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

[Dale]

My position here is the same as it was in the earlier thread and I would say(with a slight reservation) that magnetic forces can do work.A good example of this was given by Dotini in post 12.

Do you believe that classical EM accurately describes the paperclip? If so, then the power density is E.j, and any work done by B is only through it's impact on E and j.

 

[Dadface]

My post above refers to definitions,accepted nomenclatures.What word(s) would you use to name the force on the paperclip or the current carrying wire?The "magnetic" force seems to be a preferred description according to searches I have made so far.See,for example,the MIT lectures on the subject.
If we were not to call it the magnetic force then I have suggested that the electromagnetic force may be a better description,but is this being too fussy?.I imagine that most textbooks(at least up to A level standard)would still continue to refer to the "magnetic" force so as to distinguish it from the electric force between stationary charges.

 

[Dadface]

I would have to look at how the E.j approach applies to paper clip/magnet type scenarios but I think I already agreed that it is relevant to current carrying wires.In the previous thread I suggested that E needs defining since there is the E.j due to the power supply which is the source of the energy and the E.j due to the back(counter emf) which illustrates the power output.

 

[Dadface]

Do you believe that classical EM accurately describes the paperclip? If so, then the power density is E.j, and any work done by B is only through it's impact on E and j.

And vice versa any work done by E and j is only through its impact on B.Chicken and egg or a comprimise by calling it electromagnetic?

 

[Dotini]

The ideal gas law for the pressure inside the balloon, Hookes law for the tension in the balloon, and Newton's laws for the force on the can. Maxwell's equations don't figure in that kind of a scenario, at least not classically.

I'm not touching the can in any way. It moves only electrostatically. It's a textbook example of an electric field, Dale.

Respectfully,
Steve

 

[Dale]

My post above refers to definitions,accepted nomenclatures.What word(s) would you use to name the force on the paperclip or the current carrying wire?The "magnetic" force seems to be a preferred description according to searches I have made so far.See,for example,the MIT lectures on the subject.
If we were not to call it the magnetic force then I have suggested that the electromagnetic force may be a better description,but is this being too fussy?.I imagine that most textbooks(at least up to A level standard)would still continue to refer to the "magnetic" force so as to distinguish it from the electric force between stationary charges.

Force is different from work. I would call it a magnetic force or a Lorentz force. But the magnetic part of the Lorentz force does no work, at least not directly.

 

[Dale]

I'm not touching the can in any way. It moves only electrostatically. It's a textbook example of an electric field, Dale.

The balloon is charged and has an E field, but since the can is a conductor it won't exert a net force on it. At least, I think not. Do you have a reference?

To Q-reeus, see, I do ask other people to provide references, not just you.

 

[Dale]

And vice versa any work done by E and j is only through its impact on B.Chicken and egg or a comprimise by calling it electromagnetic?

No, not vice versa, but the compromise wording is fine.

 

[Dotini]

The balloon is charged and has an E field, but since the can is a conductor it won't exert a net force on it. At least, I think not. Do you have a reference?

When an uncharged object is placed near a charged object its charges rearrange themselves. Those charges attracted to the charged object move towards the charged object and those charges repelled move away. This effect is known as polarization.

Since the balloon is negatively charged and the Coke can is acting as if it is positively charged, the Coke can is attracted to the balloon. The force of attraction is so strong, in fact, that the Coke can begins to roll towards the balloon!
http://phun.physics.virginia.edu/demos/electrostatics.html

Respectfully submitted,
Steve

 

[Dale]

http://phun.physics.virginia.edu/demos/electrostatics.html

Cool reference, thanks. I will go back and re answer the earlier post you made now that I understand what you were saying.

 

[Dale]

I can create a charged object by rubbing a balloon on my hair. And then do work with it by rolling an aluminum can around on the floor. No magnet is required in this instance, as the work is done by an electric field.

Is this agreeable?

Yes, the work is done by the E field, given by E.j as usual.

 

[Q-reeus]

Tell me how I am twisting this out of shape:
Originally Posted by Q-reeus: "So a long straight magnetized rod enclosed within a similarly shaped solenoid should completely demagnetize when the solenoid generates a B field Bs equal in magnitude and of the same sign as that of the magnet's initial B field Bm. ... This manifestly does not happen."

For the same manifestly obvious reasons as before - you take it out of context and proceed to give it an erroneous slant. You failed to quote both the preceding and following portions of my entry there. Do that, and the claim I am trying to 'overthrowing Maxwell's eqn's' is seen as distortion at best. Anyway, how about you explaining the stability of atoms using just Maxwell's eqn's - or are you indeed ignorant of the classical prediction that electron's must in-spiral to the nucleus in a very brief time? ME's really do have their limitations.

