# Difference between revisions of "H-theorem"

Boltzmann's H-theorem states that the entropy of a closed system can only increase in the course of time, and must approach a limit as time tends to infinity.

$\sigma \geq 0$

where $\sigma$ is the entropy source strength, given by (Eq 36 Chap IX Ref. 2)

$\sigma = -k \sum_{i,j} \int C(f_i,f_j) \ln f_i d {\mathbf u}_i$

where the function C() represents binary collisions. At equilibrium, $\sigma = 0$.

## H-function

Boltzmann's H-function is defined by (Eq. 5.66 Ref. 3):

$H=\iint f({\mathbf V}, {\mathbf r}, t) \ln f({\mathbf V}, {\mathbf r}, t) ~ d {\mathbf r} d{\mathbf V}$

where ${\mathbf V}$ is the molecular velocity. A restatement of the H-theorem is

$\frac{dH}{dt} \leq 0$