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19 Second Law Thermo

19 Second Law Thermo. Heat Engines and 2 nd Law Thermodynamics Hk: 27, 35. Second Law of Thermodynamics. Heat flows spontaneously from a substance at a higher temperature to a substance at a lower temperature. The reverse situation does not occur spontaneously.

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19 Second Law Thermo

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  1. 19 Second Law Thermo • Heat Engines and 2nd Law Thermodynamics • Hk: 27, 35.

  2. Second Law of Thermodynamics • Heat flows spontaneously from a substance at a higher temperature to a substance at a lower temperature. • The reverse situation does not occur spontaneously. • Ex. Hot drink cools but does not reheat

  3. Heat Engines • Device that uses heat to perform work • Hot Reservoir (e.g. steam) • Cool Reservoir (e.g. pool of water) • Efficiency is work done per unit of input heat (e = W/QH) • Ex. A heat engine does 100J of work when given 300J from the hot reservoir. The efficiency is 100J/300J = 0.33 = 33%.

  4. Heat Engine Flow Chart

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  6. Summary • Heat engines use part of heat and reject rest, efficiency = work out/heat in • 2nd Law of Thermodynamics prescribes that efficiency cannot be 100% • /

  7. 19-2Refrigerators and the SecondLaw of Thermodynamics

  8. Refrigerators and the Second Law of Therm.

  9. Omit 19-3Equivalence of the Heat-Engineand Refrigerator Statements

  10. 19-4The Carnot Engine

  11. Carnot Theorem

  12. The Carnot Cycle Heat Engine

  13. 19-5Heat Pumps Omit

  14. 19-6Irreversibility and Disorder

  15. Irreversible Processes Introduce Disorder. Example: Container of gas collides inelastically with wall. Translational kinetic energy is added to general thermal energy of gas

  16. 19-7Entropy

  17. Entropy • Entropy is a measure of the disorder of a system • Low entropy (ordered) • High entropy (disordered) • Reversible Process: Entropy is constant • Irreversible Process: Entropy increases

  18. Second Law in terms of Entropy • the total entropy of the universe does not change when a reversible process occurs. • the total entropy does increase when an irreversible process occurs.

  19. Omit 19-8Entropy and the Availabilityof Energy

  20. Omit 19-9Entropy and the Probability

  21. Summary: • Heat engines accept heat and output work and heat. Refrigerators reverse this process. Carnot engine is most efficient engine. • 2nd Law of Therm. dictates the direction of natural processes. • Irreversible processes increase disorder. • Entropy is a measure of disorder and increases for any irreversible process.

  22. 0 Heat engines typically have efficiencies in the range • 3% to 5% • 20% to 60% • 80% to 90%

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