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

[harrylin]

I want to apply it to permanent magnets because permanent magnets are governed by the laws of classical EM and E.j is a general result for the matter power density in ALL situations covered by classical EM.

Perhaps you mean that a reduction in magnetic field energy implies a reduction in the corresponding electric current. Yes indeed; I'm now halfway with working out the example of post #166, and it's included in the draft.

 

[harrylin]

[..] The main OP question has been answered specifically multiple times and I thank all of you're efforts so far. It's much appreciated![..]

Those answers disagree with each other and consist largely of loose remarks. As I've stated many times, my detailed answer (in which I work out my example) comes tomorrow.

 

[Miyz]

Those answers disagree with each other and consist largely of loose remarks. As I've stated many times, my detailed answer (in which I work out my example) comes tomorrow.

Um I stay with the most logical answers...

+ Satisfied with the results how things turned out... If you have something else and would like to support it... Go ahead! We all here to learn & Discuss.

 

[harrylin]

"the rate of heat dissipation per unit volume in a conductor (the ohmic heating rate) is E.j"

The discussions in these threads is exactly not about energy that is lost in ohmic heating - that is at the detriment of mechanical work!

I now verified several of my textbooks (W.D.Day, Slater&Frank) which confirm that my first reading of Dalespam's reference was correct. In the context of Poynting according to those references, E.j stands for the dissipated energy inside the volume under consideration. The meaning of the full equation is that the energy that dissipates as heat inside a volume equals the energy that enters it minus the stored energy that is extracted from it. Evidently the topic "Poynting's theorem" deserves its own thread, which I'll start next (I'll repeat this there, please don't comment here!).

 

[Dale]

It is all energy transferred to matter, including (but not limited to) Ohmic dissipation. Do you find any that specifically say it is only Ohmic and not mechanical, or do they just mention Ohmic in the derivation and don't even mention mechanical?

Please look at the derivation of that term from the Lorentz force law, which I linked to earlier.

 

[harrylin]

------------------Follow-up of my post #166--------------------

For the following analysis I will restrict myself to mere high school classical physics.

Perhaps the simplest case to model two permanent magnets is two single current loops 1 and 2. Let's assume current loops of 1 m circumference, made of 1 mm diameter wire; distance d between the loops ca.1 cm (diameter << d). As pictured, the currents flow through the wires (---) into the screen on the left (x), and flow out of the screen on the right (0):

1 x----0

2 x----0

Here the wires are attracted to each other by the Lorentz force: F = IxB . L

However, for calculation we can approximate this particular example with two straight wires of 1 m length by opening the loops above the screen and straightening them out:

1 --------->--------- I₁

2 --------->--------- I₂

Now we have the basic configuration of the definition of the Ampere:

F = 2.10-7 I₁ I₂ L / d

For this case I obtain F= 2.10-7 * 100 * 1 / 0.01 = 2 mN.

Suppose that the top magnet is connected to an external mechanism that is actuated by it. We allow the top magnet to move towards the bottom magnet over a small distance, for example from 11 mm to 9 mm. We will assume that the average currents are approximately 1 A (more follows).

Then the work done by the Lorentz force on the top magnet and whatever it is driving, according to [1] and [2]:

F.d ≈ 2.10^-3 * 2.10^-3 J = 4 μJ.

That settles, I think, the question concerning "force doing work" according to the considered literature.

However, in my personal definition not a force but an entity "does work".
In that context, we can distinguish two main cases for the current flow.

1. A common electromagnet, let's say with a current source:
Following Lenz-Faraday-Maxwell we find that the motion will induce an electric field in each wire that opposes the motion; however the current prevents the diminishing of the magnetic field energy. Thus the field provides the acting force but the energy is passing through.
Mechanical equivalent: A spring or rope with which a tractor pulls a trailer.

I would not say that such a thing "does work", but the literature does. I would say that it "transmits work", but that's my beef.

