Heat flows from higher to lower temperature.
As noted earlier, internal energy may be transferred through a material from higher to lower temperature by a process known as heat conduction . The rate at which heat is transferred through a material body is known empirically to be proportional to the temperature difference across the body. For a rectangular body it is also known to scale in proportion to the cross sectional area of the body perpendicular to the temperature gradient and to scale inversely with the distance over which the temperature difference exists. The geometry is illustrated in figure 22.3 and the thermal conductivities of common materials are shown in table 22.3 .