Maxwell speed distribution

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

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

The mean speed is given by

and the root-mean-square speed by


According to the Shivanian and Lopez-Ruiz model [2], consider an ideal gas composed of particles having a mass of unity in the three-dimensional () space. As long as there no privileged direction when in equilibrium, we can take any direction in space and study the discrete time evolution of the velocity distribution in that direction. Let us call this axis . We can complete a Cartesian system with two additional orthogonal axis . If represents the probability of finding a particle of the gas with velocity component in the direction comprised between and at time , then the probability to have at this time a particle with a velocity will be . 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 . 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 in the direction at time : (1) Only those particles with an energy greater than at time can contribute to this velocity in the direction , that is, all those particles whose velocities verify ; (2) The new velocities after collisions are equally distributed in their permitted ranges, that is, particles with velocity can generate maximal velocities , then the allowed range of velocities measures , and the contributing probability of these particles to the velocity will be . Taking all together we finally get the expression for the evolution operator . This is:

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 conserves the energy and the null momentum of the gas over time. Moreover, for any initial velocity distribution, the system tends towards its equilibrium, i.e. towards the Maxwellian Velocity Distribution (MVD). This means that

Let us sketch now all these properties. First, we introduce the norm of positive functions (one-dimensional velocity distributions) in the real axis as

Then we have the following exact results:

Theorem 1[edit]

For any with , we have .

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

Theorem 2[edit]

The mean value of the velocity in the recursion is conserved in time. In fact, it is null for all :


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

Theorem 3[edit]

For every with , we have

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[edit]

The one-parametric family of normalized Gaussian functions , , , are fixed points of the operator . In other words, .

Theorem 5[edit]

For distributions with , suppose that , and is a normalized continuous distribution, then is a fixed point of the operator .


As a consequence of the former theorems, and by simulation of many examples, the following conjecture can be stated:

For any with , with finite and verifying , the limit is the fixed point , with . That is, the asymptotic steady state is the Gaussian distribution with the same mean energy than the initial out-of-equilibrium state .


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 , with the Boltzmann constant and the temperature of the gas, and introducing the mass of the particles, let us observe that the MVD (above presented) is recovered in its format:

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


Related reading

External resources[edit]