When there are no interactions between the molecules, the internal energy is independent
of their separation and hence independent of the volume of the sample
(Section 2.2a). Therefore, for a perfect gas we can write πT = 0. The statement πT = 0
(that is, the internal energy is independent of the volume occupied by the sample)
can be taken to be the definition of a perfect gas, for later we shall see that it implies
the equation of state pV ∝ T. If the attractive forces between the particles dominate
the repulsive forces, then the internal energy increases (dU > 0) as the volume of the
sample increases (dV > 0) and the molecules attract each other less strongly; in this
case a plot of internal energy against volume slopes upwards and πT > 0 (Fig. 2.24).