wall to its surface facing the room.<br /><br />The <i><b>T(x)</b></i> is a temperature at this point as a function from this distance.<br /><br />To be equal to <i><b>T<sub>home</sub></b></i> at the surface facing the room and to be <i><b>T<sub>out</sub></b></i> at the surface facing the outside and to be linearly changing from one value to another inside the wall of the thickness <i><b>L</b></i>, the temperature inside the wall at distance <i><b>x</b></i> from the surface facing the room should be<br /><br /><i><b>T(x)=T<sub>home</sub>+(T<sub>out</sub>−T<sub>home</sub>)·x/L</b></i><br /><br />From this we can determine the <i>heat flux</i> through the wall at a distance <i><b>x</b></i>, using the Fourier's law of thermal conduction:<br /><br /><i><b>Q(x,A) = −k·A·</b>d<b>T(x)/</b>d<b>x =<br /><br />= −k·A·(T<sub>out</sub>−T<sub>home</sub>)/L =<br /><br />= −k·A·δ/L</b></i><br /><br />That is exactly how much heat we need to maintain the difference between temperature in the room and outside.<br /><br /><br /><br /><i>Problem 2</i><br /><br /><br /><br />Calculate the heating requirement of the room with only one concrete <br />wall facing outside with no windows, assuming the following:<br /><br />(a) the temperature in the room must constantly be <i>T<sub>room</sub>=20°C</i><br /><br />(b) the temperature outside is also constant <i>T<sub>out</sub>=5°C</i><br /><br />(c) the thickness of the concrete wall facing outside is <i>L=0.2m</i><br /><br />(d) the area of the wall facing outside is <i>A=12m²</i><br /><br />(e) <i>heat conductivity</i> of concrete is <i>k=0.6W/(m·°K)</i><br /><br /><br /><br /><i>Solution</i><br /><br /><br /><br />Using the above, we can determine the heat flux through the wall at a distance <i><b>x</b></i>:<br /><br /><i><b>Q(x,A) = −k·A·δ/L</b></i><br /><br />(for any distance <i><b>x</b></i> from the surface of the wall that faces the room)<br /><br />Outside surface of the wall is at 5°C, inside is at 20°C.<br /><br />So, <i><b>δ=15°</b></i>.<br /><br />Therefore, the <i>heat flux</i> through the wall (at any distance from the inside surface) will be<br /><br /><i><b>Q = 0.6·12·15/0.2 = 540W</b></i><br /><br />That is exactly how much heat we need to maintain the temperature in the room.<br /><br /><br /><br /><i>Problem 3</i><br /><br /><br /><br />Our task is to determine the law of cooling of a relatively small hot <br />object immersed in the cool infinitely large reservoir with liquid or <br />gaseous substance.<br /><br />We assume the volume of substance this object is immersed in to be <br />"infinite" to ignore its own change of temperature related to heat <br />emitted by our hot object.<br /><br />This law of cooling should be expressed in terms of object's temperature <i><b>T</b></i> as a function of time <i><b>t</b></i>, that is, we have to find the function <i><b>T(t)</b></i>.<br /><br />Assumptions:<br /><br />(a) the initial temperature of the object at time <i><b>t=0</b></i> is assumed to be <i><b>T<sub>0</sub></b></i><br /><br />(b) the object has a shape of a thin flat square (so, its temperature is changing simultaneously in all its volume) of size <i><b>L</b></i>x<i><b>L</b></i> and mass <i><b>m</b></i><br /><br />(c) the <i>specific heat capacity</i> of the object's material is <i><b>C</b></i><br /><br />(d) the temperature of the substance surrounding our object is constant and equals <i><b>T<sub>s</sub></b></i><br /><br />(e) the <i>convective heat transfer coefficient</i> of the substance around our object is <i><b>h</b></i>.<br /><br /><br /><br /><i>Solution</i><br /><br /><br /><br />Consider a small time interval from <i><b>t</b></i> to <i><b>t+</b></i>Δ<i><b>t</b></i>.<br /><br />The temperature of an object is <i><b>T(t)</b></i>, while the temperature of the substance around it is constant <i><b>T<sub>s</sub></b></i>.<br /> Then the amount of heat transferred from our object to the substance <br />around it per unit of time per unit of its surface area is proportional <br />to the difference in temperatures with the <i>convective heat transfer coefficient</i> as a factor:<br /><br /><i><b>q = −h·</b></i>[<i><b>T(t)−T<sub>s</sub></b></i>]<br /><br />Since total surface area of our thin flat square, ignoring its thickness, is <i><b>2L²</b></i> and the time interval we consider is Δ<i><b>t</b></i>, the total amount of heat transferred by our object through its surface during this time period is<br /><br />Δ<i><b>Q(t) = −2L²·h·</b></i>[<i><b>T(t)−T<sub>s</sub></b></i>]<i><b>·</b></i>Δ<i><b>t</b></i><br /><br />This is amount of heat taken away by the substance from our object during a time interval Δ<i><b>t</b></i>.<br /><br />The same amount of heat is lost by the object, taking its temperature from <i><b>T(t)</b></i> to <i><b>T(t+</b></i>Δ<i><b>t)</b></i>.<br /><br />As we know, changes of heat and temperature are proportional and related to the object mass and specific heat capacity:<br /><br />Δ<i><b>Q(t) = C·m·</b></i>Δ<i><b>T</b></i><br /><br />where Δ<i><b>T = T(t+</b></i>Δ<i><b>t)−T(t)</b>.</i><br /><br />Equating the amount of heat lost by our object to the amount of heat <br />carried away by convection of the substance around it, we have come to <br />an equation<br /><br /><i><b>−2L²·h·</b></i>[<i><b>T(t)−T<sub>s</sub></b></i>]<i><b>·</b></i>Δ<i><b>t =<br /><br />= C·m·</b></i>[<i><b>T(t+</b></i>Δ<i><b>t)−T(t)</b></i>]<br /><br />Dividing both parts by Δ<i><b>t</b></i>, diminishing this time interval to zero and using a derivative by time <i><b>t</b></i> to express the limit, we get the following differential equation<br /><br /><i><b>−2L²·h·</b></i>[<i><b>T(t)−T<sub>s</sub></b></i>]<i><b> =<br /><br />= C·m·</b>d<b>T(t)<span style="font-size: medium;">/</span></b>d<b>t</b></i><br /><br />To solve it, let's make two simple substitutions:<br /><br />(a) <i><b>A = 2h·L²/(C·m)</b></i><br /><br />(b) <i><b>X(t) = T(t) − T<sub>s</sub></b></i><br /><br />Then, since <i><b>T<sub>s</sub></b></i> is constant,<br /><br /><i>d<b>T(t)<span style="font-size: medium;">/</span></b>d<b>t = </b>d<b>X(t)<span style="font-size: medium;">/</span></b>d<b>t</b></i><br /><br />and our differential equation looks like<br /><br /><i>d<b>X(t)<span style="font-size: medium;">/</span></b>d<b>t = −A·X(t)</b></i><br /><br />To solve this, we convert it as follows:<br /><br /><i>d<b>X(t)<span style="font-size: medium;">/</span>X(t) = −A·</b>d<b>t</b></i><br /><br /><i>d</i>[<i>ln<b>(X(t))</b></i>]<i><b> = −A·</b>d<b>t</b></i><br /><br />Integrating:<br /><br /><i>ln<b>(X(t))</b></i>]<i><b> = −A·t + B</b></i>,<br /><br />where <i><b>B</b></i> is any constant, defined by initial condition<br /><br /><i><b>X(0)=T(0)−T<sub>s</sub>=T<sub>0</sub>−T<sub>s</sub></b></i>.<br /><br />From the last equation:<br /><br /><i><b>X(t) = e<sup>B</sup>·e<sup>−A·t</sup></b></i><br /><br />Using initial condition mentioned above,<br /><br /><i><b>X(t) = (T<sub>0</sub>−T<sub>s</sub>)·e<sup>−A·t</sup></b></i><br /><br />Since <i><b>X(t)=T(t)−T<sub>s</sub></b></i><br /><br /><i><b>T(t) − T<sub>s</sub> = (T<sub>0</sub>−T<sub>s</sub>)·e<sup>−A·t</sup></b></i><br /><br />or<br /><br /><i><b>T(t) = T<sub>s</sub> + (T<sub>0</sub>−T<sub>s</sub>)·e<sup>−A·t</sup></b></i><br /><br />where <i><b>A = 2h·L²/(C·m)</b></i><br /><br />So, the difference in temperature between a hot object and infinitely <br />large surrounding substance is exponentially decreasing with time.<br /><br />Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-21346508164714122752019-06-25T14:41:00.000-07:002019-06-25T14:46:12.921-07:00Unizor - Physics4Teens - Energy - Heat Transfer - Radiation<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/ABTz7UVEuQo" width="480"></iframe><br /><br /><br/><i>Notes to a video lecture on http://www.unizor.com</i><br/><br/><u>Heat Transfer - Radiation</u><br /><br /><i>Heat radiation</i> IS NOT the same as <i>radioactivity</i>. Though, under certain circumstances (like an explosion of an atomic bomb) the heat radiation and radioactivity are both present. When we discuss the <i>heat radiation</i> we talk about a process that occurs in any object with a temperature greater than absolute zero, while <i>radioactivity</i> occurs in extreme cases of very high energy output.<br/><br/>Heat transfer through <i>radiation</i> is totally different from <i>conduction</i> and <i>convection</i>. The most important property of heat transfer by <i>radiation</i> is that heat transfer occurs without any visible material conduit that carries the heat, like molecular movement in two other cases.<br/><br/>Let's start from an example.<br/>The brightest example is our Sun, as a source of heat energy. Between Sun and Earth there is no visible material conduit, yet the heat comes to Earth and is a source of life on our planet.<br/><br/>The fundamental concept that lies in the foundation of a process of <i>heat radiation</i> is a concept of a <i><b>field</b></i>.<br/>The <i>field</i> is a region of space, where certain forces act on certain objects without visible material medium.<br/>As an example of the <i>field</i>, consider <i>gravity</i>. The Sun keeps planets on their orbits, the Earth keeps the Moon circling around, people are walking on the ground without flying away to stars etc. We did not know much about WHY the <i>gravitational field</i> exist, yet we did study its behavior, the forces involved and the laws of motion in this field.<br/><br/>There are other fields.<br/><i>Magnetic field</i> around our planet, acting on a compass, forces the arrow to point North.<br/><i>Electric field</i> exists around electrically charged objects, so other electrically charged objects are attracted to or repulsed from it.<br/><br/>In Physics we successfully study these fields, but complete understanding of WHY they have the properties that we observed is not completely clear. So, we will concentrate on properties, answering the question HOW?, not on a more fundamental question WHY?.<br/><br/>Let's start with a particular field called <i>electro-magnetic</i>. Very simplified description of this field is as follows.<br/><br/>Any electron creates an <i>electric field</i> around itself. Moving electrons, which we call <i>electric current</i> or <i><b>electric field that changes in time</b></i>, also create a <i>magnetic field</i> around them. So, <i><b>changing in time electric field creates magnetic field that changes in space</b></i>. It's an experimental fact, and we have the whole theory about properties of these fields.<br/><br/>Consider an experiment, when you move a metal rod or any other electrical conductor in a <i>magnetic field</i> or change a <i>magnetic field</i> around any electrical conductor, thus creating a <i><b>magnetic field that changes in time</b></i>. You will observe that there is an electric current in the conductor, thus creating an <i><b>electric field that changes in space</b></i>. It's an experimental fact, and we also have the whole theory about properties of this process.<br/><br/>So, <i>electric field</i> creates <i>magnetic field</i>, which, in turn, creates <i>electric field</i> etc. This is a loop of energy conversion that propagates extremely fast, with a speed of light, about 3·10<sup>8</sup>m/sec.<br/><br/>The combination of electric and magnetic forces form <i>electro-magnetic field</i> that propagates much faster than its physical medium - electrons. So, the propagation of the <i>electro-magnetic field</i> seems to be a self-sufficient process, occurring without the medium. This is a very brief and unsatisfactory explanation of the nature of the <i>electro-magnetic field</i>. We will not go much further in this explanation, but rather concentrate on the properties of the <i>electro-magnetic field</i>.<br/><br/>Assuming that we accept the existence of the <i>electro-magnetic field</i> and, however uncomfortably we feel about it, but accept that there is no need for a medium to propagate this field, we can talk about frequency of electro-magnetic transformations, that can be considered similar to oscillation of molecules in a solid. Inasmuch as the oscillations of molecules in a metal are propagated, thus transferring heat energy from hot area to cold one, oscillations of the electric and magnetic components of the <i>electro-magnetic field</i> transfers energy.<br/>This energy transfer by <i>electro-magnetic field</i> is called <i><b>radiation</b></i>.<br/><br/>As in a case of oscillating molecules in a solid, carrying more heat energy when oscillation is more intense (higher frequency), the <i>electro-magnetic field</i> oscillation carries energy with higher frequencies being more "energetic" than lower.<br/><br/>Interestingly, receptors in our skin feel the temperature of a solid object, that is, we feel the intensity of oscillation of its molecules. Similarly, we feel the warm rays of Sun on our skin, that is, we feel the intensity of electro-magnetic oscillation of the electro-magnetic field.<br/>What's more remarkable, we see the light. Apparently, electro-magnetic oscillations in certain frequency range act upon censors in our eyes, thus we see the light. Moreover, in this visible range of frequencies different frequencies of electro-magnetic oscillation produce effect of different colors in our eyes.<br/><br/>As you see, the light and heat of radiation have the same source - the oscillation of electro-magnetic field, the only difference is the frequency. In other words, the <i>light</i> and <i>heat radiation</i> are manifestations of the same process of transferring energy by the oscillations of the components of the <i>electro-magnetic field</i>.<br/><br/>An object does not have to have a temperature of the Sun to emit <i>heat radiation</i>. All objects that have temperature higher than absolute zero emit thermal radiation of some frequencies. Usually, the whole spectrum of frequencies of electro-magnetic oscillations is emitted by objects. Lower frequencies (usually called infrared) are felt by skin receptors, higher frequencies are visible by an eye. Frequencies higher than those visible by a human eye are called ultraviolet. Even higher frequencies are called X-rays, which can be produced by special equipment and, depending on intensity and time of exposure, can represent a health hazard. Even higher intensity and high frequencies are called gamma rays, and they are produced in extreme cases like nuclear explosion or nuclear reactor meltdown, and they are extremely dangerous and are usually meant, when the term <i>radioactivity</i> is used. All frequencies can be observed using some scientific instruments.<br/><br/>Any object, placed in the outer space will emit its heat energy through radiation until its temperature will reach absolute zero. Our Sun emits huge amounts of energy in all spectrum of frequencies in all directions and, eventually, run out of heat energy and go dark.<br/><br/>The intensity of radiation, that is amount of heat radiated per unit of time per unit of area of an object depends, as in other cases of heat transfer, on the temperature of an object and temperature of surrounding environment.<br/>In the complete vacuum with no other source of energy the radiation intensity of an object is proportional to the fourth degree of its absolute temperature in °K:<br/><i><b>q = </b>σ<b>·T<sup>4</sup></b></i> where<br/><i>σ = 5.67·10<sup>−8</sup> W/(m<sup>2</sup>·°K<sup>4</sup>)</i><br/>is the Stefan-Boltzmann constant.<br/>This is the Law of Stefan-Boltzmann. Its derivation is complex and is outside of the scope of this course.<br/><br/>Radiation is not only emitted by objects with temperatures above absolute zero, but also can be absorbed by them and even reflected. While ability to absorb the heat is common for other heat transfer types (conduction and convection), reflection is a specific property of <i>heat radiation</i>. More precisely, it's a specific property of oscillations of the electro-magnetic field.<br/>Obvious application of this property is the usage of mirrors that reflect the oscillations of the electro-magnetic field in a very broad spectrum of frequencies, including the visible light.<br/>An example of absorbed radiation is a slice of bread toasted in the electric toaster. It absorbs the thermal radiation emitted by electric coils, that changes the bread's structure.<br/><br/>Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-9125641408687227262019-06-21T14:21:00.001-07:002019-06-21T14:21:58.347-07:00Unizor - Physics4Teens - Energy - Heat Transfer - Convection<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/WfhCtIcRJTU" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Heat Transfer - Convection</u><br /><br />As in a case of conduction, we start with a statement: <b>heat</b> is a form of <b>internal energy</b> that is related to molecular movement.<br />However, while heat transfer during the process of <i>conduction</i> occurs between molecules oscillating around their relatively fixed positions and transferring their internal energy by "shaking" the neighboring molecules, <i>convection</i> occurs when molecules are free to travel in different directions and carry their internal energy with them.