Thermodynamics of radiation pressure and photon momentum (Part 2)

We derive some of the properties of blackbody radiation using thermodynamic identities. A few of the results reported earlier (in Part 1 of the present paper) will be re-derived here from a different perspective. We show that the fluctuations of thermal radiation can be expressed as the sum of two contributions: one resulting from classical considerations in the limit when Planck's constant h goes to zero, and a second one that is rooted in the discrete nature of electromagnetic particles (photons). Johnson noise and the Nyquist theorem will be another topic of discussion, where the role played by blackbody radiation in generating noise within an electronic circuit will be emphasized. In the context of thermal fluctuations, we also analyze the various sources of noise in photodetection, relating the statistics of photocounts to energy-density fluctuations associated with blackbody radiation. The remaining sections of the paper are devoted to an analysis of the thermodynamic properties of a mono-atomic gas under conditions of thermal equilibrium. This latter part of the paper aims to provide a basis for comparisons between a photon gas and a rarefied gas of rigid identical particles of matter.