Can a magnet's magnetic field perform work on another magnet?

Q-reeus

Took a look at Jackson 2nd Ed'n, 6.2 'Energy in the Magnetic Field'. The ab-initio assumption is made work is done on any loop current 'to maintain the current constant' against a changing flux through the circuit. OK for real currents flowing through real conducting circuits, but when made a blanket generalization for magnetic media, a strange position given his familiarity with the QM nature of an intrinsic moment. That same position is then translated holus-bolus into 6.8 where Poynting theorem is derived. Can find no account in between where such a classically behaving circuit is contrasted with that of an intrinsic moment. None. Guess that's why you just kept ducking every request of mine to show how E.j type work can be done on an intrinsic moment - Jackson has nothing to say - ipso-facto - neither then can you. Awe and reverence for authority figures. Often works out as good policy, but not always. Myself and others here have given references on the net, even pointing to some respected authority figures here in PF, that dispute Jackson's approach of it seems treating intrinsic magnetic moments as though needing to be maintained against some induced emf as though a classical circulating current. Ho hum. To each their own guru it seems.

cabraham

My "rubbish" comment was ambiguous, so I will restate it to clarify. A moving magnetic field does generate an E field per Faraday Induction Law. That is not "rubbish" at all. The "rubbish" I was referring to was the theory that said induced E field is what exerts the lifting force on the lower magnet.

When the lower magnet is on the floor about to ascend due to lifting force acting upon it, there is initially no motion, hence no induced E field & force. So the force lifting the magnet cannot be induced E field generated force. That theory is pure rubbish. The lift occurs before the induction, so induction & E force cannot account for the lift. Cause must always precede effect, & there are no exceptions or conditions to that rule.

Once again, after the magnet starts moving, an E field is indeed generated per Faraday's Induction Law, no argument there. That's been known since the mid 19th century. I'm glad we generally agree, & I agree that much of this is semantics. To me, the force lifting the magnet is Fm = qvXB. If we expand the system boundary region, another source energized said B field somewhere in history. That source received its energy from another source earlier. The disagreement truly is semantic. At least we agree on that as well. Best regards.

Claude

DaleSpam

You can certainly dispute his editorial choices about what to discuss in his text, but my point is that Jackson (and the authors of all other classical EM textbooks of which I am aware) clearly treats permanent magnets classically. I.e. permanent magnets are part of classical EM.

I haven't ducked it. I have consistently answered it.

1) Permanent magnets are described by classical EM (ref Jackson)
2) Poynting's theorem follows from the laws of classical EM (ref derivations)
3) Work done on matter is E.j in all classical EM (ref Lorentz force law)
4) Therefore work done on a permanent magnet is E.j

I don't particularly have a guru; my opinion is just based on the best evidence I have available and subject to revision as new evidence becomes available. It isn't my fault if your argument doesn't measure up by that standard.

DaleSpam

Thanks for the clarification. Given our previous discussion, I thought that it was probably just something like that.

I agree with this. The B field does indeed produce the force. I hope I never said otherwise. It is only the work that I was describing, and that is given by E.j. Whether we say that it was B via E.j or E.j via B is less important (IMO) than that we agree on the amount of work done and the forces involved.

Q-reeus

As I stated earlier, fully true only insofar as they act as sources of EM fields, or respond to such fields - with the important exclusion of E field curl component (re non-action on intrinsic moments). There is no way a permanent magnet could exist if governed solely by classical EM. Surely you don't dispute that.
Agreed.
Agreed. But PM's are not 'all classical'.
The inevitable non sequitur. Only in a strictly formal sense - by pretending Amperian 'currents' are real currents. And even then, there are problems. But for the sake of not distracting things further, I will simply agree to a presumed formal but not real E.j type 'work' being done, whereas in fact all appreciable interactions are magnetic in origin for PM's.
The marathon's not yet over.

cabraham

Once again our agreement is near complete. But how can B produce force which displaces the mass a distance upward increasing its PE, w/o doing the work? That is illogical. So we agree B produces the Fm Lorentz force that lifts the magnet. That Fm force times height raised is the work done. B did the work. But, E.J is needed to energize B. Once again, it all depends where the system boundary is arbitrarily drawn.

I'm satisfied that we have consensus except for the matter of arbitrarily choosing system boundaries, & minor semantics. I just find it surprising that so many critics agree on B providing the force, but not doing the work. How can the force which lifts not be doing the work? Strange it is indeed. Oh well, no use dwelling on it. Cheers.

Claude

DaleSpam

Because B doesn't act on the mass, it acts on the charges which are in motion relative to the mass. The force B exerts on the charges is always perpendicular to their motion, per the Lorentz force law, and therefore does not transfer energy.

