Method To find a starting point for a calculation in thermodynamics, it is often a
good idea to go back to first principles and to look for a way of expressing the quantity
we are asked to calculate in terms of other quantities that are easier to calculate.
We saw in Section 2.2a that the internal energy of a perfect gas depends only on the
temperature and is independent of the volume those molecules occupy, so for any
isothermal change, ΔU = 0. We also know that in general ΔU = q + w. The question
depends on being able to combine the two expressions. We have already derived
a number of expressions for the work done in a variety of processes, and here we
need to select the appropriate ones.