Stephen A. Lawrence wrote: > > A permanent magnet acts like a dipole box and not like a spinning > > charged ring. A permanent magnet acts like a spinning charged ring > > that does not slow down as it supplies energy.
> But this isn't quite correct, is it? The internal currents in a > permanent magnet apparently _do_ change, quite a lot, when an external > field is applied to it. It is quite correct and a very good approximation of a dipole. For a typical neo magnet internal mu is very close to mu_zero and internal currents do not change in an easily measureable manner. I can look up the numbers if you wish but changes will typically be less than 1%. > In particular, the "induced" magnetic field exhibited by a piece of > unmagnetized iron when it's in a magnetic field, as well as the changes > in the magnetic field of a permanent magnet when it's in an external > field, are due to changes in the internal currents (or whatever black > magic one believes is taking place inside the magnet to give it the field). Permanent magnets are very poor soft materials and thus do not have large internal variable alignment currents. IIRC strontium based ceramics give about less than 1.5 x mu_zero and oriented barium ceramics give about 3 x mu_zero in the aligned direction. Typical soft materials have large internal alignment currents with mu from 5 x mu_zero to 10^6 x mu_zero. > Remember, E and B fields (apparently!) follow the law of superposition, > which means overlapping fields themselves don't interact; they just > sum. They really don't just sum, it's more complex than that in real geometries with real materials, mu and saturation. Thats why we need finite element programs. > But we're all quite familiar with the way the B field is > "conducted" through iron filings (or ferrite transformer cores, for that > matter). There's no "conduction" going on, really -- what we're seeing > is the sum of the original B field, and the induced B field in the iron > filings or transformer core. This is close but oversimplified. > > The "black box" I hypothesized, which produces a dipole field which is > _fixed_, is rather unrealistic, really -- any real magnet changes its > field as a result of external fields being applied to it. Not true, it is a close approximation at frequencies where eddy currents can be ignored. Special magnets can be constructed to minimize eddy currents and they are generally insignificant in oxide ceramic magnets. >(Perhaps the > box contains a wire loop driven by a current source...) The part which > I, personally, don't understand is what determines _how_ the field of a > piece of iron changes when an external field is applied to it -- the > fact that it _does_ _change_ as the applied field changes, on the other > hand, makes sense, and is, in fact, inevitable if there are really tiny > currents of some sort inside the magnet causing the field. The currents are almost entirely electron spin dipoles aligned within in changing domains which reorient to aid the applied field. Standard magnetic materials have only a small contribution from orbital dipoles. Unusual materials with large spin-orbit coupling do exist but even here the electron spin dipoles dominate. >> Here's another good one: Does a free-falling charge radiate? > >> > > > > As an experimentalist, I would suggest that the answer might > > be found by studying black holes. > > > Crikey, what kind of lab setup do you have?? Couldn't resist that black hole on Ebay last month. :-) It would be fun to play God wouldn't it. > > I do not have much confidence in many of the theories of > > modern physics and prefer to direct my efforts to doing > > experiments suggested by a more conservative approach > > to how much we think we know. > > > When it comes to black holes, I certainly agree with you! :-) George Holz Varitronics Systems
