Charles's law

Charles's law states that, at constant pressure, the volume of a fixed quantity of gas is directly proportional to its absolute (Kelvin) temperature: V/T = constant. Equivalently, a gas expands by a fixed fraction of its volume for each degree of warming. Plotted as V against temperature it is a straight line.

The law's most profound feature appears when the straight line is extrapolated backward to V = 0. Every gas, regardless of identity, reaches zero volume at the same temperature, −273.15 °C. Since a volume cannot be negative, this marks a natural floor — the first quantitative hint of absolute zero, decades before thermodynamics gave it a deeper meaning.

Jacques Charles found the proportionality around 1787 but never published; Joseph Louis Gay-Lussac rediscovered it and published in 1802, which is why the relation is sometimes called Gay-Lussac's law. Like Boyle's law it is exact only for an ideal gas and breaks down as a real gas approaches condensation.

Discovered unpublished by Jacques Charles around 1787 and published by Joseph Louis Gay-Lussac in 1802; the extrapolation to absolute zero was later sharpened by William Thomson (Lord Kelvin) into the thermodynamic temperature scale.