# PlanetPhysics/Ideal Gas Law

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The equation that characterizes any amount of a gas is

${\displaystyle pV=nRT}$

where ${\displaystyle p}$ is the pressure, ${\displaystyle V}$ the volume, ${\displaystyle n}$ the number of the gas moles and ${\displaystyle T}$ the absolute temperature of the gas; ${\displaystyle R}$ the universal gas constant (approximately 8.314472 ${\displaystyle {\frac {{\mbox{Pa}}\cdot {\mbox{m}}^{3}}{{\mbox{mol}}\cdot {\mbox{K}}}}}$).

The gas law is accurately valid for an ideal gas, but a good approximation for real gases.

The law contains the following gas laws:

• Boyle--Mariotte law (${\displaystyle pV=}$\, constant when ${\displaystyle n}$ and ${\displaystyle T}$ are constants)
• Gay--Lussac law (${\displaystyle {\frac {V}{T}}=}$\, constant when ${\displaystyle n}$ and ${\displaystyle p}$ are constants)
• Avogadro's law (in equal conditions, equal volumes of different gases contain the same number of molecules)

### Alternative definition

The ideal gas law can also be defined using Boltzmann's constant:

${\displaystyle pV=NkT}$

The symbols are those defined above, with the difference that ${\displaystyle N}$  is the number of gas molecules, and ${\displaystyle k}$  is the Boltzmann constant, ${\displaystyle k\approx 1.38\times 10^{-23}{\mbox{JK}}^{-1}}$ . This form is often preferred by physicists who consider the number of molecules to be more fundamental than the number of moles.