Page 271 - Schaum's Outline of Theory and Problems of Applied Physics

256 THERMODYNAMICS [CHAP. 21

21.10. The heat a refrigerator absorbs from its contents is
(a) less than it gives off
(b) the same amount it gives off
(c) more than it gives off
(d) any of the above, depending on its design

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21.11. Four kilojoules of heat is given off by a gas when it is compressed from 0.08 to 0.05 m under a pressure of 200 kPa.
The internal energy of the gas
(a) decreases
(b) is unchanged
(c) increases
(d) any of the above, depending on the initial and final temperatures

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21.12. A Carnot engine that absorbs heat at 300 C and exhausts heat at 100 C has an efficiency of
(a) 33% (c) 65%
(b) 35% (d) 67%

21.13. If a Carnot engine absorbs 10 kJ of heat per cycle when it operates between 500 and 400 K, the work it does per
cycle is
(a) 2 kJ (c) 8 kJ
(b) 2.5 kJ (d) 10 kJ

21.14. To have an efficiency of 40 percent, a heat engine that exhausts heat at 350 K must absorb heat at no less than
(a) 210 K (c) 875 K
(b) 583 K (d) 1038 K
21.15. The coefficient of performance of a refrigerator that gives off 3 J of heat for every joule of mechanical energy input is

(a) 1 (c) 3
(b) 2 (d) 4

Supplementary Problems

21.1. Why is it impossible for a ship to use the internal energy of seawater to operate its engine?

21.2. One kilojoule of heat is added to a gas as it expands from 8 to 10 L at a constant pressure of 2 bar. What is the change
in the internal energy of the gas?
21.3. A 2400-hp diesel locomotive burns 160 gal/h of fuel. If the heat of combustion of the diesel oil used is 1.2 × 10 5
Btu/gal, find the efficiency of the engine.
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21.4. Steam enters a turbine engine at 550 C and emerges at 90 C. The engine has an actual overall efficiency of 35 percent.
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What percentage of its ideal efficiency is this?
21.5. The six-cylinder, four-cycle gasoline engine of a car has pistons 3.4 in. in diameter whose stroke (length of travel) is
2
4 in. If the mean effective pressure on the pistons during the power stroke is 70 lb/in. , find the number of horsepower
developed by the engine when it operates at 2000 rev/min.
21.6. A four-cylinder, four-stroke diesel engine develops 100 kW at 66 rev/s. The pistons of the engine are 108 mm in
diameter and their travel is 127 mm. Find the average pressure on the pistons during the power stroke.