Introduction

This chapter deals with the dynamics of electrically conducting fluids, usually called plasmas. The emphasis is on concepts that are of direct relevance to astrophysics. The basic ideas that are covered here will be used in several chapters of Vols. II and III.

The Mean Field and Collisions in Plasma

Several astrophysical systems are made of fully ionised gases, usually called plasmas, in which electromagnetic interactions between the constituents play a vital role. In this chapter, we treat fully ionised plasma as having two components: electrons of charge -e and positive ions of charge Ze. We begin by discussing several assumptions and approximations that will be inherent in our description.

The effective use of statistical methods in the study of neutral gases relies on the fact that the interaction between constituent particles are of short range and random in nature. This assumes that the gas is sufficiently rarefied and can be treated as ideal. To treat a plasma as a fluid, it is necessary to impose a corresponding condition that, however, has some important conceptual differences from that of neutral gases.

The condition for a plasma to be treated as ideal requires that the random kinetic energy of the particles be large compared with the electrostatic potential energy between two particles, that is, kBT ≫ e2/r ∼ e2nfrac13;, where T is the temperature, n is the number density of particles, and r ≃ n-⅓ is the mean interparticle distance.