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    Thermal radiation can be treated thermodynamically as a gas of photons with internal energy U = u(T) V and pressure p = u(T)/3 where u(T) is the energy density. Show that

    (a) entropy density s is given by s = 4p/T
    (b) Gibbs function = 0
    (c) heat capacity at constant volume Cv = 3s per unit volume
    (d) heat capacity at constant pressure Cp is infinite.


    My attempt:

    I've done (a), (b), but am having trouble on (c). I guess im trying to find T(Ds/DT) where capital D is partial.. but Im having touble arriving at 3s..

    any hints? Thanks!
 
 
 
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