We know that if heat is not transferred when the two systems are in contact, then it is said to be in thermal equilibrium state and their temperature is same.
Let 1g molecule of gas with volume and temperature be V and T respectively and number of molecules of 1 mol of gas (NA) = 6.02 × 1023 molecules. According to kinetic theory of gases, the pressure of gas for 1 g molecules is :
where v1, v2, v3, .....vn respectively are the speeds of each molecule, then
Equation (4) is a relation between the average kinetic energy of each molecule and temperature. It is clear that E and T are directly proportional to each other, i. e.,
\(\bar E \propto T\)
i. e., kinetic energy of each molecule of a gas is a measure of temperature and at absolute zero temperature, i.e.,
So, at absolute zero temperature, the molecules are motionless. Thus the absolute zero temperature is the temperature at which the speed of all molecules is zero.