 

[Q-reeus]

Q-reeus: "Electrical interactions of the E.j type simply do not and cannot apply in that wholly QM regime."
I agree.

But do you really? Then what of this from #18 (a quote from your own earlier posting):

Q-reeus: "I just managed to find the following, which might give pause for thought: http://physics.stackexchange.com/que...agnetic-dipole"
The derivation there is for force density, not power density. If you go through the next step then you get E.j. As I said in in my point 4 both E and j depend on B, so it does work indirectly.

That link to elsewhere was all about magnetic interaction with a fundamental magnetic dipole - i.e. electron's magnetic moment. You there claimed it still was explainable in terms of E.j energy interactions 'in the final step'. But now you agree with me? I'm confused - over where you really stand on this. Please clarify.

Q-reeus: "And it carries over to a permanent magnet as a QM glued ensemble of such."
I do not agree. If this were correct then classical mechanics would never be valid as every classical object is a "QM glued ensemble". As long as the energies and masses in your system are all much larger than the Planck scale then you are in the classical limit of QM and classical physics applies.

More nonsense. Have you not noticed my reference in #10 to the supercurrent situation? You do believe in the existence of quite macroscopic superconductivity (let's not forget superfluidity and one or two other more delicate phenomena like BE condensates)?

Q-reeus: "I notice you still haven't supplied any kind of coherent rebuttal to either #5 or #10."
Correct, and I will not until you provide some solid supporting evidence. The burden of proof is on you, not me; I will not accept it simply because you cannot be bothered to support your own claims. I have supported mine.

You only claim to. Actually, having taken a long time to make your public declaration of a 'final decision' here: https://www.physicsforums.com/showpost.php?p=4024036&postcount=348, it is only to be expected you now find it so hard to face up to having made a wrong decision. And btw, having accused me of 'wanting to overthrow established mainstream science' back in your #9, I find it a particularly ironic statement given, as my trawling backwards through that earlier thread has found, this choice opening position of yours there: https://www.physicsforums.com/showpost.php?p=4005572&postcount=54. Having yourself adopted my current 'heretical' position (for the wrong reasons though) right up to that later posting linked above, one might have expected a little more understanding - you know, a "Hey, I can understand that view - it had been mine life-long right up till a short while ago!" Nothing of the kind. So, evidently you believe in a life principle of being especially kind, gentle, and understanding toward yourself, but it's ok to put the boot into one's opposition via regular misquotes and innuendo. All's fair in love (or hate) and war seems your abiding motto DaleSpam.

 

[Q-reeus]

Classically, permanent dipoles are modeled as the limiting case of a current loop shrinking to zero size, but in a way that leaves its magnetic dipole moment constant.

That can be done sure, but it's not the whole story.

As you point out, this model is not entirely accurate ( for one there must somehow be some mysterious energy source which keeps the magnetic dipole moment constant when subjected to external field, not to mention that elementary particles like electrons have a dipole moment associated with them that only comes in quantized values), but it does correctly predict the net force and torque on permanent dipoles without introducing an additional axiom to theory (one treating magnetic dipoles as fundamental and different from the other two types of fundamental sources, point charges and currents).

I respectfully disagree. There *must* be a fundamental break from classical behavior just in order that those 'point dipoles' have unchanging magnitudes during magnetic interactions. It is the central point of my #5 and #10.

 

[Q-reeus]

Dotini, there is no contradiction between the quote you give and the fact that magnetic fields don't do work. Of course, if you compare the final state (paper clip attached to the magnet) with the initial state (paper clip separated from the magnet), you of course find that the change in magnetic-field energy is given by the work necessary to pick up the paper clip, attaching it to magnet.
This is precisely the content of Poynting's theorem, nicely explained at DaleSpam's link. This epxlains the the work is done by induced currents and electric fields during the transient (i.e., time-dependent!) situation when picking up the paper clip!

Missed responding to this before but address now - given your weight here as currently the thread's only expert/authority figure. If your claim that -dA/dt E fields acting on induced currents explains the above, I invite you, as someone familiar with both classical and QM regimes to apply that to what I brought up in #10. Do you disagree with my observation there that flux quantization in wholly superconducting circuits directly implies the failure of dW = E.j dv over an interval of magnetic interaction where supercurrent I_s is forced constant, despite a changed increment of enclosed flux and thus a changed A.I_s dl, where dl is an incremental length along the thin supercurrent? This despite that *on average* that quantity is invariant in a superconducting circuit - i.e. *on average* enclosed flux is an invariant? Moreover that as the superconducting circuit is shrunk smaller, so the interval of applied external B field variation over which I_s = constant grows in inverse proportion to enclosed area? Do you further agree or not with my observation that an electron is in a sense the ultimate endpoint of shrinking, not a classical, but a superconducting, loop current re it's magnetic interactions? And that in that case there are precisely zero conditions in which E.j is relevant to electron-as-magnetic-dipole? And that a permanent magnet is an ensemble of such fundamental dipoles - notwithstanding that *quantized* electron orbital motion contributes to magnetization here, though generally to a minor degree? (Earlier I had specified fully magnetized when referring to interactions involving permanent magnets owing to it simplifying things, but that restriction can be dropped with no consequences for the argument here.) And that induced eddy currents - which may be treated as behaving entirely classically, are in general a very minor consideration - especially for slow relative motions and for certain materials such as 'hard' ferrites? I shall be interested to see how or whether you choose to deal with hopefully all of the above questions.

