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Mastering Physics 19.32

Mastering Physics 19.32. Requires Section 19.8 Will re-score to make 19.32 extra credit. Before We Start. Why is T  K trans ? Why is C larger when there are more modes? Why does energy partition between modes?. Thermodynamic Paths. energy transfers. § 19.3–19.4. Energy Transfers.

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Mastering Physics 19.32

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  1. Mastering Physics 19.32 • Requires Section 19.8 • Will re-score to make 19.32 extra credit

  2. Before We Start • Why is T Ktrans? • Why is C larger when there are more modes? • Why does energy partition between modes?

  3. Thermodynamic Paths energy transfers § 19.3–19.4

  4. Energy Transfers Between system and surroundings • Work • Heat

  5. V2 W = pdV V1 Work From a volume change of the system

  6. Heat From a temperature difference dQ/dTTsurr – Tsys

  7. Work and Heat Depend on the path taken between initial and final states.

  8. CPS Question The work done by a thermodynamic system in a cyclic process (final state is also the initial state) is zero. True. False. Source: Y&F, Figure 19.12

  9. W Cyclic Process W 0 Is the system a limitless source of work? (Of course not.) Source: Y&F, Figure 19.12

  10. Work between States W is not uniquely determined by initial and final states p V

  11. What Are the Processes? The more work a path between two states produces, the more heat must be provided. p V

  12. Group Work • Qualitatively sketch a pV plot for each described process AB. • Volume is gradually doubled with no heat input, then heated at constant volume to the initial temperature. • System is heated at constant pressure until volume doubles, then cooled at constant volume to the initial temperature. • System is allowed to expand into a vacuum (free expansion) to twice its volume. • Volume is gradually doubled while maintaining a constant temperature.

  13. Group Work • Rank the processes in descending order of W. • Rank the processes in descending order of Q.

  14. Conservation of Energy DE of a system = work done on the system + heat added to the system

  15. Internal Energy U U SKi + SSVij i i j<i • Ki = kinetic energy of molecule i wrt com • Vij = intermolecular potential energy of i and j • Does not include potential or kinetic energy of bulk object • Each thermodynamic state has a unique U (U is a state function)

  16. CPS Question All other things being equal, adding heat to a system increases its internal energy U. True. False.

  17. CPS Question All other things being equal, lifting a system to a greater height increases its internal energy U. True. False.

  18. CPS Question All other things being equal, accelerating a system to a greater speed increases its internal energy U. True. False.

  19. CPS Question All other things being equal, doing work to compress a system increases its internal energy U. True. False.

  20. First law of Thermodynamics DU = Q – W DU is path-independent

  21. Cyclic Processes DU = U1 – U1 = 0 so Q – W = 0 so Q = W • Work output = heat input

  22. Work out = Heat in Does this mean cyclic processes convert heat to work with 100% efficiency? (Of course not.) Waste heat is not recovered.

  23. I p II V Example Problem 19.17 A thermodynamic cycle consists of two closed loops, I and II. d) In each of the loops, I and II, does heat flow into or out of the system? c) Over one complete cycle, does heat flow into or out of the system?

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