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445lec26 - PHYS 445 Lecture 26 Black-body radiation I...

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PHYS 445 Lecture 26 - Black-body radiation I 26 - 1 © 2001 by David Boal, Simon Fraser University. All rights reserved; further resale or copying is strictly prohibited. Lecture 26 - Black-body radiation I What's Important: number density energy density Text : Reif In our previous discussion of photons, we established that the mean number of photons with energy i is n i = 1 e ß i - 1 (26.1) Here, the questions that we want to address about photons are: what is their number density what is their energy density what is their pressure? Number density n Eq. (26.1) is written in the language of discrete states. The first thing we need to do is replace the sum by an integral over photon momentum states: Σ i d 3 p Of course, it isn't quite this simple because the density-of-states issue that we introduced before: for every spin state there is one phase space state for every h 3 , so the proper replacement more like Σ i (1/ h 3 ) d 3 p d 3 r The units now work: the left hand side is a number, and so is the right hand side.
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