As an example, suppose we want to find out how the internal energy varies with
temperature when the pressure rather than the volume of the system is kept constant.
If we divide both sides of eqn 2.41 (dU = πTdV + CV dT) by dT and impose the condition
of constant pressure on the resulting differentials, so that dU/dT on the left
becomes (∂U/∂T)p, we obtain