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

[Dale]

Ok, the electrical forces are the cause of work. Agreed. However! Who's the main cause of the electrical forces? This is the main question I feel is left ignored and I would say that its the magnetic fields/forces and their interaction that would eventually cause the electrical forces to do work. Without one of those factors. As I said earlier before magnetic fields would generate those electrical force that would eventually cause the work. Without one of them NO WORK would be done on a system.

Electric forces CAN NOT! be generated in this system without the existence of a magnetic field/force. The greater/weaker the field the same goes for the EF. So Mag.F + Elec.F are both proportional to each other.

Does this sound good everyone?(Correct me if I'm wrong/or not liking where this is going)

Miyz,

Sounds good to me, although I don't know about them being proportional.

 

[Q-reeus]

And your evidence for this is ...?

Just follow the logic. I will turn this around and ask you to explain how it could be otherwise - how do *you* explain the known magnetic behavior of real magnetic materials with the notion that E.j type work is even possible on such (sans eddy currents as I have explained). And btw, still waiting for your answers (that's plural) to #49. Ready to do that now? I slipped back to edit #66, so not hanging around.

 

[Dale]

The final answer is simply that under dynamic conditions E & B always coexist mutually. You can never have one w/o the other.

Agreed. And under the right conditions B can be the dominant source of both E and j. But the work done is always equal to E.j.

I am now convinced that there are those who refuse to accept that B forces can do work despite evidence to the contrary.

What evidence? Do you have any good evidence that you can cite that says the work done is not equal to E.j?

 

[Dale]

I will turn this around

Yes, that is your usual approach: in the absence of good evidence supporting your claim try to hide the fact behind demands that I provide disproof of your speculative claim.

I have provided my claim with good evidence. You have provided your contrary claim, but without good evidence. So your claims are just speculation, and I have no intention to be lured in.

 

[cabraham]

Agreed. And under the right conditions B can be the dominant source of both E and j. But the work done is always equal to E.j.

What evidence? Do you have any good evidence that you can cite that says the work done is not equal to E.j?

1st bold - of course the work done is equal to E.J. Who says otherwise? I certainly do not.

2nd bold - I've always stated that the work done IS equal to E.J. Dale, if I say something that is wrong then tell me. Please don't put words in my mouth. Here it is straight from Claude:

Just because the work done equals E.J does not mean that E or J is actually doing the work. Without E.J, the B field cannot energize & cannot do work. B does the work because the Lorentz force associated with B, i.e. the mag component of Lorentz force, acts along the direction of motion, whereas E & J do not.

But w/o E.J, there is no B field energy. I agree that the work done can be computed per E.J, but that energy is transferred to a B field, then motion is implemented, so that the motion resulting in KE results in a reduction in B field energy. Energy is always conserved.

Seeing that we agree in the fact that work done originates from E.J, we need not argue that point. We agree on E.J being the value of work done producing motion plus any incurred energy storage and/or loss.

My point is that those who claim B does no work, support their position by citing that with free charges B acts in the wrong direction to do work. There is no component of Lorentz mag force acting along an e- motion. It is normal to the motion meaning that B does no work on the e-. I accept that. But the E field which imparts energy to the e- may be induced by a time varying magnetic field. A B field imparts energy to the E field, then the E force does work on the e-.

E did the work, but B provides energy for the E field. Which one did the work is determined by the direction of the fields with regard to the motion. But neither does it alone. B has energy but cannot impart energy to the e-. E can impart energy to the e-, but cannot have said energy w/o B.

It seems we also agree on the principle that E & B are mutually inclusive under dynamic conditions. We agree that E.J is the value of energy transferred to the magnet plus storage & loss quantities. The only sticking point still under scrutiny is the definition of which field is really "doing work". I believe that is determined by the direction of the force vectors.

Sometimes it's E, other times it's B. But neither can do it alone. BR.

Claude

 

[Q-reeus]

Q-reeus: "I will turn this around"
Yes, that is your usual approach: in the absence of good evidence supporting your claim try to hide the fact behind demands that I provide disproof of your speculative claim.

This borders on defamatory.

I have provided my claim with good evidence. You have provided your contrary claim, but without good evidence. So your claims are just speculation, and I have no intention to be lured in.

BS! I'm calling your bluff on this one. For starters - face up, at long last, to actually answering those questions I posed of you in #49 - or in your strange world is that considered to being 'lured in'? When you finally admit your wildly speculative and unsupported assertion there re 'Planck scale' for one is a load of crap, you may gain at least some vestige of moral credibility for admitting to error. Further on from that, I call upon you to actually provide some substantive and logically coherent, point-by-point rebuttal to either my #61 or #66. It is not I who hides behind rhetoric on this! Fact is, all your 'good evidence' is completely misplaced and wrongly applied when it comes to considering *quantum mechanically* governed intrinsic magnetic moments. No? Well then do as I have requested and we shall see how far you get!

 

[Dale]

1st bold - of course the work done is equal to E.J. Who says otherwise? I certainly do not.

2nd bold - I've always stated that the work done IS equal to E.J.

Then I think that we do not have a substantive disagreement. We will make all of the same experimental predictions in all cases. I am somewhat disinclined to argument about interpretational issues when the facts are agreed upon.

I agree that the work done can be computed per E.J, but that energy is transferred to a B field, then motion is implemented, so that the motion resulting in KE results in a reduction in B field energy.

