Monday, April 15, 2019

Unizor - Physics4Teens - Energy - Heat - Molecular Movement

Notes to a video lecture on

Molecular Movement


In order to understand the nature of heat and temperature, we have to go inside the objects we experiment with.
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.

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.
That tiniest amount of water, that still preserves the quality of being water, is called a molecule.
Similarly, tiniest amount of any substance, that retains the qualities of this substance, is called a molecule.
Incidentally, if we divide a molecule of any substance, the result will be certain atoms, that possess completely different qualities.

In our discussion about heat we will not go deeper than the molecules because our purpose at this stage is to study the nature of heat 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 molecular and more elementary atomic level.

Any object or substance we are dealing with consists of certain number of molecules - the smallest particles that retains the qualities of this object or substance. This number of molecules, 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.
And not only in size. Since the molecules contain different, more elementary particles called atoms, the configuration of these atoms that form a molecule is different for different molecules. 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.

States of Matter

The next topic we would like to address is the states of matter.
When the word matter 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.

There are three major states of mattersolidliquid and gas.

When an object is solid or is in solid 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 crystal (or crystalline type of) solids or just slow movement, changing their relative position, but not changing the overall form for amorphous (non-crystalline) type.
Examples of solid objects are ice (crystal), steel (crystal), plastic cup (amorphous).

When an object or substance is liquid or is in liquid 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.
Examples of liquid substances are water, mercury ("quick silver"), oil.

When an object or substance is gas or is in gaseous 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.

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.

Nature of Heat

Now we are in position to talk about heat.
Heat is the energy of molecular motion inside any object or substance.
As we mentioned, molecules are in constant motion inside any object. The more intense this motion is - the more heat this object possesses. This implies that heat is mechanical energyof molecules inside the object or internal energy of the object or substance.

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 dissipation of heat. 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.

Since heat or internal energy 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.
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.

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.

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.
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.
This property is the principle, on which measuring of the intensity of molecular movement is based.

Heat and Temperature

Now let's address the issue of measuring the heat, that is amount of internal energy inside any object.
The term temperature is related to average intensity of the molecular movement inside an object or a substance. So, when we say that the temperature of an object has increased or decreased, we mean that average intensity of the molecular movement in it has increased or decreased correspondingly.

Our obvious task now is to quantitatively evaluate the temperature, thus measuring the intensity of the molecular movement inside an object.

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.

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.

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.

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.

All, which is left to establish is the scale and units of measure.
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.

The unit of measurement in each system is called a degreeand the temperature is written with an indication of the system as follows:
0°C, 20°C, -40°C for temperatures in Celsius system;
32°F, 68°F, -40°F for temperatures in Fahrenheit system;
273.15°K, 293.15°K, 233.15°K for temperatures in Kelvin system.
Above are the examples of three different temperatures in three different measurement systems, correspondingly.

The conversion formulas are:
X°F = 5(X−32)/9°C
X°C = (X+273.15)°K

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.
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.
Finally, in Kelvin system 0°K is so-called absolute zerotemperature - 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.

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