()(~MeVinthecaseofthenucleus).(b)particlesassociatedwithafield,:ifthetotalenergyofthefieldchanges,’llseethatthechemicalpotentialofsuchparticlesiszeroinequilibrium,,radiationemittedbyahotbody,orfromalaserisnotinequilibrium:,whoconsideredacavityfilledwithradiation,.-,,andwedefineuS(,T)dastheenergydensity(perunitvolume)oftheradiationwithfrequenciesbetweenand+d.uS(,T):Inequilibrium,uS(,T)isthesameeverywhereinthecavity,=const,theinternalenergyU=u(T):foranidealgas,anisothermalexpansionwouldconservethegasenergy,whereasforthephotongas,itistheenergydensitywhichisunchanged,thenumberofphotonsisnotconserved,isafunctionofandT;asurfaceforwhich()=(standingwaves)-(similartoaharmonicoscillators):.-
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