18.11 : Mecanismos de Transferência de Calor I

Just as interesting as the effects of heat transfer on a system are the methods by which the heat transfer occur. Whenever there is a temperature difference, heat transfer occurs. It may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice box. So many processes involve heat transfer that it is hard to imagine a situation where no heat transfer occurs. Yet, every heat transfer takes place by only three methods: conduction, convection, and radiation.

For example, heat transfer occurs in a fireplace by all three methods: conduction, convection, and radiation. Radiation is responsible for most of the heat transferred into the room. Heat transfer also occurs through conduction into the room, but much more slowly. Heat transfer by convection also occurs through cold air entering the room around the windows and hot air leaving the room by rising up the chimney.

Conduction is heat transfer through stationary matter by physical contact. The matter is stationary only on a macroscopic scale as thermal motion of the atoms and molecules occurs at any temperature above absolute zero. Heat transferred from a stove’s burner through the bottom of a pan to the food in the pan is transferred by conduction.