Conceptual Physics

Chapter 21 Problems

Conceptual Problems

C.1 An amount of gas in a well-insulated container is compressed reversibly and adiabatically to half its initial volume. Does the
entropy of the gas increase, decrease, or stay the same?

i. Increases

ii. Decreases

iii. Stays the same

C.2 A cup of hot coffee is placed inside a large, well-insulated container. Over a five minute period, the entropy of the cup

decreases by 100 J/K. What can you say about the entropy of the air surrounding the cup?

i. It does not change

ii. It increases less than 100 J/K

iii. It increases by exactly 100 J/K

iv. It increases by more than 100 J/K

v. It decreases

C.3 A refrigerator operates much like a heat pump, doing work to transfer heat from its interior to its exterior. If you leave a
refrigerator running with its door open in the middle of a well-insulated room, does the room temperature go up, go down, or
remain unchanged? Explain.
The room temperature i. goes up
ii. goes down
iii. is unchanged

Section Problems

Section 1 - Efficiency

1.1 An engine running at 38.3% efficiency receives 287 J of heat from the hot reservoir during each cycle. How much heat is
transferred to the cold reservoir during each cycle?
J

1.2 An engine does 189 J of net work during a cycle, using 329 J of heat in from the hot reservoir in the process. What is the
engine's percent efficiency?

%

1.3 A two-stroke engine runs at 2300 rpm, which means it performs 2300 engine cycles each minute. If the engine does

9.2×10^5 J of net work each minute, and has an efficiency of 0.43, how much heat is transferred into the engine during a

cycle?

J

1.4 During one engine cycle, a lawnmower engine does 55.2 J of net work. In the process, it expels 153 J of energy as heat.
What is the percent efficiency of the engine?
%

Section 4 - Entropy

4.1 In a reversible process called isothermal expansion, 457 J of heat are transferred into a quantity of gas in an engine from the

hot reservoir, while 352 J of heat are transferred out to the cold reservoir. The temperature of the gas is 473 K at the

beginning and end of the process. What is the change in entropy of the gas?

J/K

Section 5 - Second law of thermodynamics: entropy

5.1 Two large blocks are momentarily placed in contact. Block A has temperature 567 K and transfers 849 J of heat energy to

block B, which has temperature 288 K. The temperatures of the blocks are unchanged. What is the change in entropy of the

system consisting of the two blocks?

J/K

Copyright 2007 Kinetic Books Co. Chapter 21 Problems^397