Page 279 - Schaum's Outline of Theory and Problems of Applied Physics

264 HEAT TRANSFER [CHAP. 22

22.6. A concrete wall 6.0 m long, 3.5 m high, and 25 cm thick has a conductivity of 0.80 W/(m· C). When the wall has
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an outside temperature of 5 C and an inside temperature of 20 C, heat flows through it at
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(a) 10 W (c) 1.6 kW
(b) 1.0 kW (d) 1.7 kW
2
22.7. Heat flows through a wooden board 30 mm thick at 0.0086 W/cm when one of its sides is 20 C warmer than the
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other. The thermal conductivity of the wood is
(a) 0.013 W/(m· C) (c) 0.52 W/(m· C)
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(b) 0.13 W/(m· C) (d) 0.78 W/(m· C)
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2
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22.8. Glass wool has a thermal conductivity of 0.30 Btu/(ft ·h· F/in.). An R–3.0 layer of glass wool has a thickness of
(a) 1 in. (c) 3 in.
(b) 0.9 in. (d) 10 in.
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22.9. The temperature of the earth’s surface averages 15 C. If the earth were a blackbody, it would radiate energy at a rate of
(a) 0.0029 W/m 2 (c) 288 W/m 2
(b) 251 W/m 2 (d) 390 W/m 2

22.10. The bare skin of a certain person is at an average temperature of 33 C in a room whose temperature is 20 C.
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If e = 1, the net rate at which the person loses energy by radiation is
(a) 0.16 µW/cm 2 (c) 50 mW/cm 2
(b) 7.9 mW/cm 2 (d) 79 W/cm 2

22.11. An iron bar at 200 C radiates energy at 50 W. At 250 C the same bar will radiate at
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(a) 55 W (c) 75 W
(b) 63 W (d) 122 W

Supplementary Problems

22.1. Outdoors in the winter, why does a piece of metal feel colder than a piece of wood?

22.2. Why is it desirable to paint hot-water pipes with aluminum paint?

22.3. An icebox whose walls consist of a 100-mm thickness of pine wood is to be replaced by a more modern one using
glass wool insulation between pine inner and outer walls 12 mm thick. What thickness of glass wool will give the
same degree of insulation as before? The thermal conductivities of pine and glass wool are, respectively, 0.13 and
0.04 W/(m· C).
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2
22.4. A yacht has an aluminum hull 5 mm thick whose underwater area is 100 m . How much heat is conducted from the
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yacht’s interior to the water per hour if the interior is at a temperature of 20 C and the water is at 12 C? The thermal
conductivity of aluminum is 205 W/(m· C).
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22.5. How many Btu/day are conducted through a glass window 5 ft by 8 ft by 0.3 in. whose inner and outer faces are at
2
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the respective temperatures of 65 and 40 F? The thermal conductivity of glass is 5.5 Btu(ft · h· F/in.).
22.6. A sheet of R–0.06 plasterboard is 3 in. thick. What is the thermal conductivity of plasterboard?
8
22.7. A double-glazed window consists of two panes of glass separated by an airspace. To see why this is such a good
idea, compute the thermal resistances of two 0.2-in. glass panes (a) when they are in contact and (b) when they are
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0.3 in. apart. The thermal conductivities of glass and air are, respectively, 5.5 and 0.17 Btu(ft · h· F/in.).
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