Implicant

The classical virial expansion expresses the pressure of a many-particle system in equilibrium as a power series in the density. The virial expansion, introduced in 1901 by Heike Kamerlingh Onnes, is a generalization of the ideal gas law. He wrote that for a gas containing $N$ atoms or molecules,

$\frac{p}{k_{B}T}=\rho +B_2(T){\rho }^2+B_3(T){\rho }^3+\cdots ,$

where $p$ is the pressure, $k_B$ is the Boltzmann constant, $T$ is the absolute temperature, and $\rho \equiv N/V$ is the number density of the gas. Note that for a gas containing a fraction $n$ of $N_A$ (Avogadro's number) molecules, truncation of the virial expansion after the first term leads to $pV=n{N}_A{k}_{B}T=nRT$, which is the ideal gas law.

Writing $\beta =(k_{B}T{)}^{-1}$, the virial expansion can be written in closed form as

$\frac{\beta p}{\rho }=1+\sum _{i=1}^{\mathrm{\infty }}{B}_{i+1}(T){\rho }^i$.

The virial coefficients $B_i(T)$ are characteristic of the interactions between the particles in the system and in general depend on the temperature $T$.

See also

• Virial theorem
• Statistical mechanics