Nonlinear conductivity and entropy in the two-body Boltzmann gas

Hoover finds exact solutions of the two-particle Boltzmann equation for hard disks and hard spheres diffusing isothermally in an external field. The corresponding transport coefficient, one-particle current divided by field strength, decreases as the field increases. This nonlinear dependence of the current on the field and the corresponding nonlinear dependence of the distribution function on the current are compared to the predictions of ''single-time'' information theory. Hoover's exact steady-state distribution function, from Boltzmann's equation, is quite different from the approximate information-theory analog. The approximate theory badly underestimates the nonlinear decrease of entropy with current. The exact two-particle solutions found here should prove useful in testing and improving theories of steady-state and transient distribution functions far from equilibrium.