With the application of heat the length, area and volume of solid substance expands.

**Linear expansion of solid and coefficient of linear expansion:**

When heat is applied to a solid substance elongation of its length occurs towards a definite direction and the expansion along its length is called linear expansion.

Let the length of a bar at θ_{1} temperature be l_{1} and at θ_{2} temperature the length be 1_{2}.

Increase in length =** I_{2} – I_{1}**

And increase of temperature = **θ _{2} – θ_{1}**

The coefficient of linear expansion is denoted by a and it can be expressed as,

**α = ( I_{2} – I_{1})/ I_{1} (θ_{2} – θ_{1}) … …… ……(1)**

**= [Increase in length / (Initial length x increase of temperature)]**

In equation (1), if the initial length ** I_{1 }**= lm and increase of temperature,

**θ**= 1K, then

_{2}– θ_{1 }**α = (**increase in length.

*I*=_{2}– I_{1})So, the increase in length of a solid rod of lm for a rise of temperature 1K is defined as the coefficient of linear expansion of the material of the solid.

Its unit is K^{-1}. The coefficient of linear expansion of copper is **16.7 x 10 ^{-6} K^{-1}** means if the temperature of copper rod is raised through 1K, then its length increases by

**16.7 x 10**

^{-6}m.