I don't think that this is correct. In fact, I think that this is exactly backwards in the case of one permanent magnet doing work on another. While both magnets are stationary, there is no E field and therefore no work is done. Once one of the magnets starts moving the moving B field causes an E field and work is done. If anything the B field transfers energy to the E field and then to work. I.e. the work done by decreasing B is indirect, via its influence on E and j, as I said on the first page.

I am not sure if you agree with the facts here also and just not the interpretation or what?

 

[cabraham]

Then I think that we do not have a substantive disagreement. We will make all of the same experimental predictions in all cases. I am somewhat disinclined to argument about interpretational issues when the facts are agreed upon.

I don't think that this is correct. In fact, I think that this is exactly backwards in the case of one permanent magnet doing work on another. While both magnets are stationary, there is no E field and therefore no work is done. Once one of the magnets starts moving the moving B field causes an E field and work is done. If anything the B field transfers energy to the E field and then to work. I.e. the work done by decreasing B is indirect, via its influence on E and j, as I said on the first page.

I am not sure if you agree with the facts here also and just not the interpretation or what?

I know a moving magnet has an E field with it. But in what direction are E & B acting. Dale you cannot see your own prejudice. Instead of sketching the orientation of the force vectors, you simply re-iterate the already universally known truth that B cannot exist w/o E in dynamic conditions. I've already said yes to this.

Rather than try to understand the direction of the forces & the path of integration regarding the work done, you instead focus on the fact that B cannot do anything w/o E, but we already know that. So how about drawing us a picture showing the direction of E, B, & the motion of the magnet being lifted. Of course a moving magnet has an E field. But that E force cannot be what is lifting the magnet. Let's examine the chronology of events.

The lower magnet is at rest, the upper magnet is an electromagnet which turns on at time = 0. The lower magnet is at rest & has no E field, just B. The force between the 2 magnets lifts upward on the lower magnet. Just before the lower magnet ascended, it felt an upward force. But this force has to be due to B, not E. E is still zero as the magnet is not yet moving.

Once the magnet begins its ascent it generates an E field. But said E force cannot be what lifts the magnet in the 1st place. Since Lorentz force due to B is directed upward, & the motion is upward, it is clear that the B field is providing the Lorentz force which lifts the magnet.

I can't understand why this issue can be controversial at all! It works out when you keep track of the vector directions. Again, for the I forget how many times, if you feel otherwise, then:

DRAW A PICTURE!

Thanks & BR.

Claude

 

[Darwin123]

I know a moving magnet has an E field with it. But in what direction are E & B acting. Dale you cannot see your own prejudice. Instead of sketching the orientation of the force vectors, you simply re-iterate the already universally known truth that B cannot exist w/o E in dynamic conditions. I've already said yes to this.

Rather than try to understand the direction of the forces & the path of integration regarding the work done, you instead focus on the fact that B cannot do anything w/o E, but we already know that. So how about drawing us a picture showing the direction of E, B, & the motion of the magnet being lifted. Of course a moving magnet has an E field. But that E force cannot be what is lifting the magnet. Let's examine the chronology of events.

The lower magnet is at rest, the upper magnet is an electromagnet which turns on at time = 0. The lower magnet is at rest & has no E field, just B. The force between the 2 magnets lifts upward on the lower magnet. Just before the lower magnet ascended, it felt an upward force. But this force has to be due to B, not E. E is still zero as the magnet is not yet moving.

Once the magnet begins its ascent it generates an E field. But said E force cannot be what lifts the magnet in the 1st place. Since Lorentz force due to B is directed upward, & the motion is upward, it is clear that the B field is providing the Lorentz force which lifts the magnet.

I can't understand why this issue can be controversial at all! It works out when you keep track of the vector directions. Again, for the I forget how many times, if you feel otherwise, then:

DRAW A PICTURE!

Thanks & BR.

Claude

I think the statement that Maxwell’s equations don’t apply to the carriers in a permanent magnet is misleading. There are magnetic domains that can be defined by rigid boundaries. The electric currents don’t have to be thought of as looping around atoms. One can easily envision the electric currents as looping around on the boundary of the magnetic domains. On the length scale of the magnetic domains, the electric charge carriers and their matrix behave as classical particles. No quantum mechanics is needed.
The dynamics on an atomic scale can be ignored. The description of what happens is completely self consistent on the length scale of the magnetic domains. A magnetic domain can be considered a container of superfluid charge carrier confined by a boundary held together by rigid forces. The only forces act normal to the surface of the boundary of the charge carriers. It is these forces that do work on the charge carriers.
On the length scale of a magnetic domain, the magnetic field is not doing work on the carrier. The magnetic field applies a torque to the electric current loop that rotates the loop on the surface of the magnetic domain. The rigid body forces on the surface of the magnetic domain are doing work on the electric charge carriers. However, the rigid body forces are not part of the magnetic field.
The rigid body forces on the surface of a domain are no more “quantum mechanical” than the rigid body forces in any other solid. On an atomic level, it may be necessary to take into account the wave nature of the carriers. On the domain level, the forces can be described by phenomenological parameters.
Maxwell’s equations are usually used with constitutive relations, anyway. These constitutive relations are basically classical models that are valid on a large length scale, but which take into account the quantum mechanics on an atomic scale. Ohms law is an example of a constitutive relation. London’s Law for superconductors is also a constitutive relation.
I am not going to draw pictures. A magnetic domain can have a highly irregular shape. However, I will include some links to articles. You can see by their equations that Maxwell's equations are still being used.
Here are some links showing how the dynamics of a magnetic domain can be modeled by an effective eddy current and semiclassical physics.
http://pil.phys.uniroma1.it/~fran/jdv13i1pL15.pdf
“The contribution made by eddy currents to the effective mass
of a magnetic domain wall
JEL Bishop
Abstract. The eddy current drag on a moving domain wall is proportional, not to the
instantaneous wall velocity, but to a moving average of its recent velocities. Thus an
oscillating domain wall experiences an eddy force component in quadrature with its
velocity that may formally be attributed to a negative effective ‘eddy mass’. The frequency dependences of the eddy mass and eddy damping constant are calculated for
some simple model domain walls.”

