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Chapter 18 Second Law of Thermodynamics

Chapter 18 Second Law of Thermodynamics. 400K. heat. 300K. heat. Introduction. First law → conservation of energy. Processes that conserve energy may not occur. Some process occurs spontaneously, but its inverse process can not. It is about the second law of thermodynamics. 2.

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Chapter 18 Second Law of Thermodynamics

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  1. Chapter 18 Second Law of Thermodynamics

  2. 400K heat 300K heat Introduction First law → conservation of energy Processes that conserve energy may not occur Some process occurs spontaneously, but its inverse process can not. It is about the second law of thermodynamics 2

  3. working substance QH P W QL V Heat engines (1) High TH & Low TL (2) Run in a repeating cycle

  4. Refrigerator QH P W QL V Engine efficiency Efficiency of heat engine: Counterclockwise cycle: Coefficient of performance 4

  5. 1 1 P 2 2 V Reversible & irreversible processes Reversible process: can be done in reverse with no change in magnitude of W and Q No friction, quasi-static reverse in the same path Ideal model as well Real processes are irreversible Friction, turbulence, … no PV diagram! 5

  6. P A B C V V 2V Efficiency of engine Example1: The PV diagram shows a reversibleheat engine. Calculate the efficiency. working substance: Argon (Ar) Solution: isothermal isochoric isobaric 6

  7. Efficiency of another engine Homework: The PV diagram shows a reversibleheat engine. Calculate theefficiency. working substance: Argon (Ar) A P adiabatic isochoric B C isobaric V V 2V 7

  8. a b d c Carnot engine N. L. Carnot: make use of Carnot cycle: 8

  9. Carnot’s theorem The efficiency of a Carnot engine depends only on the temperature TL and TH. All reversible engines operating between the same two constant temperature TH and TL have the same efficiency. Any irreversible engine will have an efficiency less than this. —— Carnot’s theorem 9

  10. 2nd law of thermodynamics (1) Carnot’s theorem: Kelvin-Planck statement of the second law: No device is possible that the only effect is to transform heat completely into work. Or: There is no perfect (e=100%) heat engine. → 3rd law of thermodynamics Energy sources & thermal pollution 10

  11. False advertising Example2: An engine manufacturer claims that: The heat input the engine is 9kJ/s at 200℃, the heat output is 4kJ/s at 30 ℃. Do you believe it? Solution: The efficiency of the engine is can’t be believed The maximum possible efficiency: 11

  12. QH P W QL V 2nd law of thermodynamics (2) Coefficient of performance Clausius statement of the 2nd law: No device is possible whose only effect is to transfer heat from a low T system into a high T system. For an ideal refrigerator: 12

  13. TH TH QH QL W=QH QH+QL TL QL TL Equivalence of two statements If the Kelvin statement is violated The Clausius statement is also violated So Clausius → Kelvin, and vice-versa 13

  14. Refrigerator &Heat pump Example3:It is -7 ℃ insidean ideal refrigerator working in a 27 ℃ room. (a)How much work is done to take 1000J from food inside it? (b) How much heat flows to the room? Solution: (a)Coefficient of performance Work done: (b) Heat pumped: 14

  15. p V Entropy (1) General statement of 2nd law by using a quantity Carnot’s cycle: Heats flow into system: For any reversible cycle: 15

  16. c p . b . a d V Entropy (2) Integral between a and b not depend on path! Define a new quantity “entropy”: 16

  17. Entropy (3) 1) Entropy is a state variable. 2) Need not to know the absolute value of S Key point is ΔS, like the potential energy 3) For irreversible process: Still have: 17

  18. Mixing water Example4:1kg water at 20℃ is mixed with 1kg water at 80℃. Calculate the change in entropy. Solution: 20℃→50℃: 80℃→50℃: Net change in entropy: 18

  19. P i . . f V Free expansion Example5:Adiabatic free expansion from V to 2V. Calculate the change in entropy. Solution: isothermal Reversible process:sameT, V→ 2V ΔS for the environment? 19

  20. Entropy Increase Principle Reversible process: Irreversible process: Quantitative general statement of the 2nd law: The entropy of an isolated system never decreases. Or: The total entropy of any system plus that of its environment increases in any natural process. Also noted as the principle of entropy increase 20

  21. Order to disorder (1) Entropy: a measure of the disorder of system Natural processes tend to move toward a state of greater disorder. 21

  22. 400K 300K . . heat . . . . . . . . . . . . . . . . . . . . . . . . Order to disorder (2) ordered energy → disordered energy different average energy → same average energy ordered position → disordered position 22

  23. waste heat light solar energy sugar fat, … H2O, CO2, … O2 human body, civilizations waste heat Living beings & entropy A living being is an ordered system It needs negative entropy (by E. Schrödinger) Food & excretion → metabolism Circle of life 23

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