<br /><br />In other words, <i>conduction</i> is a pure transfer of energy on a micro level from one oscillating molecule in a relatively fixed position to another such molecule, while <i>convection</i>occurs when molecules freely fly away from their positions, carrying their internal energy with themselves, thus transferring energy on a macro level.<br /><br />It should be noted that, when dealing with solid objects, <i>conduction</i> is a prevailing way of heat transfer, while in liquids and gases the main way of heat transfer is <i>convection</i>. It does not mean that <i>conduction</i> does not occur in liquids or gases, it does, but it does not constitute the major way of heat transfer. Much more heat is transferred through the mechanism of <i>convection</i><br /><br />Here are a few examples of heat transfer through <i>convection</i>:<br />(a) heating up water in a pot; heat is carried from hot bottom of a pot up by hot (fast moving with high kinetic energy) molecules;<br />(b) circulation of air in the atmosphere from hot places to cold;<br />(c) circulation of water in oceans from hot places to cold.<br /><br />Describing <i>convection</i>mathematically is not a simple task.<br />While in case of <i>conduction</i> we can use a relatively simple Fourier's Law of Thermal Conduction<br /><i><b>q(x) = −k·</b>d<b>T(x)<span style="font-size: medium;">/</span></b>d<b>x</b></i><br />that describes the heat flow as a function of how fast the temperature between the layers of conducting material changes (<i>d<b>T(x)<span style="font-size: medium;">/</span></b>d<b>x</b></i>) and properties of the material itself (<i>conductivity coefficient <b>k</b></i>), the process of <i>convection</i> is significantly more complex, described by convection-diffusion differential equations that are beyond the scope of this course.<br /><br />However, for practical purposes we can use a similar formula that puts the amount of heat transferred by <i>convection</i>process in a liquid or gas during a unit of time through a unit of area as proportional to a difference of temperatures between the layers of liquid or gas and a <i>convective heat transfer coefficient <b>h</b></i> that depends on the physical properties of this liquid or gas:<br /><i><b>q = −h·(T<sub>2</sub>−T<sub>1</sub>)</b></i><br /><br />This formula puts amount of heat <i><b>q</b></i> going through a layer of a unit area of liquid or gas during a unit of time as proportional to a difference of temperatures between bounding surfaces of this layer <i><b>T<sub>2</sub>−T<sub>1</sub></b></i> and some physical properties of liquid or gas expressed in <i>convective heat transfer coefficient <b>h</b></i> that, in turn, depend on such properties as <i>viscosity</i>, <i>density</i>, the type of flow (turbulent or laminar) etc.<br /><br />Consider an example.<br />A round steam pipe of temperature 100°C goes through a room with air temperature 25°C. We have to calculate the amount of heat from the pipe to select an air conditioner required to neutralize the heat from a pipe and keep the room temperature at that level.<br />Assume that the pipe's length is 4m, diameter 0.2m and the <i>convective heat transfer coefficient</i> of air is 40J/(sec·m²·°C). As we know, J/sec is a unit called "watt", so we will use W instead of J/sec.<br /><br />The heat transfer per unit of time through a unit of area of a pipe is, therefore,<br /><i><b>q = 40·(100−25) = 3000(W/m²)</b></i><br />The pipe's area is<br /><i><b>A = π·0.2·4 = 2.512(m²)</b></i><br />Therefore, the pipe is producing the following amount of heat:<br /><i><b>Q = 3000·2.512 = 7536(W)</b></i><br /><br />So, we need an air conditioner that can extract 7536W of heat from the room to maintain stable temperature of 25°C.<br />Usually, the power of air conditioners is measured in BTU/hr (1 watt = 3.41 BTU/hr). So we need an air conditioner of approximately 2200 BTU/hr - a relatively small one.<br /><br />Another example.<br />Outside temperature is 40°C, inside a room we want temperature 25°C. The glass wall between a room and outside air has an area of 20m². What kind of air conditioner is needed to maintain the room temperature at 25°C, assuming the <i>convective heat transfer coefficient</i> of air is 40W/(m²·°C)?<br /><br /><i><b>Q = 40·(40−25)·20 = 12000(W)</b></i><br />This is equivalent to about 3500 BTU/hr.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-2963730674516771812019-06-20T12:36:00.001-07:002019-06-20T12:36:23.797-07:00Unizor - Physics4Teens - Energy - Heat Transfer - Conduction<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/0UaoqluO4OE" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Heat Transfer - Conduction</u><br /><br />As we know, heat is a form of internal energy that is related to molecular movement.<br /><br />For solids the molecular movement is usually restricted to molecules' oscillation around some neutral positions.<br /><br />For liquids the freedom of molecular motion is greater, but still restricted by external forces, like gravity, and surface tension. The average distance between molecules of liquids is relatively constant.<br /><br />Gas molecules are usually taking all the space available for them. Such forces as gravity also restrict their movement (otherwise, the air molecules would fly away from our planet), but still allow substantial freedom. The average distance between molecules of gas mostly depends on a reservoir the gas is in, the larger the reservoir - the larger average distance between molecules.<br /><br />Transfer of heat is transfer of molecular movement from one object or part of an object to another object or part of an object, from an object or part of an object with more intense molecular movement (relatively warmer) to an object or part of an object with less intense movement (relatively cooler).<br /><br />There are three major ways to transfer heat from a hot object to a cold one:<br /><i>Conduction</i>,<br /><i>Convection</i>,<br /><i>Radiation</i>.<br />This lecture explains a concept of <i>conduction</i>.<br /><br /><i><b>Conduction</b></i><br /><br /><i>Conduction</i> of heat energy is a transfer of molecular movement mostly applicable to solid objects - between two solid objects that touch each other, having an area of a contact, or within one object, one part of it having different temperature than another.<br />The <i>conductivity</i> is present in heat transfer in liquids and gases, but there it's usually combined with another form of heat transfer - <i>convection</i>, while in solids it's not the case, and we can study <i>conductivity</i> by itself.<br /><br />An example is a building wall, one side of which towards the outside having temperature of the air outside, while inner surface of the wall having room temperature. The heat energy constantly flows from a warmer surface of the wall to the opposite cooler one with some <i>rate of flow</i> that depends on the <i>thermal conductivity</i> of the wall material. The wall material with higher level of <i>thermal conductivity</i> will transfer more heat energy to a cooler side within unit of time per unit of area, which is usually not a desirable property of the building walls.<br /><br />The mechanism of heat transfer through <i>conductivity</i> can be explained as follows.<br />Imagine two objects (or two parts of the same object), a hot one with higher intensity of molecular movement and a cold one with lower intensity level of molecular movement, that touch each other along some surface, while completely insulated from heat around them. For example, you put a cold metal spoon into a styrofoam cup with hot tea.<br /><br />Molecules of a hot object are hitting the molecules of a cold one, thus forcing the molecules of a cold object to move faster. These faster molecules of a cold object, in turn, hit their neighbors, forcing them to move faster. This process of transferring heat energy through contacting surfaces continues until the intensity of molecular movement gradually equalizes on average. A hot object will lose some energy of molecular movement, while a cold one will gain it. As a result, the temperatures of both will equalize.<br /><br />Since the heat energy is a kinetic energy of molecular movement, that is a sort of mechanical energy, we expect that the total amount of energy for an isolated system will remain constant, whatever a hot object loses in its kinetic energy of molecular movement will be gained by a cold object. The total amount of heat energy will remain the same.<br /><br />If you put a silver spoon into a cup of hot tea, it will heat up faster than a spoon made of steel, which, in turn, will heat up much faster then a spoon made of wood. The reason for this is that the <i>thermal conductivity</i> of different materials is different.<br /><br />We can experimentally measure the thermal conductivity of different solids by having a standard rod of any solid material at certain starting temperature and heating its one end by bringing it to contact with some hot object. Measuring the temperature on the other end after different time intervals will give us a picture of growing temperature.<br /><br />Some materials with higher thermal conductivity will have the temperature at the opposite end of a rod growing faster than in case of other materials.<br />Metals have much higher heat conductivity then plastic or wood, for example. That's why the handle of a tea kettle is usually made of plastic or wood. Diamonds have one of the highest thermal conductivity, even higher than silver.<br /><br />More precise definition of <i>thermal conductivity</i> is related to a concept of <i>heat flux</i>(sometimes, called <i>heat flow density</i> or <i>thermal flux</i>, or <i>thermal flow density</i>). <b>Heat flux is an amount of heat energy flowing through a unit of area during a unit of time</b>.<br /><br />Let's examine how heat flows through a building wall made of some uniform material from a warm room to cold air outside the building.<br />Assume, the room temperature is <i><b>T<sub>room</sub></b></i> and the cold air outside the building has temperature <i><b>T<sub>air </sub></b></i>. If the thickness of a wall is <i><b>L</b></i>, the temperature inside the wall <i><b>T(x)</b></i>, as a function of the distance <i><b>x</b></i> from the surface facing outside, gradually changes from <i><b>T(0)=T<sub>air</sub></b></i> to <i><b>T(L)=T<sub>room</sub></b></i>.<br /><br />It is intuitively understandable and experimentally confirmed that amount of heat energy flowing through a unit of area of such a wall during a unit of time (<i>heat conductivity</i> of a wall) is proportional to a difference between temperatures <i><b>T<sub>room</sub></b></i>and <i><b>T<sub>air</sub></b></i> and inversely proportional to a thickness of a wall <i><b>L</b></i>:<br /><i><b>q = −k·(T<sub>room</sub> − T<sub>air </sub>) <span style="font-size: medium;">/</span> L</b></i><br />(negative sign is used because the flow of heat is opposite to a direction of temperature growth).<br /><br />The situation with <i>heat flux</i>might be compared with a water flow down a river between two points A and B. The difference in levels above the sea level of these points is similar to a difference in temperature between the inside and outside walls of a building. The distance between points A and B is similar to a thickness of a wall. It's reasonable to assume that amount of water flowing through a unit of area in a unit of time will be proportional to a difference between the levels of points A and B above the sea level and inversely proportional to a distance between these two points.<br /><br />To make this definition of the <i>heat flux</i> more precise and independent of the way how the heat flows inside the wall, let's consider a thin slice of wall parallel to both sides from a point at distance <i><b>x</b></i> from the outdoor cold side to a point at distance <i><b>x+</b></i>Δ<i><b>x</b></i>.<br />The heat flow through this thin slice of a wall, as a function of distance <i><b>x</b></i>, can be expressed similarly to the above:<br /><i><b>q(x)=−k·</b></i>[<i><b>T(x+</b></i>Δ<i><b>x)−T(x)</b></i>]<i><b> <span style="font-size: medium;">/</span></b></i>Δ<i><b>x</b></i><br /><br />Next step is, obviously, to reduce the thickness of the slice by making Δ<i><b>x</b></i> infinitesimal, that is Δ<i><b>x→0</b></i>, which leads to the following definition of the <i>heat flux</i>:<br /><i><b>q(x) = −k·</b>d<b>T(x)<span style="font-size: medium;">/</span></b>d<b>x</b></i><br />This definition was formulated by Fourier in 1822 and is called <i>Fourier's law of thermal conduction</i>.<br />The coefficient <i><b>k</b></i> is called <i>thermal conductivity</i>.<br /><br />To find the amount of heat <i><b>Q(x,A)</b></i> going through an area <i><b>A</b></i>at distance <i><b>x</b></i> from the outside wall during a unit of time, we have to multiply the <i>heat flux</i> by an area:<br /><i><b>Q(x,A) = −k·A·</b>d<b>T(x)<span style="font-size: medium;">/</span></b>d<b>x</b></i><br />As in a case of water flow along the river, if the temperature is linearly dependent on the distance from the outside wall, the derivative is constant and the flow of heat is constant. But, if the wall material is uneven, like in case of a river bed not being a straight line down, the heat flow rate will change.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-23446821878253532262019-06-12T19:55:00.001-07:002019-06-12T20:09:50.697-07:00Unizor - Physics4Teens - Energy - Measuring Heat - Problems<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/d6iBVozDJ5w" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br/><br/><u>Measuring Heat - Problems</u><br /><br /><i>Problem 1</i><br/><br/>How much heat energy is required to raise the temperature of <b><i>1 kg</i></b> of water from <i><b>20°C</b></i> to a boiling point of <i><b>100°C</b></i>?<br/>Assume the specific heat capacity of water is<br/><i><b>C</b><sub>w</sub><b> = 4184 J/(kg·°C)</b></i>.<br/><br/><i>Answer</i><br/><b><i>Q = C·m·(T</b><sub>end</sub><b>−T</b><sub>beg</sub><b>) =<br/>= 4184·1·(100−20) =<br/>= 334,720 J</i></b><br/><br/><i>Problem 2</i><br/><br/>A piece of unknown metal of mass <i><b>M<sub>m</sub></b></i> and temperature <i><b>T<sub>m</sub></b></i> was put into an isolated reservoir filled with <i><b>M<sub>w</sub></b></i> mass of water at temperature <i><b>T<sub>w</sub></b></i>. After the system of water and metal came to thermal equilibrium, its temperature became <i><b>T</b></i>.<br/>Assume that the metal is not too hot (so, water will not vaporize) and not too cold (so, the water will not freeze).<br/>Assuming that the water's specific heat capacity is known and equals to <i><b>C<sub>w</sub></b></i>, what is the specific heat capacity <i><b>C<sub>m</sub></b></i> of the unknown metal?<br/><br/><i>Answer</i><br/><br/><i><b>C<sub>w</sub>·M<sub>w</sub>·(T−T<sub>w</sub>) =<br/>= C<sub>m</sub>·M<sub>m</sub>·(T<sub>m</sub>−T)</b></i><br/>from which <i><b>C<sub>m</sub></b></i> equals to<br/><i><b>C<sub>w</sub>·M<sub>w</sub>·(T−T<sub>w</sub>)<font size=4>/</font></b></i>[<i><b>M<sub>m</sub>·(T<sub>m</sub>−T)</b></i>]<i><b></b></i><br/><br/><i>Problem 3</i><br/><br/>A burger has about <i><b>300 kcal</b></i> of energy in it.<br/><b><i>1 kcal = 4184 J</i></b>.<br/>A person, who ate it, wants to spend this energy by climbing up the stairs. A person's mass is <i><b>75 kg</b></i>, the height between the floor is <i><b>3 m</b></i>.<br/>Assume that only 25% of energy in the food can be used for climbing, while the other 75% is needed to maintain our body's internal functions.<br/>Counting from the ground floor as floor #0, to what floor can a person climb using that energy from a burger?<br/><br/><i>Answer</i><br/><b><i>E</b><sub>bur</sub><b> = 300</b>kcal<b> · 4184</b>J/kcal<b> =<br/>= 1,255,200</b>J<b></i></b><br/><b><i>E</b><sub>climb</sub><b> = 0.25·E</b><sub>bur</sub><b> =<br/>= 313,800</b>J<b></i></b><br/><b><i>E</b><sub>floor</sub><b> = 75</b>kg<b> · 9.8</b>m/sec²<b> · 3</b>m<b> =<br/>= 2205</b>J<b></i></b><br/><b><i>N = E</b><sub>climb</sub><b> / E</b><sub>floor</sub><b> ≅ 142</b> floors</i><br/><br/><i>Problem 4</i><br/><br/>An ice of mass <i><b>0.1</b>kg</i> has temperature <i><b>−10</b>°C</i>.<br/>What's the minimum amount of water <i><b>M</b></i> at temperature <i><b>20</b>°C</i> needed to melt it?<br/>Assume, specific heat capacity of water is <i><b>4183</b>J/(kg·°C)</i> and that of ice is <i><b>2090</b>J/(kg·°C)</i>. Assume also that the amount needed to melt ice at <i><b>0</b>°C</i> is <i><b>333,000</b>J/kg</i>.<br/><br/><i>Answer</i><br/><i><b>E</b><sub>warm</sub><b> = 2090·0.1·10 = 2090</b>J</i><br/><i><b>E</b><sub>melt</sub><b> = 333000·0.1 = 33300</b>J</i><br/><i><b>E</b><sub>need</sub><b> = 2090 + 33300 = 35390</b>J</i><br/><i><b>E</b><sub>water</sub><b> = 4183·M·20 = 83660·M</b></i><br/><i><b>E</b><sub>water</sub><b> = E</b><sub>need</sub></i><br/><i><b>35390 = 83660·M</b></i><br/><i><b>M = 0.423</b>kg</i><br/><br/> Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-62844118903892419332019-06-10T16:28:00.001-07:002019-06-10T16:28:10.421-07:00Unizor - Physics4Teens - Energy - Heat - Heat & Temperature<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/QQOqz5qTRXE" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Heat and Temperature</u><br /><br /><i>Specific Heat Capacity</i><br /><br />Let's discuss the relationship between <i>heat</i> and <i>temperature</i>.