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.

Consider a specimen of length dx and cross-sectional area A, with a temperature difference dq between the opposite faces.

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)

Rate of flow of heat: dH/dt = -kA[dq/dx] |

k is a constant known as the thermal conductivity of the material, measured in W m

(Notice the negative sign: the temperature will decrease as the distance down the bar increases).

The thermal conductivity is defined as rate of flow of heat between two surfaces of unit area separated by unit distance when the temperature difference between them is 1 K.