Reversible transformations

A transformation is reversible if and only if the total entropy does not change, that is $\Delta S_\text{tot} = 0$. This condition is far more selective than the first law alone: imposing it on the sum $\Delta S_\text{gas} + \Delta S_\text{term}$ and adding, in turn, a different constraint ($V$ constant, $P$ constant, $T$ constant, or $Q=0$) isolates exactly four ideal processes. These are the only transformations that can be represented as well-defined curves in the $P$-$V$ diagram, and the starting point for building heat engines.

• What makes a transformation reversible
• General formula for the entropy of the gas
• Reversible isochore
• Reversible isobar
• Reversible isotherm
• Reversible adiabat
• Summary table of the transformations