<br />From the unscientific standpoint these two concepts are almost identical. Heating an object results in increase of its temperature. Increasing a temperature of an object constitutes its heating.<br />Yet, from the strictly scientific viewpoint these two concepts are different.<br /><br /><i>Heat</i> is energy of some specific type, that can be transferred from one object to another, while <i>temperature</i> is an average kinetic energy of molecules of an object.<br />The same amount of <i>heat</i>, transferred into different objects, will result in different growth of their <i>temperatures</i>, depending on many factors, like mass, chemical composition, state etc. of these objects. Analogy of this is that same amount of fuel in different cars results in achieving very different speeds and, therefore, different kinetic energies in different cars, even if the gas pedal is pushed all the way down for all cars. It's just because cars are different and their internal structure converts fuel into movement differently.<br /><br />It has been experimentally observed that to increase a temperature of an isolated object of a unit mass by a unit of temperature is independent (within reasonable level of precision) of the initial temperature of an object, but depends only on the type of object's material, composition, state etc. In other words, amount of heat needed to increase a temperature of 1 kg of water from 20°C to 21°C is the same as from 50°C to 51°C. If, instead of water, we take copper, the amount of heat, needed to increase its temperature from 20°C to 21°C, will be the same as to increase it from 50°C to 51°C, but different than that for water.<br /><br />The above experimental fact allowed to establish a concept of <i><b>specific heat capacity</b></i> for each material as an <b>amount of heat required to increase the temperature of a unit of mass of this material (1 kg in SI) by a unit of temperature (1°C or 1°K in SI)</b>.<br /><br />Thus, <i>specific heat capacity</i> of water is, as we know, one kilocalorie per kilogram per degree - <i>1 kcal/(kg·°K</i>), that is about 4184 joules per kilogram per degree - <i>4183 J/(kg·°K</i>).<br />For copper the specific heat capacity is <i>385 J/(kg·°K</i>).<br />Gold has the specific heat capacity of <i>129 J/(kg·°K</i>).<br />Uranium's specific heat capacity is <i>116 J/(kg·°K</i>).<br />Cotton's specific heat capacity is <i>1400 J/(kg·°K</i>).<br />Hydrogen's specific heat capacity is <i>14304 J/(kg·°K</i>).<br />Generally speaking, but not always, more dense, more solid materials have less specific heat capacity than less dense or liquids, which have, in turn, less specific heat capacity than gases.<br /><br />Knowing <i>specific heat capacity</i><b><i>C</i></b> of material of an object and its mass <b><i>m</i></b>, we can easily determine amount of energy Δ<b><i>Q</i></b>needed to heat it up by Δ<b><i>T</i></b>degrees:<br />Δ<b><i>Q = C·m·</i></b>Δ<b><i>T</i></b><br />Inversely, knowing the amount of heat supplied, we can determine an increase in temperature:<br />Δ<b><i>T = </i></b>Δ<b><i>Q<span style="font-size: medium;">/</span>(C·m)</i></b><br />Notice, that increment of temperature Δ<b><i>T</i></b> and increment of heat energy Δ<b><i>Q</i></b> can be both positive or both negative, which means that an object, that has increased its temperature, has increased (gained, consumed) energy, and the object that decreased its temperature, has decreased (lost, released) energy.<br /><br /><i>Change of State</i><br /><br />Consider specific heat capacity of ice and water:<br />Ice: <i>2090 J/(kg·°K)</i><br />Water: <i>4183 J/(kg·°K)</i><br />Both these substances exist at temperature about 0°C=273°K. That means that we can heat <nobr>1 kg</nobr> of ice up to 0°C spending 2090 joules per each degree of temperature, but to increase the temperaature of the water around 0°C we have to spend 4183 joules per each degree. But, while ice is melting into water, which takes some time, both states, solid and liquid, exist side by side and the temperature of the water will not rise while the ice is not completely melted, we have to spend heat energy just on melting without actually changing the temperature of a substance, that remains around 0°C during the melting process.<br /><br />This experimental observation leads us to believe that, while graduate change of temperature for any specific state of matter linearly depends on the amount of heat supplied, change of state (like <b>melting</b> or <b>freezing</b>, or <b>evaporating</b> etc.) brings an element of non-linearity to this dependency.<br />More precisely, the temperature, as a function of amount of heat supply, in case of an object going through transformation of state from solid to liquid, looks like this (for better view, right click on the picture and open it in another tab of a browser):<br /><img src="http://www.unizor.com/Pictures/HeatTemperature.png" style="height: 150px; width: 200px;" /><br />As seen on this graph, supplying heat to ice will increase its temperature proportionally to amount of heat supplied.<br />Then, when the temperature reaches 0°C, ice will start melting and new heat will not change the temperature of ice/water mix, but will be used to change the state of matter from solid (ice) to liquid (water).<br /><br />This process of melting consumes heat without increasing the temperature until all ice is melted. Such a transformation of state that requires supply of heat energy is called <i><b>endothermic</b></i>.<br />Then, when the process of melting is complete, and all ice is transformed into water, the temperature of water will start increasing, as the new heat is supplied, but with a different coefficient of linearity relatively to the heat than in case of ice, since water's <i>specific heat capacity</i> is different than that of ice.<br />The graph above is characteristic to any heating process, where the transformation of the state of matter is involved. It is relatively the same for transforming liquid to gas (evaporation) or solid to gas (sublimation).<br /><br />In case of a reversed transformation from liquid to solid (freezing) or gas to liquid (condensation) or gas to solid (deposition) the heat energy must be taken away from a substance. It is the same amount in absolute value as was needed to supply to solid to melt it into liquid or to liquid to vaporise it into gas, or to solid to vaporize it into gas. So, in case of freezing, condensation or deposition we deal with the process of decreasing heat energy of a substance. Such a process is called <i><b>exothermic</b></i>.<br /><br />Amount of heat energy needed to transform a substance from one state to another is also experimentally determined and, obviously, depends on a substance, its mass and a kind of transformation it undergoes.<br />For example, melting of 1 kg of ice requires 333,000 joules of heat energy to be supplied. The same amount of heat energy should be extracted from the water at 0°C to freeze it into ice.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-89101864694300283532019-06-10T14:19:00.001-07:002019-06-10T14:19:39.219-07:00Unizor - Physics4Teens - Energy - Heat - Measuring<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/5ekDwNd0PCo" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Measuring Heat - Calorie</u><br /><br />Heat and temperature were known and researched by scientists long before the molecular movement was determined as their essence. As a result, attempts of measuring the amount of heat were unrelated to kinetic energy of the molecules.<br /><br />The unit of amount of heat was defined as amount of heat needed to increase the temperature of one gram of water by one degree Celsius (or Kelvin), and this unit of amount of heat was called a <i>calorie (cal)</i>.<br /><br />Obviously, this definition has its flaws. For example, the amount of heat needed to heat the same amount of water by the same temperature depends where exactly on Earth we are. The higher we are above the sea level - the less heat is needed. It also depends on the chemical purity of water. Also it's not obvious that the amount of heat needed to heat the water from 1°C to 2°C is the same as amount of heat needed to heat it from 88°C to 89°C, though within reasonable level of precision we do assume that this is true.<br /><br />With the development of molecular theory of heat and establishing relationship between heat and kinetic energy of molecules there was a need to put into correspondence existing units of heat (<i>calories</i>) and units of mechanical energy (<i>joules</i>). A simple experiment allowed to do just that.<br /><br />Imagine a standing on the ground large reservoir with known amount of water <i><b>M</b></i> of depth <i><b>H</b></i> from the surface to the bottom, kept at certain known temperature <i><b>T</b></i>°, and a relatively small stone of known mass <i><b>m</b></i>, having the same temperature <i><b>T</b></i>°, kept on the level of the surface of the water in a reservoir. This stone has certain known amount of potential energy <i><b>E=m·g·H</b></i>relatively to the bottom level of a reservoir.<br />Now we let the stone go down the reservoir to its bottom. Its potential energy relatively to the bottom of a reservoir decreases to zero. Where did the potential energy go? It's used to stir the water, thereby increasing the kinetic energy of its molecules.<br /><br />Because of this more intense movement of molecules, the water will increase its temperature. Potential energy of a stone will turn into kinetic energy of the molecular movement of water. If the temperature of the water has risen by Δ<i><b>T</b></i>°, potential energy of a stone <i><b>m·g·H</b></i> (in <i>joules</i>) equals to <i><b>M</b></i>·Δ<i><b>T</b></i> (in <i>calories</i>).<br /><br />Precise experiments like the above allowed to determine the correspondence between historical measure of the amount of heat in <i>calories</i> under different conditions and contemporary one in units of energy in SI - <i>joules</i>. This correspondence had been established approximately as<br /><i><b>1 calorie = 4.184 joules</b></i><br />but under different conditions (initial temperature, air pressure etc.) it might be equal to a slightly different value.<br /><br />Besides <i>calorie</i>, which is a relatively small amount of heat, the unit <i>kilocalorie (kcal)</i> had been introduced.<br />As is obvious from its name, <nobr><i><b>1 kilocalorie = 1000 calories</b></i>.</nobr><br /><br />The amount of energy contained in food and in some other practical cases very often is measured in <i>kilocalories</i>, which sometimes are called <i>large calories</i>, while a <i>calorie</i> is sometimes called <i>small calorie</i>. Unfortunately, the word "large" in many cases is omitted, which might cause misunderstanding.<br />From the experimental viewpoint, <i><b>1 kcal</b></i> is amount of heat needed to heat <i><b>1 kg</b></i> of water by one degree Celsius (or Kelvin).<br /><br />To avoid problems with the definition of <i>calorie</i> as amount of heat needed to warm up one gram of water by one degree Celsius, the contemporary <b>scientific definition</b> of <i>thermocalorie</i> is<br /><i><b>1 thermocalorie = 4.1833 joules</b></i>Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-79690377419782082032019-06-03T14:31:00.003-07:002019-06-03T14:32:23.557-07:00Unizor - Physics4Teens - Energy - Ideal Gas Kinetics - Problems <a href="https://youtu.be/MYEgnMaBJ0w"></a> <i>Notes to a video lecture on http://www.unizor.com</i><br/><br/><u>Problem 1</u><br /><br />How much kinetic energy have all the molecules in the room?<br/>How fast should an average size car move to have this amount of kinetic energy?<br/><br/>Assume the following:<br/>(a) the room dimensions are <i>4</i>x<i>4</i>x<i>3 meters</i> (that is, <i><b>V=48m³</b></i>);<br/>(b) normal atmospheric pressure is <i>100,000 Pascals</i> (that is, <i><b>p=100,000N/m²</b></i>);<br/>(c) a mass of an average size car is <i>2,000 kg</i> (that is, <i><b>M=2,000kg</b></i>).<br/><br/><i>Solution</i><br/><br/>From the lecture on <i>Kinetics of Ideal Gas</i> we know the relationship between the pressure on the walls of a reservoir, volume of a reservoir and total kinetic energy of gas inside this reservoir:<br/><i><b>p = (2/3)E</b><sub>tot </sub><b><font size=5>/</font>V</b></i><br/><br/>From this we derive a formula for total kinetic energy:<br/><i><b>E</b><sub>tot</sub><b> = (3/2)·p·V</b></i><br/><br/>Substituting the values for pressure and volume, we obtain<br/><i><b>E</b><sub>tot</sub><b> = (3/2)·100,000·48 =<br/>= 7,200,000</b>(joules)</i><br/><br/>The kinetic energy of a car is <br/><i><b>E = M·v²<font size=5>/</font>2</b></i><br/>Therefore, given the kinetic energy and mass, we can determine the car's speed:<br/><i><b>v = √<span style='text-decoration:overline'>2·E/M</span></b></i><br/>Substituting calculated above <i><b>E</b><sub>tot</sub><b>=7,200,000</b>(J)</i> for <i><b>E</b></i> and the value for mass <i><b>M=2,000</b>(kg)</i>, we obtain<br/><i><b>v = √<span style='text-decoration:overline'>2·7,200,000/2,000</span> ≅<br/>≅ 85</b>(m/sec)<b> ≅<br/>≅ 306</b>(km/hour)<b> ≅<br/>≅ 190</b>(miles/hour)</i><br/><br/><br/><u>Problem 2</u><br /><br />Given the temperature, pressure and volume of the air in a room, determine the number of gas molecules in it.<br/><br/>Assume the following:<br/>(a) the room dimensions are <i>4</i>x<i>4</i>x<i>3 meters</i> (that is, <i><b>V=48m³</b></i>);<br/>(b) normal atmospheric pressure is <i>100,000 Pascals</i> (that is, <i><b>p=100,000=10<sup>5</sup>N/m²</b></i>);<br/>(c) temperature is <i>20°C</i> (that is, <i><b>T=20+273=293°K</b></i>).<br/><br/><i>Solution</i><br/>Recall the combined law of ideal gas<br/><i><b>p·V<font size=5>/</font>T = k</b><sub>B</sub><b>·N = const</b></i><br/>where<br/><i><b>k</b><sub>B</sub><b> = 1.381·10<sup>−23</sup> (J/°K)</b></i> is Boltzmann's constant and<br/><i><b>N</b></i> is the number of gas molecules in a reservoir.<br/>From this we derive the number of molecules<br/><i><b>N = p·V<font size=5>/</font>(k</b><sub>B</sub><b>·T)</b></i><br/>Substituting the values,<br/><i><b>N = 10<sup>5</sup>·48<font size=5>/</font>(1.381·10<sup>−23</sup>·293) = 0.12·10<sup>28</sup></b></i><br/>It's a lot!<br/><br/>Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-36808442494385698552019-05-06T12:07:00.001-07:002019-05-06T12:07:05.904-07:00Unizor - Physics4Teens - Energy - Heat - Temperature, Pressure, Volume o...<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/jPWoBQEEH4s" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Temperature, Pressure<br />and Volume of Ideal Gas</u><br /><br /><i>Temperature</i> is an observable macro property of an object. It's related to a particular instrument we use to measure this property.<br />Let's examine the mechanism of this measurement using a classic mercury-based or alcohol-based thermometer.<br /><br />Our first step in measurement is to make a physical contact between a thermometer and an object of measurement (for example, a human body or air in a room). When accomplished, we expect that the measurement of a thermometer would correspond to a state of average kinetic energy of molecules of an object. The reason it happens (and it takes some time to happen) is that on a micro level molecules at the surface of an object are colliding with molecules at the surface of a thermometer, and exchange the kinetic energy, eventually equalizing it. The molecules close to a surface, in turn, collide with surface molecules and also eventually equalize their average kinetic energies. This process continues until the average level of kinetic energy in all parts of an object and in a thermometer equalize.<br /><br />What is important in this case is that the total amount of kinetic energy of all molecules of an object and a thermometer remains the same. So, if an object has more intense movement of molecules and a thermometer's molecules are moving slower, the kinetic energy is transferred from an object to a thermometer. If the molecules of a thermometer are, on average, faster, then the exchange of kinetic energy will be from a thermometer to an object.<br .="" br="" /> In any case, the average kinetic energy of molecules of both an object and a thermometer equalize.<br /><br />An important consideration is that the contact between an object and a thermometer changes the average level of kinetic energy in both. The process of measuring, therefore, is not completely neutral towards an object. However, what happens in most cases is that the number of molecules inside an object we measure is usually significantly greater that the number of molecules in a thermometer. As a result, equalizing the average kinetic energy of all molecules does not significantly change the level of average kinetic energy of molecules of an object, and the level of average kinetic energy of the molecules of a thermometer is a good representation of this characteristic of an object.