http://ci.nii.ac.jp/naid/110004637004
“In ferromagnetic substance, the origin of internal friction under low stress amplitude has been considered to be due to two kinds of eddy currents, the first named macroscopic eddy current was studied by Brown and its theoretically predicted contribution to internal friction was known to be consistent with the experimental data. But the second named microscopic eddy current has not yet been investigated except in demagnetized state. In this paper, the latter was calculated theoretically as a function of the magnetization and the results were compared with the experimental data obtained with the substances which have neglisibly small anisotropic energy.”
http://arxiv.org/pdf/0706.2122.pdf
“In conducting ferromagnetic materials, a moving domain wall induces eddy currents in the sample which give rise to an effective retarding pressure on the domain wall. We show here that the pressure is not just proportional to the instantaneous velocity of the wall, as often assumed in domain wall models, but depends on the history of the motion.”
“Once the solution of Maxwell equation with the appropriate boundary condition is given, the average eddy current pressure on the wall is obtained by integrating the magnetic field over y at the wall position x = 0:”

 

[Dale]

BS! I'm calling your bluff on this one. For starters - face up, at long last, to actually answering those questions I posed of you in #49 - or in your strange world is that considered to being 'lured in'?

Yes, that is what I call being lured in.

Before we worry about one of the tangents way down in post 49 you need to provide good evidence supporting your original thesis from back in post 5:

In short, permanent magnets do not obey classical EM in this important respect, and it cannot be maintained that dW = E.j dv covers the situation.

If you cannot do that then all of the subsequent discussion is an irrelevant distraction.

 

[Dale]

Instead of sketching the orientation of the force vectors, you simply re-iterate the already universally known truth that B cannot exist w/o E in dynamic conditions. ...

DRAW A PICTURE!

The thing is that I don't think that drawing a picture is useful in general. You can draw a picture, and it can help you for one scenario. But a general derivation works for all scenarios. That is why we do general derivations in the first place, to learn general principles that work for all cases regardless of the details.

I think your reliance on pictures is leading you astray. Furthermore, the pictures you draw show forces, which are not the same as work. The work is E.j, always, as you also recognize, but interpret differently.

Btw, if I were to draw a picture I would not do a permanent magnet and an electromagnet. I would stick with the OP's scenario of two permanent magnets. There you do not have an E field "energizing" a B field, as you like to say. It is the other way around.

 

[Q-reeus]

Q-reeus: ..For starters - face up, at long last, to actually answering those questions I posed of you in #49 - or in your strange world is that considered to being 'lured in'?
Yes, that is what I call being lured in.

Then I pity you as someone whose ego evidently prevails over humility and honesty.

Before we worry about one of the tangents way down in post 49 you need to provide good evidence supporting your original thesis from back in post 5:
Q-reeus: "In short, permanent magnets do not obey classical EM in this important respect, and it cannot be maintained that dW = E.j dv covers the situation."
If you cannot do that then all of the subsequent discussion is an irrelevant distraction.

Oh my - bet you did a lot of trawling before finding the one statement that makes me 'vulnerable'. I'll be honest and admit that the one part of the above quote: "permanent magnets do not obey classical EM in this important respect," was wrongly stated per se. I should have said and meant it to mean "...permanent magnets do not respond classically to Faraday's law..." And everything since should have left no doubt that was the real intent there. So with that awful admission cleared up - how about you fulfill your side of it and do what was asked in #76. Don't keep telling me I'm wrong unless you can detail just how my argument fails. It's all there - pick either #61 or #66 (probably a little better) and explain to all and sundry just exactly where it goes wrong. Is that not within your capability? Notice I said explain - and that means a reasoned, detailed argument. Go for it!

 

[Q-reeus]

Darwin123 - your lengthy piece in #79 is addressed to cabraham but it's thrust seems aimed chiefly at myself. None of your referenced quotes affects my argument. They deal with equivalent representations of how applied *magnetic* fields induce changed magnetization via domain wall motions. Chalk and cheese situation. Intrinsic spin/magnetic moment simply cannot be modeled *in respect of it's response to an applied time-changing B -> solenoidal E field* as though a classical circulating current - whether arbitrarily applied to single domains or whatever other scale one chooses. In #5 I emphasized the so-called surface currents of a magnetized body as the natural scale/boundary to choose, but that's irrelevant to what really matters here. I extend the same invitation to you as to DaleSpam - if E.j work is being done on those domains, or whatever other grouping of intrinsic moments one cares to choose, explain why the response is not weak diamagnetism! It's all set out in #61, #66.

