Difference between revisions of "Maxwell speed distribution"

From SklogWiki
Jump to: navigation, search
(Derivation)
Line 1: Line 1:
The '''Maxwell velocity distribution''' <ref>J. C. Maxwell "", British Association for the Advancement of Science '''29''' Notices and Abstracts 9 (1859)</ref>
+
The '''Maxwellian velocity distribution''' <ref> [http://books.google.com/books?id=hYIBOMxIuvEC&source=gbs_similarbooks_r&cad=2 James C. Maxwell, "The scientific papers of James Clerk Maxwell", Edited by W.D. Niven, paper number XXDover Publications, Vol. I,II, New York, USA (2003)]</ref> provides  probability that the speed of a molecule of mass ''m'' lies in the range ''v'' to ''v+dv'' is given by
<ref>[http://dx.doi.org/10.1080/14786446008642818  J. C. Maxwell "V. Illustrations of the dynamical theory of gases.—Part I. On the motions and collisions of perfectly elastic spheres", Philosophical Magazine '''19''' pp. 19-32 (1860)]</ref>
 
<ref>[http://dx.doi.org/10.1080/14786446008642902 J. C. Maxwell "II. Illustrations of the dynamical theory of gases", Philosophical Magazine '''20''' pp. 21-37 (1860)]</ref>
 
<ref>[http://dx.doi.org/10.1098/rstl.1867.0004 J. Clerk Maxwell "On the Dynamical Theory of Gases", Philosophical Transactions of the Royal Society of London '''157''' pp. 49-88 (1867)]</ref> provides  probability that the speed of a molecule of mass ''m'' lies in the range ''v'' to ''v+dv'' is given by
 
  
 
:<math>P(v)dv = 4 \pi v^2 dv \left( \frac{m}{2 \pi k_B T} \right)^{3/2} \exp (-mv^2/2k_B T) </math>
 
:<math>P(v)dv = 4 \pi v^2 dv \left( \frac{m}{2 \pi k_B T} \right)^{3/2} \exp (-mv^2/2k_B T) </math>
Line 18: Line 15:
  
 
:<math>\sqrt{\overline{v^2}} = \sqrt \frac{3}{2} v_{\rm max}</math>
 
:<math>\sqrt{\overline{v^2}} = \sqrt \frac{3}{2} v_{\rm max}</math>
 +
 +
 
==Derivation==
 
==Derivation==
  
  
Consider an ideal gas with particles of unity mass in the three-dimensional (<math>3D</math>) space. As long as there is not a privileged direction in the equilibrium, we can take any direction
+
According to the '''Shivanian & Lopez-Ruiz model''' <ref>[http://arxiv.org/abs/1105.4813 Elyas Shivanian and Ricardo Lopez-Ruiz "A New Model for Ideal Gases. Decay to the Maxwellian Distribution", arXiv:1105.4813v1 24 May (2011)]</ref>, consider an ideal gas with particles of unity mass in the three-dimensional (<math>3D</math>) space. As long as there is not a privileged direction in the equilibrium, we can take any direction in the space and to study the discrete time evolution of the velocity distribution in that direction. Let us call this axis <math>U</math>. We can complete a Cartesian system with two additional orthogonal axis <math>V,W</math>. If <math>p_n(u){\mathrm d}u</math> represents the probability of finding a particle of the gas with velocity component in the direction <math>U</math> comprised between <math>u</math> and <math>u + {\mathrm d}u</math> at time <math>n</math>, then the probability to have at this time <math>n</math> a particle with a <math>3D</math> velocity <math>(u,v,w)</math> will be <math>p_n(u)p_n(v)p_n(w)</math>. The particles of the gas collide between them, and after a number of interactions of the order of system size, a new velocity distribution is attained at time <math>n+1</math>. Concerning the interaction of particles with the bulk of the gas, we make two simplistic and realistic assumptions in order to obtain the probability of having a velocity <math>x</math> in the direction <math>U</math> at time <math>n+1</math>: (1) Only those particles with an energy bigger than <math>x^2</math> at time <math>n</math> can contribute to this velocity <math>x</math> in the direction <math>U</math>, that is, all those particles whose velocities <math>(u,v,w)</math> verify <math> u^2+v^2+w^2\ge x^2</math>; (2) The new velocities after collisions are equally  
in the space and to study the discrete time evolution of the velocity distribution in that direction. Let us call this axis <math>U</math>. We can complete a Cartesian system with two additional orthogonal axis <math>V,W</math>. If <math>p_n(u){\mathrm d}u</math> represents the probability of finding a particle of the gas with velocity component in the direction <math>U</math> comprised between <math>u</math> and <math>u + {\mathrm d}u</math> at time <math>n</math>, then the probability to have at this time <math>n</math> a particle with a <math>3D</math> velocity <math>(u,v,w)</math> will be <math>p_n(u)p_n(v)p_n(w)</math>. The particles of the gas collide between them, and after a number of interactions of the order of system size, a new velocity distribution is attained at time <math>n+1</math>. Concerning the interaction of particles with the bulk of the gas, we make two simplistic and realistic assumptions in order to obtain the probability of having a velocity <math>x</math> in the direction <math>U</math> at time <math>n+1</math>: (1) Only those particles with an energy bigger than <math>x^2</math> at time <math>n</math> can contribute to this velocity <math>x</math> in the direction <math>U</math>, that is, all those particles whose velocities <math>(u,v,w)</math> verify <math> u^2+v^2+w^2\ge x^2</math>; (2) The new velocities after collisions are equally  
 