<br /><br />As explained in the <i>Heat and Energy</i> lecture of this course, the temperature in mercury or alcohol thermometers is an observable expansion of the volume of liquid inside a thermometer. We also indicated in that lecture that this thermo-expansion is proportional to an average of squares of velocities of molecules, that is proportional to average kinetic energy of the molecules of a thermometer, which, in turn, is equalized with average kinetic energy of molecules of an object.<br /><br />Thus, by observing the expansion of liquid in a thermometer we measure the average kinetic energy of molecules of an object, which allows us to write the following equation:<br /><i><b>T ≅ </b>AVE(<b>E</b><sub>kin</sub>)<b> = E</b><sub>ave</sub></i><br />where <i><b>T</b></i> is an observable level of liquid inside a thermometer in some units (that is, <i>temperature</i>) and<br /><i>AVE(<b>E</b><sub>kin</sub>)<b> = E</b><sub>ave</sub></i> is average kinetic energy of molecules of an object.<br />This looks more natural in a form<br /><i>AVE(<b>E</b><sub>kin</sub>)<b> = E</b><sub>ave</sub><b> ≅ T</b></i><br />since average kinetic energy of molecules <i><b>E</b><sub>ave</sub></i> (micro characteristic) cannot be easily observed, while the temperature <i><b>T</b></i> (macro characteristic) can.<br /><br />So, we have established that the temperature (in some units, starting from absolute zero) and average kinetic energy of molecules are proportional. The coefficient of proportionality between the temperature and average kinetic energy of molecules remains unknown and is different for different substances.<br /><br />Obviously, the measure of liquid in a thermometer should be calibrated and, for this equation to be true, we have to assign the zero temperature to a state of an object when all its molecules are at rest, which happens when there is no source of energy around, like in open space far from stars.<br />The convenient scale is the Kelvin scale with zero temperature on this scale corresponding to this state of molecules at complete rest and a unit of measurement of the temperature is a <i>degree</i> with the distance between the temperature of melting ice and boiling water assigned as 100 degrees on this scale.<br /><br />From the previous lecture about kinetics of <i>ideal gas</i> we know the relationship between the <i>pressure</i> of ideal gas on the walls of a reservoir, the <i>volume</i>of a reservoir and <i>average kinetic energy of the molecules</i><br /><i><b>p = (2/3)N·E</b><sub>ave </sub><b><span style="font-size: large;">/</span>V</b></i><br />where<br /><i><b>p</b></i> is the gas pressure against the walls of a reservoir,<br /><i><b>N</b></i> is the number of gas molecules in a reservoir,<br /><i><b>V</b></i> volume of a reservoir,<br /><i><b>E</b><sub>ave</sub></i> is average kinetic energy of molecules,<br /><br />Alternatively, it can be written as<br /><i><b>E</b><sub>ave</sub><b> = (3/2)·p·V<span style="font-size: large;">/</span>N</b></i><br /><br />Comparing this with a derived above relationship between <i>temperature</i> (counting from the absolute zero) and <i>average kinetic energy of molecules</i>, we have established a relationship<br /><i><b>E</b><sub>ave</sub><b> = (3/2)·p·V<span style="font-size: large;">/</span>N ≅ T</b></i><br />and<br /><i><b>p·V<span style="font-size: large;">/</span>N = k·T</b></i><br />where <i><b>k</b></i> is an unknown coefficient of proportionality.<br /><br />The only thing that prevents us from determining average kinetic energy of molecules by its temperature is an unknown coefficient of proportionality <b><i>k</i></b>.<br /><br />Now we will concentrate attention on gases, as an object of measuring temperature and average kinetic energy of molecules.<br />Different gases have different molecules and different molecular mass. Using certain theory and using chemical and physical experiments, we can compare the masses of different molecules and even measure this mass in certain "units of mass" called <i>atomic mass units</i>.<br />For such a unit of molecular mass scientists used 1/12 of a mass of a single atom of carbon. So, in this system of units hydrogen molecule of 2 hydrogen atoms <i><b>H<sub>2</sub></b></i> has approximate mass of 2, oxygen molecule of two oxygen atoms <i><b>O<sub>2</sub></b></i> - approximately 32, carbon dioxide molecule <i><b>CO<sub>2</sub></b></i> had a measure of, approximately, 44 atomic mass units, etc.<br /><br />Using this measurement, we can always establish experiments with the same number of molecules of different gases. For example, if the mass of certain amount of oxygen (atomic mass of molecules <i><b>O<sub>2</sub></b></i> is 32) is 16 times greater than the mass of certain amount of hydrogen (atomic mass of molecules <i><b>H<sub>2</sub></b></i> is 2), we can assume that the number of molecules in both cases is the same.<br /><br />It has been experimentally established that, if the same number of different gas molecules are placed in reservoirs of the same volume and hold them at the same temperature, the pressure on the walls in both cases will be the same. Alternatively, if the pressure is the same, the temperature will be the same too.<br />In other words, the coefficient <b><i>k</i></b>in formula<br /><i><b>p·V<span style="font-size: large;">/</span>N = k·T</b></i><br />does not depend on the type of gas we deal with, it's a universal constant called the <i>Boltzman's constant</i>, which is equal to<br /><i><b>k</b><sub>B</sub><b> = 1.381·10<sup>−23</sup> (J/°K)</b></i><br />This was the reason to introduce a concept of <i>ideal gas</i>. All gases are, approximately, ideal to a certain degree of precision. This is related to the fact that molecules of the gas are flying with high speeds and on large distances from each other, much larger than their geometric sizes.<br /><br />Now we can write the equation between the temperature <i><b>T</b></i> (in degrees <b><i>°K</i></b> from absolute zero), average kinetic energy of molecules (in units of SI <i>joules J</i>), pressure <i><b>p</b></i> (in units of SI <i>newton/m²</i>), volume <i><b>V</b></i> (in <i>m³</i>) and number of molecules in a reservoir for <i>ideal gas</i>:<br /><i><b>E</b><sub>ave</sub><b> = (3/2)·p·V<span style="font-size: large;">/</span>N = (3/2)·k</b><sub>B</sub><b>·T</b></i><br /><br />Consequently, if we are dealing with certain fixed amount of gas (<i><b>N</b></i> molecules) then<br /><i><b>p·V<span style="font-size: large;">/</span>T = k</b><sub>B</sub><b>·N = const</b></i><br />That means that changing the pressure, volume and temperature of the same amount of gas preserves the expression <i><b>p·V<span style="font-size: large;">/</span>T</b></i><br />which is called the <b>Combined Ideal Gas Law</b>.<br /><br />For example, <u>if the absolute temperature remains the same</u>, but volume taken by certain amount of gas increases (decreases) by some factor, the pressure will decrease (increase) by the same factor, that is pressure and volume are inversely proportional to each other (<b>Boyle-Mariotte's Law</b>).<br /><br /><u>If the pressure remains the same</u>, but the volume taken by certain amount of gas increases (decreases) by some factor, the absolute temperature will increase (decrease) by the same factor, that is volume and absolute temperature are proportional to each other (<b>Charles' Law</b>).<br /><br /><u>If the volume taken by certain amount of gas remains the same</u>, but the absolute temperature increases (decreases) by some factor, the pressure will increase (decrease) by the same factor, that is absolute temperature and pressure are proportional to each other (<b>Gay-Lussac's Law</b>).<br /><br />Out of curiosity, let's use the formula<br /><i><b>E</b><sub>ave</sub><b> = (3/2)·k</b><sub>B</sub><b>·T</b></i><br />to calculate how fast the molecules of oxygen are flying in the room at some normal temperature.<br />Assume the pressure at the ground level is about <i>100,000N/m²</i> and the temperature in the room is about <i>20°C=293°K</i>. Then the average kinetic energy of a molecule of oxygen is<br /><i><b>E</b><sub>ave</sub><b> = (3/2)·1.381·10<sup>−23</sup>·293 =<br />= 6.06·10<sup>−21</sup> J</b></i><br />Mass of a molecule of oxygen <i><b>O<sub>2</sub></b></i> is <i><b>m=5.31·10<sup>−26</sup> kg</b></i><br />From the formula for kinetic energy <i><b>E=m·v²/2</b></i> we derive the average of squares of velocities of oxygen molecules as<br /><i>AVE(<b>v²</b>)<b> = 2·E</b><sub>ave </sub><b>/m =<br />= 2·6.06·10<sup>−21</sup>/5.31·10<sup>−26</sup> =<br />= 2.28·10<sup>5</sup></b></i><br />Therefore, the average speed of oxygen molecule will be equal to a square root of this number:<br /><i>AVE(<b>v</b>)<b> = 478 m/sec</b></i><br />Pretty fast moving! Take into consideration, however, that real oxygen molecules, as molecules of any real gas, are chaotically colliding with other and change the direction all the time.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-72352227384622028192019-05-01T20:26:00.001-07:002019-05-01T20:26:41.656-07:00Unizor - Physics4Teens - Energy - Heat - Kinetics of Ideal Gas<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/cIDMFCcne4E" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Kinetics of Ideal Gas</u><br /><br />In this lecture we will discuss a concept of <i>pressure</i> of gas, enclosed in some reservoir, against the walls of this reservoir.<br /><br />Usually, speaking about pressure, we have in mind an object of certain weight on a horizontal table, in which case the constant force of its weight exerts a pressure on a table equal to this weight divided by an area of the table occupied by an object or, as it's sometimes presented, the weight per unit of area.<br /><br />As an introduction to kinetics of gases, let's consider a reservoir shaped as a cube with sides aligned parallel to coordinate planes and with the length of each edge <i><b>L</b></i>.<br />Assume that there is a single molecule of gas of mass <i><b>m</b></i>flying between the opposite walls perpendicularly to them, elastically reflecting off these walls.<br />Keep in mind that, according to the Third Newton's Law, the force exerted by the wall towards the molecule, which causes the reflection of a molecule from the wall, is equal to the force exerted by a molecule towards the wall, which causes <i>pressure</i>.<br /><br />The area of each of these opposite walls equals to <i><b>L²</b></i>, which will be used to calculate the <i>pressure</i>.<br /><br />In this case of a single gas molecule, flying between the opposite walls of a reservoir, the force is variable. It exists during the time when a molecule hits the wall and then the force becomes zero until the next time this molecule hits the same wall.<br />A proper definition of <i>pressure</i>in this case is based on a concept of the average force during certain amount of time.<br /><br />Firstly, let's evaluate the average force during a single period of oscillation of a molecule between walls from a moment of time it's near one wall to a next moment it's at this position after flying to the opposite wall and returning back.<br /><br />As we know, a change of a momentum of an object equals to an impulse of the force that caused this change<br /><i><b>m·v</b><sub>end</sub><b> − m·v</b><sub>beg</sub><b> = F·τ</b></i><br />where<br /><i><b>m</b></i> is a mass of an object<br /><i><b>v</b><sub>beg</sub><b></b></i> is the velocity of an object in the beginning of the action of the force<br /><i><b>v</b><sub>end</sub><b></b></i> is the velocity of an object at the end of the action of the force<br /><i><b>F</b></i> is the force, acting on an object<br /><i><b>τ</b></i> is the time duration of the force, acting on an object.<br />Keep in mind that velocity and force are vectors in the above equation.<br /><br />In our case we consider one period of oscillation of a molecule as the time period of <i>average force</i> acted on it.<br />So, <i><b>τ</b></i> is the time between two consecutive events when a molecule is at the wall opposite to the one it collides with:<br /><i><b>τ = 2L/|v|</b></i>.<br />where <i><b>|v|</b></i> is now an absolute speed of a molecule (scalar).<br /><br />Since we are considering elastic reflection of a molecule in the opposite direction to its initial trajectory,<br /><i><b>v<sub>end</sub> = −v<sub>beg</sub></b></i><br />and the equation above can be written in scalar form<br /><i><b>2m·|v| = |F|·τ = |F|·2L/|v|</b></i><br />from which we determine the absolute average force<br /><i><b>|F| = m·|v|²<span style="font-size: medium;">/</span>L</b></i><br />Mass <i><b>m</b></i> of a molecule is, obviously, a constant during this process.<br />The average pressure of a molecule on a wall is the average force divided by the area of a wall:<br /><i><b>p = m·|v|²<span style="font-size: medium;">/</span>L³ = m·|v|²<span style="font-size: medium;">/</span>V =<br />= 2E</b><sub>kin </sub><b><span style="font-size: medium;">/</span>V</b></i><br />where <i><b>V</b></i> is the volume of a reservoir and <i><b>E</b><sub>kin</sub></i> is a <b>kinetic energy</b> of a molecule.<br /><br />Our first step towards a comprehensive theory is to consider a case of many molecules flying parallel to each other in the same reservoir as above, but with, generally speaking, different speeds. Obviously, the pressure against the wall will be greater because each molecule contributes its own force against the wall during a collision.<br />If <i><b>v<sub>i</sub></b></i> is a speed of the <i><b>i</b><sup>th</sup></i>molecule, the combined pressure will be<br /><i><b>p =</b></i> <span style="font-size: large;">Σ</span><i><b>p<sub>i</sub> =</b></i> <span style="font-size: large;">Σ</span><i><b>m·v<sub>i</sub>²<span style="font-size: large;">/</span>V =<br />= 2E</b><sub>tot </sub><b><span style="font-size: large;">/</span>V</b></i><br />where <i><b>E</b><sub>tot</sub></i> is a <b>total kinetic energy of all molecules</b> moving parallel to each other.<br /><br />Considering the number of molecules <i><b>N</b></i> remains the same, it's more convenient to express this formula in terms of an <b>average of squares of molecular speed</b> and <b>average kinetic energy</b> of molecules:<br /><i><b>v²</b><sub>ave</sub><b> = (1/N)</b></i><span style="font-size: large;">Σ</span><i><b>v<sub>i</sub>²</b></i><br /><i><b>E</b><sub>ave</sub><b> = (1/N)</b></i><span style="font-size: large;">Σ</span><i><b>E<sub>i</sub></b></i><br />Using this <b>average of squares of molecular speed</b>, the pressure is<br /><i><b>p = N·m·v²</b><sub>ave </sub><b><span style="font-size: large;">/</span>V = 2N·E</b><sub>ave </sub><b><span style="font-size: large;">/</span>V</b></i><br />which brings us to a principle of <b>proportionality between pressure of the gas against the walls of a reservoir and an average of squares of the molecular speed or to an average of kinetic energy of the molecules</b>.<br /><br />Granted, we showed this only for a case of all molecules moving between two opposite walls parallel each other and perpendicularly to the walls they collide with.<br /><br />Let's make another step towards comprehensive theory and consider the chaotic movement of all gas molecules.<br />To quantitatively approach molecular movement, we will use a model of an <i>ideal gas</i>.<br />This model assumes that molecules of <i>ideal gas</i> are point-mass objects completely chaotically moving in all directions with equal probabilities and elastically colliding at random times among themselves or with all the walls of a reservoir that contains this gas.<br /><br />We further assume that no interacting forces (like gravity or electromagnetic forces) exist between them. So, only <i>kinetic energy</i> of these molecules plays the role in evaluation of the characteristics of the ideal gas.<br /><br />Every vector of velocity of any molecule can be represented as a sum of three vectors along the XYZ-coordinates<br /><i><b><span style="text-decoration-line: overline;">v</span> = <span style="text-decoration-line: overline;">v</span><sub>x</sub>+<span style="text-decoration-line: overline;">v</span><sub>y</sub>+<span style="text-decoration-line: overline;">v</span><sub>z</sub></b></i><br /><br />So, every movement of a molecule can be represented as simultaneous movement in three different directions. Depending on which wall of the reservoir is hit by a molecule, its <i>pressure</i>against the wall is determined only by one component - the one that is perpendicular to the surface of a wall.<br /><br />If molecules inside the reservoir are moving completely chaotically, they are hitting all walls with approximately the same frequency. So, for each of the six walls of a cube we can use the same logic as above for one side, except for the walls parallel to YZ coordinate plane the pressures <i><b>p<sub>x</sub></b></i> will be proportional to the average of <i><b>v<sub>x</sub>²</b></i>, for the walls parallel to XZ coordinate plane the pressures <i><b>p<sub>y</sub></b></i> will be proportional to the average of <i><b>v<sub>y</sub>²</b></i> and for the walls parallel to XY coordinate plane the pressures <i><b>p<sub>z</sub></b></i> will be proportional to the average of <i><b>v<sub>z</sub>²</b></i>.<br /><br />Considering completely chaotic character of the molecular motion in the <i>ideal gas</i>, the three pressures <i><b>p<sub>x</sub></b></i>, <i><b>p<sub>y</sub></b></i> and <i><b>p<sub>z</sub></b></i>must be the same. Similarly, averages of <i><b>v<sub>x</sub>²</b></i>, <i><b>v<sub>y</sub>²</b></i> and <i><b>v<sub>z</sub>²</b></i> must be also the same.