There are three ways in which we categorise thermal expansion – Linear Expansion, Superficial Expansion and Cubical Expansion. These correspond to expansion in length, area and volume of a substance. For all types of thermal expansion, we define coefficients of expansion, which give an idea about the expansion tendency of materials. Higher coefficient of expansion indicates more expansion tendency of material. For step by step understanding of thermal expansion and its types, watch the video given below.

https://youtu.be/7ZKHc5J6R5Q

Expansion of Bounded Lengths & Cavities : When a material is heated and its temperature increases, the separation between the substance’s molecules increases uniformly in its volume if the lattice structure of the substance is isotropic. If we consider a length between two molecules of a substance, then this length will expand/contract with the same coefficient of substance, no matter if the material is solid or hollow. Same concept is used for expansion of cavities in solid bodies. To understand this concept watch the video below.

https://youtu.be/yd0gZrmP0XA