View source for Physics:Loschmidt constant

{{Short description|Number of particle in a given volume of an ideal gas}}
The '''Loschmidt constant''' or '''Loschmidt's number''' (symbol: ''n''<sub>0</sub>) is the number of particles ([[Physics:Atom|atom]]s or [[Physics:Molecule|molecule]]s) of an [[Physics:Ideal gas|ideal gas]] per volume (the [[Physics:Number density|number density]]), and usually quoted at [[Standard temperature and pressure|standard temperature and pressure]]. The 2014 CODATA recommended value<ref>CODATA Recommended Values of the Fundamental Physical Constants:2014 {{Nist|http://physics.nist.gov/cgi-bin/cuu/Value?n0std}}, {{arxiv|https://arxiv.org/pdf/1507.07956v1.pdf}}</ref> is {{val|2.6867811|(15)|e=25}} per cubic metre at 0&nbsp;[[Celsius|°C]] and 1&nbsp;[[Physics:Atmosphere (unit)|atm]] and the 2006 CODATA recommended value<ref>{{CODATA2006|url=http://physics.nist.gov/cgi-bin/cuu/Value?n0}}</ref> was 2.686&nbsp;7774(47){{e|25}} per cubic metre at 0&nbsp;°C and 1&nbsp;atm. It is named after the Austrian physicist Johann Josef Loschmidt, who was the first to estimate the physical size of molecules in 1865.<ref>{{cite journal | first = J. | last = Loschmidt |  title = Zur Grösse der Luftmoleküle | url = https://books.google.com/books?id=ppEAAAAAYAAJ&pg=PA395 | journal = Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften Wien | volume = 52 | issue = 2 | pages = 395–413 | year =1865}}.</ref> The term "'''Loschmidt constant'''" is also sometimes used to refer to the [[Physics:Avogadro constant|Avogadro constant]], particularly in [[Social:German language|German]] texts.

By ideal gas law, <math>p_0V = Nk_BT_0</math>, and since <math>N = n_0 V</math>, the Loschmidt constant is given by the relationship
:<math>n_0 = \frac{p_0}{k_\text{B}T_0},</math>
where ''k''<sub>B</sub> is the [[Physics:Boltzmann constant|Boltzmann constant]], ''p''<sub>0</sub> is the standard [[Physics:Pressure|pressure]], and ''T''<sub>0</sub> is the standard [[Physics:Thermodynamic temperature|thermodynamic temperature]].

Since the Avogadro constant ''N''<sub>A</sub> satisfies <math>R = N_\text{A} k</math>, the Loschmidt constant satisfies
:<math>n_0 = \frac{p_0N_\text{A}}{RT_0},</math>
where ''R'' is the [[Physics:Gas constant|ideal gas constant]].

Being a measure of [[Physics:Number density|number density]], the Loschmidt constant is used to define the [[Physics:Amagat|amagat]], a practical unit of number density for gases and other substances:
:1 amagat = ''n''<sub>0</sub> = {{val|2.6867811|e=25|u=m-3}},
such that the Loschmidt constant is exactly 1&nbsp;amagat.

== Modern determinations ==
In the CODATA set of recommended values for physical constants, the Loschmidt constant is calculated from the gas constant and the Avogadro constant:<ref>{{CODATA2002}}</ref> 
:<math>n_0 = \frac{N_\text{A}}{R} \frac{p_0}{T_0} = \frac{A_\text{r}(\mathrm e) M_\text{u} c \alpha^2}{2 R_\infty h R} \frac{p_0}{T_0},</math>
where ''A''{{sub|r}}(e) is the [[Physics:Relative atomic mass|relative atomic mass]] of the [[Physics:Electron|electron]], ''M''{{sub|u}} is the [[Physics:Molar mass constant|molar mass constant]], ''c'' is the [[Physics:Speed of light|speed of light]], ''α'' is the [[Physics:Fine-structure constant|fine-structure constant]], ''R''{{sub|∞}} is the [[Physics:Rydberg constant|Rydberg constant]] and ''h'' is the [[Physics:Planck constant|Planck constant]]. The pressure and temperature can be chosen freely and must be quoted with values of the Loschmidt constant. The precision to which the Loschmidt constant is currently known is limited entirely by the uncertainty in the value of the gas constant.

