Fig. 2.17 To achieve a change of state from
one temperature and volume to another
temperature and volume, we may consider
the overall change as composed of two
steps. In the first step, the system expands
at constant temperature; there is no change
in internal energy if the system consists
of a perfect gas. In the second step, the
temperature of the system is reduced at
constant volume. The overall change in
internal energy is the sum of the changes
for the two steps.