5. Heat Engine: Efficiency 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 13 T H T C Q H Q C W HEAT ENGINE

7. 17

9. Refrigerator: Coefficient of Performance The objective: remove heat from cold reservoir The cost: work 1st Law: Q H = W + Q C coefficient of performance K r  Q C /W = Q C /(Q H - Q C ) 22 T H T C Q H Q C W REFRIGERATOR

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 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 T H T C Q H Q C W

17. Preflight 2, cont. MAN THIS CLASS WAS SOMETHING ELSE!!!LAST PREFLIGHT WOP! WOP! Last preflight of the year!!!!!!!!!!!!!!!! Im studying. This is not helpful. because physics is magical IMMM SOO DONE WITH PHYSICSSSSSSSSSSSSSSSSS :) did you know that Charlie Brown's father was a barber?

18. 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