In one cycle of a reversible engine running between a high-temperature source and a low-temperature sink, a consequence
of the second law of thermodynamics is that QH/TH = QL/TL. This result is independent of what kind of material the engine
contains. It is instructive to check this general result for a specific model where we can calculate everything explicitly.
Consider a reversible cycle of an ideal gas of N molecules, starting at high temperature TH and volume V1.
(a) Perform a constant-temperature (isothermal) expansion to volume V2, and calculate the associated energy transfer
QH into the gas.
(b) Next perform a no-Q (adiabatic) expansion to volume V3 and temperature TL.
(c) Next perform a constant-temperature (isothermal) compression to volume V4, and calculate the associated energy
transfer QL out of the gas.
(d) Finally perform a no-Q (adiabatic) compression back to the starting state, temperature TH and volume V1.