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The 'combined gas law' is a gas law which combines Charles's law, Boyle's law, and Gay-Lussac's law. These laws each relate one thermodynamic variable to another mathematically while holding everything else constant. Charles's law states that volume and temperature are directly proportional to each other while pressure is held constant. Boyle's law asserts that pressure and volume are inversely proportional to each other at fixed temperature. Finally Gay-Lussac's law introduces a direct proportionality between temperature and pressure at constant volume. The inter-dependence of these variables is shown in the combined gas law, which states that:
This can be stated mathematically as
:
where:
:''P'' is the pressure.
:''V '' is the volume.
:''T '' is the temperature (measured in kelvin).
:''k '' is a constant with units of energy divided by temperature.
For comparing the same substance under two different sets of conditions, the law can be written as:
:
The addition of Avogadro's law to the combined gas law yields the ideal gas law.