Page 280 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 22] HEAT TRANSFER 265

22.8. A 3-kg beef roast requires 13 min to be heated from 40 to 45 C in an oven maintained at 200 C. How long will the
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same roast take to be heated from 60 to 65 C in the same oven?
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22.9. A small hole leading into a cavity behaves as a blackbody because any radiation that falls on it is trapped inside by
multiple reflections until it is absorbed. How many watts are radiated from a hole 1 cm in diameter in the wall of a
furnace whose interior temperature is 650 C?
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22.10. A blackbody is at a temperature of 500 C. What should its temperature be in order that it radiates twice as much
energy per second?
22.11. Sunspots appear dark even though they are actually very hot (typically 5000 K) because the rest of the solar surface
is even hotter (about 6000 K). Find the ratio between the radiation rates of a sunspot and the normal solar surface.

Answers to Multiple-Choice Questions

22.1. (d) 22.7. (b)

22.2. (c) 22.8. (b)

22.3. (d) 22.9. (d)

22.4. (a), (b), (d) 22.10. (b)

22.5. (b) 22.11. (c)
22.6. (b)

Answers to Supplementary Problems

22.1. Metals are much better conductors of heat than wood and therefore conduct heat away from the hand more rapidly.

22.2. Such paint gives a finish that reflects most of the light that falls on it. Since a poor absorber of radiation is also a
poor emitter of radiation, a pipe painted in this way radiates relatively little heat.

22.3. 23 mm

22.4. 118 GJ
5
22.5. 4.4 ×10 Btu
2
22.6. 6.25 Btu/(ft ·h· F/in.)
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22.7. (a) R–0.07 (b) R–4.18

22.8. 15 min
22.9. 3.23 W

22.10. 646 C
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22.11. R spot /R sun = 0.48