"When a solid is heated and its area increases then the thermal expansion is called superficial or area expansion.
Consider a solid plate of area A0 (Fig).
When it is heated, the change in area of the plate is directly proportional to (i) the original area, A0, and (ii) change in temperature, ΔT.
where β is called the coefficient of superficial or area expansion.
Definition of β : " It is defined as the increase in area per unit are per degree increase in temperature."
Unit of β : It is °C-1 or K-1.