1. The problem statement, all variables and given/known data 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. 2. Relevant equations (My expressions for G, p and S agree with wikipedia: http://en.wikipedia.org/wiki/Photon_gas) 3. 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.