2. For a true permanent magnet there is no external current source.
Following Lenz-Faraday-Maxwell we find that the motion will induce an electric field in each wire that opposes the motion by reducing the currents and the corresponding magnetic fields. Thus the magnetic fields deliver the energy of the magnetic forces that drive the external application.
Mechanical equivalent: A spring that gives off stored energy to drive a clock.

Consequently for the case of a real magnet, not only according to [1] and [2] but also according to my personal definition of "work" the magnetic field "does work", as it loses energy by providing the acting force that does work.

[1] http://en.wikipedia.org/wiki/Work_(physics)
[2] http://www.lightandmatter.com/html_books/lm/ch13/ch13.html#Section13.1

 

[Dale]

for calculation we can approximate this particular example with two straight wires of 1 m length by opening the loops above the screen and straightening them out:

1 --------->--------- I₁

2 --------->--------- I₂

Now we have the basic configuration of the definition of the Ampere:

F = 2.10-7 I₁ I₂ L / d

For this case I obtain F= 2.10-7 * 100 * 1 / 0.01 = 2 mN.

Suppose that the top magnet is connected to an external mechanism that is actuated by it. We allow the top magnet to move towards the bottom magnet over a small distance, for example from 11 mm to 9 mm. We will assume that the average currents are approximately 1 A (more follows).

Then the work done by the Lorentz force on the top magnet and whatever it is driving, according to [1] and [2]:

F.d ≈ 2.10^-3 * 2.10^-3 J = 4 μJ.

That settles, I think, the question concerning "force doing work" according to the considered literature.

What is E.j in this case?

 

[harrylin]

What is E.j in this case?

I considered in what followed two cases that could be relevant for that issue, perhaps you like to explain if/how E.j applies. As I stated in post #166:
this looks like a good example to work through, complete with numbers to calculate the force between them according to the different explanations.

 

[chingel]

Harrylin, do you agree that in the case of the two wires, the magnetic field does not apply force the wire directly, but only to the electrons? Do you agree that a magnetic field can do no work on electrons? You calculated the work done as if the force is applied directly to the wire.

The magnetic field changes the direction of the moving electron, while its speed stays the same, no work done. Then the electron hits the side of the wire, giving some of its momentum and kinetic energy to the wire structure. The magnetic field makes it possible for the electrons to hit the sides of the wire, but the wire is moved by the electron hitting it and the work is done on the wire by the kinetic energy of the electron (because if a moving electron hits a still wire, the wire will start moving and gain some kinetic energy), which makes the electron slow down (ie current would diminish unless something is pushing the electrons forward again).

 

[harrylin]

Harrylin, do you agree that in the case of the two wires, the magnetic field does not apply force the wire directly, but only to the electrons? [..]

Hi chingel, I gave my analysis complete with definitions, equations, numbers and my conclusions that follow from them. According to those definitions, a contact force that displaces a mass does work on that body - also if most of that mass is displaced by means of other forces inside that mass, and even if the work is done through a rope. If your analysis is based on different definitions, please present those definitions and your analysis (and not just with vague claims - calculation please!).

 

[Dale]

I considered in what followed two cases that could be relevant for that issue, perhaps you like to explain if/how E.j applies. As I stated in post #166:
this looks like a good example to work through, complete with numbers to calculate the force between them according to the different explanations.

I have already explained how E.j applies. E.j applies ANY TIME you are considering work done on matter in a classical EM problem. It is derived directly from Maxwell's equations and the Lorentz force law. It doesn't matter the details of the scenario, as long as it is governed by the laws of classical EM then the amount of work done is given by E.j. I can post the references again if needed.

You have at least 2 sources of E in this problem. One is the Hall effect, and the other is the motion of the wire itself. So it shouldn't be surprising that E.j is a relevant quantity.

My claim is that work done on matter is always given by E.j in any scenario governed by the laws of classical EM. This claim is proven correct by the referenced derivations from the laws of classical EM. Are you disputing this claim? Do you believe that your posted example represents a disproof by counterexample?

