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 non-zero 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:
1-6, 2-5, 3-4, 4-3, 5-2, and 6-1. The numbers 2 and 12 are the least probable because they
could only occur one way each: either 1-1, or 6-6. Thus, you are six times more likely to
throw a 7 than a 2 or 12.
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