1. Dr. Jie ZouPHY 13611 Chapter 22 Heat Engines, Entropy, and the Second Law of Thermodynamics

2. Dr. Jie ZouPHY 13612 Outline Heat engines (22.1) What is a heat engine? Thermal efficiency e of a heat engine Second law of thermodynamics (22.1) Kelvin-Planck form of the second law of thermodynamics Heat pumps and refrigerators (22.2, a brief discussion) Clausius statement of the second law of thermodynamics

3. Dr. Jie ZouPHY 13613 An example-a steam engine A schematic steam engine

4. Dr. Jie ZouPHY 13614 Heat engines Heat engine: a device that takes in energy by heat and, operating in a cyclic process, expels a fraction of that energy by means of work. Operation of a heat engine: A heat engine carries some working substance through a cyclic process during which (1) the working substance absorbs energy by heat from a high-temperature energy reservoir, (2) work is done by the engine, and (3) energy is expelled by heat to a lower- temperature reservoir. W eng = |Q h |-|Q c |: the net work W eng done by a heat engine is equal to the net energy Q net transferred to it. Schematic representation of a heat engine

5. Dr. Jie ZouPHY 13615 Thermal efficiency e of a heat engine Thermal efficiency e : the ratio of the net work done by the engine during one cycle to the energy input at the higher temperature during the cycle. In practice, all heat engines expel only a fraction of the input energy Q h by mechanical work and e < 100%. A good automobile engine: e ~ 20%. A diesel engine: e = 35% - 40%. Question: Does a “perfect” heat engine exist?

6. Dr. Jie ZouPHY 13616 Kelvin-Planck form of the second law of thermodynamics It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the input of engine by heat from a reservoir and the performance of an equal amount of work. Meaning: This statement of the second law means that, during the operation of a heat engine, W eng can never be equal to |Q h |, or, alternatively, that some energy |Q c | must be rejected to the environment. Engineering point of view: The most important implication of the second law is the limited efficiency of heat engines. A machine that operates in a cycle, taking in energy by heat and expelling an equal amount of energy by work, cannot be constructed.

7. Dr. Jie ZouPHY 13617 Example 22.1 Find the efficiency of a heat engine that absorbs 2000 J of energy from a hot reservoir and exhausts 1500 J to a cold reservoir. (A) Find the efficiency of the engine. (B) How much work does this engine do in one cycle. Answer: (A) 25.0%. (B) 5.0x10 2 J.

8. Dr. Jie ZouPHY 13618 Heat pumps and refrigerators (a brief discussion) The natural direction of heat transfer is from the hot reservoir to the cold reservoir. Refrigerators, air conditioners, and heat pumps are devices that use work to transfer heat from a cold object to a hot object. Question: Does a “perfect” heat pump exist? Schematic representation of a heat pump

9. Dr. Jie ZouPHY 13619 Clausius statement of the second law of thermodynamics It is impossible to construct a cyclical machine whose sole effect is to transfer energy continuously by heat from one object to another object at a higher temperature without the input of energy by work. Meaning: Energy does not transfer spontaneously by heat from a cold object to a hot object. This direction of energy transfer requires an input of energy to a heat pump, which is often supplied by means of electricity.

10. Dr. Jie ZouPHY 136110 Homework Ch. 22, P. 700, Problems: 1, 2, 4.