Key points (a) The change in internal energy may be expressed in terms of changes in temperature
and volume. The internal pressure is the variation of internal energy with volume at constant temperature.
(b) Joule’s experiment showed that the internal pressure of a perfect gas is zero.
(c) The change in internal energy with volume and temperature is expressed in terms of the
internal pressure and the heat capacity and leads to a general expression for the relation between
heat capacities.