The area of a solid is increased with the increase of temperature.It is called superficial expansion.
Let initial surface area of a solid at θ1 temperature = A1
When the temperature is increased to θ2 the final surface area =A2
So, Increase in temperature = θ2 – θ1
And increase in area = A2 – A1
The coefficient of superficial expansion is expressed by the symbol β
Superficial expansion, β = [(A2 – A1) / A1(θ2 – θ1)] ….. ………. ………(1)
[Increase in surface area / (Initial area x increase of temperature)]
In equation (1) if the surface area A1 = 1 m2 and the increase of temperature (θ2 – θ1) = 1K is considered, then the increase in surface area β = A2 – A1 =increase in surface area.
So, the increase in surface area of 1 m2 surface area of a solid for the rise of temperature 1K is called the coefficient of superficial expansion of the material of that solid. Its unit is K-1. The coefficient of superficial expansion of copper is 33.4 x 10-6 K-1. It means that if the temperature of a copper body is increased through 1K, then the increase in surface area of copper is 33.4 x 10-6 m2.