 

[Dale]

For the same manifestly obvious reasons as before - you take it out of context and proceed to give it an erroneous slant. You failed to quote both the preceding and following portions of my entry there. Do that, and the claim I am trying to 'overthrowing Maxwell's eqn's' is seen as distortion at best.

So do you or do you not assert that Maxwell's equations predicts the complete demagnetization of any magnet due to an equal applied field? And do you or do you not believe that prediction is contrary to observation?

Just because I quote only a portion of your post does not mean that I am misrepresenting your position.

 

[Q-reeus]

So do you or do you not assert that Maxwell's equations predicts the complete demagnetization of any magnet due to an equal applied field?

Not any magnet - a hypothetical 'magnet' comprised of classical perfectly conducting loop currents. And honestly, this was manifestly and clearly evident in my #5 - why do you keep asking the same questions to obvious answers?

And do you or do you not believe that prediction is contrary to observation?

I refer you back to #5!

Just because I quote only a portion of your post does not mean that I am misrepresenting your position.

Be honest DaleSpam - this has become a deeply personal thing, spanning a considerable time now over many past threads. You resent my style of 'irreverent' balloon pricking - especially when that balloon(s) are your's.

 

[Dale]

More nonsense. Have you not noticed my reference in #10 to the supercurrent situation? You do believe in the existence of quite macroscopic superconductivity (let's not forget superfluidity and one or two other more delicate phenomena like BE condensates)?

In superfluidity, BE condensates, and superconductivity the thermal energy is small compared to the Planck scale. My comment applies to all of them as well.

You only claim to. Actually, having taken a long time to make your public declaration of a 'final decision' here: https://www.physicsforums.com/showpost.php?p=4024036&postcount=348, it is only to be expected you now find it so hard to face up to having made a wrong decision.

The difference between that conversation and this being that vanhees71 actually presented me with good evidence supporting his point, in the form of solid references and good derivations from accepted theory.

That is something you have never done in any of our conversations. Should you ever decide to do likewise then you stand a good chance of changing my mind, but as it is your positions consistently appear to me to be both impervious to and unsupported by any good evidence.

Post some good evidence and then I will resume the discussion with you here, otherwise we are just wasting time and adding unnecessary tension to the forum.

 

[Q-reeus]

In superfluidity, BE condensates, and superconductivity the thermal energy is small compared to the Planck scale. My comment applies to all of them as well.

You have shifted ground unannounced, but not enough to salvage that bit: Here's the original passage above bit originally referred to:

Q-reeus: "And it carries over to a permanent magnet as a QM glued ensemble of such."
I do not agree. If this were correct then classical mechanics would never be valid as every classical object is a "QM glued ensemble". As long as the energies and masses in your system are all much larger than the Planck scale then you are in the classical limit of QM and classical physics applies.

A blanket statement - and confused at that. Why do you choose the Planck scale as demarcation point - for both energies and length? It's crazy and excludes everything above fundamental particle if even there. Atoms for instance would far from qualify as a fundamentally quantum object.

Q-reeus: "You only claim to. Actually, having taken a long time to make your public declaration of a 'final decision' here: https://www.physicsforums.com/showpost.ph...&postcount=348 , it is only to be expected you now find it so hard to face up to having made a wrong decision."

The difference between that conversation and this being that vanhees71 actually presented me with good evidence supporting his point, in the form of solid references and good derivations from accepted theory.

I can accept and respect that you believe that position, but...then:

That is something you have never done in any of our conversations.

You mean 'not something I accept'. The essence of my position is well enough put - and evidently Lubos Motl is similarly dismissed as 'crackpot'.

Should you ever decide to do likewise then you stand a good chance of changing my mind, but as it is your positions consistently appear to me to be both impervious to and unsupported by any evidence.

Evidence? It is fundamentally a conceptual issue one either 'gets' or not.

Post some good evidence and then I will resume the discussion with you here, otherwise we are just wasting time and adding unnecessary tension to the forum.

