For heat \(\Delta \mathrm{Q}\) transferred, the entropy of region 1 decreases according to \(\Delta \mathrm{S}_{1}=-\Delta \mathrm{Q} / \mathrm{T}_{1}\), while the entropy of region 2 increases by \(\...For heat \(\Delta \mathrm{Q}\) transferred, the entropy of region 1 decreases according to \(\Delta \mathrm{S}_{1}=-\Delta \mathrm{Q} / \mathrm{T}_{1}\), while the entropy of region 2 increases by \(\Delta \mathrm{S}_{2}=\Delta \mathrm{Q} / \mathrm{T}_{2}\). In the idealized limit of very small, but nonzero temperature difference, the flow of heat is said to be reversible because the generation of entropy is negligible.
