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CHAPTER2_SECTION2 - 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.
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