Announcements 9/27/10 5 days left to get your clicker registered Exam review session: Thurs, 8-9:30 pm, room C460 Reading assignment for Wednesday: a. a.New: Read the Colton “What is entropy?” handout (also available on website). It’s complicated, but spend at least minutes glancing over it. Don’t worry about the examples for now. We’ll go over it in class on Wednesday. – – If it seems super complicated, don’t stress too much. You don’t have to know details for the exam. b. b.On syllabus: Also spend 5-10 minutes on the book section Read the marble example (Ex. 22.7, in my edition), but don’t worry about the other example (Ex. 22.8, my edition). Your reading quizzes on Wed. will be simply: a. a.Did you spend minutes looking over the “What is Entropy” handout? b. b.Did you spend 5-10 minutes reading through section 22.8?

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

Time for some physics humor Xkcd comic: Thermodynamics song: a. a.

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

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

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?

Thermodynamics For the air in this room, right now: a. a.Microstate = ? b. b.Macrostate = ? Hold this thought until Wednesday

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

“Proof” by example, monatomic gas Path 1: A  C  B Path 2: A  D  B (D  B = isothermal) A B P V C D V1V1 2V 1 4V 1 P1P1 2P 1 Path 1: A  C + C  B Path 2: A  D + D  B Equal?

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 Wednesday I’ll explain how/why this quantity is related to microstates & macrostates

 S for “free expansion” What is V 2 ? T 2 ? P 2 ? How to find  S?  S for adiabatic? Adiabats as constant entropy contours (“isentropic” changes) Wait… isn’t “free expansion” an adiabatic process? beforeafter

 S for isothermal?  S for const. volume?  S for const. pressure?

 S of Universe  S of gas doesn’t depend on path (state variable): What about  S of surroundings? What about  S total =  S gas +  S surroundings ? A B P V

Thought Question What is required in order for the entropy of a system to decrease? a. a.It must undergo an irreversible process b. b.The system must be isolated c. c.The system must not be isolated d. d.The initial temperature must be higher than the final temp. e. e.It is never possible for a system’s entropy to decrease!