Conduction of heat energy

The transfer of heat energy from one place to another through a substance without the movement of the substance as a whole is known as conduction.
Conduction of heat is most important in the insulation of houses because much of the heat produced in the house is lost by conduction through the walls, roof, windows and floor and knowledge of materials that can be used to reduce this loss is of great importance.

If we consider a specimen of length dx and cross-sectional area A, with a temperature difference dq between the opposite faces (Figure 1), then the rate of flow of heat energy through it will depend on
(a) the cross-sectional area of the face, (A)
(b) the temperature difference between the faces (dq)
(c) the inverse of the distance between the faces (1/dx)
Therefore:


where k is a constant known as the thermal conductivity of the material, measured in W m^-1 K^-1.


(Notice the negative sign: this implies that the temperature will decrease as the distance down the bar increases). Notice also that you may use ^oC or K for the units for temperature difference. The actual temperature is not vital – simply the difference in temperature between the two faces. The value of k for various materials is given in the table below (see Data section for further values).