- #1
ed321
- 3
- 0
Homework Statement
I’m struggling to reconcile two results about the behaviour of a photon gas, any help would be appreciated:
First of all the Gibbs free energy=0, which means that dG=0=Vdp-SdT
But also p=1/3 U/V and S=4/3 U/T which means p=1/4 ST/V. Now if we call the entropy per unit volume S/V=s, p=1/4sT and differentiating both sides with respect to p at constant s we get 1=1/4s dp/dT which doesn’t agree with the result from the Gibb’s function.
Homework Equations
(My expressions for G, p and S agree with wikipedia: http://en.wikipedia.org/wiki/Photon_gas)
The Attempt at a Solution
I think I must not be differentiating p=1/4sT correctly, but I can’t see what I need to do differently.
Thanks.