The cross-sectional area of the molecule enclosure in a large circle. It is denoted by the letter σ (sigma).

In thermodynamics , a model is often considered where one molecule (a test molecule) moves, and the rest (field molecules) are motionless. If we use such a model, we can say that the effective cross section is the cross-sectional area of the cylinder covered by the molecule, such that the molecules through which it passes interact with the test molecule. I.e$\sigma =\pi ({r}_{\mathrm{one}}+{r}_{2}{)}^{2}$ where *r* _{1} and *r* _{2} is the radius of the test molecule and field molecules. But there are deviations from this simple ratio.

## Sutherland Formula

In the case of an effective scattering cross section , they are caused by the interaction of molecules with each other. These amendments are described by Sutherland's formula.$\sigma ={\sigma}_{0}(\mathrm{one}+S/T)$ , where T is the gas temperature , and S is the Sutherland constant (different for different gases). But this formula can only be used for gases under not too much pressure, since it provides only pairwise collisions, not taking into account triple or more.

## Effective ionization cross section

For the effective ionization cross section, the dependence on the velocity is more pronounced, since if the kinetic energy of the molecule is less than the ionization energy, then ionization will not occur at all, i.e., σ = 0.

## See also

Effective Cross Section