 

[harrylin]

What is E.j in this case?

I considered in what followed two cases that could be relevant for that issue, perhaps you like to explain if/how E.j applies. As I stated in post #166:
this looks like a good example to work through, complete with numbers to calculate the force between them according to the different explanations.

I have already explained how E.j applies. E.j applies ANY TIME you are considering work done on matter in a classical EM problem. It is derived directly from Maxwell's equations and the Lorentz force law. [..]

Please tell us what is E.j for cases 1 and 2! :tongue2: My example calculation used the basic EM equations. If you present your calculation, then there may be something left to discuss for this topic.

 

[Dale]

Please tell us what is E.j for cases 1 and 2! :tongue2: My example calculation used the basic EM equations. If you present your calculation, then there may be something left to discuss for this topic.

I don't know how to calculate E or j inside the wire in this case, but I don't need to. I can prove that the work is equal to E.j always. Do you disagree? If so, it is up to you to support your claim, not me. If you believe that the work is not E.j, in contradiction of the laws of classical EM, then prove it.

 

[harrylin]

I don't know how to calculate E or j inside the wire in this case, but I don't need to. I can prove that the work is equal to E.j always. Do you disagree? If so, it is up to you to support your claim, not me. If you believe that the work is not E.j, in contradiction of the laws of classical EM, then prove it.

I did not use that for my answer to the OP; I thought that perhaps your answer would complement mine. If you can't use it for a precise answer then I don't see its relevance, except perhaps what I already mentioned in post #211 and which is a point of agreement (I think). This is directly taken from the laws of classical EM.

Addendum: Note that following Lenz's law, the induced emf in the wire opposes the attractive magnetic force by reducing the magnetic field in case 2 (true permanent magnet) - it accounts for removing energy from the field that is doing the work.

And I now also think that the OP's question has been exhaustively answered.

 

[OmCheeto]

I don't know how to calculate E or j inside the wire in this case, but I don't need to. I can prove that the work is equal to E.j always. Do you disagree? If so, it is up to you to support your claim, not me. If you believe that the work is not E.j, in contradiction of the laws of classical EM, then prove it.

I wish I could comprehend 1/10 of the things going on in this thread.

But I keep thinking about neutrons and their electric neutrality and magnetic dipole moments.

I found the following on wiki:

∂W = -1/2 * 4παE² -1/2 * 4πβH²

It strikes me that two closely spaced neutrons would have no E field between them, so the equation would reduce to ∂W = -1/2 * 4πβH²

They claim "∂W is the energy change in the electromagnetic field due to the presence of the nucleon in the field." And for two neutrons with misaligned dipole moments, it strikes me that ∂W is due strictly to the H field.

Does this make any sense at all, or am I just waving my hands?

 

[Dale]

This is directly taken from the laws of classical EM

So do you agree that the work is given by E.j?

 

[Miyz]

And I now also think that the OP's question has been exhaustively answered.

So you agree that the electrical forces/fields do the work?

So do you agree that the work is given by E.j?

hahaha you and I both are looking for that final answer! :tongue:

 

[harrylin]

So you agree that the electrical forces/fields do the work?
hahaha you and I both are looking for that final answer! :tongue:

I certainly gave my "final answers"* in my "final analysis" #216, it uses the basic EM laws and you only need high school physics to verify it!

Perhaps you did not spot my answers because the text was half a page long and I did not put them upfront and in bold? :uhh: For you I will put my answers now in bold and my answer to this thread upfront!

I distinguished two cases and the answer on that other question depends on it. However, case 2 fully answered the topic question.
My final answer to this topic was: Yes, a magnet's magnetic field can perform work on another magnet.

