According to Charles' Law, at constant pressure P, the volume V is directly proportional to the temperature T:
1) V ˜ T
2) V = kT, where k is a constant
Rewriting equation 2, we have:
3) V/T = constant
Charles' Law is also commonly written as:
4) V1/T1 = V2/T2
where V1 and T1 are the original values of the gas, while V2 and T2 represent its final values.
In addition to the pressure being constant, the same caveats used in any ideal gas law apply here. These results apply for systems where an equilibruim state has been reached, the gas is not too dense, and P is around atmospheric pressure. Real gases approximate these conditions enough such that the law is applicable in every day life.