Explaining Photon Entropy: G=<E>-TS+pV

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SUMMARY

The discussion centers on the derivation of photon entropy, specifically the equation G=-TS+pV. The transition from 'TS' to (4/3) and from pV to (1/3) is clarified through the understanding of photon behavior in thermodynamics. It is established that G equals zero due to the massless nature of photons. The relationship pV = U/3 is explained as a result of geometric considerations in photon interactions, while S = 4/3 U/T is derived from Stefan's law and entropy integration.

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  • Familiarity with photon behavior and properties
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blueyellow
i have been revising but in my notes it says this about photon entropy:

G=<E>-TS+pV
=<E>-(4/3)<E>+(1/3)<E>=0

how did they go from 'TS' to (4/3)<E>
and pV to (1/3)<E>?
 
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Don't know what information you're starting with. Actually it's much easier to argue G = 0 from the beginning! (because photons have zero mass). Otherwise you might get everything by integrating over the Planck distribution. pV = U/3 is often hand-waved (1/3 of the photons hit each wall), and S = 4/3 U/T usually comes from deriving Stefan's law U ~ T4 and then integrating dS = dU/T.
 

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