Method The definition of entropy instructs us to find the energy supplied as heat
for a reversible path between the stated initial and final states regardless of the
actual manner in which the process takes place. A simplification is that the expansion
is isothermal, so the temperature is a constant and may be taken outside the
integral in eqn 3.2. The energy absorbed as heat during a reversible isothermal
expansion of a perfect gas can be calculated from ΔU = q + w and ΔU = 0, which
implies that q = −w in general and therefore that qrev = −wrev for a reversible change.
The work of reversible isothermal expansion was calculated in Section 2.3.