# Differences

This shows you the differences between two versions of the page.

Both sides previous revision Previous revision | |||

classical_phase_density [2015/08/18 20:27] nikolaj |
classical_phase_density [2015/08/18 20:29] (current) nikolaj |
||
---|---|---|---|

Line 25: | Line 25: | ||

^ $ \frac{\mathrm d}{\mathrm dt}{\hat\rho}(\pi(t),t)=0 $ ^ | ^ $ \frac{\mathrm d}{\mathrm dt}{\hat\rho}(\pi(t),t)=0 $ ^ | ||

where $\pi$ is the solution of the [[Hamiltonian equations]]. | where $\pi$ is the solution of the [[Hamiltonian equations]]. | ||

- | |||

- | === Volume in statistical physics === | ||

- | A characteristic volume $V$ may be given by an integral over the spatial part of ${\mathcal M}$. This is e.g. how $V$ arises in the statistical mechanics derivation in the classical setting of the ideal gas law $p := -\frac{\partial}{\partial V}\langle{H}\rangle = \frac{N}{V}\cdot k_B T$. See also [[https://en.wikipedia.org/wiki/Cluster_expansion|Cluster expansion]]. | ||

- | Introducing the density $n=\frac{N}{V}$, this holds true for infinite volumes. | ||

- | In the derivation via quantum gases in an infinite volume, a volume parameter is introduced in when the momenta are quantized (see [[Classical density of states]]). | ||

- | |||

- | A remark on the latter case: Note that the physical constants $\hbar$ and $c$ can be used to translate energy to frequency (or time) and further translates time to length. Using this, we can write down models involving a volume parameter $V$, defining a characteristic energy $\frac{(\hbar c)^3}{V}$. This may then e.g. be embedded via (unitless!) expressions as complicated as | ||

- | |||

- | $\frac{V}{(\hbar c)^3}\int {\mathrm d}E\, f(\frac{V}{(\hbar c)^3}E)$. | ||

=== Reference === | === Reference === |