One application of thermodynamics is the transfer of thermal energy into work in an engine. In general, this cyclic process involves an exchange of heat with two reservoirs, heat in at a high temperature and heat out at a low temperature, resulting in net positive work from the process. We use PV diagrams to describe the work done and heat exchange in each step of the cycle. There is a limit to the work you can get out of an engine compared to the net thermal energy you put in. Conservation of energy (first law of thermodynamics) says that you cannot get more energy out than you put in, i.e., you can only break even. But the second law of thermodynamics says that you cannot even break even, you can only lose. The second law of thermodynamics says that, as time goes forward, entropy (disorder) increases. It takes energy to decrease entropy. In this chapter we will pull together the ideas of work, heat, and entropy to show how everything fits together in simplified engines.

- Illustration 21.1: Carnot Engine Cycle.
- Illustration 21.2: Entropy and Probability.
- Illustration 21.3: Entropy and Heat.
- Illustration 21.4: Engines and Entropy.

- Exploration 21.1: Engine Efficiency.
- Exploration 21.2: Internal Combustion Engine.
- Exploration 21.3: Entropy, Probability, and Microstates.
- Exploration 21.4: Entropy of Expanding Ideal Gas.

- Problem 21.1: Three cycle engine.
- Problem 21.2: Otto engine.
- Problem 21.3: Brayton cycle.
- Problem 21.4: Internal combustion engine.
- Problem 21.5: Engine cycle and PV diagram.
- Problem 21.6: Match the expansion to the TS graph.
- Problem 21.7: Find the entropy of an expansion.
- Problem 21.8: Refrigerator and entropy.