9. Absolute temperature

Charles's law takes a particularly simple form if a new temperature scale is introduced instead of the Celsius scale. Let the temperature \(T\) on the new scale be related to the Celsius temperature \(t\) by the equation

\( T = t \, + \, 273^0 \)(1-6)

Then, considering that the thermal coefficient of volume expansion \(k = {1 \over 273} \, degrees^{-1}\), the Charles's law (1-2) can be written in the form:

\( V = V_0(1 + kt) = V_0 {\Big[1 + k {\left( T - {1 \over k } \right)} \Big]} = V_0kT\) (1-7)

The temperature determined by the equation (1-6) is called the absolute temperature. The absolute temperature scale is called the Kelvin scale by the name of the English scientist who introduced it.

Zero temperature on the Kelvin scale is called absolute zero temperature, \( -273^0 \, C \). None of the bodies, as we will see later (see section, 32), can be cooled down below absolute zero. That is why the absolute temperature scale is not as arbitrary as the Celsius scale.

For now we introduced an absolute temperature scale quite formally. In the chapter of molecular-kinetic theory it will be shown that the absolute temperature is directly related to the average kinetic energy of atoms and molecules. At T=0 the thermal motion of atoms and molecules stops.