For the two cases that I considered my full replies were slightly different:

1. For a pair of electromagnets that is fed by a current source: According to the definitions that we found on the web, a rope does work if a tractor pulls something with it. Then the magnetic field of such an electromagnet can do work, but not according to my personal, un-sourced definition which I also mentioned because in that special case it is ultimately the battery or other power source that "does the work" by loosing energy (I should perhaps add that if that it is a battery, then this is chemical energy that is converted to electric fields which are converted to magnetic fields that pull/push; and then one may look into the question if chemical energy is all electric fields).

2. A pair of pure permanent magnets, modeled by two zero resistance current loops (no external source): following the referenced definitions including my own, the magnetic field of a permanent magnet can perform work on another magnet by loosing magnetic field energy when it moves by magnetic force; thus the magnetic field of a permanent magnet "does the work".

I have nothing else to comment on this thread, thanks for posing the question!

* I think that this was my final answer - but one can never be fully sure of anything!

 

[Dale]

OK, it is clear that you are unwilling to answer the direct question about E.j. I am not sure why you are avoiding it, but I won't push further.

I will comment that merely because you can come up with a simple example where it is difficult to calculate E.j does not imply that E.j is not equal to the work. So I don't see that the calculation you provided contributed any useful insight to the question. What would have been better is a scenario where both E.j and the B force are easy to calculate.

 

[Q-reeus]

OK, it is clear that you are unwilling to answer the direct question about E.j. I am not sure why you are avoiding it...

What a joke - the pot calling the kettle black. And who was it that repeatedly refused to own up to a blunder first pointed out in #49, and further repeatedly refused to respond - in any sensible way - to my #72, #76, #82? The weak and evasive responses in last part of #88, and #95, amount to an outright refusal to acknowledge that ferromagnetism is a fundamentally quantum mechanical, not classical phenomenon. There is no E.j type work done on the overwhelmingly dominant contributor - intrinsic magnetic moments. Stop accusing others of being evasive - finally do own up to your blunder pointed out in #49, and further provide that detailed point-by-point rebuttal I first asked of you back in #72 and rounded out in #76.

 

[OmCheeto]

I was thinking about this a bit more today, and I decided that if magnetic fields couldn't do work, then neither could electric fields.

I saw everything as a bunch of springs.

It was a very odd image.

Thank god we don't understand how supernovas really work, as otherwise, the universe would simply be running down to nothing.

Ok to delete, and infract, and ban me. I'm way off topic.
Ja znaio schto, ja niechevo nie znaio.

 

[Dale]

What a joke - the pot calling the kettle black. And who was it that repeatedly refused to own up to a blunder first pointed out in #49, and further repeatedly refused to respond - in any sensible way - to my #72, #76, #82? The weak and evasive responses in last part of #88, and #95, amount to an outright refusal to acknowledge that ferromagnetism is a fundamentally quantum mechanical, not classical phenomenon.

Everything is fundamentally quantum mechanical. So what? That doesn't imply that classical mechanics is never applicable. In the case of ferromagnetism, it just means that you cannot use classical mechanics to derive the constituitive relationships. However, once you have the constituitive relation for your magnet, either derived from QM or empirically measured, then you can certainly use classical EM to describe the work done by one ferromagnet on another. See Jackson or any other classical EM text, permanent magnets are definitely considered part of classical EM.

There is no E.j type work done on the overwhelmingly dominant contributor - intrinsic magnetic moments.

And your evidence is ...?

 

[Q-reeus]

I was thinking about this a bit more today, and I decided that if magnetic fields couldn't do work, then neither could electric fields.
I saw everything as a bunch of springs.

