Chapter 20: Entropy and 2nd Law of Thermodynamics
Questions and Example Problems
Suniverse  0
S 
1
dQ
T
ò
W
QH

QH  QL
QH
òC  1 
TL
TH
Questions 20
a. A refrigerator is kept in a garage that is not heated in the cold winter or airconditioned in the hot summer. Does it cost more for this refrigerator to make a
kilogram of ice cubes in the winter or in the summer?
b. When water freezes from a less-ordered liquid to a more-ordered solid, its entropy
decreases. Why doesn’t this decrease in entropy violate the second law of
thermodynamics?
c. A glass of water contains a teaspoon of dissolved sugar. After a while, the water
evaporates, leaving behind sugar crystals. The entropy of the sugar crystals is less
than the entropy of the dissolved sugar because the sugar crystals are in moreordered state. Why doesn’t this process violate the second law of thermodynamics
d. Suppose you want to increase the efficiency of a heat engine. Would it be better to
increase TH or to decrease TC by an equal amount? Why?
e. What would be the efficiency of a Carnot engine operating with TH = TC? What would
be the efficiency if TC = 0K and TH were any temperature above 0K? Interpret your
answers.
Example 20.1
A gasoline engine in a large truck takes in 10,000 J of heat and delivers 2000 J of
mechanical work per cycle. The heat is obtained by burning gasoline with heat of
combustion LC = 5.0×104 J/g.
a. What is the thermal efficiency of this engine?
b. How much heat is discarded in each cycle
Example 20.2
Analyze this heat cycle to determine (a) the net work done per cycle, (b) the engine’s
thermal efficiency, and (c) the engine’s power output if it runs at 600 rpm. Assume the
gas is monatomic.
20-1
Example 20.3
The figure shows a reversible cycle through which 1 mol of a
monatomic ideal gas is taken where Vb =103 m3 and pb =10
atm. For the cycle, find
a. the energy added to the gas as heat
b. the energy leaving the gas as heat
c. the network done by the gas
d. the efficiency of the cycle
20-2