Kinetic Equation for the Dilute Boltzmann Gas in an External Field

We report a kinetic equation for an auxiliary distribution function f (k, v(1), t) which yields the intermediate scattering function I-s (k, t). To this end, the projection operator proposed by Stecki was applied. The scattering operator was given in explicit form in the limit of low density gas. The general kinetic equation was next specialized for the case of Lorentz gas.