Mean free path: Difference between revisions

From Glossary of Meteorology
(Created page with " {{TermHeader}} {{TermSearch}} <div class="termentry"> <div class="term"> == mean free path == </div> #<div class="definition"><div class="short_definition">Average dis...")
 
m (Rewrite with Template:Term and clean up)
 
Line 1: Line 1:
 
{{Term
 
|Display title=mean free path
{{TermHeader}}
|Definitions={{Definition
{{TermSearch}}
|Num=1
 
|Meaning=#Average distance a molecule travels in a gas between collisions.
<div class="termentry">
|Explanation=This concept has a meaning only to the extent that the paths of molecules are mostly straight  lines interrupted by changes in direction (collisions) over comparatively shorter distances of order  the molecular size. Molecules in a liquid are never free in this sense. The mean free path ''L'' in a  gas of a single molecular species with a [[Maxwell&ndash;Boltzmann distribution]] of speeds is  <blockquote>[[File:ams2001glos-Me7.gif|link=|center|ams2001glos-Me7]]</blockquote> where ''n'' is the number [[density]] of molecules and ''S'' is the mutual collision [[cross section]]. To the  extent that a molecule can be considered a hard sphere with diameter ''d'', ''S'' = &#x003c0;''d''<sup>2</sup>. The concept  of molecular diameter is fuzzy and each method for determining it yields different results (Kennard  1938). At [[sea level]] the mean free path in air is of order 0.1 &#x003bc;m.<br/>  
  <div class="term">
#Average distance a [[photon]] travels in a turbid medium between [[scattering]] (scattering mean  free path) events, or the average distance a photon travels before being absorbed ([[absorption]] mean  free path).<br/> The scattering mean free path in a medium is the inverse of its [[scattering coefficient]]; the  absorption mean free path is the inverse [[absorption coefficient]]; and the total mean free path is  the inverse of the sum of scattering and absorption coefficients. At visible wavelengths, the scattering  mean free path in clouds is of order 10 m.<br/> Kennard, E. H. 1938. Kinetic Theory of Gases. 97&ndash;114.  
== mean free path ==
}}
  </div>
}}
 
#<div class="definition"><div class="short_definition">Average distance a molecule travels in a gas between collisions.</div><br/> <div class="paragraph">This concept has a meaning only to the extent that the paths of molecules are mostly straight  lines interrupted by changes in direction (collisions) over comparatively shorter distances of order  the molecular size. Molecules in a liquid are never free in this sense. The mean free path ''L'' in a  gas of a single molecular species with a [[Maxwell&ndash;Boltzmann distribution]] of speeds is  <div class="display-formula"><blockquote>[[File:ams2001glos-Me7.gif|link=|center|ams2001glos-Me7]]</blockquote></div> where ''n'' is the number [[density]] of molecules and ''S'' is the mutual collision [[cross section]]. To the  extent that a molecule can be considered a hard sphere with diameter ''d'', ''S'' = &#x003c0;''d''<sup>2</sup>. The concept  of molecular diameter is fuzzy and each method for determining it yields different results (Kennard  1938). At [[sea level]] the mean free path in air is of order 0.1 &#x003bc;m.</div><br/> </div>
#<div class="definition"><div class="short_definition">Average distance a [[photon]] travels in a turbid medium between [[scattering]] (scattering mean  free path) events, or the average distance a photon travels before being absorbed ([[absorption]] mean  free path).</div><br/> <div class="paragraph">The scattering mean free path in a medium is the inverse of its [[scattering coefficient]]; the  absorption mean free path is the inverse [[absorption coefficient]]; and the total mean free path is  the inverse of the sum of scattering and absorption coefficients. At visible wavelengths, the scattering  mean free path in clouds is of order 10 m.</div><br/> </div><div class="reference">Kennard, E. H. 1938. Kinetic Theory of Gases. 97&ndash;114. </div><br/>
</div>
 
{{TermIndex}}
{{TermFooter}}
 
[[Category:Terms_M]]

Latest revision as of 10:46, 29 March 2024

  1. Average distance a molecule travels in a gas between collisions.
This concept has a meaning only to the extent that the paths of molecules are mostly straight lines interrupted by changes in direction (collisions) over comparatively shorter distances of order the molecular size. Molecules in a liquid are never free in this sense. The mean free path L in a gas of a single molecular species with a Maxwell–Boltzmann distribution of speeds is
ams2001glos-Me7
where n is the number density of molecules and S is the mutual collision cross section. To the extent that a molecule can be considered a hard sphere with diameter d, S = πd2. The concept of molecular diameter is fuzzy and each method for determining it yields different results (Kennard 1938). At sea level the mean free path in air is of order 0.1 μm.
  1. Average distance a photon travels in a turbid medium between scattering (scattering mean free path) events, or the average distance a photon travels before being absorbed (absorption mean free path).
    The scattering mean free path in a medium is the inverse of its scattering coefficient; the absorption mean free path is the inverse absorption coefficient; and the total mean free path is the inverse of the sum of scattering and absorption coefficients. At visible wavelengths, the scattering mean free path in clouds is of order 10 m.
    Kennard, E. H. 1938. Kinetic Theory of Gases. 97–114.
  1. Average distance a molecule travels in a gas between collisions.

The massless particle that, according to the quantum theory of radiation, carries the smallest discrete amount of electromagnetic energy hν, where h is Planck's constant and ν is the frequency of the associated electromagnetic wave.

Retrieved from "https://glossary.ametsoc.org/w/index.php?title=Mean_free_path&oldid=35183"
Powered by MediaWiki
Powered by WikiTeq
Powered by Semantic MediaWiki
Copyright 2024 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact permissions@ametsoc.org. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.