The ‘equipartition theorem’ of classical mechanics was introduced in Section F.5.
According to it, the average energy of each quadratic contribution to the energy is
kT. As we saw in Section F.5, the mean energy of the atoms free to move in three
dimensions is kT and the total energy of a monatomic perfect gas is NkT, or nRT
(because N = nNA and R = NAk). We can therefore write