I agree that the remaining difference is semantic, but hopefully the Lorentz force law and Poynting's theorem help you understand why other people will describe it differently than you do and perhaps help you understand that the description is reasonable, even though it differs from your own.

DaleSpam

You did state it earlier, but without evidence.

Classical EM doesn't purport to be a theory about the existence of magnets, charges, or fields. It is just a theory about their behavior. Classical EM does, in fact, accurately describe the behavior of permanent magnets, including force, energy, work, etc.

OK, as long as the equations accurately predict the outcome of experiments, that is sufficiently "real" for me. I try to remain agnostic about everything else.

cabraham

Sorry Dale but you can't have it both ways. The E force is also perpendicular to the magnet motion. Not to the charge, but to the direction the mass is moving. Using your definition, E force cannot do work either because E force is not in line with the magnet's motion. I've explained to the point of exhaustion that the work is done on the mass, not the charges. The Fm force (due to B) acts in the right direction along with the motion, but Fe does not. That "Fm is normal to charge motion & thus cannot do work" argument has been demolished beyond sufficiency. We are well beyond that. I can't believe that that argument is still being invoked. I explained that in my 1st post in the 1st thread.

All my posts are consistent with all known laws of physics. Like I said recently, the reason some differ with me is based on where the system boundaries are arbitrarily drawn. If you zoom in close, just looking at Fm & the magnets, then Fm lifts the magnet & does the work. But zoom your lens out to a bigger picture, & B gets energy from another field/source. Zoom out more & another energy source is "doing the work".

Fm acts on the magnet lifting it a finite distance. The Fm direction is along the motion. It does work, not on the charges but on the magnet mass as a whole. The direction is along, not normal. By the definition you cited, that is work being done by Fm (associated Lorentz force per B).

To me it's too easy, & I am at a loss to understand how this can even be argued. We arrived at the right answer long ago. Some just refuse to accept Fm as doing work. It's just a prejudice & nothing more. All attempts to invoke physics laws to prove that E does the work have not withstood scrutiny. BR.

Claude

DaleSpam

The E field is not perpendicular to the current, therefore it does work on the charges.

This is where we disagree. EM forces only act on charges, not masses. I don't know if this is just another semantic disagreement or not, but if it is not then your position seems problematic to say the least.

harrylin

As I already gave up on this topic, seeing all this I just have to laugh along with you - but in despair :

Q-reeus

Ha ha. Harald, we all agree magnets just keep on working regardless of how each see's it. Guess that's all that really matters in the end.

Miyz

People! Why can't you all agree that the magnetic fields induce the force that does the work. Call it what you want, but the magnetic field generated a force.

I still don't understand why we haven't finished this yet.

Based on all the laws of Electrodynamics its known as a fact that magnetic fields induce electrical fields and that electrical fields INDUCE magnetic fields. As Claude said before they are of the same coin. Each facing a different side.

They are both equal to each other. In what way? In a way that without one of them, the other can't exist or do anything!

Peace!

Miyze,

cabraham

Very true indeed. E & B (or E & H if you like) cannot exist independently if the energy is time varying (power is non-zero). Ampere's Law (AL) tells us that the curl of H equals the displacement current plus the conduction current. The displacement current is dD/dt, & the conduction current is J = σE As we know, D = εE. In a conductor we have conduction current, in an insulator we have displacement current. In either case, E is non-zero if H has a curl. If H is zero, so is E.

Faraday's Law (FL) tells us that the curl of E equals the time rate of change of B. A static B has no E, & an ir-rotational E has no B. But in these cases we've examined, induced E fields are always rotational, hence there has to be a time varying B associated. The B fields enclose a current, hence an E field is present. E forces align with charge carrier velocity.

So whenever one says, this field is "doing the work", another can counter with "no way, the other field is doing it", then produce equations showing how the work cannot be done without the other. They are right, of course, but they don't acknowledge the chicken-egg paradox they are falling victim to. Which comes first is endless.

Fe = qE acts in the wrong direction to lift the magnet, & it cannot be the one doing work. Fm = qvXB is in the right direction lifting the magnet, doing work equal to mgh. Although the Fm is not doing work on charges, it deflects charges w/o changing their KE, & internal bonding forces yank the lattice particles in the direction of the displaced electrons. These forces are E & SN.

I covered this issue in the current loop thread. I was told that my tethering treatise was irrelevant. So I am at a point where it is pointless to continue. I will clarify if asked, but if not asked, then I bid all of you a great & happy weekend, enjoy the pro football games, that is what I will be doing. Cheers.

Claude

Miyz

Good words used in good time.

I probably won't enjoy any games or such! I'll self teach my self about the wonders of or physical world.

Take care Claude!