On a hot day the metal pendulum of a grandfather clock expands, causing the clock to run slowly. Many practical devices, such as the mercury thermometer, work on the principle of thermal expansion. It is important for engineers to take thermal expansion into account when designing structures. You have probably noticed expansion joints in roadway bridges which allow the bridge to expand in hot weather without damage. Most solids expand when heated.
Thermal energy causes the atoms of all solids above absolute zero to vibrate.
Raising the temperature causes the atoms to increase the amplitude of their vibrations. This increase in amplitude increases the average separation between the atoms and the material expands. Let the length of a solid be L and its temperature change be T . If the change in temperature is not too large, the change in length L is proportional to the original length and the change in temperature.