Convection

Convection (Fig. 15-7) is similar to conduction in that heat leaves an object as it comes in contact with something else. In the case of convection, the transfer of heat happens by means of a moving stream of a fluid (liquid or gas) rather than another object. Our skin may be warmed or cooled by convection when it is exposed to warm or cool air passing by it. The air molecules pass by the molecules on the surface of our skin and absorb heat, and we feel cooler. The same thing happens when we run cold water over our skin. The amount of convection depends upon how rough the surface is, its orientation to the stream of fluid, the direction of the stream's flow, the type of fluid in the stream, and whether the flow is free or is forced. When there is a large difference between the air temperature and the skin temperature, plus more air or water movement, more heat will be transmitted by convection.

Convection can also heat, as well as cool. A hot bath warms us thoroughly as the heat from the water is trans

ferred by convection to our skin. Hot air from a room's heating system flowing past us will also warm our skin.

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