[Physics] Are all reversible processes adiabatic


My understanding is that in a reversible process there is no increase in entropy, it remains constant. So $\Delta S = 0$ no?

And since it's reversible we know from the second law that $\Delta S = \frac{\delta Q_{\mathrm{rev}}}{T}$.

So doesn't a process being reversible mean that $\delta Q_{\mathrm{rev}} = 0$ ?

Best Answer

The formula for the change in entropy $$ \mathrm{d}S = \frac{\delta Q_\mathrm{rev}}{T} $$ refers to the entropy change of the system. For a reversible process the total change in entropy of the system and its surroundings will be $0$. The reason for this is that for a reversible process temperature of the system and its surroundings must only differ infinitesimally (otherwise an infinitesimal change could not reverse the process) and clearly the heat transferred to the system is the negative of the heat transferred to the surroundings so \begin{align} \mathrm{d}S_\mathrm{sys} + \mathrm{d}S_\mathrm{sur} & = \frac{\delta Q_\mathrm{sys}}{T} + \frac{\delta Q_\mathrm{sur}}{T}\\ & = \frac{\delta Q_\mathrm{sys} - \delta Q_\mathrm{sys}}{T}\\ & = 0 \end{align}