When the final volume is greater than the initial volume, as in an expansion, the
logarithm in eqn 2.10 is positive and hence w < 0. In this case, the system has done
work on the surroundings and there is a corresponding reduction in its internal
energy. (Note the cautious language: we shall see later that there is a compensating
influx of energy as heat, so overall the internal energy is constant for the isothermal
expansion of a perfect gas.)