FTFS Chap02 P080

# FTFS Chap02 P080 - Chapter 2 Basic Concepts of...

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Chapter 2 Basic Concepts of Thermodynamics Review Problems 2-80E 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. 2-81E 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. 2-82E 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 2-36

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Chapter 2 Basic Concepts of Thermodynamics 2-83E 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 2-84E 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. 2-85 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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FTFS Chap02 P080 - Chapter 2 Basic Concepts of...

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