Announcements – 9/23/11. Prayer Wednesday is last lecture on Thermodynamics Reading assignment for Wed is posted to class website: the “What is entropy” handout in Supplementary Material section at bottom Results of doodle.com voting: Exam review will be Fri 9/30/11, 4 pm, room C460. Pearls
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Done last time:
V2 = V1 (P1/P2)1/g = 0.4562 m3
Error recognized since I knew |Qh| – |Qc| had to be positive
The issue: I overspecified the parameters. Specifically, P3 cannot be 200 kPa. P2V2 = P3V3 P3 = 136.87 kPa
state B; TB = 650K
state A; TA = 300K
*Other books’ terminology: reversible vs totally reversible.
“C” for “Carnot”
HW 11-5 – 11-7: find efficiency for a specific Carnot cycle
Optional HW: eC derived for a general Carnot cycle
Th = max temp of cycle
Tc = min temp of cycle
Clausius: Heat energy does not spontaneously flow from cold to hot.
Kelvin-Plank: You can’t fully convert all heat to work.
What if you could make heat go from coldhot?
What if you could make a perfect engine? Then use it to power a refrigerator.
Then do this:
Bottom line: you could build a system to do that, but it couldn’t be built from an engine/heat reservoirs that look like this:
(Obvious? It’s really: “Only a totally reversible…”)
Why not this?
Which work is bigger? Can you see the problem?
Image from Wikipedia
Image from http://web.me.unr.edu/me372/Spring2001/The%20Brayton%20Cycle%20with%20Regeneration.pdf
(who apparently got it from a textbook, but I’m not sure which one)