== First determinations ==
Loschmidt did not actually calculate a value for the constant which now bears his name, but it is a simple and logical manipulation of his published results. [[Biography:James Clerk Maxwell|James Clerk Maxwell]] described the paper in these terms in a public lecture eight years later:<ref name="Maxwell">{{cite journal | last = Maxwell | first = James Clerk |  url = http://web.lemoyne.edu/~giunta/maxwell.html | title = Molecules | journal = Nature | volume = 8 | pages = 437–441 | year = 1873 | doi = 10.1038/008437a0 |bibcode = 1873Natur...8..437. | issue=204 | doi-access = free }}</ref>
<blockquote>
Loschmidt has deduced from the dynamical theory the following remarkable proportion:—As the volume of a gas is to the combined volume of all the molecules contained in it, so is the mean path of a molecule to one-eighth of the diameter of a molecule.
</blockquote>
To derive this "remarkable proportion", Loschmidt started from Maxwell's own definition of the [[Physics:Mean free path|mean free path]] (there is an inconsistency between the result on this page and the page cross-referenced to the mean free path; here appears an additional factor 3/4):
:<math>\ell = \frac{3}{4n_0\pi d^2},</math>
where ''n''{{sub|0}} has the same sense as the Loschmidt constant, that is the number of molecules per unit volume, and ''d'' is the effective diameter of the molecules (assumed to be spherical). This rearranges to
:<math>\frac{1}{n_0} = \frac{16}{3} \frac{\pi\ell d^2}{4},</math>
where 1/''n''{{sub|0}} is the volume occupied by each molecule in the gas phase, and ''π''ℓ''d''{{sup|2}}/4 is the volume of the cylinder made by the molecule in its trajectory between two collisions. However, the true volume of each molecule is given by ''πd''{{sup|3}}/6, and so ''n''{{sub|0}}''πd''{{sup|3}}/6 is the volume occupied by all the molecules not counting the empty space between them. Loschmidt equated this volume with the volume of the liquified gas. Dividing both sides of the equation by ''n''{{sub|0}}''πd''{{sup|3}}/6 has the effect of introducing a factor of ''V''{{sub|liquid}}/''V''{{sub|gas}}, which Loschmidt called the "condensation coefficient" and which is experimentally measurable. The equation reduces to
:<math>d = 8\frac{V_\text{l}}{V_\text{g}}\ell</math>
relating the diameter of a gas molecule to measurable phenomena. 

The number density, the constant which now bears Loschmidt's name, can be found by simply substituting the diameter of the molecule into the definition of the mean free path and rearranging:
:<math>n_0 = \left(\frac{V_\text{g}}{V_\text{l}}\right)^2 \frac{3}{256\pi\ell^3}.</math>
Instead of taking this step, Loschmidt decided to estimate the mean diameter of the molecules in air. This was no minor undertaking, as the condensation coefficient was unknown and had to be estimated–it would be another twelve years before Raoul Pictet and [[Biography:Louis Paul Cailletet|Louis Paul Cailletet]] would liquify nitrogen for the first time. The mean free path was also uncertain. Nevertheless, Loschmidt arrived at a diameter of about one nanometre, of the correct [[Order of magnitude|order of magnitude]].

Loschmidt's estimated data for air give a value of ''n''{{sub|0}}&nbsp;= {{val|1.81|e=24|u=m-3}}. Eight years later, Maxwell was citing a figure of "about 19 million million million" per cm{{sup|3}}, or {{val|1.9|e=25|u=m-3}}.<ref name="Maxwell" />

== See also ==
* [[Physics:Avogadro's law|Avogadro's law]]
* List of scientists whose names are used in physical constants

== References ==
{{Reflist}}

[[Category:Amount of substance]]
[[Category:Physical constants]]

{{Sourceattribution|Loschmidt constant}}