The distribution function
for species
satisfies

The left-hand side is equal to the total time derivative (in six-dimensional phase space) of the distribution function. The term on the right-hand side treats collisional effects due to fields generated by other particles within a Debye radius . Collisional effects are only important in this small range because of Debye shielding of the long-range electrostatic force in a plasma. The characteristic frequency of collisions is denoted by .

Due to the presence of many oppositely charged particles around a central positive test charge in a plasma, the scalar electrostatic potential falls off as , where the extra exponential factor is due to Debye shielding. The Debye length is given by , where is the electron plasma frequency, with .