 

[Miyz]

Ow my ow my this thread just became a mess of words.

 

[Miyz]

Dale, Claude, Q-reeus.

YOU'RE ARGUING on something that should have a basic answer.
I've contacted (Mod note: Name deleted) to help me out.

Now here is the e-mail:"On 8/22/12 7:24 AM, miyze adam wrote:
Thank you professor from you're example's I've concluded that magnetic field/force can do no work on a free charge or a loop of wire.
However, in case of a permanent magnet's magnetic field/force applied on another magnet. Is there work being done?

>>>YES
In general can magnet's do work on each other?

>>>Well, if work is being done, it is NOT the magnetic field that does it. However, a moving magnet generates also an ELECTRIC field, and the electric field can do work.
Thank you Professor.

Regards,

Miyze.
"

That was his reply to my previous e-mail. My question are in black and his answers are in red.
I wanted to post this ASAP. However, got busy with collage... Anyways you all have to understand that the MAIN FORCE! that does work is certainly the electrical force. I agree with that point but what GENERATES THAT ELECTRICAL FORCES? the magnetic field! Now are the magnetic field doing work? CERTAINLY.

Now as I've agreed with Claude previously is that without the Mag.fields the electrical forces can't do work WHY? because they are not created by the magnetic field. Simple.

Now Dale they are all proportional to each other! The greater the magnetic field the higher the electrical forces so yea their proprtional to each. Mf = Ef, Its as if I'm a student and the more homework the proferssor assagin's the more work IM DOING( Student = electric force/field, Prof = Magnetic force/field.) Simple as that. So yea I think I've found out the simplest answer to this thread that we can all agree upon? Yes! Magnetic field/force can do work to create the electrical field thatould evnetually do work on the other MANGET.
Miyz,

 

[harrylin]

Dale, Claude, Q-reeus.

YOU'RE ARGUING on something that should have a basic answer.
I've contacted (Mod note: Name deleted) to help me out.

Now here is the e-mail:

"On 8/22/12 7:24 AM, miyze adam wrote:

Thank you professor from you're example's I've concluded that magnetic field/force can do no work on a free charge or a loop of wire.
However, in case of a permanent magnet's magnetic field/force applied on another magnet. Is there work being done?

>>>YES

In general can magnet's do work on each other?

>>>Well, if work is being done, it is NOT the magnetic field that does it. However, a moving magnet generates also an ELECTRIC field, and the electric field can do work.
[..]

Magnetic field/force can do work to create the electrical field thatould evnetually do work on the other MANGET.

Yes, that's how I also understood it (see my post #52). Note that his opinion is just one of the opinions in this thread. Remains the question of how this electric field comes about in a permanent magnet. And much disagreement seems also to be about that. Are these due to currents that heat the magnets (eddy currents)?

 

[Q-reeus]

However, in case of a permanent magnet's magnetic field/force applied on another magnet. Is there work being done?
>>>YES
In general can magnet's do work on each other?
>>>Well, if work is being done, it is NOT the magnetic field that does it. However, a moving magnet generates also an ELECTRIC field, and the electric field can do work.

All seems to in black and..red. Sorry but there needs to be some context applied there. What was Prof. Griffith's definition of 'work being done' exactly? Could be he meant just conversion of mechanical to heat via eddy current generation, in which case I would agree. Maybe email again Miyz, and get specifics on just what he meant exactly by 'work being done'. I may be in a minority of one here but stick to the position no electrical work is done on the chief source of permanent magnetism - electron intrinsic magnetic moments. Period. Considering asking him specifically if and how electrical work can be done on an intrinsic electron magnetic moment. That would be interesting. Then again maybe I should just go and ring-in Lubos Motl here! Whatever. I can agree there is 'virtual work' being done by pretending an intrinsic moment responds diamagnetically to a -dA/dt E field, but it does not so respond. So where next. What's on TV right now?

 

[Dale]

Oh my - bet you did a lot of trawling before finding the one statement that makes me 'vulnerable'.

Hardly. That is the exact quote that I objected to in post 6 and have been requesting some evidence to support ever since.

I'll be honest and admit that the one part of the above quote: "permanent magnets do not obey classical EM in this important respect," was wrongly stated per se.

Since you now agree that permanent magnets obey Maxwell's equations then you must agree that the work done on a permanent magnet is necessarily given by E.j. (It is possible to take a stance like cabraham's where you agree that the work is equal to the quantity E.j but say that it work is not "done by" E. But in any case the amount of work done must be equal to E.j)

I should have said and meant it to mean "...permanent magnets do not respond classically to Faraday's law..." And everything since should have left no doubt that was the real intent there.

Just for clarification. By "respond classically" I believe that you mean something like "respond as though composed of a loop of current in a zero-resistance, zero-susceptibility conductor"? If not, can you clarify what you intend by "respond classically"?

If that is what you mean then I agree with the statement, although I wouldn't use the phrase "respond classically" to describe that model.

It's all there - pick either #61 or #66 (probably a little better) and explain to all and sundry just exactly where it goes wrong. Is that not within your capability? Notice I said explain - and that means a reasoned, detailed argument. Go for it!

OK.

It is an experimental fact that ferromagnetic and ferrimagnetic media do not respond to an induced magnetic field as though composed of microscopic perfectly conducting classical loop currents.

