conduction, convection and radiation. . . Conduction Conduction is the mechanism of transfer of heat between two adjacent parts of a body because of their temperature difference.
Suppose, one end of a metallic rod is put in a flame, the other end of the rod will soon be so hot that you cannot hold it by your bare hands. Here, heat transfer takes place by conduction from the hot end of the rod through its different parts to the other end. Gases are poor thermal conductors, while liquids have conductivities intermediate between solids and gases. Heat conduction may be described quantitatively as the time rate of heat flow in a material for a given temperature difference.
Consider a metallic bar of length L and uniform cross-section A with its two ends maintained at different temperatures. This can be done, for example, by putting the ends in thermal contact with large reservoirs at temperatures, say, T C and T D , respectively (Fig. . ).
Let us assume the ideal condition that the sides of the bar are fully insulated so that no heat is exchanged between the sides and the surroundings. After sometime, a steady state is reached; the temperature of the bar decreases uniformly with distance from T C to T D ; ( T C > T D ). The reservoir at C supplies heat at a constant rate, which transfers through the bar and is given out at the same rate to the reservoir at D. It is found experimentally that in this steady state, the rate of flow of heat (or heat current) H is proportional to the temperature difference ( T C – T D ) and the area of cross-section A and is inversely proportional to the length L : H = KA C D ( .
) The constant of proportionality K is called the thermal conductivity of the material. The greater the value of K for a material, the more rapidly will it conduct heat. The SI unit of K is J s – m – K – or W m – K – .