<br /><br />Since <i><b><span style="text-decoration-line: overline;">v</span> = <span style="text-decoration-line: overline;">v</span><sub>x</sub>+<span style="text-decoration-line: overline;">v</span><sub>y</sub>+<span style="text-decoration-line: overline;">v</span><sub>z</sub></b></i>, according to three-dimensional equivalent of Pythagorean Theorem,<br /><i><b>v² = v<sub>x</sub>² + v<sub>z</sub>² + v<sub>z</sub>²</b></i><br />and, similarly, for averages<br /><i><b></b>AVE(<b>v²</b>)<b> = v²</b><sub>ave</sub><b> =<br />=</b>AVE(<b>v<sub>x</sub>²</b>)<b>+</b>AVE(<b>v<sub>y</sub>²</b>)<b>+</b>AVE(<b>v<sub>z</sub>²</b>)</i><br /><br />From the equality of averages for the ideal gas we come to the following equalities:<br /><i><b>v²</b><sub>ave</sub><b> = 3·</b>AVE(<b>v<sub>x</sub>²</b>)</i><br /><i><b></b>AVE(<b>v<sub>x</sub>²</b>)<b> = (1/3)·v²</b><sub>ave</sub><b></b></i><br /><br />Therefore,<br /><i><b>p<sub>x</sub> = N·m·</b>AVE(<b>v<sub>x</sub>²</b>)<b><span style="font-size: large;">/</span>V =<br />= (1/3)N·m·v²</b><sub>ave </sub><b><span style="font-size: large;">/</span>V =<br />= (2/3)N·E</b><sub>ave </sub><b><span style="font-size: large;">/</span>V</b></i><br /><br />Similarly,<br /><i><b>p<sub>y</sub> = N·m·</b>AVE(<b>v<sub>y</sub>²</b>)<b><span style="font-size: large;">/</span>V =<br />= (1/3)N·m·v²</b><sub>ave </sub><b><span style="font-size: large;">/</span>V =<br />= (2/3)N·E</b><sub>ave </sub><b><span style="font-size: large;">/</span>V</b></i><br />and<br /><i><b>p<sub>z</sub> = N·m·</b>AVE(<b>v<sub>z</sub>²</b>)<b><span style="font-size: large;">/</span>V =<br />= (1/3)N·m·v²</b><sub>ave </sub><b><span style="font-size: large;">/</span>V =<br />= (2/3)N·E</b><sub>ave </sub><b><span style="font-size: large;">/</span>V</b></i><br /><br />And, since pressures on all walls are the same in the ideal gas,<br /><i><b>p = (1/3)N·m·v²</b><sub>ave </sub><b><span style="font-size: large;">/</span>V =<br />= (2/3)N·E</b><sub>ave </sub><b><span style="font-size: large;">/</span>V = (2/3)E</b><sub>tot </sub><b><span style="font-size: large;">/</span>V</b></i><br /><br />The expression of a <i>pressure</i> in terms of average kinetic energy and volume is more general than in terms of mass, average of squares of molecular speed and volume, as it encompasses a case with molecules of different masses.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-76393475022064104892019-04-17T06:40:00.001-07:002019-04-17T06:40:36.534-07:00Unizor - Physics4Teens - Energy - Temperature and Kinetic Energy<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/0wO3LmBTN7c" width="480"></iframe><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Heat, Temperature &<br />Kinetic Energy of Molecules</u><br /><br />The goal of this lecture is to demonstrate that <i>temperature</i>, as a measure of <i>heat</i>, is proportional to <i>average of a square of molecular speed</i>.<br /><br /><i>Temperature</i> is what we see on a thermometer that measures the intensity of the molecular movement. Let's examine this process of measurement using the classical mercury-based thermometer.<br /><br />The thermometer shows different values of temperature based on physical property of mercury to expand, as its molecules are moving faster. The reason for this is that, as the molecules move faster, they cover longer distance before they are stopped by collision with other molecules or the walls or by a surface tension forces. The faster molecules push against obstacles with greater forces, and that is the reason why the volume of hot mercury is larger than the volume of the same mass of cold mercury.<br /><br />Let's quantify this increase in volume.<br />There is a force of resistance that acts against the movement of the molecule, as it pushes its way through, that stops this movement. In some way it's analogous to throwing a stone vertically up against the force of gravity. The force of gravity eventually stops the movement upward, and the stone falls back on the ground.<br /><br />The height of the trajectory of a stone depends on the initial speed it has at the beginning of movement on the ground level. Assume, the initial speed is <i><b>V</b></i>. Then the kinetic energy in the beginning of motion is <i><b>M·V²/2</b></i>, where <i><b>M</b></i> is a mass of a stone. The potential energy relative to the ground in the beginning of motion is zero.<br />At the highest point at height <i><b>H</b></i>the kinetic energy is zero but potential is <i><b>M·g·H</b></i>, where <i><b>g</b></i> is a free fall acceleration.<br /><br />As we know, the total mechanical energy is constant in this case, from which follows that kinetic energy in the beginning of motion (with potential energy zero) equals to potential energy at the highest point (with kinetic energy zero):<br /><i><b>E<sub>kin</sub> = M·V²/2 = M·g·H</b></i><br />From this follows<br /><i><b>H = V²/(2g)</b></i><br />So, the height of the rise and the square of initial velocity are proportional to each other, independently of mass.<br /><br />Obviously, one mercury molecule does not do much, but when the heat is relatively evenly distributed among all molecules, their combined efforts push the surface of the mercury in the tube sufficiently noticeably, and the height it rises is proportional to average of a square of molecular speed.<br /><br />The thermometers are different, their design and construction differ, they all must be calibrated, but one thing remains - a principle of <b>proportionality between geometrical expansion (that is, temperature, as we read it on a thermometer's scale) and an average of squares of the molecular speed</b>.<br /><br />It is important to point out that this proportionality should be understood in absolute terms. That is, if the speed is zero, the temperature is zero as well. So, we are talking about temperature scales with zero representing <i>absolute zero</i> on the Kelvin's scale, like in space far from any source of energy.<br />The coefficient of proportionality, of course, depends on the units of measurement.<br /><br />So, if the average speed of molecules doubles, the temperature on the Kelvin's scale rises by a factor of 4.<br /><br />Inversely, if an object is at temperature 0°K (absolute zero), the kinetic energy of its molecule is zero, that is no molecular movements.<br />If we observe that the temperature has risen by a factor of 4, we can safely assume that the average speed of molecules doubles.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-25128332571872825572019-04-15T10:52:00.001-07:002019-04-15T10:52:03.781-07:00Unizor - Physics4Teens - Energy - Heat - Molecular Movement<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/AO9b5Mn9Eao" width="480"></iframe><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Molecular Movement</u><br /><br /><i><b>Molecules</b></i><br /><br />In order to understand the nature of <i>heat</i> and <i>temperature</i>, we have to go inside the objects we experiment with.<br />If we divide a drop of water into two smaller drops, each of these smaller drops will still be water. Let's continue dividing a smaller drop into even smaller and smaller. There will be a point in this process of division, when further division is not possible without changing a nature of the object, it will no longer be water.<br /><br />From what we know now, the water consists of hydrogen and oxygen - two gases, connected in some way. In our process of division of a drop of water, we will reach such a point that, if we divide this tiniest drop of water, the result will be certain amount of hydrogen and certain amount of oxygen, but no water.<br />That tiniest amount of water, that still preserves the quality of being water, is called a <b><i>molecule</i></b>.<br />Similarly, tiniest amount of any substance, that retains the qualities of this substance, is called a <b><i>molecule</i></b>.<br />Incidentally, if we divide a <i>molecule</i> of any substance, the result will be certain <n><i>atoms</i>, that possess completely different qualities.<br /><br />In our discussion about <i>heat</i> <u>we will not go deeper than the <i>molecules</i></u> because our purpose at this stage is to study the nature of <i>heat</i> as it relates to different objects and substances, so the preservation of the qualities of these objects and substances is important. That's why in this part of a course we will not cross the border between <i>molecular</i> and more elementary <i>atomic</i> level.<br /><br />Any object or substance we are dealing with consists of certain number of <i>molecules</i> - the smallest particles that retains the qualities of this object or substance. This number of <i>molecules</i>, by the way, for regular objects we see and use in practical life, is extremely large because the size of molecules is extremely small. We cannot see individual molecules with a naked eye. Only special equipment, different in different cases, can help us to see individual molecules. And, being so tiny, molecules of different substances are different in size among themselves.<br />And not only in size. Since the molecules contain different, more elementary particles called <i>atoms</i>, the configuration of these <i>atoms</i> that form a <i>molecule</i> is different for different <i>molecules</i>. Thus, a molecule of water contains two atoms of hydrogen and one atom of oxygen that connects hydrogen atoms into some three-dimensional construction. A molecule of protein consists of many different types of atoms and its structure and size are quite different from the molecule of water.<br /><br /><i><b>States of Matter</b></i><br /><br />The next topic we would like to address is the <i><b>states of matter</b></i>.<br />When the word <i>matter</i> is used in physics, it means any object or substance that occupies certain space and has certain mass, thus consisting of certain molecules interacting among themselves.<br /><br />There are three major <i>states of matter</i>: <b>solid</b>, <b>liquid</b> and <b>gas</b>.<br /><br />When an object is <b>solid</b> or is in <b>solid</b> state, it means that it retains its shape and form regardless of surrounding environment, not intended to change its form. The molecules of this object are strongly connected to each other. Their movement relative to each other is rather restricted. This movement can be oscillating around some point, maintaining an orderly three-dimensional structure, for <i>crystal</i> (or <i>crystalline</i> type of) solids or just slow movement, changing their relative position, but not changing the overall form for <i>amorphous</i> (<i>non-crystalline</i>) type.<br />Examples of solid objects are ice (crystal), steel (crystal), plastic cup (amorphous).<br /><br />When an object or substance is <b>liquid</b> or is in <b>liquid</b> state, it takes the shape of a vessel or reservoir it's in. The connection between the molecules in case of liquid is strong enough to hold the molecules together, but not strong enough to preserve the overall shape.<br />Examples of liquid substances are water, mercury ("quick silver"), oil.<br /><br />When an object or substance is <b>gas</b> or is in <i>gaseous</i> state, it takes as much space as it is available. Connections between the molecules are weak and they fly in all directions in completely chaotic fashion. Examples of gases are air, helium, oxygen.<br /><br />Some examples above represent objects or substances that contain only one type of molecules, like ice or mercury, or helium. Some other examples are objects or substances that contain more than one type of molecules mixed together, like steel, oil or air.<br /><br /><i><b>Nature of Heat</b></i><br /><br />Now we are in position to talk about <b><i>heat</i></b>.<br /><i>Heat</i> is the <i>energy of molecular motion</i> inside any object or substance.<br />As we mentioned, molecules are in constant motion inside any object. The more intense this motion is - the more <i>heat</i> this object possesses. This implies that <i>heat</i> is <i>mechanical energy</i>of molecules inside the object or <i>internal energy</i> of the object or substance.<br /><br />As we know, mechanical energy can be transferred from one object to another, like during the collision of two billiard balls. Similarly, mechanical energy of one molecule can be transferred to its neighbors, from them - to their neighbors etc. This is a process of <i>dissipation of heat</i>. All what's necessary for this is the relative proximity of the molecules. This is exactly the way how heat is transferred from one body to another, from flame to pot, from pot to water, from water to vegetables in it, making soup.<br /><br />Since <i>heat</i> or <i>internal energy</i> of an object is related to motion of its molecules, and increased heat means faster movement of the molecules, and, as we see, different states of matter are related to the strength of connection between the molecules, we can expect that the state of an object (solid, liquid, gas) might change with increasing or decreasing its internal energy by supplying or taking away the source of energy.<br />Indeed, it's true. Heat the ice - it will transform into water. Heat the water - it will transform into vapor. Heat the steel - it will melt. Freeze the helium - it will transform into liquid helium. Freeze the mercury - it will solidify.<br /><br />As we see, the same molecules can form objects in different states. It only depends on the amount of internal energy, that is amount of heat, the object possesses.<br /><br />Even without transformation from one state to another, heat causes certain changes in the object visible without any special instruments. We all know that mercury thermometer is working based on the property of mercury to expand as the temperature is rising.<br />This is a general property of most of the objects - to change physical dimensions with increase or decrease of amount of heat (internal energy) carried by their molecules during their constant motion.<br />This property is the principle, on which measuring of the intensity of molecular movement is based.<br /><br /><i><b>Heat and Temperature</b></i><br /><br />Now let's address the issue of measuring the heat, that is amount of internal energy inside any object.<br /><i>The term <b>temperature</b> is related to average intensity of the molecular movement inside an object or a substance. So, when we say that the <i>temperature</i> of an object has increased or decreased, we mean that average intensity of the molecular movement in it has increased or decreased correspondingly</i>.<br /><br />Our obvious task now is to quantitatively evaluate the temperature, thus measuring the intensity of the molecular movement inside an object.<br /><br />It would be great, if we knew kinetic energy of each molecule at each moment of time and average it up to get the temperature in the units of energy. Alas, it's impossible. We have to find some easier method, not necessarily 100% accurate, but sufficient for day-to-day practical purposes.<br /><br />Convenient instrument for this is a classic thermometer, whose indications are directly related to a change in physical size of objects with change of intensity of the molecular motion inside them.<br /><br />A simple thermometer consists of a small reservoir with mercury and thin tube coming from it, so the mercury level in the tube will go up with increase of intensity of the molecular motion of the mercury or down, when the intensity decreases.<br /><br />If we want to measure the temperature of any object, we bring it in contact with our thermometer and, when the temperatures equalize, which might take some time, the level of mercury in a tube of a thermometer will correspond to intensity of the molecular movement inside the object.<br /><br />All, which is left to establish is the scale and units of measure.<br />There are three major systems of measurement of temperature: Celsius, Fahrenheit and Kelvin. Celsius system is used everywhere, except United States and its territories. Fahrenheit system is used in United States and its territories. Kelvin system is used everywhere in scientific research and equations of Theoretical Physics.<br /><br />The unit of measurement in each system is called a <i>degree</i>and the temperature is written with an indication of the system as follows:<br />0°C, 20°C, -40°C for temperatures in Celsius system;<br />32°F, 68°F, -40°F for temperatures in Fahrenheit system;<br />273.15°K, 293.15°K, 233.15°K for temperatures in Kelvin system.<br />Above are the examples of three different temperatures in three different measurement systems, correspondingly.<br /><br />The conversion formulas are:<br />X°F = 5(X−32)/9°C<br />X°C = (X+273.15)°K<br /><br />In the Celsius system the temperature 0°C corresponds to the temperature of melting ice at the sea level on Earth. Temperature of 100°C is the temperature of boiling water at the sea level on Earth. This range is divided into 100 degrees making up a scale.<br />Degrees in the Fahrenheit system are also connected to some natural processes. 