14.5 Conduction  (Page 4/8)

How does the rate of heat transfer by conduction change when all spatial dimensions are doubled?

Some electric stoves have a flat ceramic surface with heating elements hidden beneath. A pot placed over a heating element will be heated, while it is safe to touch the surface only a few centimeters away. Why is ceramic, with a conductivity less than that of a metal but greater than that of a good insulator, an ideal choice for the stove top?

Loose-fitting white clothing covering most of the body is ideal for desert dwellers, both in the hot Sun and during cold evenings. Explain how such clothing is advantageous during both day and night.

(a) Calculate the rate of heat conduction through house walls that are 13.0 cm thick and that have an average thermal conductivity twice that of glass wool. Assume there are no windows or doors. The surface area of the walls is $\text{120}{\text{m}}^2$ and their inside surface is at $\text{18.}\mathrm{0º}\text{C}$ , while their outside surface is at $5\text{.00º}\text{C}$ . (b) How many 1-kW room heaters would be needed to balance the heat transfer due to conduction?

The rate of heat conduction out of a window on a winter day is rapid enough to chill the air next to it. To see just how rapidly the windows transfer heat by conduction, calculate the rate of conduction in watts through a $3\text{.}{\text{00-m}}^2$ window that is $0\text{.635 cm}$ thick (1/4 in) if the temperatures of the inner and outer surfaces are $5\text{.00ºC}$ and $-\text{10}\text{.}\mathrm{0º}\text{C}$ , respectively. This rapid rate will not be maintained—the inner surface will cool, and even result in frost formation.

Calculate the rate of heat conduction out of the human body, assuming that the core internal temperature is $\text{37}\text{.}\mathrm{0º}\text{C}$ , the skin temperature is $\text{34}\text{.}\mathrm{0º}\text{C}$ , the thickness of the tissues between averages $1\text{.00 cm}$ , and the surface area is $1\text{.}\text{40}{\text{m}}^2$ .