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Natural Science Forum / Physics / Research / August 2007photon entropy
Is the photon entropy independent of its frequency? does it remain constant? I am trying to work out the photon gas entropy using such a postulate. Is this well justified? Please cite sources wherever possible Thanks, Goombaj > Is the photon entropy independent of its frequency? does it remain > constant? I am trying to work out the photon gas entropy using such a > postulate. Is this well justified? Please cite sources wherever > possible Entropy is not a property of a particle, it is a statistical property of ensembles of particles (or other things). However, provided the wording of the question is sorted out, it can be answered by analogy: the "entropy of a photon" depends on its frequency just like the "entropy of a massive particle" depends on its energy. The idea here is that, if you want, you can treat a photon in the same way as a massive particle, but taking the mass -> 0 limit in the end. So, can you answer your own question if you replace "photon" by "massive particle"? Now, calculating the entropy of a photon gas can be a tricky thing. It involves dealing with statistical mechanics of a relativistic system. Here's another question to ponder. Do thermodynamic properties of a photon gas depend on the number of photons in it? The question is intentionally vague. :-) Igor > > Is the photon entropy independent of its frequency? does it remain > > constant? I am trying to work out the photon gas entropy using such a > > postulate. Is this well justified? Please cite sources wherever > > possible > Entropy is not a property of a particle, it is a statistical property of > ensembles of particles (or other things). This is the point. Consequently, the entropy does not depend on the energy of the individual particles, but of the set (spectrum) of energies they can assume, see Einstein, 'Specific heat of solids...', 1907. I also would like to recommend you Einstein's 1905 paper, where he argues that a gas of photons behaves like a dilute gas of atoms. > ...Now, calculating the entropy of a photon gas can be a tricky thing. It > involves dealing with statistical mechanics of a relativistic system. > Here's another question to ponder. Do thermodynamic properties of a > photon gas depend on the number of photons in it? The question is > intentionally vague. :-) Thus, consider the chemical potential to find it out ;-) Best wishes, Peter Agreed. Sorry me previous post was vague. unintentionally :( For most systems, say the ideal classical gas, if some more particles are introduced into the (isolated) system then the increase in entropy depends on the number of particles introduced and also on the total energy added via these particles. For photons, it doesnt matter how many photons one introduces into the system, all that matters is the energy added. Moreover, the entropy/ <no. of particles> is a constant independent of temperature or any other physical parameters. its a pure number! I think its strange. Is it just because the chemical potential is zero? THanks, Goombaj p.s. I hope this time I've been more precise. > > > Is the photon entropy independent of its frequency? does it remain > > > constant? I am trying to work out the photon gas entropy using such a [quoted text clipped - 20 lines] > Best wishes, > Peter |
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