0°F is the temperature of freezing of some chemical solution, while 100°F is approximately the temperature of a human body.<br />Finally, in Kelvin system 0°K is so-called <i>absolute zero</i>temperature - the temperature of outer space far from any source of energy. The unit of one degree in Kelvin system equals to that of one degree in Celsius.</n>Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-81429910006220681492019-04-02T19:04:00.001-07:002019-04-11T05:00:52.589-07:00Unizor - Physics4Teens - Energy - Potential Energy - Gravity<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/WwG9DBUplko" width="480"></iframe><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Energy of Gravity</u><br /><br />An object of mass <b><i>m</i></b> is raised on certain height <b><i>h</i></b> above the surface of the Earth.<br />Analyze its potential energy, as a function of height <b><i>h</i></b>.<br />Do not assume that the force of gravity is constant, but rather use the Newton's Law of Universal Gravitation.<br />Assume that the radius of Earth is <b><i>R</i></b> and its mass is <b><i>M</i></b>.<br /><br /><i>Solution</i>:<br /><br />If <b><i>x</i></b> is the distance from the center of the Earth to an object, the force of gravity equals to<br /><b><i>F(x) = G·M·m<span style="font-size: medium;">/</span>x²</i></b><br />(where <b><i>G</i></b> is a <i>gravitational constant</i> approximately equaled to <i>6.674·10<sup>-11</sup>N·m<sup>2</sup>·kg<sup>−2</sup></i>).<br /><br />Potential energy of an object at height <b><i>h</i></b> above the ground is the work performed by the force of gravity as an object falls down to the surface of the Earth from the distance <b><i>R+h</i></b> from the center to the distance <b><i>R</i></b>.<br /><br />Since the force of gravity varies, we should use the calculus to calculate this work.<br />An infinitesimal increment of work <i>d<b>W(x)</b></i>, assuming the force <b><i>F(x)</i></b> acts at a distance <i>d<b>x</b></i>, equals to<br /><i>d<b>W(x) = F(x)·</b>d<b>x</b></i><br /><br />Integrating this by <i><b>x</b></i> from <i><b>R+h</b></i>to <i><b>R</b></i>, we get the full work and the potential energy of an object at height <i><b>h</b></i> above the ground:<br /><b><i>E<sub>pot</sub> = W(h) = <span style="font-size: large;">∫</span><sup><sup>R</sup></sup><sub><sub>R+h</sub></sub>F(x)·</i></b><i>d<b>x =<br />= G·M·m·</b></i>[<b><i>1/R − 1/(R+h)</i></b>]<br /><br />Obviously, when an object is at the ground level (<b><i>h=0</i></b>), its potential energy equals to zero and, as it moves higher (<i><b>h</b></i> is increasing), its potential energy grows to its maximum value of <b><i>G·M·m<span style="font-size: medium;">/</span>R</i></b>, as the object moves farther and farther from the ground.<br /><br />When the height of the object <b><i>h</i></b>is significantly smaller than the radius of the Earth <nobr>(<b><i>h << R</i></b>),</nobr> our formula for potential energy can be approximated with a simpler expression:<br /><b><i>E<sub>pot</sub> = W(h) =<br />= G·M·m·</i></b>[<b><i>1/R − 1/(R+h)</i></b>]<b><i> =<br />= G·M·m·h<span style="font-size: medium;">/</span></i></b>[<b><i>R·(R+h)</i></b>]<b><i> ≅<br />≅ G·M·m·h<span style="font-size: medium;">/</span>R² = m·g·h</i></b>,<br />where <b><i>g=G·M<span style="font-size: medium;">/</span>R²</i></b> is the free fall acceleration at the ground level, approximately equaled to<br /><b><i>g ≅ 9.81 m/sec²</i></b><br />and <b><i>P = m·g</i></b> is the weight of an object at the ground level, assumed constant for small heights <b><i>h</i></b>.<br />In this case the potential energy is simply a product of weight (force of gravity) and distance - a classical expression for work performed by a constant force:<br /><b><i>E<sub>pot</sub> = W(h) ≅ P·h</i></b><br /><br />Let's also analyze the kinetic energy of the object.<br />Firstly, we assume that our object starts its free falling from the height <i><b>H</b></i> above the ground with zero initial velocity.<br /><br />Its potential energy at any height <i><b>h</b></i> we know from the above calculation:<br /><b><i>E<sub>pot</sub> = G·M·m·</i></b>[<b><i>1<span style="font-size: medium;">/</span>R − 1<span style="font-size: medium;">/</span>(R+h)</i></b>]<br />Its kinetic energy depends on its speed <i><b>V</b></i>:<br /><i><b>E<sub>kin</sub> = m·V²<span style="font-size: medium;">/</span>2</b></i><br />The problem is, we don't know how the speed depends on the height <i><b>h</b></i>.<br /><br />What we do know is the dependency of the force of gravity <i><b>F</b></i> on the height <i><b>h</b></i>:<br /><i><b>F = G·M·m<span style="font-size: medium;">/</span>(R+h)²</b></i><br />We also know that speed <i><b>V</b></i> is the first derivative of height <i><b>h</b></i>by time <i><b>t</b></i> and the acceleration is the second derivative:<br /><i><b>V(t) = h'(t)</b></i><br /><i><b>a(t) = h"(t)</b></i><br />Therefore, according to the Newton's Second Law,<br /><i><b>F = m·a = G·M·m<span style="font-size: medium;">/</span>(R+h)²</b></i><br />Now we have an expression for acceleration <i><b>a</b></i>:<br /><i><b>a = G·M<span style="font-size: medium;">/</span>(R+h)²</b></i><br />Let's underscore that acceleration is the second derivative of distance <i><b>h</b></i>, as a function of time <i><b>t</b></i>:<br /><i><b>h"(t) = G·M<span style="font-size: medium;">/</span></b></i>[<b><i>R+h(t)</i></b>]<b><i>²</i></b><br /><br />We will not attempt to solve this differential equation to get a function <i><b>h(t)</b></i>, take its derivative to get the speed and substitute into a formula for kinetic energy.<br />Instead, we will apply a clever trick.<br />Let's multiply the last expression by <i><b>2h'(t)</b></i>:<br /><i><b>2h'(t)h"(t) =<br />= 2G·M·h'(t)<span style="font-size: medium;">/</span></b></i>[<b><i>R+h(t)</i></b>]<b><i>²</i></b><br />The reason for this operation is the following.<br />The left side of this equation is the derivative of the square of the speed:<br /><i>d/dt</i>{[<i><b>h'(t)</b></i>]²}<i><b> = 2h'(t)·h"(t)</b></i><br />and on the right side of this equation we find another derivative:<i>d/dt</i>{<i><b>2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(t)</b></i>]}<i><b> =<br />= 2G·M·h'(t)<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(t)</b></i>]<i><b>²</b></i><br />Therefore, we have the equality of the derivatives:<br /><i>d/dt</i>{[<i><b>h'(t)</b></i>]²}<i><b> =</b></i><br />= <i>d/dt</i>{<i><b>2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(t)</b></i>]}<br /><br />If derivatives are equal, the functions differ by a constant:<br />[<i><b>h'(t)</b></i>]²<i><b> = 2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(t)</b></i>] <i><b>+ C</b></i><br />The constant <i><b>C</b></i> can be determined, using the conditions at the beginning of motion at time <i><b>t=0</b></i>:<br /><i><b>h(0) = H</b></i> and <i><b>h'(0) = 0</b></i><br />Therefore,<br />[<i><b>h'(0)</b></i>]²<i><b> = 2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(0)</b></i>] <i><b>+ C</b></i><br />[<i><b>0</b></i>]²<i><b> = 2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+H</b></i>] <i><b>+ C</b></i><br /><i><b>C = − 2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+H</b></i>]<br />Now we can write<br />[<i><b>h'(t)</b></i>]²<i><b> =<br />= 2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(t)</b></i>]<i><b>−2G·M<span style="font-size: medium;">/</span></b></i>[<i><b>R+H</b></i>]<br />From this we can express the kinetic energy<br /><i><b>E<sub>kin</sub> = m·V²(t)<span style="font-size: medium;">/</span>2 =<br />= m·</b></i>[<i><b>h'(t)</b></i>]<i><b>²(t)<span style="font-size: medium;">/</span>2 =<br />= G·M·</b></i>{<i><b>1<span style="font-size: medium;">/</span></b></i>[<i><b>R+h(t)</b></i>]<i><b>−1<span style="font-size: medium;">/</span></b></i>[<i><b>R+H</b></i>]}<br /><br />Since our task was to express the energy in terms of height above the ground, we can simply write it as<br /><i><b>E<sub>kin</sub> = G·M·</b></i>[<i><b>1<span style="font-size: medium;">/</span></b></i>(<i><b>R+h</b></i>)<i><b>−1<span style="font-size: medium;">/</span></b></i>(<i><b>R+H</b></i>)]<br /><br />Finally, let's find the full mechanical energy of an object falling from the sky.<br /><b><i>E<sub>pot</sub> = G·M·m·</i></b>[<b><i>1<span style="font-size: medium;">/</span>R−1<span style="font-size: medium;">/</span>(R+h)</i></b>]<br /><i><b>E<sub>kin</sub> = G·M·</b></i>[<i><b>1<span style="font-size: medium;">/</span></b></i>(<i><b>R+h</b></i>)<i><b>−1<span style="font-size: medium;">/</span>(R+H)</b></i>]<br /><b><i>E<sub>full</sub> = E<sub>pot</sub> + E<sub>kin</sub> =<br />= G·M·</i></b>[<i><b>1<span style="font-size: medium;">/</span>R−1<span style="font-size: medium;">/</span></b></i>(<i><b>R+H</b></i>)]<br />As we see, the full mechanical energy is constant, it does not depend on the height. As an object falls from the sky, its potential energy is decreasing, but kinetic energy increasing, and the sum of both types of energy remains constant.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-64530033689238085032019-02-25T20:57:00.001-08:002019-02-25T20:57:38.229-08:00Unizor - Physics4Teens - Energy - Potential Energy of a Spring<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/RTL8ESfVkas" width="480"></iframe><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Energy of Spring</u><br /><br />The energy discussed in the previous introductory lecture was an energy related to constant force acting on an object. In this lecture we will address a more difficult case of a variable force of a spring acting horizontally on an object according to the Hooke's Law.<br />Let's remind the Hooke's Law. It states that the force of a spring, fixed on one end, onto a point-object attached to its free end is proportional to the length the spring is extended or compressed from its neutral position.<br />The corresponding formula for the force <b><i>F</i></b> of a spring onto an object is<br /><b><i>F = −k·L</i></b><br />where <b><i>L</i></b> is a displacement of the spring's free end and <b><i>k</i></b> is a <i>coefficient of elasticity</i> - a specific characteristic of "stiffness" of each spring.<br />The minus sign signifies that the direction of the force is opposite to a displacement. The frame of reference is associated with the neutral position of a spring and the X-axis is directed towards spring's stretching. Then, for positive displacement <b><i>L</i></b>(stretching) the force is directed towards neutral point (that is, negative), while for negative displacement (compression) the force also directed towards a neutral position, (that is, positive).<br /><br />At this point we recommend to refresh the material in the "Work and Elasticity" lecture of the "Mechanical Work" chapter of the "Mechanics" part of this course.<br />As stated in that lecture, the total amount of work <b><i>W</i></b> to compress (or stretch) a spring with elasticity <b><i>k</i></b> by length <b><i>L</i></b>equals<br /><b><i>W = k·L²/2</i></b><br />regardless of how exactly we compress or stretch a spring.<br /><br />Assume now that we have compressed a spring by the length <b><i>L</i></b>, performing work equaled to <b><i>W=k·L²/2</i></b>. At a spring's end we have attached an object of mass <b><i>m</i></b>. Then we let the spring go by itself with the object attached to its end.<br /><br />Intuitively, we think that, at first, the spring should return back to its neutral position. During this time the object's speed will increase because the force of elasticity of a spring will push it.<br /><br />But then, when a spring reaches its neutral position, the object, having certain speed, will continue its move by inertia, stretching the spring. During this time the force of elasticity of a spring will slow down the movement of the object until it stops at the end of the stretching process.<br /><br />At that time the force of elasticity will pull the object back to a neutral position, increasing its speed.<br /><br />At the neutral position object continues moving by inertia, compressing a string, until the force of elasticity stops its movement when the spring will be in the compressed position, as in the beginning of our process.<br /><br />At this point the process repeats itself and can continue like this indefinitely.<br /><br />Well, our intuition is correct, a spring with an object attached to it will oscillate indefinitely. Let's analyze quantitatively these oscillations.<br /><br />Our first goal is to find the dependency of the object's position on time. This can be done using the Newton's Second Law, taking into account the initial position of the object at the end of a compressed spring with no initial speed.<br />Let <b><i>S(t)</i></b> be a function describing the position of the object at the end of a spring relatively to its neutral position with stretching being a positive direction. Then <b><i>S(0)=−L</i></b> and <b><i>S'(0)=0</i></b>. The acceleration is the second derivative of <b><i>S(t)</i></b>, that is <nobr><b><i>a=S"(t)</i></b>.</nobr> The force of elasticity is proportional to <b><i>S(t)</i></b> as <b><i>F=−k·S(t)</i></b>. Now the Newton's Second Law <b><i>F=m·a</i></b> looks like this differential equation:<br /><b><i>−k·S(t) = m·S"(d)</i></b><br />with two initial conditions:<br /><b><i>S(0) = −L</i></b><br /><b><i>S'(0) = 0</i></b><br />This differential equation is fully solvable and its general solution is<br /><b><i>x(t) = C<sub>1</sub>·cos(t·√<span style="text-decoration-line: overline;">k/m</span>) +<br />+ C<sub>2</sub>·sin(t·√<span style="text-decoration-line: overline;">k/m</span>)</i></b><br /><br />Constants <b><i>C<sub>1</sub></i></b> and <b><i>C<sub>2</sub></i></b> can be determined using the initial conditions:<br /><b><i>C<sub>1</sub> = −L</i></b><br /><b><i>C<sub>2</sub> = 0</i></b><br />Therefore,<br /><b><i>S(t) = −L·cos(t·√<span style="text-decoration-line: overline;">k/m</span>)</i></b><br /><br />This function describes simple sinusoidal oscillations with amplitude <b><i>L</i></b> and period<br /><b><i>T=2π√<span style="text-decoration-line: overline;">m/k</span></i></b>.<br /><br />We can also calculate the speed of an object as the first derivative of distance:<br /><b><i>V(t) = S'(t) = </i></b><i>d<b>S(t)/</b>d<b>t =<br />= L·√<span style="text-decoration-line: overline;">k/m</span>·sin(t·√<span style="text-decoration-line: overline;">k/m</span>)</b></i><br /><br />During the first quarter of this time an object accelerates under the influence of the elasticity of a spring, moving from the extremely compressed position on a spring to its neutral position. During the next quarter of the period the spring's elasticity slows it down, while it moves to extremely stretched position of a spring. During the next quarter of the period elasticity of a spring accelerates it again to a string's neutral position. Finally, the elasticity slows the object down, as it moves to the initial most compressed position.<br /><br />Let's calculate the work that spring performs during the first quarter of its period from complete compression to a neutral point by accelerating the object from initial speed <b><i>V(0)=0</i></b>to its maximum at a spring's neutral position <b><i>V(T/4)</i></b>.<br />It can be done by two methods.<br /><br />1. By integrating by displacement of a spring's end from value <b><i>−L</i></b> (full compression) to value <b><i>0</i></b> (neutral position)<br /><b><i>W<sub>[−L,0]</sub> =<br />= <span style="font-size: large;">∫</span><sup><sup>0</sup></sup><sub><sub>−L</sub></sub>F(S)·</i></b><i>d<b>S =<br />= <span style="font-size: large;">∫</span><sup><sup>0</sup></sup><sub><sub>−L</sub></sub>(−k·S)·</b>d<b>S =<br />= −k·S²/2<span style="font-size: large;">|</span><sup><sup>0</sup></sup><sub><sub>−L</sub></sub> = k·L²/2</b></i><br /><br />2. By integrating by time from <b><i>t=0</i></b> to <b><i>t=T/4=(π/2)·√<span style="text-decoration-line: overline;">m/k</span></i></b>(calculations are more complex but lead to the same result)<br /><b><i>W<sub>[0,T/4]</sub> =<br />= <span style="font-size: large;">∫</span><sub><sub>0</sub></sub><sup><sup>T/4</sup></sup>F(S(t))·</i></b><i>d<b>S(t) =<br />= <span style="font-size: large;">∫</span><sub><sub>0</sub></sub><sup><sup>T/4</sup></sup>F(S(t))·(</b>d<b>S(t)/</b>d<b>t)·</b>d<b>t =<br />= <span style="font-size: large;">∫</span><sub><sub>0</sub></sub><sup><sup>T/4</sup></sup>F(S(t))·V(t)·</b>d<b>t =<br />= <span style="font-size: large;">∫</span><sub><sub>0</sub></sub><sup><sup>T/4</sup></sup>k·L·cos(t·√<span style="text-decoration-line: overline;">k/m</span>)·<br />·L·√<span style="text-decoration-line: overline;">k/m</span>·sin(t·√<span style="text-decoration-line: overline;">k/m</span>)·</b>d<b>t =<br />= <span style="font-size: large;">∫</span><sub><sub>0</sub></sub><sup><sup>T/4</sup></sup>k·L²·cos(t·√<span style="text-decoration-line: overline;">k/m</span>)·<br />·sin(t·√<span style="text-decoration-line: overline;">k/m</span>)·</b>d<b>(t·√<span style="text-decoration-line: overline;">k/m</span>) =<br />= <span style="font-size: large;">∫</span><sub><sub>0</sub></sub><sup><sup>T/4</sup></sup>k·L²·sin(t·√<span style="text-decoration-line: overline;">k/m</span>)·<br />·</b>d<b>(sin(t·√<span style="text-decoration-line: overline;">k/m</span>)) =<br />= k·L²sin²((1/2)π√<span style="text-decoration-line: overline;">m/k</span>·√<span style="text-decoration-line: overline;">k/m</span>)/2</b></i><br />After cancellation of factors and taking into account that <b><i>sin(π/2)=1</i></b> we obtain the same expression for work:<br /><b><i>W<sub>[0,T/4]</sub> = k·L²/2</i></b><br /><br />What we have proven now is that the same amount of work done to compress a spring (<b><i>W=k·L²/2</i></b>) is performed by a spring when it returns to its neutral position.