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In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a result of one or more successive collisions with other particles.
Mean free path is the average distance that a particle will travel without collision. For a fast moving particle (that is, one moving much faster than the particles it is moving through) the kinetic diameter is given by, [2] = where, d is the kinetic diameter, r is the kinetic radius, r = d/2, l is the mean free path, and n is the number ...
The mean free path of a molecule in a gas is the average distance between its collision with other molecules. This is inversely proportional to the pressure of the gas, given constant temperature. In air at STP the mean free path of molecules is about 96 nm.
Schematic drawing of a molecule in a cylindrical pore in the case of Knudsen diffusion; are indicated the pore diameter (d) and the free path of the particle (l).Knudsen diffusion, named after Martin Knudsen, is a means of diffusion that occurs when the scale length of a system is comparable to or smaller than the mean free path of the particles involved.
The Knudsen number is a dimensionless number defined as =, where = mean free path [L 1], = representative physical length scale [L 1].. The representative length scale considered, , may correspond to various physical traits of a system, but most commonly relates to a gap length over which thermal transport or mass transport occurs through a gas phase.
The mean free time for a molecule in a fluid is the average time between collisions. The mean free path of the molecule is the product of the average speed and the mean free time. [ 1 ] These concepts are used in the kinetic theory of gases to compute transport coefficients such as the viscosity .
mean free path, the average distance between two subsequent collisions of the electron (ion) with plasma components: , =, ¯,, where , ¯ is an average velocity of the electron (ion) and , is the electron or ion collision rate.
The inelastic mean free path of electrons can roughly be described by a universal curve that is the same for all materials. [1] [3] The knowledge of the IMFP is indispensable for several electron spectroscopy and microscopy measurements. [4]