Explain linear expansion. Write its unit.

Linear expansion refers to the increase in the length of a solid material when its temperature is increased.
The increase in length $(\Delta l)$ is directly proportional to the original length $(l)$ and the change in temperature $(\Delta T)$.
$\Delta l \propto l$ and $\Delta l \propto \Delta T$
Combining these,we get $\Delta l \propto l \Delta T$.
$\frac{\Delta l}{l} = \alpha_{l} \Delta T$,where $\alpha_{l}$ is the coefficient of linear expansion.
Rearranging,$\Delta l = \alpha_{l} l \Delta T$.
The coefficient $\alpha_{l}$ is a characteristic property of the material and depends on the type of material.
The unit of $\alpha_{l}$ is $(^{\circ}C)^{-1}$ or $K^{-1}$.
If $l_{1}$ is the initial length at temperature $T_{1}$ and $l_{2}$ is the final length at temperature $T_{2}$,then:
$l_{2} - l_{1} = \alpha_{l} l_{1} (T_{2} - T_{1})$
$l_{2} = l_{1} [1 + \alpha_{l} (T_{2} - T_{1})]$