SI & Units of Time: UNIZOR.COM - Physics4Teens - Units in Physics - Base Units

Notes to a video lecture on http://www.unizor.com

SI & Units of Time


Short Introduction to SI

Christopher Columbus has landed in the Bahamas in 1492, but was sure it's somewhere in Asia.
Historians attribute this mistake to a difference between Roman miles
(1 Roman mile ≅ 1.48 km)
and nautical ones
(1 nautical miles ≅ 1.85 km).

In September of 1999 NASA has lost a Mars Climate Orbiter spacecraft because its different components were made based on different units of measurement - most were made in metric units, but some were based on old English units.

There were other incidents, when incompatibility of components was caused by different units of measurement.

There is no need to explain the necessity to have common measuring units, so people can meaningfully communicate with each other.

In contemporary Physics we have an international system of units of measurement called SI.
A few lectures on this topic would conclude the course "Physics 4 Teens".

The SI units are divided into two categories - base units and derived units.

Base units are the units to measure the following quantitative physical characteristics:

time,
length,
mass,
electric current,
temperature,
quantity (of matter),
luminosity.

The most important aspect of defining physical units of measurement is to connect them to processes that to a high degree of certainty are constant in our world, like the frequency of electromagnetic oscillations emitted by a specific element or the speed of light in vacuum.

This approach is a reverse of an old one, when the units of measurement are defined subjectively.

Previously, the unit of time, the second, was defined as some fraction of a cycle of some concrete clock stored in a concrete place.
Analogously, the unit of length, the meter, was defined as the length of a concrete metal rod stored under some conditions in some laboratory in Paris.
This approach does not qualify for precise measurements because these "standard" second or meter are subject to change as the time goes on, conditions are slightly changing etc.

Instead, after measuring some objectively constant process, we define a unit of time or a unit of length as based on that process.


Time Units

The unit of time in SI is a second.
Historically, units of time were based on some astronomical observation, like a period of rotation of a Moon around our Earth or a period of rotation of the Earth around the Sun.
The problem was, these definitions are not constant because the rotations and their periods do change.

To make this unit of time really standard and independent of changing conditions of astronomical objects or concrete man-made clocks, the contemporary definition of the second in simple terms is as follows.

Physicists experimented with atoms of cesium-133 (⁵⁵Cs¹³³). Cesium is a soft metal with very low melting temperature of 28.4°C (83.1°F).
Their observation involved putting atoms of cesium-133 into a microwave beam of electromagnetic oscillations until the atoms vibrate in resonance. They have found the resonant frequency of these oscillations to be 9,192,631,770 oscillations per second. This is the frequency of photons that excite the outer electrons of a cesium atom to jump to an excited orbit.

Since the physical properties of this kind can be considered constant everywhere within a broad spectrum of conditions, it was decided that the good definition of a second can be based on this property.

The definition of a second in SI is that this is a period of time during which the number of oscillations of electromagnetic field that exited the atom of cesium-133, when it switches between the excited and ground level, is equal to 9,192,631,770.

This is a theoretical definition. From a practical standpoint, there is a device called a cesium clock that is the most precise time measuring instrument developed. This device was constructed and refined in 1950's by British physicist Essen.
Its precision was at the time one second in about 300 years. With subsequent modernizations, which also involved the analysis of a spin of cesium nucleus, the precision of cesium clock now is one second in more than a million years. The cesium clock is a very complicated device, it's specification is beyond the level of this course.

The abbreviation of the second in SI is s (sometimes, sec is used).

Derived from the second time unit are its fractions
1 millisecond (ms) = 10⁻³ sec
1 microsecond (μs) = 10⁻⁶ sec
1 nanosecond (ns) = 10⁻⁹ sec
1 picosecond (ps) = 10⁻¹² sec
as well as old familiar units
1 minute (min) = 60 sec
1 hour (hr) = 3600 sec