# Page:Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt.pdf/11

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We ask: how large is the probability of the last-mentioned state relative to the original state? Or, what is the probability that at some point in time all n independently moving points in a volume v0 have by chance ended up in the volume v?

For this probability, which is a "statistical probability" one obtains the value:

${\displaystyle W=\left({\frac {v}{v_{0}}}\right)^{n}\ ;}$

one derives from this, applying Boltzmann's principle:

${\displaystyle S-S_{0}=R\left({\frac {n}{N}}\right)\lg \left({\frac {v}{v_{0}}}\right).}$

It's noteworthy that for this derivation, from which the Boyle-Gay-Lussac law and the identical law of osmotic pressure can be easily derived thermodynamically [1], there is no need to make any assumption regarding the way the molecules move.

## Interpretation of the Volume Dependence of the Entropy of Monochromatic Radiation using Boltzmann's Principle

In paragraph 4 we found for the dependence of Entropy of the monochromatic radiation on volume the expression:

${\displaystyle S-S_{0}={\frac {E}{\beta \nu }}\lg \left({\frac {v}{v_{0}}}\right).}$

This formula can be recast as follows:

${\displaystyle S-S_{0}={\frac {R}{N}}\lg \left[{\left({\tfrac {v}{v_{0}}}\right)}^{{\tfrac {N}{R}}{\tfrac {E}{\beta \nu }}}\right]}$
1. If E is the energy of the system, then one obtains:
${\displaystyle -d(E-TS)=pdv=TdS=TR{\frac {n}{N}}{\frac {dv}{v}}}$;
therefore
${\displaystyle pv=R{\frac {n}{N}}T.}$