[Physics] What are thermal energy distributions

photoelectric-effectquantum mechanicsstatistical mechanicsthermal-radiation

I am trying to understand the photoelectric-effect deeply. My teacher used the Planck's law and integrated it to deduce the Stefan-Boltzmann law. He somehow showed some quantum-physical characteristic — something that intensity did not increase the energy of photon as expected classically but the stopping voltage.

Now let's take a step back. He started with Planck's law and I want to understand how it is connected to other thermal equilibriums such as Bose-Einstein distribution, Fermi-Dirac distribution and Maxwell-Boltzmann distribution.

What are the thermal energy distributions? How to remember them? Some mnemonics? Are they somehow connected? I know BE and FD are the quantum-physical descriptions while MB is a classical approximation but I don't how Planck's law is related to them, how?

Wikipedia about Planck's law

As an energy distribution, it is one of a family of thermal equilibrium distributions which include the Bose–Einstein distribution, the Fermi–Dirac distribution and the Maxwell–Boltzmann distribution.

Best Answer

I just remember $$ \frac{1}{\exp(\beta (E-\mu)) \pm 1}$$ You can work out the sign from the fact that Bose-Einstein distributions can diverge (so they go with the - sign), whereas Fermi-Dirac is bounded (so they go with the + sign). Maxwell-Boltzmann applies to classical systems, so quantum statistics don't matter, so take the limit that the two distributions are the same (so drop the $\pm1$).

These expressions represent the average number of particles occupying a state with energy $E$. The chemical potential $\mu$ is just a knob that lets you adjust the overall density. You can also think of it as (roughly) the energy it takes to add a particle to the system. To find the total number of particles in the system you have to sum this over all of the energy levels. You can use this information to find all kinds of thermal averages. For example:

$$ \text{total energy} = \sum_{\text{all energies}} \left(\text{distribution function}\times\text{number of states with a given energy}\right)$$

This is essentially what is going on in Planck's law, only the sum is left off. Planck's law is the Bose-Einstein distribution (with $\mu=0$ because photons can be freely created and destroyed) multiplied by the number of states with an energy in a small range about $E$. This tells you how much energy there is in the photons with energies in that range.