1. Physics 101: Lecture 28, Pg 1 Physics 101: Lecture 28 Thermodynamics II l Today’s lecture will cover Textbook Chapter 15.6-15.9 Final Check Final Exam Room Assignment! Bring ID! Be sure to check all of our grades are correct! Final Exam review Wed. at usual lecture time

2. Physics 101: Lecture 28, Pg 2 Final Exam l 45 Problems l Roughly even distribution l Study è Old hour exams è Discussion Quizzes è Homework è Practice problems 07

3. Physics 101: Lecture 28, Pg 3Recap: è 1st Law of Thermodynamics è energy conservation Q =  U + W Heat flow into system Increase in internal energy of system Work done by system V P l U depends only on T (U = 3nRT/2 = 3pV/2) l point on p-V plot completely specifies state of system (pV = nRT) l work done is area under curve l for complete cycle  U=0  Q=W 09

4. Physics 101: Lecture 28, Pg 4 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

5. Physics 101: Lecture 28, Pg 5 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

6. Physics 101: Lecture 28, Pg 6 Preflight Lect 27 Consider a hypothetical device that takes 1000 J of heat from a hot reservoir at 300K, ejects 200 J of heat to a cold reservoir at 100K, and produces 800 J of work. Does this device violate the first law of thermodynamics ? 1. Yes 2. No correct l W (800) = Q hot (1000) - Q cold (200) l Efficiency = W/Q hot = 800/1000 = 80% 80% efficient 20% efficient 25% efficient 16

7. Physics 101: Lecture 28, Pg 7 17

8. Physics 101: Lecture 28, Pg 8 Heat Engine ACT l Can you get “work” out of a heat engine, if the hottest thing you have is at room temperature? 1) Yes2) No T H 300K T C = 77K QHQH QCQC W HEAT ENGINE 19

9. Physics 101: Lecture 28, Pg 9 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

10. Physics 101: Lecture 28, Pg 10 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

11. Physics 101: Lecture 28, Pg 11ACT A hot (98 C) slab of metal is placed in a cool (5C) bucket of water. What happens to the entropy of the metal? A) IncreaseB) SameC) Decreases What happens to the entropy of the water? A) IncreaseB) SameC) Decreases What happens to the total entropy (water+metal)? A) Increase B) Same C) Decreases  S = Q/T  S = Q/T water – Q/T metal Heat leaves metal: Q<0 Heat enters water: Q>0 29

12. Physics 101: Lecture 28, Pg 12 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

13. Physics 101: Lecture 28, Pg 13 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

14. Physics 101: Lecture 28, Pg 14 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

15. Physics 101: Lecture 28, Pg 15 Example Consider a hypothetical refrigerator that takes 1000 J of heat from a cold reservoir at 100K and ejects 1200 J of heat to a hot reservoir at 300K. 1. How much work does the refrigerator do? 2. What happens to the entropy of the universe? 3. Does this violate the 2nd law of thermodynamics? Answers: 200 J Decreases yes THTH TCTC QHQH QCQC W Q C = 1000 J Q H = 1200 J Since Q C + W = Q H, W = 200 J  S H = Q H /T H = (1200 J) / (300 K) = 4 J/K  S C = -Q C /T C = (-1000 J) / (100 K) = -10 J/K  S TOTAL =  S H +  S C = -6 J/K  decreases (violates 2 nd law) 39

16. Physics 101: Lecture 28, Pg 16 Preflight LECT 28 Consider a hypothetical device that takes 1000 J of heat from a hot reservoir at 300K, ejects 200 J of heat to a cold reservoir at 100K, and produces 800 J of work. Does this device violate the second law of thermodynamics ? 1. Yes 2. No correct l W (800) = Q hot (1000) - Q cold (200) l Efficiency = W/Q hot = 800/1000 = 80% l Max eff = 1-T c /T h =1 - 100/300 = 67%  S H = Q H /T H = (-1000 J) / (300 K) = -3.33 J/K  S C = +Q C /T C = (+200 J) / (100 K) = +2 J/K  S TOTAL =  S H +  S C = -1.33 J/K  (violates 2 nd law) 41 -1000J/300K = -3.33 J/K, 200J/100K = 2 J/K Total = -1.33 J/K (Should not be negative) It's just like the last preflight...isn't it?

17. Physics 101: Lecture 28, Pg 17 Preflight 2 Last preflight EVER! Yea! This class has never been dull \ But I find that my head is so full \ of equations, I dread \ I can't hold up my head. \ I blame gravitational pull. I'm gonna be real honest with you here. I have no idea what the answer to the question is. I'm tired, Its time for finals, and I just want my question put up on the board. PLEASE! put this in the lecture, I've been waiting all semester!!

18. Physics 101: Lecture 28, Pg 18 Preflight 28, cont. Today is our last day of class. \ I very much hope that I pass. \ Although it's December, \ All I can remember \ Is force is acceleration times mass. Shell-dog and I are wondering what "ACT" means in our lecture practice problems. I'm too heartbroken that physics is almost over to explain my answer.

19. Physics 101: Lecture 28, Pg 19 Preflight 3 Which of the following is forbidden by the second law of thermodynamics? 1. Heat flows into a gas and the temperature falls 2. The temperature of a gas rises without any heat flowing into it 3. Heat flows spontaneously from a cold to a hot reservoir 4. All of the above Answer: 3 43

20. Physics 101: Lecture 28, Pg 20 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 Second Law: Entropy always increases! l Carnot Cycle: Reversible, Maximum Efficiency e = 1 – T c /T h 50