Stephen A. Lawrence wrote: > What's more, a magnetic field _does_ _no_ _work_, _ever_. It's > typically hard to see exactly what's really happening with a permanent > magnet, but this law is always followed: the force exerted by a > magnetic field on a charged particle is always perpendicular to its > motion, and hence cannot impart energy to it. > > As to a citation, check any E&M text. The "standard" reference on this > is probably Jackson, titled something like "Electrodynamics". > Griffiths' text on the same subject, "Intro to Electrodynamics", is > generally considered more accessible, however.
Yes , I have both of these books and have noticed the " magnetic field does no work" line in Griffiths and other texts. This is the interpretation in all texts that I have seen. I find that this statement is somewhat misleading and most students probably find it confusing as the interaction of two magnets can obviously do work. Force * distance is work. What is being done here is a somewhat arbitrary classification of electromagnetic forces into electric and magnetic forces. The Lorentz force law F=q(E+VxB) actually hides what I think of as a longitudinal magnetic force included in the original Ampere law inside the qE term. The earlier Ampere law included this interaction between the magnetic fields of moving charges as an explicit term. A straight forward application of the Lorentz law gives the same total force of a magnetic field on current flow around a complete circuit as the Ampere law but not the correct distribution of forces along the wire. This has been shown in many experiments including my own but is largely ignored in texts. Tom Phipps has written about the problems with the Lorentz force law in Infinite Energy and in his book "Heretical Verities". He and others have suggested adding a longitudinal term which provides invariance and matches experiment. >If you want to look at a far, far simpler case which can be understood >without knowing how magnetic domains work, take a look at my "A little >brain teaser" post from last night. It's a tightly constrained gedanken >experiment in which the rules of the game guarantee that the B field >does no work, yet it shows the same "Darn the magnet obviously just >performed work!!" effect. In that case, it's obvious where the energy >comes from, though. I posted it as a "puzzle", and I didn't originally >intend to post the solution for a day or two, but that plan no longer >looks so hot, so I'll just say that in the gedanken experiment the >spinning ring slows down. Total energy of the system -- rotational >energy + linear kinetic energy -- must be conserved, and the right-hand >rule quickly leads to the conclusion that as the ring moves through the >field toward the box it experiences a torque opposite its direction of spin. Yes the ring slows down, but the dipole field also provides energy. How does the spin of the ring slow down? The only force that will slow the ring must be longitudinal to the electron motion. If you want to save the Lorentz force law you will have to include dynamic electric fields along the ring caused by movement through the changing magnetic fields. Think of it as being caused by voltage induced by magnetic induction. In general when magnets are attracted to each other and move closer, total field energy increases and the energy comes from the individual dipoles. That would be the electron spin and orbital motion dipoles. George Holz Varitronics Systems
