Coulomb Collisions

Charged particles interact according to the Coulomb force law, according to which the interaction strength diminishes slowly with distance. Charged particles, moving in a plasma, constantly interact with other charged particles via this long-range force. For this reason the path of a particle is not a broken one (with discontinuous derivatives) as is the case with neutral particles, but a smooth curve (Fig. 43). However, a detailed analysis shows that thermal motion and transport processes can still be described in terms of a collision theory by introducing the concept of Coulomb collisions. This means that the actual curved path of the particles can be replaced by a series of broken lines (the dashed line in Fig. 43). Each time the direction of a particle changes by approximately 90° as a result of continuous interactions we say that the particle has experienced a Coulomb collision; this corresponds to a change in direction in the dashed line.