announcements 2/2/11 exam review session (tentative): friday, 3-4:30 pm a. a.i will send email...
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Announcements 2/2/11 Exam review session (tentative): Friday, 3-4:30
pma. I will send email tomorrow with final date/time,
and room location. (Vote today if you haven’t yet!)
Reading assignment for Friday: a. Section 22.8 – Especially the marble example
(Ex. 22.7, in my edition), but not the “Adiabatic Free Expansion: One Last Time” example (Ex. 22.8, in my edition).
b. 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.
Reading quiz Which of the following is a version of the
Second Law of Thermodynamics?a. The entropy of any system decreases
in all real processesb. The entropy of any system increases in
all real processesc. The entropy of the Universe decreases
in all real processesd. The entropy of the Universe increases
in all real processes
Time for some physics humor Xkcd comic:
Thermodynamics song:a. http://www.uky.edu/~holler/CHE107/media/first_second_la
w.mp3
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 lefta. microstate = A spades, K spd, Q spd, J spd, 10 spdb. macrostate = ?c. How many microstates make up that macrostate?
Hand on righta. microstate = 2 spades, 3 diam, 7 heart, 8 clubs, Q
diamb. macrostate = ?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. Microstate = ?b. Macrostate = ?
Hold this thought until Friday
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”
AB
P
V
B
A
dQ
T
“Proof” by example, monatomic gas Path 1: ACB Path 2: ADB
(DB = isothermal)
ln ln 2
ln ln 2
ln ln 4
ln1ln 2
C CV
V C A V
A A
B BP
P B C P
C C
D DP
P D A P
A A
B BB Don
D D
nC dTdQnC T T nC
T T
nC dTdQnC T T nC
T T
nC dTdQnC T T nC
T T
nRT V VworkdQ QdQ nR
T T T T T
A
BP
V
C
D
V1 2V1 4V1
P1
2P1
Path 1: AC + CB
Path 2: AD + DB
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!
B
AB
A
dQS
T
Advertisement: On Friday I’ll (try to) explain how/why this quantity is related to microstates & macrostates
S for “free expansion”
What is V2? T2? P2? How to find S?
S for adiabatic?
Adiabats as constant entropy contours (“isentropic” changes)
Wait… isn’t “free expansion” an adiabatic process?
before after