A term extended surface is commonly used in reference to a solid that experiences energy transfer by conduction and convection between its boundary and surroundings. A temperature gradient in x direction sustains heat transfer by conduction internally, at the same time; there is heat dissipation by convection into an ambient at Ta from surface at temperature Ts given as

Q=hAs(Ts-Ta)

When the temperature Ts and Ta are fixed by design considerations, there are only two ways to increase the heat transfer rate:

i) To increase the convection coefficient h,

ii) To increase the surface are A.

In the situations, in which an increase in h is not practical or economical, the heat transfer rate can be improved by increasing surface area. Generally, the fins are used on the surfaces where the heat transfer coefficient is very low. For e.g. In a car radiator the outer surface of the tubes is finned because the heat transfer coefficient for air at the outer surface is much smaller than that of water flow inside the tubes. Similarly, the electrical transformers and motors in which the generated heat is dissipated to air by providing fins on its outer surface.