I agree. Ferromagnetic media do not respond to an induced magnetic field as though composed of microscopic perfectly conducting classical loop currents, particularly not if you mean in a zero-resistance zero-susceptibility material.

This is not, in fact, how ferromagnetic media are modeled in classical EM. What must be supplied in classical EM is a constituitive relationship between M and either H or B. For example, see here. For a ferromagnetic material over a reasonably small range of H (where hysteresis does not occur) you could approximate the constituitive relationship as something like $M(H)=M_0+k H$. The constituitive relationships cannot be derived under classical EM, and are simply determined empirically (or calculated from other theories like QM) and are used as conditions on the fields. However, once you have the constituitive relationships describing the matter, then classical EM applies and Maxwell's "macroscopic" equations may be used to accurately describe the interaction, including that of two permanent magnets.

This is discussed in depth in Jackson's "Classical Electrodynamics" starting on pages 13-16 and continued in chapter 6.

 

[Dale]

YOU'RE ARGUING on something that should have a basic answer.

It has an answer ("basic" is hard to define). But whether or not something has an answer does not mean that people will agree on it. I think you are expecting too much by assuming 1) that if an answer is correct then everyone will agree on it and 2) that the correct answer corresponds to your preconceptions.

Btw, it sounds like your professor agrees with my position. My own opinion, as always, is open to revision based on further evidence and information, but as of today none of the arguments presented by Q-reeus or cabraham seem to carry the weight of evidence of Maxwell's equations.

However, cabraham at least recognizes the validity of Maxwell's equations and agrees on the amount of work being done and simply interprets the equations differently, so I find his position the most palatable of the contrary ones.

Now Dale they are all proportional to each other! The greater the magnetic field the higher the electrical forces so yea their proprtional to each.

The property you are describing here is linearity. Maxwell's equations are indeed linear. That isn't the same as proportional.

 

[Miyz]

It has an answer ("basic" is hard to define). But whether or not something has an answer does not mean that people will agree on it. I think you are expecting too much by assuming 1) that if an answer is correct then everyone will agree on it and 2) that the correct answer corresponds to your preconceptions.

Btw, it sounds like your professor agrees with my position. My own opinion, as always, is open to revision based on further evidence and information, but as of today none of the arguments presented by Q-reeus or cabraham seem to carry the weight of evidence of Maxwell's equations.

However, cabraham at least recognizes the validity of Maxwell's equations and agrees on the amount of work being done and simply interprets the equations differently, so I find his position the most palatable of the contrary ones.

The property you are describing here is linearity. Maxwell's equations are indeed linear. That isn't the same as proportional.

Prof.David said its the moving magnetic field that generates the electric field. Now where in Maxwell's equations exactly that states that their not proportional?

Dale, do you agree with my final point? However, you could agree that magnetic field/force's value = the electrical field/force generated.

That simply means M = E so if one value is 100 the other would be the same. I'm not sure what you mean by linearity, and hope you get my point. That we can finally AGREE upon.

 

[Miyz]

Does my post (#85) makes sense and do you agree?

Yes/No, with a simple explanation why...
Miyz,

 

[Dale]

Prof.David said its the moving magnetic field that generates the electric field. Now where in Maxwell's equations exactly that states that their not proportional?

The professor is referring to Faraday's law which states:
$$\nabla \times \mathbf E = -\frac{\partial \mathbf B}{\partial t}$$

When you say they are proportional then you are saying:
$$\mathbf E = k \mathbf B$$

Which is not at all equivalent to Faraday's law.

What I was saying about linearity is that if you have E and B such that they satisfy Maxwell's equations (E and B are not proportional) then if you double E then you will also double B and get a valid solution to Maxwell's equations. This is part of what it means for system of differential equations to be linear.

 

[Miyz]

The professor is referring to Faraday's law which states:
$$\nabla \times \mathbf E = -\frac{\partial \mathbf B}{\partial t}$$

When you say they are proportional then you are saying:
$$\mathbf E = k \mathbf B$$

Which is not at all equivalent to Faraday's law.

What I was saying about linearity is that if you have E and B such that they satisfy Maxwell's equations (E and B are not proportional) then if you double E then you will also double B and get a valid solution to Maxwell's equations. This is part of what it means for system of differential equations to be linear.

Or vice versa, if you double B , E will also double based on the equation. Right Dale?

 

[Q-reeus]

Q-reeus: "I'll be honest and admit that the one part of the above quote: "permanent magnets do not obey classical EM in this important respect," was wrongly stated per se".
Since you now agree that permanent magnets obey Maxwell's equations then you must agree that the work done on a permanent magnet is necessarily given by E.j. (It is possible to take a stance like cabraham's where you agree that the work is equal to the quantity E.j but say that it work is not "done by" E. But in any case the amount of work done must be equal to E.j)

You misconstrue my position re ME's - I clarified in the part you excerpted from above, then include below, but fail to put it together. You continue to fall back though in #89 to outright misrepresentation: "However, cabraham at least recognizes the validity of Maxwell's equations" - implying in the context that I do not. I call upon you to withdraw those remarks as outright false.

Q-reeus: "I should have said and meant it to mean "...permanent magnets do not respond classically to Faraday's law..." And everything since should have left no doubt that was the real intent there."
Just for clarification. By "respond classically" I believe that you mean something like "respond as though composed of a loop of current in a zero-resistance, zero-susceptibility conductor"? If not, can you clarify what you intend by "respond classically"?

