# Chemical potential (statistical physics)

1. Apr 23, 2005

### Edgardo

Hi all,

I have a question about the chemical potential $\mu$ from statistical physics:
Why is the chemical potential $\mu$ for a photon gas (or phonon gas) zero?

Edgardo

2. Apr 24, 2005

### shyboy

That is because a number of both phonons and photons can fluctuate freely without changing the free energy of the system. So their average number is determined by the system state.

3. Apr 24, 2005

### dextercioby

The # of photons is not a thermodynamical variable for a photon gas in thermal equilibrium (grand canonical ensemble).It can be 0,finite or even infinite.The only thing we can compute is its average...

Daniel.

P.S.Equilibrium thermodynamics tells us that $\mu$ and $N$ are conjugate variables.