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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.
Universal curve for the electron inelastic mean free path in elements based on equation (5) in. [1] If a monochromatic , primary beam of electrons is incident on a solid surface, the majority of incident electrons lose their energy because they interact strongly with matter , leading to plasmon excitation, electron-hole pair formation, and ...
Free molecular flow describes the fluid dynamics of gas where the mean free path of the molecules is larger than the size of the chamber or of the object under test. For tubes/objects of the size of several cm, this means pressures well below 10 −3 mbar .
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.
As a rule of thumb there should be 20 or more particles per cubic mean free path for accurate results. [citation needed] The evolution of the system is integrated in time steps, , which are typically on the order of the mean collision time for a particle. At each time step all the particles are moved and then a random set of pairs collide.
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 .
The attenuation coefficient of a volume, denoted μ, is defined as [6] =, where Φ e is the radiant flux;; z is the path length of the beam.; Note that for an attenuation coefficient which does not vary with z, this equation is solved along a line from =0 to as:
It is both the mean distance over which a high-energy electron loses all but 1 ⁄ e of its energy by bremsstrahlung, [1] and 7 ⁄ 9 of the mean free path for pair production by a high-energy photon. It is also the appropriate length scale for describing high-energy electromagnetic cascades.