Suppose that two heat engines are connected in series, such that the heat exhaust of the first engine is used as the heat input of the second engine as shown in figure. The efficiencies of the engines are η₁ and η₂, respectively. The net efficiency of the combination is given by

Question

G1 Physics · Accepted Answer

Let the first engine receive heat Q₁. It produces W₁=η₁Q₁ and rejects Q₂=(1-η₁)Q₁ to the second engine. The second produces W₂=η₂Q₂=η₂(1-η₁)Q₁. Hence ηnet=(W₁+W₂)/Q₁=η₁+(1-η₁)η₂. Option C.