Charles's law

At constant pressure, a fixed amount of gas has a volume directly proportional to its absolute temperature. It predicts that heating the gas expands it in proportion to its temperature in kelvins.

Ideal gas law: An equation of state relating pressure, volume, amount, and absolute temperature for an ideal gas. Charles's law follows by holding pressure and gas amount fixed in this broader equation.
Volume: The amount of three-dimensional space occupied by a substance or object. The law tracks how much space a gas occupies as temperature changes.
Jacques Charles: A French scientist and balloonist associated with early measurements of gas expansion with temperature. His name became attached to the volume–temperature relation, though he did not publish it.
Hot-air balloon: An aircraft that gains buoyancy by heating air inside an envelope. Heating the enclosed air expands it and lowers its density at roughly ambient pressure.
Absolute temperature: Temperature measured from absolute zero, usually in kelvins. Volume is proportional to kelvins, not to Celsius or Fahrenheit readings.
Pressure: Force exerted per unit area by particles or other matter on a surface. Pressure must remain fixed for the stated volume–temperature proportionality.
Joseph Louis Gay-Lussac: A French chemist who published quantitative studies of gases and thermal expansion. His 1802 work reported the proportional expansion relation and credited Charles's earlier observations.
Constant-pressure gas thermometer: A thermometer that infers temperature from the volume of gas maintained at constant pressure. Its operating principle directly uses volume's proportionality to absolute temperature.
Kinetic theory of gases: A model that explains gas properties through the motion and collisions of microscopic particles. It explains how increased molecular speed raises pressure unless expansion keeps pressure steady.
Thermodynamic temperature: A physical measure of temperature defined on an absolute scale with zero at absolute zero. Its absolute scale makes direct proportionality meaningful.

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