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Announcements 9/26/11

Announcements 9/26/11. Exam review session: Friday, 4 pm, room C460 Reading assignment for Wednesday: Section 22.8 – Especially read the marble example (Ex. 22.7, in my edition), but don’t worry about the “Adiabatic Free Expansion: One Last Time” example (Ex. 22.8, in my edition).

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Announcements 9/26/11

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  1. Announcements 9/26/11 • Exam review session: Friday, 4 pm, room C460 • Reading assignment for Wednesday: • Section 22.8 – Especially read the marble example (Ex. 22.7, in my edition), but don’t worry about the “Adiabatic Free Expansion: One Last Time” example (Ex. 22.8, in my edition). • The “What is entropy?” handout posted to website – Read up through Example 1. Please spend at least ~10 minutes glancing over it, or you will likely be really confused in class on Friday. xkcd

  2. 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

  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 = ? (Just called the “state”) The state the air is in will be “very close” to the one that has the most number of microstates. Next time: Entropy of a state  #Microstates in the state The state the air is in will be “very close” to the one with the highest entropy. Hold this thought until next time

  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 Wed I will explain how/why this quantity is related to microstates & macrostates

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

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

  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)

  13. Thermodynamics Song • http://www.uky.edu/~holler/CHE107/media/first_second_law.mp3 (takes 4:13)

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