 
distributed in their permitted ranges, that is, particles with velocity <math>(u,v,w)</math> can generate maximal velocities <math>\pm U_{max}=\pm\sqrt{u^2+v^2+w^2}</math>, then the allowed range of velocities <math>[-U_{max},U_{max}]</math> measures <math>2|U_{max}|</math>, and the contributing probability of these particles to the velocity <math>x</math> will be <math>p_n(u)p_n(v)p_n(w)/(2|U_{max}|)</math>. Taking all together we finally get the expression  
 
distributed in their permitted ranges, that is, particles with velocity <math>(u,v,w)</math> can generate maximal velocities <math>\pm U_{max}=\pm\sqrt{u^2+v^2+w^2}</math>, then the allowed range of velocities <math>[-U_{max},U_{max}]</math> measures <math>2|U_{max}|</math>, and the contributing probability of these particles to the velocity <math>x</math> will be <math>p_n(u)p_n(v)p_n(w)/(2|U_{max}|)</math>. Taking all together we finally get the expression  
 
for the evolution operator <math>T</math>. This is:
 
for the evolution operator <math>T</math>. This is:
Line 99: Line 97:
 
MVD = p_{\alpha}(u)p_{\alpha}(v)p_{\alpha}(w)=\left({m\alpha\over\pi}\right)^{3\over 2}\,e^{-m\alpha (u^2+v^2+w^2)} \;\;\; with \;\;\;  \alpha=(2k\tau)^{-1}.
 
MVD = p_{\alpha}(u)p_{\alpha}(v)p_{\alpha}(w)=\left({m\alpha\over\pi}\right)^{3\over 2}\,e^{-m\alpha (u^2+v^2+w^2)} \;\;\; with \;\;\;  \alpha=(2k\tau)^{-1}.
 
</math>
 
</math>
 +
 +
Moreover, the increasing of the entropy is found during all the decay process. This gives rise to the celebrated [[H-theorem]] <ref> [http://books.google.pn/books/about/Lectures_on_Gas_Theory.html?id=-I7QzCXnstEC Ludwig Boltzmann, "Lectures on Gas Theory", Translated by S.G. Brush, Dover Publications, New York, USA (1995)]</ref>.
  
 
==References==
 
==References==
 
<references/>
 
<references/>
 +
 
;Related reading
 
;Related reading
*[http://dx.doi.org/10.1080/002068970500044749 J. S. Rowlinson "The Maxwell-Boltzmann distribution", Molecular Physics '''103''' pp. 2821 - 2828 (2005)]
+
 
*[http://arxiv.org/abs/1105.4813 Elyas Shivanian and Ricardo Lopez-Ruiz "A New Model for Ideal Gases. Decay to the Maxwellian Distribution", arXiv:1105.4813v1 24 May (2011)]
+
*[http://www.ingentaconnect.com/content/tandf/tmph/2005/00000103/F0030021/art00003 J. S. Rowlinson "The Maxwell-Boltzmann distribution", Molecular Physics '''103''' pp. 2821 - 2828 (2005)]
 +
 
 
==External resources==
 
==External resources==
 +
 
*[ftp://ftp.dl.ac.uk/ccp5/ALLEN_TILDESLEY/F.24  Initial velocity distribution] sample FORTRAN computer code from the book [http://www.oup.com/uk/catalogue/?ci=9780198556459 M. P. Allen and D. J. Tildesley "Computer Simulation of Liquids", Oxford University Press (1989)].
 
*[ftp://ftp.dl.ac.uk/ccp5/ALLEN_TILDESLEY/F.24  Initial velocity distribution] sample FORTRAN computer code from the book [http://www.oup.com/uk/catalogue/?ci=9780198556459 M. P. Allen and D. J. Tildesley "Computer Simulation of Liquids", Oxford University Press (1989)].
 
[[category: statistical mechanics]]
 
[[category: statistical mechanics]]

Revision as of 18:12, 19 July 2011

The Maxwellian velocity distribution [1] provides probability that the speed of a molecule of mass m lies in the range v to v+dv is given by

