Magnetohydrodynamics (MHD) is the study of the motion of electrically
conducting fluids
(liquids and gases) in the presence of a magnetic field. The subject is
also called
hydromagnetics, magneto-fluid dynamics, or magnetodynamics. When the
conducting fluid is
compressible it is referred to as magneto-gas dynamics.
Magnetohydrodynamics, as the name implies, is the result of the fusing
of two branches of
physics--electromagnetic theory and hydrodynamics (see FLUID MECHANICS).
Electromagnetic
theory is governed by MAXWELL'S EQUATIONS, and hydrodynamics is governed
by Navier-Stokes
fluid dynamic equation. The combination of these two sets of equations
constitutes the
subject of magnetohydrodynamics.
One of the most interesting aspects of MHD is the interaction between
the field and the
motion of the fluid. Electric currents induced in the field as a result
of its motion modify
the field, and at the same time their flow in the magnetic field
produces mechanical forces
that modify the motion.
DEVELOPMENT OF MHD
The principles of MHD were known to Michael Faraday and his
contemporaries as early as the
1830s. In 1831, Faraday experimented with mercury flowing in a glass
tube between the poles
of a magnet to demonstrate the MHD principle. The magnetic fields
available, however, were
not strong enough, and Faraday was unable to observe any MHD effects. He
also proposed the
use of tidal currents in the Earth's magnetic field for power
generation. Extensive
experiments with ionized gas as a conducting fluid have been performed
only recently.
In the early 1930s, the British physicist Sydney Chapman and V. C. A.
Ferraro developed the
theory of magnetic storms. This theory first drew attention to the
importance of mutual
interaction of the mechanical and magnetic forces in conducting fluids.
In 1937, studies were
made on the flow of conducting fluids across a magnetic field. In 1940,
Hannes Alfven of
Sweden introduced the idea of the "guided center" in discussing the
drift of gyrating
particles in a magnetic field. Finally in 1942, Alfven reported a new
type of wave motion,
which now bears his name. By combining Maxwell's equations with the
fundamental equation of
hydrodynamics, Alfven was able to predict the existence of a new type of
wave, which he named
the magnetohydrodynamic wave.