## PHY452H1S Basic Statistical Mechanics. Lecture 4: Maxwell distribution for gases. Taught by Prof. Arun Paramekanti

Posted by peeterjoot on January 17, 2013

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# Disclaimer

Peeter’s lecture notes from class. May not be entirely coherent.

# Review: Lead up to Maxwell distribution for gases

For the random walk, after a number of collisions , we found that a particle (labeled the th) will have a velocity

We argued that the probability distribution for finding a velocity was as in

# What went wrong?

However, we know that this must be wrong, since we require

Where our argument went wrong is that when the particle has a greater than average velocity, the effect of a collision will be to slow it down. We have to account for

- Fluctuations “random walk”
- Dissipation “slowing down”

There were two ingredients to diffusion (the random walk), these were

- Conservation of particles
We can also think about a conservation of a particles in a velocity space

where is a probability current in this velocity space.

- Fick’s law in velocity space takes the form

The diffusion results in an “attempt” to flatten the distribution of the concentration as in (Fig 2).

We’d like to add to the diffusion current an extra frictional like term

We want something directed opposite to the velocity and the concentration

This gives

Can we find a steady state solution to this equation when ? For such a steady state we have

Integrating once we have

supposing that at , integrating once more we have

This is the *Maxwell-Boltzmann* distribution, illustrated in (Fig3).

The concentration has a probability distribution.

Calculating from this distribution we can identify the factor.

This also happens to be the energy in terms of temperature (we can view this as a definition of the temperature for now), writing

Here

# Equilibrium steady states

Fluctuations Dissipation

This is a specific example of the more general *Fluctuation-Dissipation theorem*.

**Generalizing to 3D**

Fick’s law and the continuity equation in 3D are respectively

As above we have for the steady state

Integrating once over all space

This is three sets of equations, one for each component of

So that our steady state equation is

Computing the average 3D squared velocity for this distribution, we have

For each component the normalization kills off all the contributions for the other components, leaving us with the usual ideal gas law kinetic energy.

# Phase space

Now let’s switch directions a bit and look at how to examine a more general system described by the phase space of generalized coordinates

Here

- , where is the physical space dimension.

The motion in phase space will be governed by the knowledge of how each of these coordinates change for all the particles. Assuming a Hamiltonian and recalling that , gives us

we have for the following set of equations describing the entire system

Example, 1D SHO

This has phase space trajectories as in (Fig4).

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