Mean Free Path

Also found in: Dictionary, Acronyms, Wikipedia.

Mean free path

The average distance traveled between two similar events. The concept of mean free path is met in all fields of science and is classified by the events which take place. The concept is most useful in systems which can be treated statistically, and is most frequently used in the theoretical interpretation of transport phenomena in gases and solids, such as diffusion, viscosity, heat conduction, and electrical conduction. The types of mean free paths which are used most frequently are for elastic collisions of molecules in a gas, of electrons in a crystal, of phonons in a crystal, and of neutrons in a moderator. See Kinetic theory of matter

Mean Free Path

(l ), the mean length of the path traversed by a particle between two successive collisions with other particles. The concept of mean free path is used extensively in calculations of various transfer processes, such as viscosity, heat conduction, diffusion, and electrical conduction.

According to the kinetic theory of gases, molecules move uniformly and rectilinearly from collision to collision. If a molecule traverses an average path v in 1 sec, undergoing in the process v elastic collisions with similar molecules, then

ī = v/v = 1/nσ√2

where n is the number of molecules per unit volume (the density of the gas) and σ is the effective cross section of the molecule. As the density of the gas (its pressure) increases, the mean free path decreases, since the number of collisions v per sec increases. A rise in temperature or in the intensity of motion of the molecules leads to a certain decline in cr and consequently to an increase in σ. For ordinary molecular gases under normal conditions (at atmospheric pressure and 20°C), l ~ 10-5 cm, which is approximately 100 times greater than the average distance between molecules.

In many cases the concept of mean free path is also applicable to particles whose motion and interaction conform to the laws of quantum mechanics (such as conduction electrons in a solid, neutrons in weakly absorbing mediums, and photons in stars), but the calculation of the mean free path for such particles is more difficult.

mean free path

[′mēn ¦frē ′path]
(acoustics)
For sound waves in an enclosure, the average distance sound travels between successive reflections in the enclosure.
(physics)
The average distance traveled between two similar events, such as elastic collisions of molecules in a gas, of electrons or phonons in a crystal, or of neutrons in a moderator.
References in periodicals archive ?
Before describing the experimental approach, we first present the key fluid properties for computing the slip correction factor, the viscosity and the mean free path.
Sondheimer, "The mean free path of electrons in metals," Advances in Physics, vol.
The gamma ray mass attenuation coefficient, half-value layer, mean free path, effective atomic number, and effective electron densities have been calculated using the WinXCom program in the energy range 1keV-100GeV of GAGCO and CMO scintillator materials.
2 The electron mean free path as a Schwarzschild radius
where [[lambda].sub.G0] is thermal conductivity of the free air, II is the porosity, l is mean free path of heat carriers and, [beta] is a constant that depends on the type of gas and temperature.
where [[lambda].sub.Au] is the mean free path of electrons in Au, [I.sub.i] the XPS intensity of element i, and [S.sub.i] the sensitivity factor of element i.
Each conducting channel exhibits either intrinsic conductance ([G.sub.Q]) and ohmic conductance ([G.sub.o]) or scattering conductance which is dependent on mean free path A and tube length (/), where [G.sub.o] = ([2q.sup.2]/h)([lambda]/l) and [G.sub.Q] = [2q.sup.2]/h [congruent to] 1/12.9 K[OMEGA] (h is the planks constant and q is the charge of an electron and mean free path [lambda] which is diameter dependent).
In nanoporous materials the characteristic dimension of the domain is comparable to those of heat carriers, such as the mean free path of air or the average wavelength of phonon (2), (38).
where [bar.[lambda]] is mean free path of gas molecules and D is pore throat diameter
With a glass mean free path of just 3.2 mm, EcoGuard Solar Boost-LP mirrors attain an average solar transmission of 95 percent.
The mean free path for a proton with given energy which picks up a charge is simply [DELTA]r = 1 / [sigma.sub.01]N where N is the number of atoms per unit volume which is proportional to the pressure.
There are two different length scales, wavelength and mean free path, of electron and phonon affecting the electrical and thermal transport.

Site: Follow: Share:
Open / Close