Temperature Units: UNIZOR.COM - Physics4Teens - Units in Physics - Base SI Units
Channel: Zor Shekhtman on UNIZOR Education
Date: 2023-04-21

UNIZOR.COM - Physics through a Looking Glass of Mathematician
Notes to a video lecture on http://www.unizor.com

For a short introduction to the International System of units (SI) see the previous lecture "SI Intro & Time" within this topic.

Temperature Units

As a prerequisite to this lecture, we suggest to familiarize yourself with the lecture "Temperature, Pressure and Volume of Ideal Gas" of the topic "Molecular Movement" in the chapter "Energy" of this course.

As we have established in that lecture, the temperature of some substance is a measure of kinetic energy of molecules of this substance. If we set a temperature of a substance with no movements of its molecules to zero, average kinetic energy of moving molecules of this substance is proportional to a substance temperature measured in some units. The coefficient of proportionality between the temperature and average kinetic energy of molecules is different for different substances under different conditions.

So, first of all, we have to assign the zero temperature to a state of a substance (or 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.

Secondly, we have to choose a unit of temperature.
Until recently the most popular scale of temperatures was Celsius (°C) with 0°C assigned to a temperature of melting ice and 100°C assigned to a boiling water with a unit of measurement being 1/100th of a difference between these two points on a scale.
This same unit was used in a scale that starts with the absolute zero temperature called Kelvin scale.

Therefore, Kelvin scale for temperature was with zero temperature corresponding to a state of molecules at complete rest and a unit of measurement of the temperature was a degree with the distance between the temperature of melting ice and boiling water assigned as 100 degrees on this scale.

In 2019 this definition of units of measurement for temperature was revised in favor of defining a unit of temperature based on some physical constant.

For this purpose was chosen Boltzmann's constant introduced in the lecture referenced above that stated the inter-dependency of the main characteristics of ideal gas - pressure (p), volume (V), number of molecules (N) and absolute temperature (T) - the Combined Ideal Gas Law:
p·V/T = kB·N
where
kB = 1.380649·10^(−23) (J/K)
is the Boltzmann's constant.

In the spirit of other redefinition of units in SI to be defined based on some physical constants, the value of Boltzmann's constant was postulated to be EXACTLY equal to 1.380649·10−23 (J/K), which fixed the value of the unit of Kelvin temperature called kelvin (not a degree, as in Celsius).

Now the temperature of a substance with no molecular movement will be 0K.
Temperature of melting ice will be 273.15K (equivalent to 0°C).
Temperature of boiling water will be 373.15K (equivalent to 100°C).