Physics 207, Lecture 29, Dec. 13.ppt

Physics 207, Lecture 29, Dec. 13
Agenda: Finish Ch. 22, Start review, Evaluations
Heat engines and Second Law of thermodynamics
Carnot cycle
Reversible/irreversible processes and Entropy
Assignments:
Problem Set 11, Ch. 22: 6, 7, 17, 37, 46 (Due, Friday, Dec. 15, 11:59 PM)
Friday, Review
Heat Engines
Example: The Stirling cycle
V
P
TC
TH
Va
Vb
1
2
3
4
We can represent this cycle on a P-V diagram:
Gas
T=TH
Gas
T=TH
Gas
T=TC
Gas
T=TC
1
1
2
3
4
x
start
Heat Engines and the 2nd Law of Thermodynamics
A heat engine goes through a cycle (start and stop at the same point, same state variables)
1st Law gives
U = Q + W =0
What goes in e out
1st Law gives
Qh = Qc + Wcycle (Q’s > 0)
So (cycle work on world)
Qnet=|Qh| - |Qc| = -Wsystem = Wcycle
Hot reservoir
Cold reservoir
Engine
Qh
Qc
Wcycle
Engine
Efficiency of a Heat Engine
How can we define a “figure of merit” for a heat engine?
Define the efficiency e as:
Consider two heat engines:
Engine I:
Requires Qin = 100 J of heat added to system to get W=10 J of work (done on world in cycle)
Engine II:
To get W=10 J of work, Qout = 100 J of heat is exhausted to the pare eI, the efficiency of engine I, to eII, the efficiency of engine II.
(A) eI < eII
(B) eI > eII
(C) Not enough data to determine
Lecture 29: Exercise 1Efficiency
Reversible/irreversible processes andthe best engine, ever
Reversible process:
Every state along some path is an equilibrium state
The system can be returned to its initial conditions along the same path
Irreversible process;
Process which is not reversible !
All real physical processes are irreversible
. energy is lost through friction and the initial conditions cannot be reached along the same path
However, some processes are almost reversible
If they occur slowly enough (so that system is almost in equilibrium)
Carnot Cycle
Named for Sadi Carnot (1796- 1832)
Isothermal expansion
Adiabatic expansion
pression
pression
The Carnot cycle
The