Understanding the Second Law of Thermodynamics: Processes, Efficiency, and Applications

1. Chapter 20 The Second Law of Thermodynamics

2. Introduction • Melting butter on a hot cob of corn is a delicious way to transform a solid to a liquid. • Nature seems to trend this process everywhere you look. Fancy houses of cards fall down, your house gets dirty all by itself, liquids evaporate, solids melt, order to disorder. • In fact, you must invest energy to force order from disorder (the clean house is my “favorite” example). 

3. Directions for thermodynamic processes • As we already mentioned, the maximum useful outcome will come from a reversible process (like taking a building apart piece by piece) instead of an irreversible process (like imploding the same building with explosives). • We also see the natural tendency of nature favors disorder over order. (It is hard to build a tower, but easier to knock it down with explosives.)

4. Q20.2 An ideal gas is taken around the cycle shown in this pV–diagram, from a to b to c and back to a. Process b c is isothermal. Which of the processes in this cycle could be reversible? A. a b B. b c C. c a D. two or more of A., B., and C. E. none of A., B., or C.

5. A20.2 An ideal gas is taken around the cycle shown in this pV–diagram, from a to b to c and back to a. Process b c is isothermal. Which of the processes in this cycle could be reversible? A. a b B. b c C. c a D. two or more of A., B., and C. E. none of A., B., or C.

6. Q20.3 An ideal gas is taken around the cycle shown in this pV–diagram, from a to c to b and back to a. Process c b is adiabatic. Which of the processes in this cycle could be reversible? A. a c B. c b C. b a D. two or more of A., B., and C. E. none of A., B., or C.

7. A20.3 An ideal gas is taken around the cycle shown in this pV–diagram, from a to c to b and back to a. Process c b is adiabatic. Which of the processes in this cycle could be reversible? A. a c B. c b C. b a D. two or more of A., B., and C. E. none of A., B., or C.

8. Heat engines • As heat flows from a reservoir at higher temperature to a sink at lower temperature, work may be removed. • Even if no work is removed, maximum engine efficiencies never reach 100% and depend on Th and Tc .

9. Q20.4 During one cycle, an automobile engine takes in 12,000 J of heat and discards 9000 J of heat. What is the efficiency of this engine? A. 400% B. 133% C. 75% D. 33% E. 25%

10. A20.4 During one cycle, an automobile engine takes in 12,000 J of heat and discards 9000 J of heat. What is the efficiency of this engine? A. 400% B. 133% C. 75% D. 33% E. 25%

11. Q20.5 During one cycle, an automobile engine with an efficiency of 20% takes in 10,000 J of heat. How much work does the engine do per cycle? A. 8000 J B. 6400 J C. 2000 J D. 1600 J E. 400 J

12. A20.5 During one cycle, an automobile engine with an efficiency of 20% takes in 10,000 J of heat. How much work does the engine do per cycle? A. 8000 J B. 6400 J C. 2000 J D. 1600 J E. 400 J

13. Analyze heat engine • A gasoline engine in a truck takes in 10,000 J of heat and delivers 2000 J of mechanical work per cycle. The heat is obtained by burning gasoline with heat of combustion Lc = 5.0 x 104 J/g. • What is the thermal efficiency of this engine? • How much heat is discarded in each cycle? • How much gasoline is burned I each cycle? • If the engine goes through 25 cycles per second, what is its power output in watts?

14. The internal-combustion engine • A fuel vapor can be compressed, then detonated to rebound the cylinder, doing useful work.

15. The Otto cycle and the Diesel cycle • A fuel vapor can be compressed, then detonated to rebound the cylinder, doing useful work.

16. Refrigerators • A refrigerator is essentially a heat engine running backwards.

17. Air conditioning, the clever placement of an air conditioner

18. The Second Law stated in practical terms • You can’t make a machine that does nothing but move heat from a cold item to a hot sink.

19. Q20.6 A copper pot at room temperature is filled with room-temperature water. Imagine a process whereby the water spontaneously freezes and the pot becomes hot. Why is such a process impossible? A. It violates the first law of thermodynamics. B. It violates the second law of thermodynamics. C. It violates both the first and second laws of thermodynamics. D. none of the above

20. A20.6 A copper pot at room temperature is filled with room-temperature water. Imagine a process whereby the water spontaneously freezes and the pot becomes hot. Why is such a process impossible? A. It violates the first law of thermodynamics. B. It violates the second law of thermodynamics. C. It violates both the first and second laws of thermodynamics. D. none of the above

21. The Carnot Cycle • A thought experiment envisioning the most efficient heat engine that might be created. • Reversible processes of isothermal expansion, adiabatic expansion, isothermal compression, then finally adiabatic compression.

22. Q20.7 A Carnot engine takes heat in from a reservoir at 400 K and discards heat to a reservoir at 300 K. If the engine does 12,000 J of work per cycle, how much heat does it take in per cycle? A. 48,000 J B. 24,000 J C. 16,000 J D. 9000 J E. none of the above

23. A20.7 A Carnot engine takes heat in from a reservoir at 400 K and discards heat to a reservoir at 300 K. If the engine does 12,000 J of work per cycle, how much heat does it take in per cycle? A. 48,000 J B. 24,000 J C. 16,000 J D. 9000 J E. none of the above

24. Analysis of Carnot Cycles • Follow Example 20.2 and Figure 20.14. • Follow Example 20.3.

25. Reverse a standard Carnot Cycle to make a refrigerator • Follow Example 20.4 illustrated by Figure 20.15.

26. Entropy and order (or disorder) • As mentioned in the introduction, disorder is the natural direction of the universe. The firecracker explosion at right is a solid burning very quickly. • Follow Example 20.5. • Follow Example 20.6.

27. Entropy in gas expansion • Follow Conceptual Example 20.7. • Follow Example 20.8 aided by Figure 20.19. • Follow Example 20.9.

28. Entropy is not a conserved quantity • An irreversible process can be modeled by many small Carnot Cycles. • Consider Example 20.10.

29. Entropy may be treated microscopically • Tossing a coin helps to picture the outcomes of a statistic event. To model atoms or molecules, we need ~1023 “coins” with many possible outcomes (not just “heads” or “tails”).

30. A gas as the microscopic, statistical model • Follow Example 20.11, aided by Figure 20.25.