1. Physics 101: Lecture 26, Pg 1 Physics 101: Lecture 26 Thermodynamics II Final

2. Physics 101: Lecture 26, Pg 2 THTH TCTC QHQH QCQC W HEAT ENGINE THTH TCTC QHQH QCQC W REFRIGERATOR system l system taken in closed cycle   U system = 0 l therefore, net heat absorbed = work done Q H - Q C = W (engine) Q C - Q H = -W (refrigerator) energy into green blob = energy leaving green blob Engines and Refrigerators 11

3. Physics 101: Lecture 26, Pg 3 THTH TCTC QHQH QCQC W HEAT ENGINE The objective: turn heat from hot reservoir into work The cost: “waste heat” 1st Law: Q H -Q C = W efficiency e  W/Q H =W/Q H = (Q H -Q C )/Q H = 1-Q C /Q H Heat Engine: Efficiency 13

4. Physics 101: Lecture 26, Pg 4 17

5. Physics 101: Lecture 26, Pg 5 THTH TCTC QHQH QCQC W REFRIGERATOR The objective: remove heat from cold reservoir The cost: work 1st Law: Q H = W + Q C coeff of performance K r  Q C /W = Q C /W = Q C /(Q H - Q C ) Refrigerator: Coefficient of Performance 22

6. Physics 101: Lecture 26, Pg 6 New concept: Entropy (S) l A measure of “disorder” l A property of a system (just like p, V, T, U) è related to number of number of different “states” of system l Examples of increasing entropy: è ice cube melts è gases expand into vacuum l Change in entropy: è  S = Q/T »>0 if heat flows into system (Q>0) »<0 if heat flows out of system (Q<0) 25

7. Physics 101: Lecture 26, Pg 7 Second Law of Thermodynamics l The entropy change (Q/T) of the system+environment  0 è never < 0 è order to disorder l Consequences è A “disordered” state cannot spontaneously transform into an “ordered” state è No engine operating between two reservoirs can be more efficient than one that produces 0 change in entropy. This is called a “Carnot engine” 31

8. Physics 101: Lecture 26, Pg 8 Carnot Cycle l Idealized Heat Engine è No Friction   S = Q/T = 0 è Reversible Process »Isothermal Expansion »Adiabatic Expansion »Isothermal Compression »Adiabatic Compression 32

9. Physics 101: Lecture 26, Pg 9 THTH TCTC QHQH QCQC W HEAT ENGINE The objective: turn heat from hot reservoir into work The cost: “waste heat” 1st Law: Q H -Q C = W efficiency e  W/Q H =W/Q H = 1-Q C /Q H  S = Q C /T C - Q H /T H  0  S = 0 for Carnot Therefore, Q C /Q H  T C / T H Q C /Q H = T C / T H for Carnot Therefore e = 1 - Q C /Q H  1 - T C / T H e = 1 - T C / T H for Carnot e = 1 is forbidden! e largest if T C << T H Engines and the 2nd Law 36

10. Physics 101: Lecture 26, Pg 10 Summary l First Law of thermodynamics: Energy Conservation  Q =  U + W l Heat Engines è Efficiency = = 1-Q C /Q H l Refrigerators è Coefficient of Performance = Q C /(Q H - Q C ) Entropy  S = Q/T l 2 nd Law: Entropy always increases! l Carnot Cycle: Reversible, Maximum Efficiency e = 1 – T c /T h 50