Editing Cross section (physics) (section)

== Collision among gas particles ==
[[File:CrossSectionFig1.svg|thumb|upright=1.5|'''Figure 1.''' In a gas of particles of individual diameter {{math|2''r''}}, the cross section {{math|''σ''}}, for collisions is related to the particle number density {{math|''n''}}, and mean free path between collisions {{math|''λ''}}.]]

In a [[gas]] of finite-sized particles there are collisions among particles that depend on their cross-sectional size. The average distance that a particle travels between collisions depends on the density of gas particles. These quantities are related by
: <math>\sigma = \frac{1}{n \lambda},</math>
where
: {{math|''σ''}} is the cross section of a two-particle collision ([[SI]] unit: m<sup>2</sup>),
: {{math|''λ''}} is the [[mean free path]] between collisions (SI unit: m),
: {{math|''n''}} is the [[number density]] of the target particles (SI unit: m<sup>−3</sup>).

If the particles in the gas can be treated as hard spheres of radius {{math|''r''}} that interact by direct contact, as illustrated in Figure 1, then the effective cross section for the collision of a pair is
: <math>\sigma = \pi \left(2r\right)^2</math>

If the particles in the gas interact by a force with a larger range than their physical size, then the cross section is a larger effective area that may depend on a variety of variables such as the energy of the particles.

Cross sections can be computed for atomic collisions but also are used in the subatomic realm. For example, in [[nuclear physics]] a "gas" of low-energy [[neutron]]s collides with nuclei in a reactor or other nuclear device, with a [[neutron cross section|cross section that is energy-dependent]] and hence also with well-defined [[mean free path]] between collisions.