<br /><br />That's why we say that the spring accumulates <i>potential energy</i>, when it's compressed, and releases it, when it returns back to neutral position.<br /><br />Now, let's see where this energy, released by a spring during its return to a neutral position, goes. Our assumption is that it is transformed into kinetic energy of the object attached to its free end. Let's check it out.<br /><b><i>E<sub>kin</sub> = m·V²(T/4)/2 =<br />= m·L²·(k/m)·sin²(π/2)/2 =<br />= k·L²/2</i></b><br />So, kinetic energy of the object, when a spring is in the neutral position, exactly equals to a spring's potential energy when it's fully compressed.<br />Compressing a spring results in transferring some external energy to its potential energy. Then, as the spring returns to its neutral position, it releases this potential energy into a kinetic energy of the object attached to its end. At the neutral position the potential energy of a spring is zero, but kinetic energy of the object equals to potential energy of a spring in a compressed position.<br /><br />Our last step is to calculate <i>full energy</i> of the system (<i>potential energy</i> of a spring and <i>kinetic energy</i> of the object attached to it) at any moment of time <b><i>t</i></b>.<br /><b><i>E<sub>pot</sub>(t) = k·S²(t)/2 =<br />= k·L²·cos²(t·√<span style="text-decoration-line: overline;">k/m</span>)/2</i></b><br /><b><i>E<sub>kin</sub> = m·V²(t)/2 =<br />= m·L²·k/m·sin²(t·√<span style="text-decoration-line: overline;">k/m</span>)/2</i></b><br /><b><i>E<sub>full</sub> = E<sub>pot</sub> + E<sub>kin</sub> = k·L²/2</i></b><br />(since <i>sin²(φ)+cos²(φ)=1</i>)<br /><br />As we see, the <i>full energy</i>remains constant, it does not depend on the attached object's mass, only on the properties of the spring - coefficient of <i>elasticity</i> <b><i>k</i></b> and its initial displacement from the neutral position.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-92198475701089309432019-02-25T13:09:00.001-08:002019-02-25T13:09:02.740-08:00Unizor - Physics4Teens - Energy - Potential Energy - Introduction<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/R0kvn8nnHKA" width="480"></iframe><br /><br /><i><br /></i><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Potential Energy - Introduction</u><br /><br /><i>Potential energy</i> is a quantitative characteristic of an object's position relative to other objects or environment that are a source of some forces. These forces would result in motion of the object under consideration (if no counteracting forces present) and, therefore, would result in performing certain work related to this motion.<br /><br />Typical example of <i>potential energy</i> is related to a position of some object in the gravitational field.<br />Since a planet Earth attracts all objects, an object raised to a certain height above the ground will fall, if no support is provided. The motion of falling object is the result of work of the forces of gravity. Therefore, by definition, an object raised above the ground has certain amount of <i>potential energy</i>equal to work performed by the forces of gravity to move it down to the ground.<br /><br />In a simplified case of a small object of mass <b><i>m</i></b>, raised on small (relatively to the size of the Earth) height <b><i>h</i></b>, the force of gravity, acting on this object, is constant and is equal to the object's weight <nobr><b><i>P = m·g</i></b></nobr>, where <nobr><b><i>g ≅ 9.8m/sec²</i></b></nobr> is the acceleration of the free fall near the Earth's surface.<br />If no support is provided, the object will fall straight down. Thus, the constant force of gravity <b><i>P</i></b> would perform work, pushing the object down at a distance <b><i>h</i></b>, thus performing work<br /><b><i>W = P·h = m·g·h</i></b><br /><br />Let's examine the relationship between an object's <i>potential energy</i> and its <i>kinetic energy</i> as it falls down from certain initial height.<br />Assume an inertial frame of reference associated with Earth and X-coordinate directed straight up perpendicularly to the ground with ground level be at coordinate <b><i>X=0</i></b>.<br />Initial position of the object is, therefore, at X-coordinate equal to <b><i>X=h</i></b>. The falling object moves with constant acceleration <b><i>−g</i></b> (negative, since it moves from positive <b><i>X=h</i></b> to <b><i>X=0</i></b>). Initial speed of the object is zero.<br />Therefore, the time-dependent motion <b><i>X(t)</i></b> can be described by the kinematic equation<br /><b><i>X(t) = h − g·t²/2</i></b><br />The object's velocity is directed vertically down and, as a function of time, is the first derivative of the <b><i>X(t)</i></b>:<br /><b><i>V(t) = X'(t) = −g·t</i></b><br />(which is obvious from the fact that the motion is with constant acceleration <b><i>−g</i></b> and zero initial speed)<br /><br />As object falls down, its height diminishes, which means that its <i>potential energy</i> also diminishes. At any given moment of time <b><i>t</i></b> this energy equals to<br /><b><i>E<sub>pot</sub>(t) = m·g·X(t) = m·g·(h−g·t²/2)</i></b><br /><br />At the same time the <i>kinetic energy</i> of the object is increasing, since its speed <b><i>V(t)</i></b>is increasing. At any given moment of time <b><i>t</i></b> this energy equals to<br /><b><i>E<sub>kin</sub>(t) = m·V²(t)/2 = m·g²·t²/2</i></b><br /><br />Remarkably, the <i>full mechanical energy</i> of this object, which is a sum of its <i>potential energy</i> and <i>kinetic energy</i> is constant:<br /><b><i>E<sub>full</sub> = E<sub>pot</sub> + E<sub>kin</sub> = m·g·h</i></b>,<br />which is its <i>potential energy</i> in the beginning of motion (when its initial speed and, therefore, <i>kinetic energy</i> are zero).<br /><br />The total time in motion from the initial position above the ground at height <b><i>h</i></b> to the ground can be obtained by resolving the equation <b><i>X(t)=0</i></b> for variable <b><i>t</i></b>:<br /><b><i>X(t) = h − g·t²/2 = 0</i></b><br /><b><i>t<sub>fall</sub> = √<span style="text-decoration-line: overline;">2h/g</span></i></b><br />At that time of touching the ground the <i>potential energy</i> of an object equals to zero and its <i>kinetic energy</i> equals to<br /><b><i>E<sub>kin</sub>(t<sub>fall</sub>) = m·V²(t<sub>fall</sub>)/2 = m·g²t²<sub>fall</sub>/2 = m·g·h</i></b><br /><br />In the beginning the potential energy is at its maximum and kinetic energy at zero. At the end the potential energy is at zero, while kinetic energy at its maximum, but their sum is always the same. So, it appears that, as an object falls, its potential energy is transformed into kinetic energy, preserving their sum - the <i>full mechanical energy</i>.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-10341770902094528212019-02-22T12:39:00.001-08:002019-02-22T12:39:55.771-08:00Unizor - Physics4Teens - Energy - Problems on Kinetic Energy<iframe allowfullscreen="" frameborder="0" height="270" src="https://www.youtube.com/embed/Hp8BHnRkO1A" width="480"></iframe><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><u>Problems on Kinetic Energy</u><br /><br /><i>Problem A</i><br /><br />A car engine accelerates a car of mass <b><i>m</i></b> from the state at rest to some maximum speed during the time <b><i>T</i></b> with constant acceleration <b><i>a</i></b> along a straight line.<br />Ignore loss of mass due to burning fuel.<br />What is the kinetic energy <b><i>E<sub>kin</sub></i></b>of a car at the end of this period of acceleration and what is the work <b><i>W</i></b> performed by the car engine during this time?<br /><br /><i>Solution</i>:<br /><br /><b><i>V</i></b><i><sub>end</sub><b> = a·T</b></i><br /><b><i>E</i></b><i><sub>kin</sub><b> = m·V²</b><sub>end</sub><b> /2 = m·a²·T²/2</b></i><br /><br /><b><i>W = F·S = (m·a) · (a·T²/2) =<br />= m·a²·T²/2</i></b><br /><b><i>W = E</i></b><i><sub>kin</sub><b> = m·a²·T²/2</b></i><br /><br /><i>Problem B</i><br /><br />A driver of a car, going with speed <b><i>V</i></b>, sees an obstacle and strongly presses on breaks, so its wheels stop spinning. The car slows down to complete stop by the force of friction only at a distance <b><i>S</i></b> from the spot when it started to break.<br />What is the coefficient of kinetic friction <b><i>μ</i></b> of car's wheels against the ground?<br />Assume, the free fall acceleration is <b><i>g</i></b>.<br /><br /><i>Solution</i><br /><br />Let <b><i>m</i></b> be an unknown mass of the car.<br />Let <b><i>W</i></b> be a work of the force of friction to stop the car.<br />Let <b><i>E</i></b><i><sub>kin</sub><b></b></i> be a kinetic energy of the car in the beginning of breaking. The ending kinetic energy is zero, since the speed at the end is zero. So, the initial kinetic energy should be equal to the work required to stop the car.<br />Let <b><i>F</i></b> be a constant force of friction equal to the car's weight multiplied by a coefficient of kinetic friction.<br /><b><i>E</i></b><i><sub>kin</sub><b> = m·V²/2 = W</b></i><br /><b><i>W = F</i></b><i><sub friction="" sub=""><b>·S</b></sub></i><br /><b><i>F = W/S = m·g·μ</i></b><br /><b><i>μ = W/(m·g·S) = m·V²/(2·m·g·S)</i></b><br /><b><i>μ = V²/(2·g·S)</i></b><br />Notice independence of the car's mass.<br /><br /><br /><i>Problem C</i><br /><img src="http://www.unizor.com/Pictures/RotationalEnergy.png" style="height: 270px; width: 200px;" /><br />A point-object <b><i>A</i></b> of mass <b><i>m</i></b> is freely rotating on a thread of a length <b><i>L</i></b> that forms an angle <b><i>φ</i></b>with vertical.<br />What is its kinetic energy?<br /><br /><i>Solution</i><br /><br /><b><i>T</i></b> is a force of tension of the thread. We can represent it as a sum of two components - <b><i>T<sub>v</sub></i></b>that balances the weight <b><i>P</i></b> and <b><i>T<sub>h</sub></i></b> that keeps an object on a circular trajectory (centripetal force).<br /><b><i>T<sub>v</sub> = m·g</i></b><br /><b><i>T<sub>h</sub> = T<sub>v</sub>·tan(φ) = m·g·tan(φ)</i></b><br />This centripetal force causes centripetal acceleration<br /><b><i>a = V²/R</i></b><br />(see a chapter <i>Mechanics - Superposition of Forces - Resultant Forces - Example 2</i> in this course)<br />where <b><i>V</i></b> is a linear speed of an object and <b><i>R</i></b> is a radius of a circular trajectory, which is equal to <b><i>L·sin(φ)</i></b>.<br />Therefore,<br /><b><i>T<sub>h</sub> = m·a = m·V²/R</i></b><br />Hence,<br /><b><i>m·g·tan(φ) = m·V²/R</i></b><br />Kinetic energy is<br /><b><i>E = m·V²/2 =<br />= m·g·tan(φ)·R/2 =<br />= m·g·L·tan(φ)·sin(φ)/2</i></b><br /><br /><br /><i>Problem D</i><br /><br />An ideal pendulum of mass <b><i>m</i></b>with a thread length <b><i>L</i></b> performs small <i>harmonic</i> oscillations with its angle of deviation from the vertical <b><i>φ</i></b> conforming to the equation<br /><b><i>φ(t) = φ<sub>0</sub>·cos(√<span style="text-decoration-line: overline;">g/L </span>·t)</i></b><br />where <b><i>φ<sub>0</sub></i></b> is the initial deviation from the vertical, <b><i>g</i></b> - free fall acceleration and <b><i>t</i></b> - time.<br />What is its kinetic energy at the lowest point of its trajectory?<br /><br /><i>Solution</i>:<br /><br />We recommend to refresh the properties of a pendulum in the "Mechanics" part of this course in the chapter "Pendulum, Spring".<br />Let's find the period of oscillations.<br />Since function <b><i>y=cos(x)</i></b> has a period <b><i>2π</i></b>, function <b><i>y=cos(k·x)</i></b>has period <b><i>2π/k</i></b>.<br />In our case the period of oscillation <b><i>T</i></b> is equal to<br /><b><i>T = 2π·√<span style="text-decoration-line: overline;">L/g </span></i></b><br />The angular velocity is a derivative of the angle of deviation <b><i>φ(t)</i></b>:<br /><b><i>ω(t) = </i></b><i>d<b>φ(t)/</b>d<b>t =<br />= −φ<sub>0</sub>·√<span style="text-decoration-line: overline;">g/L </span>·sin(√<span style="text-decoration-line: overline;">g/L </span>·t)</b></i><br />At the lowest point of a trajectory the time equals to 1/4 of a period.<br />Therefore, we can calculate the angular velocity at this point by substituting<br /><b><i>t = T/4 = (1/4)·2π·√<span style="text-decoration-line: overline;">L/g </span>=<br />= (π/2)·√<span style="text-decoration-line: overline;">L/g </span></i></b><br />which gives the angular speed at this point as<br /><b><i>ω(T/4) =<br />= −φ<sub>0</sub>·√<span style="text-decoration-line: overline;">g/L </span>·sin(√<span style="text-decoration-line: overline;">g/L </span>·T/4) =<br />= −φ<sub>0</sub>·√<span style="text-decoration-line: overline;">g/L </span>·<br />·sin(√<span style="text-decoration-line: overline;">g/L </span>·(π/2)·√<span style="text-decoration-line: overline;">L/g </span>) =<br />= −φ<sub>0</sub>·√<span style="text-decoration-line: overline;">g/L </span></i></b><br />The linear speed of a rotating object equals to its angular speed, multiplied by a radius.<br />Therefore, linear speed <b><i>V</i></b> at the lowest point equals to<br /><b><i>V = L·ω(T/4) =<br />= −φ<sub>0</sub>·√<span style="text-decoration-line: overline;">g·L </span></i></b><br />The kinetic energy at this point is<br /><b><i>E = m·V²/2 = m·g·L·φ<sub>0</sub>²/2</i></b><br /><b>IMPORTANT NOTE</b>:<br />The following equation for small <i>harmonic</i> oscillation used in this problem<br /><b><i>φ(t) = φ<sub>0</sub>·cos(√<span style="text-decoration-line: overline;">g/L </span>·t)</i></b><br />was obtained in "Mechanics" part of this course in chapter "Pendulum, Spring" by simplification of the differential equation that described the motion of the pendulum. In particular, it was assumed that for small oscillations the angle of deviation from the vertical <b><i>φ</i></b>and its sine <b><i>sin(φ)</i></b> are approximately equal. The exact value of the kinetic energy of a pendulum at its lowest point can be derived using its <i>potential energy</i> at the top position, which we will address in the next chapter, and which is equal to <b><i>2m·g·L·sin²(φ<sub>0</sub>/2)</i></b>. If in this expression we replace <b><i>sin(φ<sub>0</sub>)</i></b>with <b><i>φ<sub>0</sub></i></b> (which is acceptable for small angles), we will get the formula for kinetic energy we obtained above.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-62303294637641957662019-01-22T12:57:00.001-08:002019-01-22T12:57:08.197-08:00Unizor - Physics4Teens - Mechanics - Work and Gravity<br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Work and Gravity</u><br /><br /><br /><br />Let's consider a force to lift an object above the planet against the <br />force of gravity. In simple cases, when the height is relatively small <br />compared with the size of a planet, the force of gravity is considered <br />to be constant. In this case we will assume that we lift to substantial <br />height and have to take into consideration the Law of Gravity that tells<br /> that the force of gravity is proportional to masses of objects involved<br /> (a planet of mass <b><i>M</i></b> and an object of mass <b><i>m</i></b> that we lift) and inversely proportional to a distance <b><i>r</i></b> between the objects:<br /><br /><b><i>F = G·M·m <span style="font-size: medium;">/</span>r²</i></b><br /><br />where <b><i>G</i></b> - a <i>gravitational constant</i>.<br /><br /><br /><br />Our task is to find the work <b><i>W</i></b> needed to lift an object of mass <b><i>m</i></b> from a surface of a planet of mass <b><i>M</i></b> and radius <b><i>R</i></b> to height <b><i>H</i></b> above its surface.<br /><br /><br /><br />The easiest approach is to represent all parameters as functions of the distance <b><i>r</i></b> between a center of a planet and an object.<br /><br />Then the force of gravity as a function of <b><i>r</i></b> is<br /><br /><b><i>F(r) = G·M·m <span style="font-size: medium;">/</span>r²</i></b><br /><br />An infinitesimal increment (<i>differential</i>) of work equals<br /><br /><i>d<b>W(r) = F(r)·</b>d<b>r =<br /><br />= G·(M·m <span style="font-size: medium;">/</span>r²)·</b>d<b>r</b></i><br /><br /><br /><br />Let's integrate the differential of work on an interval of <b><i>r</i></b> [<b><i>R,R+H</i></b>]:<br /><br /><b><i><span style="font-size: large;">∫</span></i></b><sub><sub>[R,R+H]</sub></sub><i>d<b>W(r) =<br /><br />= G·M·m·<span style="font-size: large;">∫</span></b></i><sub><sub>[R,R+H]</sub></sub><b><i>(1/r²)</i></b><i>d<b>r =<br /><br />= G·M·m·</b></i>[<b><i>1/R − 1/(R+H)</i></b>]<b><i> =<br /><br />= G·M·m·H <span style="font-size: medium;">/</span> </i></b>[<b><i>R·(R+H)</i></b>]<br /><br /><br /><br />For small (relatively to radius of a planet <b><i>R</i></b>) height <b><i>H</i></b> the expression in curly brackets above is approximately equal to<br /><br /><b><i>G·M·m <span style="font-size: medium;">/</span></i></b><b><i>R² = m·g</i></b><br /><br />where <b><i>g = G·M <span style="font-size: medium;">/</span></i></b><b><i>R²</i></b> is an acceleration of the free falling on a planet's surface.