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Maximizing Efficiency: Heat Engines and the Second Law of Thermodynamics

Explore the principles of heat engines and the Second Law of Thermodynamics, including efficiency calculations and entropy concepts. Learn how heat flows, work is performed, and why efficiency cannot reach 100%. Discover the significance of irreversible processes and the increase in disorder. This comprehensive guide covers everything from heat engine flow charts to the Carnot cycle and the role of entropy in determining system disorder. Improve your understanding of thermodynamics and the fundamental laws governing energy transformations.

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Maximizing Efficiency: Heat Engines and the Second Law of Thermodynamics

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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

  5. 0

  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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