ABSTRACT
Magnetohydrodynamics, in its most general form, deals with the motion of compressible conducting fluids in the presence of magnetic fields. Any such conducting fluid can be regarded as being made up of an equal number of positive and negative charges. Their motion can be described by kinetic theory, together with the laws of thermodynamics. In this representation, the interaction with matter is entirely taken into account by what is often called the Joule ‘heating’ term. This is a misnomer. The term describes energy transfer to matter in the form of particles. In some circumstances, for example in a resistor, the energy transferred from field to particles is randomized and leads simply to an increase in the internal energy of the matter consisting of the particles in this model. It is then appropriate to talk of Joule heating. The interpretation of the Poynting vector as representing the energy flux in the fields as a function of position can be controversial.
