Convection is the transfer of thermal energy due to the motion of the substancethat contains the thermal energy. Although the conduction process can occur in liquidsand gases as well as solids, convection may have a much larger effect in liquids andgases where the molecules are free to migrate. 55Table 4.1 Thermal Conductivity of Common MaterialsMetalsThermalConductivity(J/s m oC)NonmetalsThermalConductivity(J/s m oC)Silver 420 Brick 0.84Copper 380 Glass 0.84Aluminum 237 Water 0.56Brass 120 Cork 0.042Iron 80 Wood 0.04Nickel <80 Air 0.023A hot cup of coffee transfers thermal energy as heat to the surrounding air byconvection. The heated air over the coffee expands and rises. The rising heated air isreplaced by cooler, denser air, which, in turn, expands and rises. This circulation of airis an example of convection. Convection occurs because as the temperature isincreased, most materials expand. This means that they become less dense. Thus, thewarmer air near the hot coffee is buoyed up to the ceiling just as a piece of cork, whichis less dense than water, rises to the surface of the water. When home furnaces werefirst introduced, most home heating systems relied on convection to circulate thewarmer air. Now, the natural process
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