There is something to that viewpoint - i.e. it seems we are dealing with nothing more than a redistribution of energy in a nominally closed system. Example - two fully magnetized bar magnets at some initial separation/orientation have a certain mutual magnetic potential energy. After interacting by motion, the final state has a new value of mutual magnetic potential energy. The difference is ascribed chiefly to mechanical work done, with a typically tiny E.j contribution from eddy currents.[To the extent one magnet alters the magnetization of the other, an additional internal energy change needs factoring in] I made that point early on, but it's not quite the whole story. If instead of -m.B style magnetic potential energy I had substituted dW = H.dB style magnetic field energy, a certain 'paradox' arises - there are more or less 'springs' present, to use your analogy. But it's an 'internal' thing so to speak, and by formulating in terms of -m.B, the problem is not evident and has no appreciable bearing on the accessible energy exchanges. To expand on that is for another thread.
[Just to make clear; the m in -m.B above refers to the summed moments over a given magnet, while the B refers to the external applied field, acting on m, owing to the other magnet. Perhaps better to here substitute μ₀H for B, on the understanding H always refers to an externally applied magnetic field. It's always true that, summed over all relevant volume and time, H.dB field energy equates to -m.B potential energy, with B inclusive of all contributions, both internal and external.]

 

[Q-reeus]

Q-reeus: "What a joke - the pot calling the kettle black. And who was it that repeatedly refused to own up to a blunder first pointed out in #49, and further repeatedly refused to respond - in any sensible way - to my #72, #76, #82? The weak and evasive responses in last part of #88, and #95, amount to an outright refusal to acknowledge that ferromagnetism is a fundamentally quantum mechanical, not classical phenomenon."

Everything is fundamentally quantum mechanical. So what?

So everything in relation to this topic.

That doesn't imply that classical mechanics is never applicable.

And who's suggesting otherwise? And btw why do you continue to evade the issue raised in #49? I'd say my ego-over-honesty/humility assessment is being continually born out.

In the case of ferromagnetism, it just means that you cannot use classical mechanics to derive the constituitive relationships. However, once you have the constituitive relation for your magnet, either derived from QM or empirically measured, then you can certainly use classical EM to describe the work done by one ferromagnet on another. See Jackson or any other classical EM text, permanent magnets are definitely considered part of classical EM.

This is nothing more than an abbreviated repeat of your evasive #88. That classical mechanics (why not classical EM?) cannot be used to derive those constitutive relations is everything here. Classical perfectly conducting loop currents are drastically different in behavior to real intrinsic magnetic moments. Face up to that, and your position crumbles.

Q-reeus: "There is no E.j type work done on the overwhelmingly dominant contributor - intrinsic magnetic moments."
And your evidence is ...?

And just what kind of 'evidence' do you need here? I've explained beginning at first entry in #5 why not. If you really can't figure that out after all my previous input, not much is expected to change now. However for the remote prospect of some genuine breakthrough, I will again point you to #45, #61, #66, #117. The latter summarizes the former. [In light of my #233 remarks, full consistency requires a modification of last part of #117 to read "mechanical energy change + -m.μ₀H type magnetic potential energy change = 0" rather than "mechanical energy change + magnetic field energy change = 0". Which was already covered in #191] Instead of trying to put me on the back foot all the time, finally do what I asked way back - how do you find that electrical work is done on an intrinsic magnetic moment? Explain in your own words how that is even possible - it's what I've been asking from way back in #72.

 

[harrylin]

I forgot to include a link to the thread that I started on interpretation of E.j:
https://www.physicsforums.com/showthread.php?t=632696

 

[Q-reeus]

I forgot to include a link to the thread that I started on interpretation of E.j:
https://www.physicsforums.com/showthread.php?t=632696

Harald, there are better examples of the Poynting theorem that explicitly express reactive (i.e. 'stored' or 'static/quasi-static') field energy in the formula; e.g. "my.ece.ucsb.edu/bobsclass/201C/Handouts/Chap1.pdf" see (1.31) on p9 there. Anyway, crucially one has to distinguish between purely formal results treating fictitious Amperian currents as classical real rho*v current densities, and the actual case in magnetic media involving intrinsic moments that definitely do *not* respond to an applied E as though real classical currents.

