This was quoted from the
Wikipedia Magnetohydrodynamics page. One assumes that
brantc actually did read the webpage he quotes from, but maybe he simply overlooked this passage on the very same page:
Paragraph 1
"MHD applies quite well to astrophysics since over 99% of baryonic matter in the universe is made up of plasma including stars, the interplanetary medium (space between the planets), the interstellar medium (space between the stars), nebulae and jets. Many astrophysical systems are not in local thermal equilibrium, and therefore require additional kinematic treatment to describe all the phenomena in the system (see astrophysical plasma)."
So to begin with, the very same page from which
brantc lifted his quote explicitly contradicts the claim being foisted by him, namely that MHD does not apply to astrophysical plasmas. However, his failure here runs deeper than that because there is a second implication from this webpage which is equally false, namely that MHD requires a collisional plasma. That too is not true, although in order to run down the truth, one must do research at a slightly higher level than reading wikipedia pages.
Paragraph 2
"Because MHD does not explicitly treat individual particle motions, it may at first be thought that it is of little use in collisionless plasmas. However, MHD is always a correct description of the large-scale bulk dynamics of a fluid, with or without internal collisions, so long as the fluid cannot support a significant electric field in its own reference frame."
Magnetic reconnection, Priest & Forbes, section 1.7 "Relevance of MHD to Collisionless Systems", page 38.
So what does it mean, "no significant electric field in its own reference frame"? It means simply that there is no significant charge separation in the plasma. Does that make physical sense? Apparently it does ...
Paragraph 3
"For example, because the Debye length is very small, the positive and negative charge densities of the plasma very nearly cancel. Because each separate charge density is enormous, this cancellation is essential, otherwise the electric fields generated would be enormous and lead to rapid motion of of the electrons, which would immediately short out those fields and restore the quasi-neutral balance. However, because charge densities of the individual species are so large, even after substantial cancellation there is more than enough net charge to produce the electric fields to enforce this near cancellation. The cancellation is termed 'charge neutrality' and means no more than that the positive and negative charge densities are nearly equal.
Plasma Physics for Astrophysics, Russell Kulsrud, section 1.1 "How Do We Describe a Plasma and its Electromagnetic fields?", page 12.
Points to be emphasized so far:
- MHD is applicable to collisionless plasmas
- MHD is applicable to astrophysical plasmas
So now let us consider this ...
It is
brantc who has not only turned basic physics around here, but in fact now asserts, whether he knows it or not, that the entire discipline of hydrodynamics is entirely wrong, which I am sure will come as a great surprise to the generations of scientists & engineers who seem to have overlooked that juicy tidbit of information. The fact that MHD does not explicitly treat particles in a plasma is of no more consequence than the fact that hydrodynamics does not explicitly treat molecules of water but is still able to represent the physics of water waves with remarkable clarity & correctness. Look back up the page to the indents that I have labeled
paragraph 2 &
paragraph 3 which also respond to this very point.
The assertion that MHD is physically invalid because it does not explicitly treat particle properties is itself a false assertion.
Now let us consider reality. The "electromagnetic theory of fixed conductors" is more simply the theory of currents flowing in wires. No current flows in a wire until an electric field appears, and that electric field comes before everything else because fixed conductors are
fixed. The electrons in the wire are pretty much sitting around waiting for an electric field to happen.
Not so for a plasma. We already know that the plasma cannot have a background electric field in it because if it did the mobile electrons in the plasma would short out the field (see
paragraph 3 above). The electric field cannot come first. The magnetic field can & does come first because it is unavoidably generated by the physical motion of the charged particles of the plasma. This is "dynamo theory", the generation of magnetic fields by the bulk motion of an electrically conducting fluid or plasma (e.g.,
Fluid Dynamics and Dynamos in Astrophysics and Geophysics; Andrew M. Soward,
et al., editors; CRC Press, 2005;
Brandenburg, 2009). The electric fields that accelerate charged particles in a plasma almost invariably arise from the time variable magnetic field generated in the plasma by a dynamo process, which then either generates an electric field by induction or by magnetic reconnection.
So the quote from Priest & Forbes is correct in is attribution of primary electric fields in the electromagnetic theory of fixed conductors, but primary magnetic field in astrophysical plasmas.
Indeed so, and this is exactly where the energy comes from in MHD, the kinetic energy of the particles. That is the energy which dynamo generates the magnetic field, which in turn generates electric fields by induction and/or reconnection, which in turn accelerates particles in the plasma (on must be aware of feedback & other interdependencies; since the particle energy is the source of particle acceleration, only a subset of particles can be accelerated). The fact that NHD treats the particles in bulk rather than as individual particles is on no consequence to this point.
