The transfer of heat from one molecule to an adjacent molecule while the particles remain in fixed positions relative to each other is conduction. For example, if a piece of pipe has a hot fluid on the inside and a cold fluid on the outside, heat is transferred through the wall of the pipe by conduction. This is illustrated in Figure 2-1. The molecules stay intact, relative to each other, but the heat is transferred from molecule to molecule by the process of conduction. This type of heat transfer occurs in solids or, to a much lesser extent, within fluids that are relatively stagnant.
The rate of flow of heat is proportional to the difference in temperature through the solid and the heat transfer area of the solid, and inversely proportional to the thickness of the solid. The proportionality constant, k, is known as the thermal conductivity of the solid. Thus, the quantity of heat flow may be expressed by the following equation:
The thermal conductivity of solids has a wide range of numerical values, depending upon whether the solid is a relatively good conductor of heat, such as metal, or a poor conductor, such as glass-fiber or calcium silicate. The latter serves as insulation.