Turning work into heat is easy—just rub two objects together. Work from the friction force increases the objects’ thermal energy and their temperature. Heat energy is then transferred from the warmer objects to the cooler environment. FIGURE 21.3 is the energy-transfer diagram for this process. The conversion of work into heat is 100% efficient in that all the energy supplied to the system as work is ultimately transferred to the environment as heat.

FIGURE 21.3 Work can be transformed into heat with 100% efficiency.

The reverse—transforming heat into work—isn’t so easy. Heat can be transformed into work in a one-time process, such as an isothermal expansion of a gas, but at the end the system is not restored to its initial state. To be practical, a device that transforms heat into work must return to its initial state at the end of the process and be ready for continued use. You want your car engine to turn over and over for as long as there’s fuel.

Interestingly, no one has ever invented a “perfect engine” that transforms heat into work with 100% efficiency and returns to its initial state so that it can continue to do work as long as there is fuel. Of course, that such a device has not been invented is not a proof that it can’t be done. We’ll provide a proof shortly, but for now we’ll make the hypothesis that the process of FIGURE 21.4 is somehow forbidden.

FIGURE 21.4 There are no perfect engines that turn heat into work with 100% efficiency.

Notice the asymmetry between Figures 21.3 and 21.4. The perfect transformation of work into heat is permitted, but the perfect transformation of heat into work is forbidden. This asymmetry parallels the asymmetry of the two processes in Figure 21.2. In fact, we’ll soon see that the “perfect engine” of Figure 21.4 is forbidden for exactly the same reason: the second law of thermodynamics.