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Chapter 12
The Laws of Thermodynamics
Quick Quizzes
1.
(b). The work done on a gas during a thermodynamic process is the
negative
of the area
under the curve on a
PV
diagram. Processes in which the volume decreases do positive
work on the gas, while processes in which the volume increases do negative work on the
gas. The work done on the gas in each of the four processes shown is:
,
W
,
W
, and
Thus, the correct ranking (from most negative to most positive) is a,c,b,d.
5
4.00
10 J
a
W
=−
×
5
3.00
b
=+
×
5
3.00
c
×
5
4.00
d
W
×
2.
A
is isovolumetric,
B
is adiabatic,
C
is isothermal,
D
is isobaric.
3.
(c). The highest theoretical efficiency of an engine is the Carnot efficiency given by
1
cc
eT
h
T
. Hence, the theoretically possible efficiencies of the given engines are:
A
700 K
10
1000 K
e
=
.
3
0
0
,
B
500 K
1
0.375
800 K
e
=
, and
C
300 K
600 K
e
=
.
5
0
0
and the correct ranking (from highest to lowest) is C, B, A.
4.
(b).
0
r
Q
S
T
=
∆=
and
Q
in an adiabatic process. If the process was reversible, but not
adiabatic, the entropy of the system could undergo a nonzero change. However, in that
case, the entropy of the system’s surroundings would undergo a change of equal
magnitude but opposite sign, and the total change of entropy in the universe would be
zero. If the process was irreversible, the total entropy of the universe would increase.
0
=
5.
The number 7 is the most probable outcome because there are six ways this could occur:
16, 25, 34, 43, 52, and 61. The numbers 2 and 12 are the least probable because they
could only occur one way each: either 11, or 66. Thus, you are six times more likely to
throw a 7 than a 2 or 12.
431
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CHAPTER 12
Answers to Even Numbered Conceptual Questions
2.
Either statement can be considered an instructive analogy. We choose to take the first
view. All processes require energy, either as energy content or as energy input. The kinetic
energy which it possessed at its formation continues to make Earth go around. Energy
released by nuclear reactions in the core of the Sun drives weather on Earth and
essentially all processes in the biosphere. The energy intensity of sunlight controls how
lush a forest or jungle can be and how warm a planet is. Continuous energy input is not
required for the motion of the planet. Continuous energy input is required for life because
energy tends to be continuously degraded, as energy is transferred by heat into lower
temperature sinks. The continuously increasing entropy of the Universe is the index to
energytransfers completed.
4.
Shaking opens up spaces between the jelly beans. The smaller ones have a chance of
falling down into spaces below them. The accumulation of larger ones on top and smaller
ones on the bottom implies an increase in order and a decrease in one contribution to the
total entropy. However, the second law is not violated and the total entropy of the system
increases. The increase in the internal energy of the system comes from the work required
to shake the jar of beans (that is, work your muscles must do, with an increase in entropy
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 Spring '08
 pralle
 Thermodynamics, Work

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