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Chapter 14: Q21Q (page 390)
Explain why air temperature readings are always taken with the thermometer in the shade.
In the presence of direct sunlight, the thermometer's reading is higher than the normal air temperature.
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(II) A 100-W light bulb generates 95 W of heat, which is dissipated through a glass bulb that has a radius of 3.0 cm and is 0.50 mm thick. What is the difference in temperature between the inner and outer surfaces of the glass?
A leaf of area \({\bf{40}}\;{\bf{c}}{{\bf{m}}^{\bf{2}}}\) and mass \({\bf{4}}{\bf{.5 \times 1}}{{\bf{0}}^{{\bf{ - 4}}}}\;{\bf{kg}}\) directly faces the Sun on a clear day. The leaf has an emissivity of 0.85 and a specific heat of \({\bf{0}}{\bf{.80}}\;{\bf{kcal/kg}} \cdot {\bf{K}}\) (a) Estimate the energy absorbed per second by the leaf from the Sun, and then (b) estimate the rate of rise of the leaf’s temperature. (c) Will the temperature rise continue for hours? Why or why not? (d) Calculate the temperature the leaf would reach if it lost all its heat by radiation to the surroundings at 24°C. (e) In what other ways can the heat be dissipated by the leaf?
(II) How long does it take the Sun to melt a block of ice at 0°C with a flat horizontal area \({\bf{1}}{\bf{.0}}\;{{\bf{m}}^{\bf{2}}}\) and thickness 1.0 cm? Assume that the Sun’s rays make an angle of 35° with the vertical and that the emissivity of ice is 0.050
A house thermostat is normally set to 22°C, but at night it is turned down to 16°C for 9.0 h. Estimate how much more heat would be needed (state as a percentage of daily usage) if the thermostat were not turned down at night. Assume that the outside temperature averages 0°C for the 9.0 h at night and 8°C for the remainder of the day, and that the heat loss from the house is proportional to the temperature difference inside and out. To obtain an estimate from the data, you must make other simplifying assumptions; state what these are.
Estimate the rate at which heat can be conducted from the interior of the body to the surface. As a model, assume that the thickness of tissue is 4.0 cm, that the skin is at 34°C and the interior at 37°C, and that the surface area \({\bf{1}}{\bf{.5}}\;{{\bf{m}}{\bf{2}}}\) is Compare this to the measured value of about 230 W that must be dissipated by a person working lightly. This clearly shows the necessity of convective cooling by the blood.
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