Editing Kinetic theory of gases (section)

=== Mean free path ===
{{Main|Mean free path}}
In kinetic theory of gases, the [[Mean free path#Kinetic theory|mean free path]] is the average distance traveled by a molecule, or a number of molecules per volume, before they make their first collision. Let <math> \sigma </math> be the collision [[Cross section (physics)#Collision among gas particles|cross section]] of one molecule colliding with another. As in the previous section, the number density <math> n </math> is defined as the number of molecules per (extensive) volume, or <math> n = N/V </math>. The collision cross section per volume or collision cross section density is <math> n \sigma </math>, and it is related to the mean free path <math>\ell</math> by<math display="block">\ell = \frac {1} {n \sigma \sqrt{2}} </math>

Notice that the unit of the collision cross section per volume <math> n \sigma </math> is reciprocal of length.