Huh!? An obvious yes to that, given it has been stated quite clearly many times from #5 onwards - no sudden change now. More later.

I agree. Ferromagnetic media do not respond to an induced magnetic field as though composed of microscopic perfectly conducting classical loop currents, particularly not if you mean in a zero-resistance zero-susceptibility material.

Oh? Then given your stand you are in deep trouble without realizing it.

This is not, in fact, how ferromagnetic media are modeled in classical EM. What must be supplied in classical EM is a constituitive relationship between M and either H or B. For example, see here.

A strange reference, would have expected something more comprehensive and to the point, like that given back in #66.

For a ferromagnetic material over a reasonably small range of H (where hysteresis does not occur) you could approximate the constituitive relationship as something like M(H)=M0+kH. The constituitive relationships cannot be derived under classical EM, and are simply determined empirically (or calculated from other theories like QM) and are used as conditions on the fields. However, once you have the constituitive relationships describing the matter, then classical EM applies and Maxwell's "macroscopic" equations may be used to accurately describe the interaction, including that of two permanent magnets.

And the constiutive relationship B = μ₀H(1+χ) when applied to magnetic media makes it very clear such media cannot be treated as tiny classical current loops - so we come full circle on that one.

This is discussed in depth in Jackson's "Classical Electrodynamics" starting on pages 13-16 and continued in chapter 6.

That and similar treatments afaik never ask or answer the question of just how or where the E.j 'electrical work' done on an intrinsic moment appears. I have asked you often enough now and you continue to duck it - if as you maintain E.j work is literally done on a magnet, which is a collection of intrinsic moments, explain why those moment magnitudes are totally unaltered. Can you? When two fully and uniformly magnetized bar magnets, pointed end-to-end, are allowed to draw together, explain in your own words why there is zero change in the fictitious Amperian circulating currents in both magnets. Sure, and this is something I have never denied, there is a formal quantity amounting to integrating E = -dA/dt over time on those surface Amperian currents that numerically gives the work done in the attraction process. And nothing of substance changes in this matter when more realistic changes in material magnetization are considered - one is still dealing overwhelmingly with intrinsic moments - whether or not they may reorient in response to forces and torques owing to external *magnetic* fields.

But as I have always maintained, there is no actual *electrical* E.j type work done on those fictitious 'currents' for the simple reason they do not respond by changing. Such seemingly magical behavior cannot be classically accounted for - there are no tiny feedback circuitry & batteries maintaining I = constant against the E in E.j The magnetic response is that of a system of microscopic fictitious, perfectly rigid 'true magnetic dipoles'. Hence the formal E.j quantity is rightly attributable to magnetic energy change - the character of which one might ultimately owe to somewhat mysterious 'work done' by QM 'forces' maintaining intrinsic spin constant. Or just accept quantization of the intrinsic moment, if thought of as a tiny loop current, switches-off any response to a solenoidal E field. Recalling an electron is both a quantized charge and a quantized magnetic moment, it is quite free to respond to any applied E *as a point charge*, hence eddy-currents. I'm about done arguing this - too much energy wasted fighting misrepresentation, evasiveness, and continually cycling worn out arguments. Must go.

 

[Dale]

You misconstrue my position re ME's - I clarified in the part you excerpted from above, then include below, but fail to put it together. You continue to fall back though in #89 to outright misrepresentation: "However, cabraham at least recognizes the validity of Maxwell's equations" - implying in the context that I do not. I call upon you to withdraw those remarks as outright false.

If you accept Maxwells equations then you must accept that the work done on matter is E.j. The former implies the latter.

 

[OmCheeto]

The more I know, the less I understand...

You need not change any magnetic field. If big magnets are confusing you, think of 2 elementary particles with no charge, but with magnetic moment. There are magnetic forces between them. These forces are not perpendicular to velocities and so they do work.

[ref]

The magnetic field does NO work on a charged particle. In fact, the only thing a magnet can do work on is another magnet or a ferromagnetic material.

[ref]

$$\mathbf{F} = \mu \left( \frac{\partial \mathbf{B}}{\partial z} \right)$$
It is this last force that attracts magnets to each other, or iron filings to a magnet.

[ref]

I have spent the last 16 hours trying to figure this out on my own, but my math is in dreadful disrepair. I spent about two hours going through my Halliday and Resnick(1986), and could find no reference to magnet - magnet interactions, except for a single equation:

F=(3μ₀/2∏)μ²/r²

I could not find anything in their description of Maxwell's equations that implies that a pair of magnets cannot do work on each other.

time varying E field: n/a
time varying B field: n/a
E field on a gaussian surface: n/a
B field is zero: n/a

And the Lorentz force law seems to be completely non-applicable also.

Standard disclaimer: Everything I claim to know on this subject, I learned today. And being somewhat senile, will forget by morning.

 

[Miyz]

The more I know, the less I understand...

[ref]

[ref]

[ref]

I have spent the last 16 hours trying to figure this out on my own, but my math is in dreadful disrepair. I spent about two hours going through my Halliday and Resnick(1986), and could find no reference to magnet - magnet interactions, except for a single equation:

F=(3μ₀/2∏)μ²/r²

I could not find anything in their description of Maxwell's equations that implies that a pair of magnets cannot do work on each other.

time varying E field: n/a
time varying B field: n/a
E field on a gaussian surface: n/a
B field is zero: n/a

And the Lorentz force law seems to be completely non-applicable also.

