Chapter 2
Basic Concepts of Thermodynamics
Review Problems
280E
The efficiency of a refrigerator increases by 3% per
°
C rise in the minimum temperature. This
increase is to be expressed per
°
F, K, and R rise in the minimum temperature.
Analysis
The magnitudes of 1 K and 1
°
C are identical, so are the magnitudes of 1 R and 1
°
F.
Also, a
change of 1 K or 1
°
C in temperature corresponds to a change of 1.8 R or 1.8
°
F.
Therefore, the increase in
efficiency is
(a)
3%
for each K rise in temperature, and
(b), (c)
3/1.8 =
1.67%
for each R or
°
F rise in temperature.
281E
The boiling temperature of water decreases by 3
°
C for each 1000 m rise in altitude. This decrease
in temperature is to be expressed in
°
F, K, and R.
Analysis
The magnitudes of 1 K and 1
°
C are identical, so are the magnitudes of 1 R and 1
°
F.
Also, a
change of 1 K or 1
°
C in temperature corresponds to a change of 1.8 R or 1.8
°
F.
Therefore, the decrease in
the boiling temperature is
(a)
3 K
for each 1000 m rise in altitude, and
(b), (c)
3
×
1.8 =
5.4
°
F = 5.4 R
for each 1000 m rise in altitude.
282E
The average body temperature of a person rises by about 2
°
C during strenuous exercise. This
increase in temperature is to be expressed in
°
F, K, and R.
Analysis
The magnitudes of 1 K and 1
°
C are identical, so are the magnitudes of 1 R and 1
°
F.
Also, a
change of 1 K or 1
°
C in temperature corresponds to a change of 1.8 R or 1.8
°
F.
Therefore, the rise in the
body temperature during strenuous exercise is
(a)
2 K
(b)
2
×
1.8 =
3.6
°
F
(c)
2
×
1.8 =
3.6 R
236
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Basic Concepts of Thermodynamics
283E
Hypothermia of 5
°
C is considered fatal. This fatal level temperature change of body temperature is
to be expressed in
°
F, K, and R.
Analysis
The magnitudes of 1 K and 1
°
C are identical, so are the magnitudes of 1 R and 1
°
F.
Also, a
change of 1 K or 1
°
C in temperature corresponds to a change of 1.8 R or 1.8
°
F.
Therefore, the fatal level
of hypothermia is
(a)
5 K
(b)
5
×
1.8 =
9
°
F
(c)
5
×
1.8 =
9 R
284E
A house is losing heat at a rate of 3000 kJ/h per
°
C temperature difference between the indoor and
the outdoor temperatures. The rate of heat loss is to be expressed per
°
F, K, and R of temperature
difference between the indoor and the outdoor temperatures.
Analysis
The magnitudes of 1 K and 1
°
C are identical, so are the magnitudes of 1 R and 1
°
F.
Also, a
change of 1 K or 1
°
C in temperature corresponds to a change of 1.8 R or 1.8
°
F.
Therefore, the rate of heat
loss from the house is
(a)
3000 kJ/h
per K difference in temperature, and
(b), (c)
3000/1.8 =
1667 kJ/h
per R or
°
F rise in temperature.
285
The average temperature of the atmosphere is expressed as
T
atm
= 288.15 – 6.5
z
where
z
is altitude in
km. The temperature outside an airplane cruising at 12,000 m is to be determined.
Analysis
Using the relation given, the average temperature of the atmosphere at an altitude of 12,000 m is
determined to be
T
atm
= 288.15  6.5z
= 288.15  6.5
×
12
=
210.15 K
= 
63
°
C
Discussion
This is the “average” temperature. The actual temperature at different times can be different.
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 Spring '07
 Dr.Kinne
 Thermodynamics

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