 | Heat flows at a rate proportional to the difference in temperature. Thermal equilibrium occurs when temperatures are equal. Heat is exchanged through conduction, convection, and radiation. In free convection, fluid moves owing to differences in density caused by temperature changes. In forced convection fluid flow is driven, such as by wind, pumps, or fans. Convection is much more effective than conduction in heat transfer involving fluids, such as air or water. Radiation travels at the speed of light and can pass through empty space. Conduction and convection are far slower and occur only through matter. All objects at temperatures above absolute zero emit and absorb thermal radiation. The average temperature of Earth and the other planets depends on the balance between emitted thermal radiation and radiation absorbed from the Sun.
|  | | By the end of this chapter you should be able to
|  | explain the concept of thermal equilibrium;
| | explain why hot objects cool at a decreasing rate when left in a constant-temperature environment;
| | explain the similarities and differences among conduction, convection, and radiation;
| | calculate the power of heat conduction through simple solid shapes such as rods and walls;
| | explain the meaning of the wind chill index in terms of heat transfer; and
| | relate the blackbody curve to the color temperature of a light source such as a compact fluorescent bulb.
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|  | 24A: Heat transfer
24B: Visualizing convection
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