Convection is the transfer of thermal energy due to the motion of the substance
that contains the thermal energy. Although the conduction process can occur in liquids
and gases as well as solids, convection may have a much larger effect in liquids and
gases where the molecules are free to migrate.
A hot cup of coffee transfers thermal energy as heat to the surrounding air by
convection. The heated air over the coffee expands and rises. The rising heated air is
replaced by cooler, denser air, which, in turn, expands and rises. This circulation of air
is an example of convection. Convection occurs because as the temperature is
increased, most materials expand. This means that they become less dense. Thus, the
warmer air near the hot coffee is buoyed up to the ceiling just as a piece of cork, which
is less dense than water, rises to the surface of the water. When home furnaces were
first introduced, most home heating systems relied on convection to circulate the
warmer air. Now, the natural process