Maxwell–Boltzmann distribution について

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In statistics the Maxwell–Boltzmann distribution is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann.
It was first defined and used in physics (in particular in statistical mechanics) for describing particle speeds in idealized gases where the particles move freely inside a stationary container without interacting with one another, except for very brief collisions in which they exchange energy and momentum with each other or with their thermal environment. Particle in this context refers to gaseous particles (atoms or molecules), and the system of particles is assumed to have reached thermodynamic equilibrium.〔''Statistical Physics'' (2nd Edition), F. Mandl, Manchester Physics, John Wiley & Sons, 2008, ISBN 9780471915331〕
While the distribution was first derived by Maxwell in 1860 on heuristic grounds,〔See:
* Maxwell, J.C. (1860) ("Illustrations of the dynamical theory of gases. Part I. On the motions and collisions of perfectly elastic spheres," ) ''Philosophical Magazine'', 4th series, 19 : 19-32.
* Maxwell, J.C. (1860) ("Illustrations of the dynamical theory of gases. Part II. On the process of diffusion of two or more kinds of moving particles among one another," ) ''Philosophical Magazine'', 4th series, 20 : 21-37.〕 Boltzmann later carried out significant investigations into the physical origins of this distribution.
A particle speed probability distribution indicates which speeds are more likely: a particle will have a speed selected randomly from the distribution, and is more likely to be within one range of speeds than another. The distribution depends on the temperature of the system and the mass of the particle.〔University Physics – With Modern Physics (12th Edition), H.D. Young, R.A. Freedman (Original edition), Addison-Wesley (Pearson International), 1st Edition: 1949, 12th Edition: 2008, ISBN (10-) 0-321-50130-6, ISBN (13-) 978-0-321-50130-1〕
The Maxwell–Boltzmann distribution applies to the classical ideal gas, which is an idealization of real gases. In real gases, there are various effects (e.g., van der Waals interactions, vortical flow, relativistic speed limits, and quantum exchange interactions) that make their speed distribution sometimes very different from the Maxwell–Boltzmann form. However, rarefied gases at ordinary temperatures behave very nearly like an ideal gas and the Maxwell speed distribution is an excellent approximation for such gases. Thus, it forms the basis of the kinetic theory of gases, which provides a simplified explanation of many fundamental gaseous properties, including pressure and diffusion.〔Encyclopaedia of Physics (2nd Edition), R.G. Lerner, G.L. Trigg, VHC publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1, ISBN (VHC Inc.) 0-89573-752-3〕
== Distribution function ==

The Maxwell–Boltzmann distribution is the function
:

f(v) = \sqrt\right)^3}\, 4\pi v^2 e^},

where

m

is the particle mass and

kT

is the product of Boltzmann's constant and thermodynamic temperature.
This probability density function gives the probability, per unit speed, of finding the particle with a speed near

v

. This equation is simply the Maxwell distribution (given in the infobox) with distribution parameter

a=\sqrt

. In probability theory the Maxwell–Boltzmann distribution is a chi distribution with three degrees of freedom and scale parameter

a=\sqrt

.
The simplest ordinary differential equation satisfied by the distribution is:
:

k T v f'(v)+f(v) \left(m v^2-2 kT\right)=0,

f(1)=\sqrt} e^} \left(\frac\right)^

or in unitless presentation:
:

a^2 x f'(x)+\left(x^2-2 a^2\right) f(x)=0,

f(1)=\frac} e^}}.

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