 

[harrylin]

Harald, there are better examples of the Poynting theorem that explicitly express reactive (i.e. 'stored' or 'static/quasi-static') field energy in the formula; e.g. "my.ece.ucsb.edu/bobsclass/201C/Handouts/Chap1.pdf" see (1.31) on p9 there. Anyway, crucially one has to distinguish between purely formal results treating fictitious Amperian currents as classical real rho*v current densities, and the actual case in magnetic media involving intrinsic moments that definitely do *not* respond to an applied E as though real classical currents.

Please clarify matters in the thread on that topic - that will be helpful for later discussions! It's surely a matter of interpretation. If I did not make a serious error in my post #216, any alternative analysis that is also based on classical EM should yield the same answers. And as I elaborated, at first sight "E.j" looks compatible with my analysis; I'm just not sure about an always correct soundbite for "E.j" and if the symbols always have the same meaning in different contexts. I just had a quick look at your reference; the dedicated thread is certainly meant to discuss "the ambiguity of the Poynting theorem"!

So, I will now really not comment anymore here except if someone points out a serious error or if an alternative detailed case analysis is given.

 

[Q-reeus]

Please clarify matters in the thread on that topic - that will be helpful for later discussions! It's surely a matter of interpretation. If I did not make a serious error in my post #216, any alternative analysis that is also based on classical EM should yield the same answers. And as I elaborated, at first sight "E.j" looks compatible with my analysis; I'm just not sure about an always correct soundbite for "E.j" and if the symbols always have the same meaning in different contexts. I just had a quick look at your reference; the dedicated thread is certainly meant to discuss "the ambiguity of the Poynting theorem"!

So, I will now really not comment anymore here except if someone points out a serious error or if an alternative detailed case analysis is given.

Not interested in buying into that other thread - sorry. However, I did go back and read your #216, and the relevant section is:

2. For a true permanent magnet there is no external current source.
Following Lenz-Faraday-Maxwell we find that the motion will induce an electric field in each wire that opposes the motion by reducing the currents and the corresponding magnetic fields. Thus the magnetic fields deliver the energy of the magnetic forces that drive the external application.
Mechanical equivalent: A spring that gives off stored energy to drive a clock.

Consequently for the case of a real magnet, not only according to [1] and [2] but also according to my personal definition of "work" the magnetic field "does work", as it loses energy by providing the acting force that does work.

You are seemingly saying there is no real difference between electromagnet case 1 (iron-free coils), and permanent magnets? Or at least it reads to me like you are saying the induced -dA/dt E owing to relative motion between magnets 'reduces the currents'. Am I reading this wrong? Please go right back to #5 and contemplate the implications. Upon saturation, the fictitious Amperian currents do not further respond to any level of increased external B. Strange you may think - Lenz's law say's they aught to. But then we are dealing in actuality with a collection of intrinsic moments, not real currents. It matters much. Even when far from saturation, any response of the Amperian 'currents' is owing to changes in orientation of intrinsic moments under magnetic forces/torques. So what do you say to... oh, forgot - bound yourself to an oath of silence! Too bad.

 

[harrylin]

[..] You are seemingly saying there is no real difference between electromagnet case 1 (iron-free coils), and permanent magnets? [..]oh, forgot - bound yourself to an oath of silence! Too bad.

Hehe I'm reasonable - and of course a clarification is reasonable. I summarized the cases that I analysed in post #216 again in post #228.

With "a real magnet" I was referring to my conclusion from case 2, permanent magnets without iron (permanent=without external current source). On purpose I kept it simple, avoiding complications by magnetization etc. And indeed I took it for granted that if the magnetic field of a permanent magnet without iron can do work, this will also be valid for magnets with iron.

 

[Dale]

And btw why do you continue to evade the issue raised in #49? I'd say my ego-over-honesty/humility assessment is being continually born out.

And your continued iniability to produce any evidence suppporting your objectionable claims bears out my assessment of your bias and ignorance. Your beliefs are unsupported by and uninfluenced by evidence.

It isn't up to me to answer every nonsense objection you raise when you cannot even demonstrate that the topic is a valid concern. If YOU think that there is something wrong with some aspect of classical mechanics, then it is up to YOU to show good evidence why. I won't waste time attacking your half-baked whimsical speculations.

