##  [Charles's Law](/charless-law-0) 

 Definition

The empirical relation that, for a fixed amount of gas at constant pressure, the volume is directly proportional to its absolute temperature, often stated as V/T = constant (with T in Kelvin).

 

 

 

 

 

 





## Principle

Principle

At constant pressure and moles, heating a gas increases its average molecular kinetic energy and thus its volume so that V/T remains constant in the ideal approximation; the law encodes thermal expansion behavior of gases under isobaric conditions.

 

 

 

 

 





## Demonstration

Demonstration

Heating a balloon of helium at ambient pressure causes its volume to increase approximately linearly with temperature when measured in Kelvin; experimentally, measuring volume at several temperatures and plotting V versus T yields a straight line intercepting the origin if extrapolated to absolute zero under ideal assumptions.

 

 

 

 

## Misapplication

Misapplication

Using degrees Celsius instead of Kelvin, extrapolating the linear relation below temperatures where the gas liquefies, or assuming the law holds when pressure cannot be held constant (e.g., sealed rigid container).

 

 

 

 

 





## Consequence

Consequence

Provides a simple method to relate thermal changes to volumetric expansion at constant pressure, supports the definition and measurement of absolute temperature scales and underlies practical devices like gas thermometers.

 

 

 

 

## Reversal

Reversal

The converse is Gay-Lussac's/pressure law: at fixed volume and amount, pressure is proportional to temperature; reversing the process (cooling at constant P) shrinks volume accordingly until non-ideal or phase-change boundaries intervene.

 

 

 

 

 





## Boundary

Boundary

Valid for ideal or sufficiently dilute gases at pressures where no condensation occurs and when the pressure is maintained constant; excludes regimes with phase changes, chemical reactions, or significant non-ideal behavior.

 

 

 

 

 





## Semantic Tension

Semantic Tension

Tension between the ideal linear V–T proportionality and real-gas phenomena (thermal expansion coefficients varying with pressure, phase transitions); also between naming conventions (Charles vs. Gay-Lussac) concerning constant-P vs constant-V statements.

 

 

 

 

 





## Synthesis

Synthesis

Charles's law states that for a fixed amount of ideal gas at constant pressure, volume scales linearly with absolute temperature, providing a direct macroscopic connection between thermal energy and volumetric response in the isobaric ideal-gas limit.