Page 272 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 21] THERMODYNAMICS 257

21.7. A Carnot engine absorbs 200 kJ of heat at 500 K and exhausts 150 kJ. What is its exhaust temperature?

21.8. A Carnot engine absorbs 1 MJ of heat at 327 C and exhausts heat to a reservoir at 127 C. How much work does
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it do?
21.9. A Carnot engine whose efficiency is 35 percent absorbs heat at 500 C. What must its intake temperature be if its
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efficiency is to be 50 percent with the same exhaust temperature?
5
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21.10. A Carnot engine absorbs 500 Btu of heat at 500 F and performs 1 ×10 ft·lb of work. What is its exhaust temperature?
21.11. Three designs for a refrigerator to operate between −20 and 30 C are proposed. Design A is claimed to need 100 J
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of work per kilojoule of heat extracted, design B to need 200 J, and design C to need 300 J. Which design would
you choose and why?
21.12. Describe the operating cycle of a four-stroke diesel engine with the help of a p-V diagram.

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21.13. An ideal refrigerator extracts heat from a freezer at −20 C and exhausts it at 50 C. How many joules of heat are
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extracted per joule of work input?
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21.14. How much work must an ideal refrigerator perform to make 1 kg of ice at −10 C from 1 kg of water at 20 C when
20 C is also its exhaust temperature?
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21.15. The energy efficiency ratio (EER) of a refrigerator or air conditioner is the ratio between the heat in Btu extracted per
hour and the power in watts the machine uses. Find the relationship between EER and coefficient of performance.
21.16. What horsepower rating is needed for the motor of a 5-ton air conditioner whose coefficient of performance is 2.5?

Answers to Multiple-Choice Questions

21.1. (b) 21.9. (b)

21.2. (d) 21.10. (a)

21.3. (c) 21.11. (c)
21.4. (a), (b), (c) 21.12. (b)

21.5. (a) 21.13. (a)

21.6. (a) 21.14. (b)

21.7. (c) 21.15. (b)
21.8. (d)

Answers to Supplementary Problems

21.1. There would be no suitable low-temperature reservoir to absorb the waste heat from the engine.

21.2. The internal energy increases by 600 J.