1. The problem statement, all variables and given/known data I think I understand the first 3 steps of the Carnot cycle but not the 4th. 2. Relevant equations The cycle here: http://en.wikipedia.org/wiki/Carnot_cycle#Stages 3. The attempt at a solution I understand that in stage 1, the gas expands by taking in heat from the hot reservoir and expanding to maintain constant temperature of the high temp reservoir. Then in stage 2, the gas is allowed to do work on the surroundings by expanding more, this time cooling as it expands because it's not taking in any heat from the hot reservoir as it's adiabatic. For stage 3, the cold reservoir takes away heat from the gas allowing it to compress it at constant temperature - the gas would cool if it was in contact with the cold reservoir at constant volume, but allowing it to compress keeps it at constant temperature. Then in stage 4, the gas apparently compresses because the surroundings do work on it. But where is this work coming from? If the gas is at the cool temperature, it will be near enough at the same temperature as the surroundings so why would there be a pressure difference to allow it to compress?