What you see as ego is simply a resistance on my part to accept your unsubstantiated claims which contradict the good evidence that I currently have available to me. Simply look at my interactions with others who have provided good evidence to support their claims which contradicted mine.

Btw, I have lost track of which issue in 49 is the one that you were particularly concerned about?

That classical mechanics (why not classical EM?) cannot be used to derive those constitutive relations is everything here.

I don't know why. In every theory of matter, including the standard model, there are properties of the matter that are determined only by experiment and not by the theory. Why should it be OK for QED to experimentally determine the charge of an electron but not OK for classical EM to experimentally determine the constituitive relationship for a ferromagnet?

I am not claiming that classical EM is a theory of everything, merely that it accurately describes the work done when one ferromagnet interacts with another.

Classical perfectly conducting loop currents are drastically different in behavior to real intrinsic magnetic moments. Face up to that, and your position crumbles.

How does my position crumble? I have never taken the position that classical ferromagnets are modeled as loop currents in a perfecly conducting material. That was your strawman position, and I have already agreed that the strawman position is wrong, but it was never my position.

And just what kind of 'evidence' do you need here?

I need a rigorous derivation from accepted laws of physics, or reliable experimental evidence by reference from mainstream scientific literature. That should always be considered an acceptable request on this forum.

 

[Q-reeus]

And your continued iniability to produce any evidence suppporting your objectionable claims bears out my assessment of your bias and ignorance. Your beliefs are unsupported by and uninfluenced by evidence.

It isn't up to me to answer every nonsense objection you raise when you cannot even demonstrate that the topic is a valid concern. If YOU think that there is something wrong with some aspect of classical mechanics, then it is up to YOU to show good evidence why. I won't waste time attacking your half-baked whimsical speculations.

What you see as ego is simply a resistance on my part to accept your unsubstantiated claims which contradict the good evidence that I currently have available to me. Simply look at my interactions with others who have provided good evidence to support their claims which contradicted mine.

That kind of polemic reads like a de facto admission of having no real answers.

Btw, I have lost track of which issue in 49 is the one that you were particularly concerned about?

The first one there, as if you didn't know. Try 'Planck scale'.

I don't know why. In every theory of matter, including the standard model, there are properties of the matter that are determined only by experiment and not by the theory. Why should it be OK for QED to experimentally determine the charge of an electron but not OK for classical EM to experimentally determine the constituitive relationship for a ferromagnet? I am not claiming that classical EM is a theory of everything, merely that it accurately describes the work done when one ferromagnet interacts with another.

You specifically maintain that work on permanent magnets is not just mostly (which it isn't), but exclusively of the actual E.j type, yet continually shy away from defending it against my counterclaims, as I have continually asked.

Q-reeus: "Classical perfectly conducting loop currents are drastically different in behavior to real intrinsic magnetic moments. Face up to that, and your position crumbles."

How does my position crumble? I have never taken the position that classical ferromagnets are modeled as loop currents in a perfecly conducting material. That was your strawman position, and I have already agreed that the strawman position is wrong, but it was never my position.

There are only three possible models to use for magnetized media (ignoring for the moment eddy currents which are peripheral):
Gilbert model based on magnetic monopoles formed into rigid dipoles
Classical Amperian loop currents which have to be perfectly conducting
Intrinsic magnetic moments. [better not forget orbital contribution - significant only in ferrites. And which behave very much as for intrinsic moments, and far from classical Amperian currents.]

Once you accept that only the latter fits the facts, your whole position crumbles as stated.

Q-reeus: "And just what kind of 'evidence' do you need here?"
I need a rigorous derivation from accepted laws of physics, or reliable experimental evidence by reference from mainstream scientific literature. That should always be considered an acceptable request on this forum.

The only reliable experimental evidence needed in this instance is the fact of magnetic saturation. Supplemented with a head that can think independently.

