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Physics 1220/1320

Physics 1220/1320. Thermodynamics & Electromagnetism Chapter 19 -20. Ideal Gas:. Phase Diagram: Substance expanding on melting. Ideal Gas to real gas: Phase transitions Here: liquid/gas. Phase diagrams – 3-dim pVT system. Examples of quasi-static processes: isothermal constant T

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Physics 1220/1320

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  1. Physics 1220/1320 Thermodynamics & Electromagnetism Chapter 19 -20

  2. Ideal Gas: Phase Diagram: Substance expanding on melting

  3. Ideal Gas to real gas: Phase transitions Here: liquid/gas

  4. Phase diagrams – 3-dim pVT system

  5. Examples of quasi-static processes: isothermal constant T isobaric constant p isochoric constant V adiabatic no heat flow

  6. 1st Law of Thermodynamics Conservation of energy:

  7. State Functions

  8. Graphical representation of quasi-static processes

  9. State Functions Ideal gas - Isothermal processes

  10. g = Cp/Cv= (5/2R)/(3/2R)=5/3

  11. TVg-1 = const.

  12. Second Law of Thermodynamics: gives direction to processes No system can undergo a process where heat is absorbed and convert the heat into work with the system ending in the state where it began: No perpetuum mobile.

  13. Heat engines: heaters and refrigerators b/c heat cannot flow from a colder to a hotter body w/o a cost (work). iow e=100% is not possible! Reversible vs irreversible processes.

  14. Early heat engines:

  15. 4 stroke or Otto engine intake stroke compression stroke ignition power stroke exhaust stroke Bottom right Ta, bottom left Tb Use T*Vg-1 = const. law for the two adiabatic processes For r=8, g = 1.4 e=56%

  16. Diesel Cycle Typical rexp ~ 15, rcomp ~ 5

  17. Carnot Cycle The best-efficiency cycle 2 reversible isothermal and 2 reversible adiabatic processes.

  18. There are many other useful state functions: the thermodynamic potentials Enthalpy H = U + PV Free energy at const P, T G = U + PV - TS Free energy at const. T F = U – TS Entropy S etc. The first law revised (for a p-V-T system): DU= TdS - pdV

  19. Entropy: Cost of Order – macroscopic interpretation: reversible and irreversible processes Total entropy change zero: reversible Use dQ = S dT in first law!

  20. Entropy ~ microscopic interpretation w is no. of possible states

  21. The 3rd Law of Thermodynamics 3rd Law: It is impossible to reach absolute zero.

  22. ‘Fun’ mnemonic about thermodynamic laws: 1st Law ‘You can’t win, you can only break even’ 2nd Law ‘You can break even only at absolute zero’ 3rd Law ‘You cannot reach absolute zero’ Moral: one can neither win nor break even The American Scientist , March 1964, page 40A The laws of thermodynamics give processes a direction

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