Mean Free Path

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Mean Free Path

(symbol l, Missing Image:img src="SP7_m_files/lambdasm.gif", L) 

1. Of any particle, the average distance that a particle travels between successive collisions with the other particles of an ensemble.
In vacuum technology, the ensemble of particles of interest comprises only the molecules in the gas phase. </dd>
2. Specifically, the average distance traveled by the molecules of a perfect gas between consecutive collisions with one another. It may be determined roughly from either of the formulas

Missing Image:img alt="l=3u/pc = 3v/c" src="SP7_m_files/173mean1.gif" height="17" width="125"
Missing Image:img alt="l=1/ sqrt[2 pi n dE2]" src="SP7_m_files/173mean2.gif" height="26" width="100"
where l is the mean free path; u (lower case

Mu) is the dynamic viscosity; v is the kinematic viscosity; p (lower case Rho) is the density; c is the molecular speed (a function of the gas temperature); n is the number of molecules per unit volume; and d is the molecule diameter. </dd>
Given the mean free path l0 at a level where the pressure is p0 , the temperature is T0 (�K), and the acceleration of gravity is g0, then its value at any other level is

l = l0 p0 Tg / pT0 g0
where p, T, and g are the pressure, temperature, and acceleration of

gravity, respectively, at the new level. See mixing length. </dd>
3. For any process the reciprocal of the cross section per unit volume for that process. </dd>


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use