Standard disclaimer: Everything I claim to know on this subject, I learned today. And being somewhat senile, will forget by morning.

Reasonable & Interesting point.

 

[Dale]

And the Lorentz force law seems to be completely non-applicable also.

Maxwells equations describe the fields interactions with themselves. The Lorentz force law describes the interaction between the fields and matter. You cannot have any work done (or any other influence) on matter without the Lorentz force law.

 

[OmCheeto]

Maxwells equations describe the fields interactions with themselves. The Lorentz force law describes the interaction between the fields and matter. You cannot have any work done (or any other influence) on matter without the Lorentz force law.

Sounds like I have 16 more hours of studying the Lorentz force in front of me, as the wiki quick synopsis doesn't seem to indicate this:

Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes called Laplace force),

We're talking about two permanent magnets. No wire involved.

the electromotive force in a wire loop moving through a magnetic field (an aspect of Faraday's law of induction),

We're talking about two permanent magnets. No wire loop or movement involved.

and the force on a particle which might be traveling near the speed of light (relativistic form of the Lorentz force).

This might be relevant, as my 16 hours of study indicated that magnetic dipole moments might be relativistically based.
Please don't ask for a reference. I stopped at scores of web sites yesterday.
But laymen like myself are always trying to put 2 + 2 together, even though we have forgotten our math.

For the neutron, this suggests that there is internal structure involving the movement of charged particles, even though the net charge of the neutron is zero. If g=2 were an expected value for the proton and g=0 were expected for the neutron, then it was noted by early researchers that the the proton g-factor is 3.6 units above its expected value and the neutron value is 3.8 units below its expected value. This approximate symmetry was used in trial models of the magnetic moment, and in retrospect is taken as an indication of the internal structure of quarks in the standard model of the proton and neutron.

[ref]

Well, something's going on that's creating a dipole magnet. Must be whirling charges of some kind.

(15 minutes of Lorentz Force wiki study)

In real materials the Lorentz force is inadequate to describe the behavior of charged particles, both in principle and as a matter of computation. The charged particles in a material medium both respond to the E and B fields and generate these fields. Complex transport equations must be solved to determine the time and spatial response of charges, for example, the Boltzmann equation or the Fokker–Planck equation or the Navier–Stokes equations. For example, see magnetohydrodynamics, fluid dynamics, electrohydrodynamics, superconductivity, stellar evolution. An entire physical apparatus for dealing with these matters has developed. See for example, Green–Kubo relations and Green's function (many-body theory).

[ref]

This is going to take longer than 16 hours. :grumpy:

hmmm...

I'm a former electrician by trade, and thought I knew how diodes worked. One day, I entered university, and it seemed there was more to their life then just a simple bias. They were filled with dopes and holes and what-not. Later, I joined a science forum and probed a bit deeper, into the quantum world of diodes. And being that I had no comprehension of the quantum world, I decided I did not know how diodes worked.

 

[gabbagabbahey]

We're talking about two permanent magnets. No wire involved.

The force and torque on an ideal magnetic dipole (permanent dipoles are the main source of magnetism in permanent magnets, so to understand a magnet you first need to understand an ideal magnetic dipole) can be computed by considering the Lorentz force on a current loop and taking the limit as the loop is shrunk to zero size while its magnetic moment is held constant (Classically, something must hold the magnetic moment constant, and this unkown something is what must do the work when the dipole is placed in an external magnetic field).

Some posters in this thread apparently believe that even classically, magnetic dipoles are fundamentally different than current loops (and according to the links/quotes provided in their arguments, they are not alone in that belief). I would argue that since the dynamics of a dipole are easily computed from the Lorentz force law applied to a current loop, that classically, magnetic dipoles should be treated on the same footing as any other current distribution (and most of the Electrodynamics textbooks I've seen do exactly that). Treating them as being fundamentally different just adds (unnessecarily!) another axiom to the classical theory of Electrodynamics.

 

[vanhees71]

That's what I keep posting in this strange thread over and over. The current density associated with magnetization is given by
$$\vec{j}_{\text{mag}}=c \vec{\nabla} \times \vec{M}.$$
Together with this definition also for the interaction between permanent magnets the Poynting theorem holds, and this clearly says that the electric field is responsible for the work done on charges. Of course there is the energy density of the electromagnetic field, $$T_{\text{em}}^{00}=\epsilon=\frac{1}{2}(\vec{E}^2+\vec{B}^2),$$
which in this three-dimensional notation is split into an electric and a magnetic part, and of course there is energy exchange between the electric and magnetic parts. Still, the power done on the charges is given by
$$P=\int \mathrm{d}^3 \vec{x} \vec{E} \cdot \vec{j}.$$
Thus, only the electric field is responsible for the exchange between energy of the electromagnetic field and charges.

 

[OmCheeto]

The force and torque on an ideal magnetic dipole (pemanent diploes are the main source of magnetism in permanent magnets, so to understand a magnet you first need to understand an ideal magnetic dipole) can be computed by considering the Lorentz force on a current loop and taking the limit as the loop is shrunk to zero size

All the way to zero? Or just approaching? You can't have a cross product on a point can you? That would negate Lorentz's B-field factor.