<br /><br />By definition, <b><i>m·g</i></b> is the weight <b><i>P</i></b> of an object on a surface of a planet, which we consider a constant force in this approximation.<br /><br />So, our formula for work for small height above the planet is reduced to simple expression<br /><br /><b><i>W = P·H</i></b><br /><br />which is a base "force times distance" expression that defines the work <br />in simple case of constant force acting along a trajectory.<br /><br /><br /><br />For large height <b><i>H</i></b> we cannot ignore the change of gravity <br />as an object moves far from the planet, and the exact formula must be <br />used to calculate the work.<br /><br /><br /><br />As in other cases, the work depends only on characteristics of <br />interacting objects (their masses in this case) and the result of work <br />(lifting on certain height), not the way how we achieve this result.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-8647100944692850352019-01-22T12:34:00.001-08:002019-01-22T12:34:27.458-08:00Unizor - Physics4Teens - Mechanics - Work and Elasticity<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Work and Elasticity</u><br /><br /><br /><br />Let's consider a slightly more complicated case of a variable force.<br /><br />A good example of this is the work performed by a force to stretch or <br />compress a spring, in which case a force linearly depends on a length a <br />spring is stretched or compressed.<br /><br /><br /><br />Consider a spring with elasticity coefficient <b><i>k</i></b> that force <b><i>F</i></b> compresses by length <b><i>S</i></b>.<br /><br />Consider further that a spring is compressed by the same length in equal<br /> time interval, that is its end, where the force is applied, is <br />uniformly moves under the force of compression with constant speed <b><i>V</i></b>.<br /><br />Our task is to find the amount of work needed to compress a spring by the length <b><i>L</i></b>.<br /><br /><br /><br />In this case we can calculate the length of compression as a function of time<br /><br /><b><i>S(t) = V·t</i></b><br /><br />According to the Hook's Law, the force of compression is proportional to the length of compression:<br /><br /><b><i>F = k·S</i></b><br /><br />Since the force and the length of compression are time-dependent, this can be written as<br /><br /><b><i>F(t) = k·S(t) = k·V·t</i></b><br /><br /><br /><br />An infinitesimal increment (<i>differential</i>) of work equals<br /><br /><i>d<b>W(t) = F(t)·</b>d<b>S(t) = F(t)·V·</b>d<b>t =<br /><br />= k·V·t·V·</b>d<b>t = k·V²·t·</b>d<b>t</b></i><br /><br /><br /><br />The time interval <b><i>T</i></b> needed to compress a spring by length <b><i>L</i></b> with speed of compression <b><i>V</i></b> equals to <b><i>T = L<span style="font-size: medium;">/</span>V</i></b><br /><br />Let's integrate the differential of work on a time interval [<b><i>0,T</i></b>]:<br /><br /><b><i><span style="font-size: large;">∫</span></i></b><sub><sub>[0,T]</sub></sub><i>d<b>W(t) = k·V²·<span style="font-size: large;">∫</span></b></i><sub><sub>[0,T]</sub></sub><b><i>t·</i></b><i>d<b>t =<br /><br />= k·V²·T²<span style="font-size: medium;">/</span>2 = k·L²<span style="font-size: medium;">/</span>2</b></i><br /><br /><br /><br />Remarkably, the work does not depend on speed <b><i>V</i></b> of compression, only on the length <b><i>L</i></b> the spring is compressed and its elasticity <b><i>k</i></b>.<br /><br /><br /><br />We can easily generalize this by getting rid of dependency on the speed of compression in our calculations.<br /><br />Instead of all parameters being functions of time <b><i>t</i></b>, let's use the length of compression <b><i>S</i></b> as a base variable.<br /><br />Since <b><i>F(S) = k·S</i></b>,<br /><br /><i>d<b>W(S) = F(S)·</b>d<b>S = k·S·</b>d<b>S</b></i><br /><br />We can integrate it on an interval <b><i>S</i></b>∈[<b><i>0,L</i></b>] getting<br /><br /><b><i><span style="font-size: large;">∫</span></i></b><sub><sub>[0,L]</sub></sub><i>d<b>W(S) = k·<span style="font-size: large;">∫</span></b></i><sub><sub>[0,L]</sub></sub><b><i>S·</i></b><i>d<b>S =<br /><br />= k·L²<span style="font-size: medium;">/</span>2</b></i><br /><br /><br /><br />As you see, this is a more general (and simpler!) derivation of the same<br /> formula that does not depend at all on how we compress the spring, only<br /> on its elasticity and a length of compression.<br /><br /><br /><br />So, total amount of work <b><i>W</i></b> to compress a spring with elasticity <b><i>k</i></b> by length <b><i>L</i></b> equals<br /><br /><b><i>W = k·L²<span style="font-size: medium;">/</span>2</i></b><br /><br />regardless of how exactly we compress a spring.<br /><br />Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0tag:blogger.com,1999:blog-3741410418096716827.post-1247834147562597282019-01-14T12:07:00.001-08:002019-01-14T12:07:24.467-08:00Unizor - Physics4Teens - Energy - Kinetic Energy - Introduction<br /><br /><br /><br /><br /><br /><i>Notes to a video lecture on http://www.unizor.com</i><br /><br /><br /><br /><u>Kinetic Energy - Introduction</u><br /><br /><br /><br />Kinetic energy is a quantitative characteristic of an object's motion, <br />that signifies that this motion can result in some work, when our object<br /> interacts with surrounding environment.<br /><br /><br /><br />As an example, consider an object of mass <b><i>m</i></b> uniformly moving within some inertial frame along a straight line trajectory with speed <b><i>v</i></b>.<br /><br /><br /><br />Assume that at some moment of time there appears a constant force <b><i>F</i></b> that acts against its motion. For example, a constant air resistance.<br /><br /><br /><br />As a result of this interaction with air molecules, our object will slow<br /> down because of air resistance acting against its motion until it <br />stops. So, the motion of our object caused some work - moving molecules <br />of air away from the trajectory, after which the motion of our object no<br /> longer exists, it stops completely.<br /><br />Obviously, instead of air resistance, we can consider friction or <br />gravity, or any other force, assumed constant for this experiment.<br /><br /><br /><br />Let's calculate the work done by this force, as it acts against the motion of our object and causes its deceleration.<br /><br /><br /><br />First of all, we calculate the deceleration <b><i>a</i></b>:<br /><br /><b><i>a = F/m</i></b><br /><br />Then, knowing deceleration, we get the time <b><i>t</i></b> our force acts until an object stops:<br /><br /><b><i>t = v/a = m·v/F</i></b><br /><br />Now the distance <b><i>S</i></b> our object travels until a full stop:<br /><br /><b><i>S = a·t²/2 = F·t²/(2m) =<br /><br />= F·m²·v²/(2m·F²) =<br /><br />= m·v²/(2F)</i></b><br /><br />Finally, the work <b><i>W</i></b> performed by our force <b><i>F</i></b> during the distance <b><i>S</i></b>:<br /><br /><b><i>W = F·S = m·v²/2</i></b><br /><br /><br /><br />What's most remarkable about this formula is that the work performed by force <b><i>F</i></b> does not depend on this force, but only on the characteristic of an object (mass <b><i>m</i></b>) and its motion (speed <b><i>v</i></b>).<br /><br /><br /><br />So, it appears that there is something specific for an object (its <i>mass</i>) and its motion (<i>speed</i>) - the amount of work to bring this object to a state of rest, and this quantity of work equals to <b><i>m·v²/2</i></b>.<br /><br />This quantitative characteristic of an object in motion is called its <b>kinetic energy</b>.<br /><br /><br /><br />Let's slightly complicate the problem. The constant force <b><i>F</i></b> slows down our object not to a full stop, but to final speed <b><i>v<sub>end</sub></i></b>. What will be the work force <b><i>F</i></b> would perform?<br /><br /><br /><br />As before,<br /><br /><b><i>a = F/m</i></b><br /><br /><b><i>t = (v−v<sub>end</sub>)/a = m·(v−v<sub>end</sub>)/F</i></b><br /><br /><b><i>S = v·t − a·t²/2 =<br /><br />= m·v·(v−v<sub>end</sub>)/F −<br /><br />− F·m²·(v−v<sub>end</sub>)²/(2m·F²) =<br /><br />= m·v·(v−v<sub>end</sub>)/F −<br /><br />− m·(v−v<sub>end</sub>)²/(2F) =<br /><br />= (m/2F)·(v²−v²<sub>end</sub>)</i></b><br /><br />Finally, the work <b><i>W</i></b> performed by our force <b><i>F</i></b> during the distance <b><i>S</i></b>:<br /><br /><b><i>W = F·S = m·(v²−v²<sub>end</sub>)/2 =<br /><br />= m·v²/2 − m·v²<sub>end</sub>)/2</i></b><br /><br /><br /><br />This formula indicates that the work needed to change the state of a moving object from a state with one value of its <b>kinetic energy</b> to another equals to a difference between the values of its <b>kinetic energy</b> at these two states.<br /><br /><br /><br />Absolutely analogous calculations can prove that the work of a constant force that accelerates an object of mass <b><i>m</i></b> from a state of rest to speed <b><i>v</i></b> equals to<br /><br /><b><i>W = m·v²/2</i></b> for any force,<br /><br />and the work needed to accelerated an object from speed <b><i>v<sub>beg</sub></i></b> to <b><i>v</i></b> equals to<br /><br /><b><i>W = m·v²/2 − m·v²<sub>beg</sub>/2</i></b><br /><br /><br /><br />In other words, <b>work</b>, that results from the interaction of moving object with surrounding environment, and its <b>kinetic energy</b> are intimately related. One can be converted into another and vice versa.<br /><br /><br /><br />Work performed by different forces is, by definition, <i>additive</i>.<br /><br />It means that, if force <b><i>F<sub>1</sub></i></b> acted on a distance <b><i>S<sub>1</sub></i></b> performing work <b><i>W<sub>1</sub>=F<sub>1</sub>·S<sub>1</sub></i></b> and force <b><i>F<sub>2</sub></i></b> acted on a distance <b><i>S<sub>2</sub></i></b> performing work <b><i>W<sub>2</sub>=F<sub>2</sub>·S<sub>2</sub></i></b>, then the total amount of work performed by both forces is a sum of their individual work.<br /><br /><br /><br />Immediate consequence of this is that <b>kinetic energy</b> of a system of objects, each having its own mass and moving with its own speed, equals to a sum of their individual <b>kinetic energies</b>.<br /><br />That is, <b>kinetic energy</b> is <i>additive</i>.<br /><br /><br /><br />If <b><i>N</i></b> objects of masses <b><i>m<sub>1</sub></i></b>, <b><i>m<sub>2</sub></i></b>, ... <b><i>m<sub>N</sub></i></b> are moving with speeds <b><i>v<sub>1</sub></i></b>, <b><i>v<sub>2</sub></i></b>, ... <b><i>v<sub>N</sub></i></b> then their <b>total kinetic energy</b> equals to<br /><br /><b><i>W = </i><span style="font-size: medium;">Σ</span></b><sub>i∈[1,N]</sub><b>(<i>m</i></b><i><sub>i</sub><b>·v²</b><sub>i</sub></i><b>)/<i>2</i></b><br /><br /><br /><br />As a final example, consider a case when an object of mass <b><i>m</i></b> initially moves along the X-axis. The X-component of its velocity vector is <b><i><span style="text-decoration: overline;">V<sub>x</sub></span></i></b> and its Y-component of a velocity vector <b><i><span style="text-decoration: overline;">V<sub>y</sub></span></i></b> is zero. Then its speed <b><i>V</i></b> (a scalar) equals to its X-component <b><i>V<sub>x</sub></i></b> (a scalar) and its <b>kinetic energy</b> is<br /><br /><b><i>E<sub>k</sub> = m·V²<span style="font-size: medium;">/</span>2</i></b><br /><br />where <b><i>V = V<sub>x</sub></i></b><br /><br /><br /><br />Assume that the force <b><i>F</i></b> acts at an angle <b><i>φ</i></b> to the X-axis.<br /><br />In this case we can represent the vector of force <b><i><span style="text-decoration: overline;">F</span></i></b> as a sum of two perpendicular vectors of force: <b><i><span style="text-decoration: overline;">F<sub>x</sub></span></i></b> acting along the X-axis and <b><i><span style="text-decoration: overline;">F<sub>y</sub></span></i></b> acting along the Y-axis.<br /><br />Obviously,<br /><br /><b><i><span style="text-decoration: overline;">F </span></i></b> = <b><i><span style="text-decoration: overline;">F<sub>x</sub></span></i></b> + <b><i><span style="text-decoration: overline;">F<sub>y</sub></span></i></b><br /><br /><br /><br />These two perpendicular to each other forces, applied to our object, <br />give it a vector of acceleration that also can be represented as a sum <br />of two perpendicular vectors<br /><br /><b><i><span style="text-decoration: overline;">a<sub>x</sub></span> = <span style="text-decoration: overline;">F<sub>x</sub></span> <span style="font-size: medium;">/</span>m</i></b><br /><br /><b><i><span style="text-decoration: overline;">a<sub>y</sub></span> = <span style="text-decoration: overline;">F<sub>y</sub></span> <span style="font-size: medium;">/</span>m</i></b><br /><br /><br /><br />Let's assume that the force <b><i><span style="text-decoration: overline;">F </span></i></b> acts for a duration of time <b><i>t</i></b>.<br /><br />During this time the force <b><i><span style="text-decoration: overline;">F </span></i></b><br /> performs certain work and the velocity of an object will change. We <br />will compare the amount of work performed by the force with a change in <br />the kinetic energy of an object.<br /><br /><br /><br />Considering the initial speed of an object along the X-axis was <b><i>V<sub>x</sub></i></b> and acceleration <b><i>a<sub>x</sub></i></b>, the distance covered by our object along the X-axis equals to<br /><br /><b><i>S<sub>x</sub> = V<sub>x</sub>·t + a<sub>x</sub>·t² <span style="font-size: medium;">/</span>2</i></b><br /><br />At the same time our object moved along the Y-axis with initial speed <b><i>0</i></b> and acceleration <b><i>a<sub>y</sub></i></b>, covering the distance<br /><br /><b><i>S<sub>y</sub> = a<sub>y</sub>·t² <span style="font-size: medium;">/</span>2</i></b><br /><br /><br /><br />In terms of components of the force <b><i><span style="text-decoration: overline;">F </span></i></b> the distances along the coordinates are<br /><br /><b><i>S<sub>x</sub> = V<sub>x</sub>·t + F<sub>x</sub>·t² <span style="font-size: medium;">/</span>(2m)</i></b><br /><br /><b><i>S<sub>y</sub> = F<sub>y</sub>·t² <span style="font-size: medium;">/</span>(2m)</i></b><br /><br /><br /><br />The total work performed by force <b><i><span style="text-decoration: overline;">F </span></i></b>, considering work is additive and can be summarized in each direction, is<br /><br /><b><i>W = F<sub>x</sub>·S<sub>x</sub> + F<sub>y</sub>·S<sub>y</sub> =<br /><br />= F<sub>x</sub>·V<sub>x</sub>·t + F<sub>x</sub>²·t²<span style="font-size: medium;">/</span>(2m) +<br /><br />+ F<sub>y</sub>²·t²<span style="font-size: medium;">/</span>(2m)</i></b><br /><br /><br /><br />Now let's calculate the kinetic energy of an object at the end of time period <b><i>t</i></b>.<br /><br />The X-component of the velocity will be equal to<br /><br /><b><i>V<sub>x</sub><sup>t</sup> = V<sub>x</sub> + a<sub>x</sub>·t = V<sub>x</sub> + F<sub>x</sub>·t <span style="font-size: medium;">/m</span></i></b><br /><br />The Y-component of the velocity will be equal to<br /><br /><b><i>V<sub>y</sub><sup>t</sup> = a<sub>y</sub>·t = F<sub>y</sub>·t <span style="font-size: medium;">/m</span></i></b><br /><br /><br /><br />The object's kinetic energy at the end of the time period <b><i>t</i></b>, equals to<br /><br /><b><i>E<sub>k</sub><sup>t</sup> = m·(V<sup>t</sup>)² <span style="font-size: medium;">/2</span></i></b><br /><br />where <b><i>V<sup>t</sup></i></b> is the speed at time <b><i>t</i></b>.<br />Using the Pythagorean Theorem, we can represent <b><i>(V<sup>t</sup>)²</i></b> as<br /><br /><b><i>(V<sup>t</sup>)² = (V<sub>x</sub><sup>t</sup>)² + (V<sub>y</sub><sup>t</sup>)²</i></b>.<br />Therefore, the kinetic energy can be summarized from X- and Y-components and can be calculated as a sum of:<br /><br /><b><i>E<sub>x</sub><sup>t</sup> = m·(V<sub>x</sub><sup>t</sup>)² <span style="font-size: medium;">/2</span></i></b><br /><br /><b><i>E<sub>y</sub><sup>t</sup> = m·(V<sub>y</sub><sup>t</sup>)² <span style="font-size: medium;">/2</span></i></b><br /><br /><br /><br />The increment of the kinetic energy is<br /><br />Δ<b><i>E = E<sub>x</sub><sup>t</sup> + E<sub>y</sub><sup>t</sup> − E<sub>k</sub></i></b><br /><br />All we have to do is to compare amount of work <b><i>W</i></b> performed by force <b><i><span style="text-decoration: overline;">F </span></i></b> and increment of kinetic energy Δ<b><i>E</i></b> and make sure that they are equal.<br /><br /><br /><br />Indeed,<br /><br />Δ<b><i>E = m·(V<sub>x</sub>+F<sub>x</sub>·t <span style="font-size: medium;">/</span>m)²<span style="font-size: medium;">/</span>2 +<br /><br />+ m·(F<sub>y</sub>·t <span style="font-size: medium;">/</span>m)²<span style="font-size: medium;">/</span>2 − m·V<sub>x</sub>²<span style="font-size: medium;">/</span>2 =<br /><br />= F<sub>x</sub>·V<sub>x</sub>·t + F<sub>x</sub>²·t²/(2m) +<br /><br />+ F<sub>y</sub>²·t²/(2m) = W</i></b><br /><br /><br /><br /><b>Therefore, the work performed by a force acting on an object during <br />certain period of time equals to an increment of a kinetic energy of <br />this object</b>.Unizorhttp://www.blogger.com/profile/06592791874048701921noreply@blogger.com0