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Reading quiz

Reading quiz. Which of the following is a version of the Second Law of Thermodynamics? The entropy of any system decreases in all real processes The entropy of any system increases in all real processes The entropy of the Universe decreases in all real processes

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Reading quiz

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  1. Reading quiz • Which of the following is a version of the Second Law of Thermodynamics? • The entropy of any system decreases in all real processes • The entropy of any system increases in all real processes • The entropy of the Universe decreases in all real processes • The entropy of the Universe increases in all real processes

  2. Time for some physics humor • Xkcd comic: • Thermodynamics song: • http://www.uky.edu/~holler/CHE107/media/first_second_law.mp3

  3. Second Law • Clausius: Heat spontaneously flows from hot to cold, not the other way around • Why? Order. • Which hand is more likely?

  4. Microstates vs Macrostates • Hand on left • microstate = A spades, K spd, Q spd, J spd, 10 spd • macrostate = ? • How many microstates make up that macrostate? • Hand on right • microstate = 2 spades, 3 diam, 7 heart, 8 clubs, Q diam • macrostate = ? • How many microstates make up that macrostate? • The most common macrostates are those that…

  5. Probability  Heat flow • You separate a deck into two halves: one is 70% red, 30% black; the other is 30% red, 70% black. What will happen if you randomly exchange cards between the two?

  6. Thermodynamics • For the air in this room, right now: • Microstate = ? • Macrostate = ? Hold this thought until Friday

  7. A New State Variable • State variables we know: P, V, T, Eint • Observation: doesn’t depend on path  Something is a state variable! • Assumption: path is well defined, T exists whole time  “Internally reversible” P B A V

  8. P C B 2P1 P1 A D V V1 2V1 4V1 Equal? “Proof” by example, monatomic gas • Path 1: ACB • Path 2: ADB (DB = isothermal) • Path 1: AC + CB • Path 2: AD + DB

  9. Entropy: S • Assume S = 0 is defined somewhere. (That’s actually the Third Law, not mentioned in your textbook.) • Integral only defined for internally reversible paths, but… • S is a state variable! …so it doesn’t matter what path you use to calculate it! Advertisement: On Friday I’ll (try to) explain how/why this quantity is related to microstates & macrostates

  10. DS for “free expansion” before after • What is V2? T2? P2? • How to find DS? DS for adiabatic? • Adiabats as constant entropy contours (“isentropic” changes) • Wait… isn’t “free expansion” an adiabatic process?

  11. DS for isothermal? DS for const. volume? DS for const. pressure?

  12. DS of Universe • S of gas doesn’t depend on path (state variable): • What about DS of surroundings? • What about DStotal = DSgas + DSsurroundings? P B A V (See HW problem 12-4)

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