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

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

See also

• Boltzmann equation
• Second law of thermodynamics

References

1. L. Boltzmann "", Wiener Ber. 63 pp. 275- (1872)
2. Sybren R. De Groot and Peter Mazur "Non-Equilibrium Thermodynamics", Dover Publications
3. Robert Zwanzig "Nonequilibrium Statistical Mechanics", Oxford University Press (2001)