P(v)dv = 4 \pi v^2 dv \left( \frac{m}{2 \pi k_B T} \right)^{3/2} \exp (-mv^2/2k_B T)

where T is the temperature and k_B is the Boltzmann constant. The maximum of this distribution is located at

v_{\rm max} = \sqrt{\frac{2k_BT}{m}}

The mean speed is given by

\overline{v} = \frac{2}{\sqrt \pi} v_{\rm max}

and the root-mean-square speed by

\sqrt{\overline{v^2}} = \sqrt \frac{3}{2} v_{\rm max}


Derivation

According to the Shivanian & Lopez-Ruiz model [2], consider an ideal gas with particles of unity mass in the three-dimensional (3D) space. As long as there is not a privileged direction in the equilibrium, we can take any direction in the space and to study the discrete time evolution of the velocity distribution in that direction. Let us call this axis U. We can complete a Cartesian system with two additional orthogonal axis V,W. If p_n(u){\mathrm d}u represents the probability of finding a particle of the gas with velocity component in the direction U comprised between u and u + {\mathrm d}u at time n, then the probability to have at this time n a particle with a 3D velocity (u,v,w) will be p_n(u)p_n(v)p_n(w). The particles of the gas collide between them, and after a number of interactions of the order of system size, a new velocity distribution is attained at time n+1. Concerning the interaction of particles with the bulk of the gas, we make two simplistic and realistic assumptions in order to obtain the probability of having a velocity x in the direction U at time n+1: (1) Only those particles with an energy bigger than x^2 at time n can contribute to this velocity x in the direction U, that is, all those particles whose velocities (u,v,w) verify  u^2+v^2+w^2\ge x^2; (2) The new velocities after collisions are equally distributed in their permitted ranges, that is, particles with velocity (u,v,w) can generate maximal velocities \pm U_{max}=\pm\sqrt{u^2+v^2+w^2}, then the allowed range of velocities [-U_{max},U_{max}] measures 2|U_{max}|, and the contributing probability of these particles to the velocity x will be p_n(u)p_n(v)p_n(w)/(2|U_{max}|). Taking all together we finally get the expression for the evolution operator T. This is:



p_{n+1}(x)=Tp_n(x) = \int\int\int_{u^2+v^2+w^2\ge x^2}\,{p_n(u)p_n(v)p_n(w)\over 2\sqrt{u^2+v^2+w^2}} \; {\mathrm d}u{\mathrm d}v{\mathrm d}w\,.

Let us remark that we have not made any supposition about the type of interactions or collisions between the particles and, in some way, the equivalent of the Boltzmann hypothesis of molecular chaos would be the two simplistic assumptions we have stated on the interaction of particles with the bulk of the gas. In fact, the operator T conserves in time the energy and the null momentum of the gas. Moreover, for any initial velocity distribution, the system tends towards its equilibrium, i.e. towards the Maxwellian Velocity Distribution (MVD). This means that


\lim_{n\rightarrow\infty} T^n \left(p_0(x)\right) \rightarrow p_f(x)=MVD\;(1D\;case)\,.

Let us sketch now all these properties.

First, we introduce the norm ||\cdot|| of positive functions (one-dimensional velocity distributions) in the real axis as


\vert\vert p\vert\vert=\int_{-\infty}^{+\infty} p(x) dx.

Then we have the following exact results:

Theorem 1

For any p with ||p||=1, we have ||Tp||=||p||.

This can be interpreted as the conservation of the number of particles or in an equivalet way of the total mass of the gas.

Theorem 2

The mean value of the velocity in the recursion p_n=T^np_0 is conserved in time. In fact, it is null for all n:


<x,Tp>=<x,T^2p>=<x,T^3p>=\cdots=<x,T^np>=\cdots=0\,,

where


<f,g>=\int_{-\infty}^{+\infty}f(x)g(x){\mathrm d}x\,.

It means that the zero total momentum of the gas is conserved in its time evolution under the action of T.

Theorem 3

For every p with ||p||=1, we have


<x^2,p>=<x^2,Tp>=<x^2,T^2p>=<x^2,T^3p>=\cdots=<x^2,T^np>=\cdots \,.

It means that the mean energy per particle is conserved and in consequence, by Theorem 1, the total energy of the gas is conserved in time.

Theorem 4

The one-parametric family of normalized gaussian functions p_{\alpha}(x)=\sqrt{\alpha\over\pi}e^{-\alpha x^2}, \alpha\ge 0, ||p_{\alpha}||=1, are fixed points of the operator T. In other words, Tp_{\alpha}=p_{\alpha}.

Conjecture

As a consequence of the former theorems and by simulation of many exmaples, it can be claimed the following conjecture:

For any p with ||p||=1, with finite <x^2,p> and verifying \lim_{n\rightarrow\infty} ||T^np(x)-\mu(x)||=0, the limit \mu(x) is the fixed point p_{\alpha}(x)=\sqrt{\alpha\over\pi}\,e^{-\alpha x^2}, with \alpha=(2\,<x^2,p>)^{-1}.

Conclusion

In physical terms, it means that for any initial velocity distribution of the gas, it decays to the Maxwellian distribution, which is just the fixed point of the dynamics. Recalling that <x^2,p>=k\tau, with k the Boltzmann constant and \tau the temperature of the gas, and introducing the mass m of the particles, let us observe that the MVD (above presented) is recovered in its 3D format:


MVD = p_{\alpha}(u)p_{\alpha}(v)p_{\alpha}(w)=\left({m\alpha\over\pi}\right)^{3\over 2}\,e^{-m\alpha (u^2+v^2+w^2)} \;\;\; with \;\;\;  \alpha=(2k\tau)^{-1}.

Moreover, the increasing of the entropy is found during all the decay process. This gives rise to the celebrated H-theorem [3].

References

Related reading

External resources