His equation would reduce to F = qE

I suppose that might work on a single dipole, but we're talking about magnets. These are billions of dipoles, all interconnected in a 3 dimensional lattice.

I suppose we could simplify the problem by setting up a pair of two Fe₂O₃ magnet pairs, and analyze the forces between them. Might be fun.

while its magnetic moment is held constant (Classically, something must hold the magnetic moment constant, and this unkown something is what must do the work when the dipole is placed in an external magnetic field).

I would call that a quantum effect, of which I have no knowledge.

Some posters in this thread apparently believe that even classically, magnetic dipoles are fundamentally different than current loops (and according to the links/quotes provided in their arguments, they are not alone in that belief). I would argue that since the dynamics of a dipole are easily computed from the Lorentz force law applied to a current loop, that classically, magnetic dipoles should be treated on the same footing as any other current distribution (and most of the Electrodynamics textbooks I've seen do exactly that). Treating them as being fundamentally different just adds (unnessecarily!) another axiom to the classical theory of Electrodynamics.

I'm pretty sure I read yesterday something to the effect that a "classical current loop" description of dipoles does not work.

But I'd be willing to pick up an Electrodynamics text if that will set me straight.

 

[OmCheeto]

Together with this definition also for the interaction between permanent magnets the Poynting theorem holds, and this clearly says that the electric field is responsible for the work done on charges.

Poynting theorem is not valid in electrostatics or magnetostatics - in these instances the electric and magnetic fields are not changing in time

[ref]

I believe we are talking about magnetostatics.

And could you provide a reference to your equations. As I've said, I've forgotten nearly all my math, and many of the equations I've seen over the last 20 hours use different symbols for the same equations.

How I interpret $$\vec{j}_{\text{mag}}=c \vec{\nabla} \times \vec{M}.$$

the magnitude of the current density = the speed of light times curl magnetic field vector

?

No explanation required, but I spent quite a while tracing back the origin of "E.j", which apparently is the abbreviated proof that the magnetic field does no work. :uhh:

If only I could use that at work...
Om's boss; What's the reason for this?
Om; E.j
Om's boss; What's E.j?
Om; You don't know? Pfft!

 

[Miyz]

That's what I keep posting in this strange thread over and over. The current density associated with magnetization is given by
$$\vec{j}_{\text{mag}}=c \vec{\nabla} \times \vec{M}.$$
Together with this definition also for the interaction between permanent magnets the Poynting theorem holds, and this clearly says that the electric field is responsible for the work done on charges. Of course there is the energy density of the electromagnetic field, $$T_{\text{em}}^{00}=\epsilon=\frac{1}{2}(\vec{E}^2+\vec{B}^2),$$
which in this three-dimensional notation is split into an electric and a magnetic part, and of course there is energy exchange between the electric and magnetic parts. Still, the power done on the charges is given by
$$P=\int \mathrm{d}^3 \vec{x} \vec{E} \cdot \vec{j}.$$
Thus, only the electric field is responsible for the exchange between energy of the electromagnetic field and charges.

Other then Om's points I'd like to say that the magnetic field/foce does work but INDIRECLTY. You could disagree but think about it, the electrical are generated how?
A changing magnetic field creates an electric field. Now that's something.
Maxwell–Faraday equation.

Now if you don't agree Van, just state you're reason and maybe I or another memeber could discuss this matter? Certainly its the magnetic field that now the MAIN force that causes this. If it can't do work well ok, I'll just hire another force to do it for me :tongue:

 

[gabbagabbahey]

All the way to zero? Or just approaching? You can't have a cross product on a point can you?

All the way to zero (for an ideal or point dipole). There's no problem computing the cross product of two vector fields at a point, provided both the vector fields are defined there.

That would negate Lorentz's B-field factor.

His equation would reduce to F = qE

I'm not sure why you think this.

I suppose that might work on a single dipole, but we're talking about magnets. These are billions of dipoles, all interconnected in a 3 dimensional lattice.

I suppose we could simplify the problem by setting up a pair of two Fe₂O₃ magnet pairs, and analyze the forces between them. Might be fun.

The microscopic details of what goes on inside a material are horrendously complicated, and not really well described by classical theory. Usually all we care about are macroscopic effects, and so you use the dipole moment per unit volume averaged over many thousands of atoms, the so-called magnetization $\mathbf{M}$ to calculate the macroscopic fields and forces.

In the presence of an external magnetic field, each magnetic dipole associated with the spin of an unpaired electron in a material will experience a torque which tends to align them with the external field (the force per atom holding the unpaired electron to its atom is typically much larger than this torque, so the electron stays bound to its atom/molecule and the external field then tends to flip the entire atom/molecue) this is the mechanism behind paramagnetism. Other internal forces and heat make it so this alignment is never 100%, and what you end up with is some average dipole moment per unit volume. Other processes such as diamagnetism (flipping of the orbital magnetic moment due to the change in speed of the orbiting electron) and ferromagnetism also will result in some average dipole moment per unit volume.

The net force and torque on an object in an external magnetic field can then be calculated by summing up (integrating) the force on each tiny bit of Magnetization.

I'm pretty sure I read yesterday something to the effect that a "classical current loop" description of dipoles does not work.

If you can find that source again, maybe you can reference some more of it. I've never seen a classical situation where treating dipoles as current loops does not give the correct force and torque on a magnet.