 

[Dale]

That kind of polemic reads like a de facto admission of having no real answers.

Your inability to produce any evidence reads like a de facto admission of having no real question. It doesn't bother me in the slightest that I have no real answer to no real question.

You specifically maintain that work on permanent magnets is not just mostly (which it isn't), but exclusively of the actual E.j type, yet continually shy away from defending it against my counterclaims, as I have continually asked.

I have provided evidence supporting my position in the form of references to rigorous derivations from accepted laws of physics. Why should I defend against your counterclaims when there is no evidence to support them?

The first one there, as if you didn't know. Try 'Planck scale'.

What is wrong with that? Whenever you are calculating some quantized observable there is always a factor involving Planck's constant, the Planck scale I am referring to. If any of your energies/masses/sizes etc. are small compared to the appropriate Planck-scale factor then quantum effects are important, if they are all large compared to the factor then quantum effects are not important.

The only reliable experimental evidence needed in this instance is the fact of magnetic saturation. Supplemented with a head that can think independently.

The constituitive relationships for ferromagnets include magnetic saturation also, so that is not evidence against the applicability of classical EM here. I can post a link later if you need a reference.

 

[Q-reeus]

Hehe I'm reasonable - and of course a clarification is reasonable.

Good show. Reasonableness in a thoroughly unreasonable thread is a welcome relief.

I summarized the cases that I analysed in post #216 again in post #228.

With "a real magnet" I was referring to my conclusion from case 2, permanent magnets without iron (permanent=without external current source). On purpose I kept it simple, avoiding complications by magnetization etc. And indeed I took it for granted that if the magnetic field of a permanent magnet without iron can do work, this will also be valid for magnets with iron.

OK having now looked at what you are saying in the light of #228, I both agree and disagree with that position. Agreed that for permanent magnets the energy exchanges are necessarily of magnetic nature so in that sense the magnetic field is doing the work. Disagree though that this can be modeled using zero resistance loop currents. Such currents must respect Lenz's law and that always means diamagnetism, not the observed ferromagnetism. The latter is dominated by contribution from intrinsic moments - which never change magnitude - only orientation under action of magnetic fields. Not agreeing?

 

[Q-reeus]

Q-reeus: "That kind of polemic reads like a de facto admission of having no real answers."
Your inability to produce any evidence reads like a de facto admission of having no real question. It doesn't bother me in the slightest that I have no real answer to no real question.

If you do decide to quit this tit-for-tat nonsense, think about a serious response to just one question. How can E.j type work be performed on an intrinsic magnetic moment. Answer in your own words.

Q-reeus: "The first one there, as if you didn't know. Try 'Planck scale'."

What is wrong with that? Whenever you are calculating some quantized observable there is always a factor involving Planck's constant, the Planck scale I am referring to. If any of your energies/masses/sizes etc. are small compared to the appropriate Planck-scale factor then quantum effects are important, if they are all large compared to the factor then quantum effects are not important.

Still wishing to maintain correctness? Just read the very first sentence here: http://en.wikipedia.org/wiki/Planck_scale

The constituitive relationships for ferromagnets include magnetic saturation also, so that is not evidence against the applicability of classical EM here.

It is if you understand the issue - your claim exclusively (or even substantially) real E.j work is done on permanent magnets. Bunk.

 

[harrylin]

[..] Agreed that for permanent magnets the energy exchanges are necessarily of magnetic nature so in that sense the magnetic field is doing the work.

Nice to finally get a response of my example. :tongue2:

Disagree though that this can be modeled using zero resistance loop currents. [..]

I thought to use the Ampere model of permanent magnets: https://en.wikipedia.org/wiki/Force_between_magnets#Amp.C3.A8re_model
According to you this does not lead to an erroneous answer to the question, and that's what matters.
However, if you think to offer a better classical model, please redo the analysis with it.

PS: I rephrased this message to highlight the essentials when Q-reeus replied.