In addition, the total change of entropy in both thermal reservoirs over Carnot cycle is zero too, since the inversion of a heat transfer direction means a sign inversion for the heat transferred during isothermal stages: − − Q H T H − − Q C T C = Δ Δ S r , H + Δ Δ S r , C = 0 Here we denote the entropy change for a thermal reservoir by Δ Δ S r , i = − − Q i / T i , where i is either H for a hot reservoir or C for a cold one.