Charles' Law, in physics, a principle that deals with the effect of heat on the expansion of gases. The law states:

If the pressure of a gas remains constant, the volume of the gas will increase as the temperature increases.

Thus if the temperature increases, the gas takes up more space. If the temperature decreases, the gas takes up less space. The principle was first formulated by the French physicist Jacques Alexandre Cesar Charles in 1787.

Charles' law is stated this way in formula form:

where V1 equals the original volume, V2 equals the new volume, T1 equals the original temperature, and T2 equals the new temperature.

In using Charles' law, temperatures must be converted to the Kelvin scale, in which the zero point is absolute zero (-273.15° C.).