If you don't want unnecessary tension then avoid making blanket accusations and distorting my words - you well know I don't take that lying down. And how about you actually dealing with certain questions I posed earlier - answering them may help you.
From #42:

Anyway, how about you explaining the stability of atoms using just Maxwell's eqn's - or are you indeed ignorant of the classical prediction that electron's must in-spiral to the nucleus in a very brief time? ME's really do have their limitations.

And you know the context to that one - so don't twist it to suit.
Finally, kindly this round, actually do answer my question in the earlier part of #43 - will you now clarify what seems to be a contradictory position?

 

[Miyz]

I'm lost with all this... Please if someone can simply present an answer.
Yes/No, supporting a simple explanation?
I've lost myself with all this confusion.However, noticed that Van & Dale are still referring the same answer used in my previous thread.

Interesting.Miyz.

 

[Dotini]

You can pick up a paper clip with a magnet. You can pick up another magnet with a magnet. That much is clear. Beyond that, authorities differ.

Respectfully,
Steve

 

[harrylin]

Shocking and disconcerting that physicists and perhaps even textbooks are in disagreement! [..]

As we discovered in the original thread, it's basically a matter of words. Some people use a (IMHO strange) definition of "work", such that the use of field energy for displacing an object is not "work" done by that field.

I guess that if I pull on an object with a rope and ask if I "did work", some may answer yes, others may answer "no, the rope did the work" and again others may answer that depending on the system boundaries that we regard, if it was me or the rope that did the work. I would say that I performed work on the object by means of the rope.

I'm lost with all this... Please if someone can simply present an answer.
Yes/No, supporting a simple explanation?

See above; so I simply say Yes, in agreement with Robert Gardner.

My more complete answer (based on the elaborate explanations of others):
Yes, the magnetic field does work by means of induced currents and electric fields.

However it's unclear to me where/how such currents occur in permanent magnets. Does this mean that they heat up? If this was explained somewhere, can someone please provide a link?

 

[Q-reeus]

See above; so I simply say Yes, in agreement with Robert Gardner.

Good for you Harald - but then you muddy the waters with:

My more complete answer (based on the elaborate explanations of others):
Yes, the magnetic field does work by means of induced currents and electric fields.

What induced currents? [it's true there are c(∇×M) 'induced currents' in the case of that soft iron paper clip, where induced magnetization is significant. Fundamentally though, such 'currents' owing to changing orientation of fundamental spin moments are fictitious entities. In the case of fully magnetized permanent magnets, there is no room to invoke such at all] Have you read through my little piece addressed to vanhees71 in #45? And btw this can be extended not just to permanent magnets but anywhere that magnetic media are involved - e.g. transformers, and real motors incorporating 'soft' iron armatures etc. We are talking about how energy is redistributed in an interacting system, so 'work being done' is as you say perhaps a little ambiguous. What can be said is there is for sure no electrical E.j work being done on those fundamental magnetic entities - electron as magnetic moment. The only consistent classical way of treating them is as perfectly rigid (sans SR length contraction) fictional magnetic dipoles with magnetic charges at each end. That's in terms of their impact on the *externally accessible* system energetics - magnetic susceptibility of the media. There is a somewhat 'delicate' issue raised in my #7 if one follows it through ('excess internal energy'), but it's really a matter for a separate thread that I may later open probably in QM section not here.

 

[harrylin]

Good for you Harald - but then you muddy the waters with:
[...]
What induced currents? [..]

Good - so now we are two who are asking that same question!

I did not study your exchange which appears to somewhat challenge "standard science", instead I wait with interest on more explanation by those who make such claims.

 

[Miyz]

Then the conclusion of this thread is that magnet's can do work on each other.
But what force exactly is the cause of work?

 

[Miyz]

Anyone?!

 

[Dale]

You are not going to get a consensus conclusion on this thread. My conclusion is the same as the previous thread since this is still a situation covered by the laws of classical EM. Others have firm differing opinions but no good evidence to support them.

You should form your own opinion either based on evidence and the laws of physics or based on unsubstantiated speculation, as you prefer.

 

[cabraham]

You are not going to get a consensus conclusion on this thread. My conclusion is the same as the previous thread since this is still a situation covered by the laws of classical EM. Others have firm differing opinions but no good evidence to support them.

You should form your own opinion either based on evidence and the laws of physics or based on unsubstantiated speculation, as you prefer.

So what's your conclusion? What is the basis? I just would like to know.

Claude

 

[Miyz]

So what's your conclusion? What is the basis? I just would like to know.

Claude

His first post is the main conclusion. It's the same as the preview thread that I've started, unfortunately Dale and I disagree. I honestly do not believe that the same concept on a "loop" is applied to a "permanent magnet".

Miyz,

 

[Miyz]

However, we could make this conclusion more debatable. Maybe?