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

[K^2]

That's just wrong K...

The magnetic field does create/generate an electrical field. A basic contradiction of you're words is a motor, without B, the electrical force that does work on the loop is not generate and no work is done! And based on Faraday-Maxwell's equation that states: "A changing magnetic field creates an electric field".

Magnetic field can generate electric field. But who said B changes? Again. Hall effect. B is constant. No induced E. The E that does the work is due to externally applied voltage differential. However, the amount of work done can depend on whether B is there or not.

My point is, it doesn't matter what the source of E is, that's the field that's doing the work. Saying that the only way work can be done in a system with a B field is done by the induced E is wrong, because E due to sources can also be doing work in the system.

 

[Miyz]

Magnetic field can generate electric field. But who said B changes? Again. Hall effect. B is constant. No induced E. The E that does the work is due to externally applied voltage differential. However, the amount of work done can depend on whether B is there or not.

My point is, it doesn't matter what the source of E is, that's the field that's doing the work. Saying that the only way work can be done in a system with a B field is done by the induced E is wrong, because E due to sources can also be doing work in the system.

The Hall Effect, is something I'm most certainly not interested in. If you're using it to support you're argument then that's fine. However, I asked the question does a magnetic field do any work on a manget? The reply: No. OK, what do they do? Well, they create an E Field that eventually does the work.

Now weather they are changing or I'm not sure, but if you bring two magnets close to each other and try to repel them a force is acted on BOTH of them.( I wanted to understand what is that force and what caused it? And that question is answered.)

Saying that without B, E can do no work = True in case of a MOTOR & TWO MAGNETS!
Hall effect is irrelevant to my OP question.

Miyz,

 

[Miyz]

But who said B changes?

Yea, I think when two magnets are close to each other attracting/repelling, the magnetic field is changing and E is induced.(Not sure correct me if I'm wrong with supporting fact + equations please.)

Miyz,

 

[harrylin]

Why is this going way out of hand? Seriously. You all are going way to deep.

I ask all of you now to just answer my OP question please.

I gave you the answer in my posts #138 and #148. Did you not understand it? I will summarize it even simpler here. My only regret is that this is basic stuff from my physics curriculum which I forgot over the years - and apparently I'm not the only one with amnesia!

I think the most suitable answer is that the magnetic field's generate/create electrical fields that would do the work. Based on Maxwell's equations. [...]

Once more: that turned out to be wrong - and herewith my excuses for letting myself go along with such error! Not only is the energy provided by the magnetic field, the basic textbook solution for calculating the force only accounts for the magnetic field force.

Here it is once more, with more step by step explanation:

- Magnetic fields are modeled as due to small current loops inside the magnets.
- The simplest model of two magnets consists of two current loops.
Analysis:
- Magnetic attraction is due to the inhomogeneous magnetic fields of the magnets:
http://en.wikipedia.org/wiki/Force_between_magnets#Magnetic_force_due_to_non-uniform_magnetic_field (skip the Gilbert model and stick to the Ampere model)
- You can find all the equations you need there and in the further links
- The only force in basic textbook calculations of magnetic attraction is this magnetic force
- It is proportional to the square of the magnetic field strengths and it is still present when the magnets are moving
- Therefore, the magnetic force pulls the magnets together (surprised? )
- It therefore does work (see: https://en.wikipedia.org/wiki/Work_(physics))

 

[cabraham]

I've no need to look at sketches in this case. Notice the q in the Lorentz Force law? Where there is no charge, there is no Lorentz force. Where there is no moving charge/current, there is no magnetic Lorentz force (qvxB). The Lorentz force acts directly on the charge carriers in the loop, not on the loop itself.

Some mechanism within the loop (in the case of a physical loop connected to a battery, that mechanism is the electric force generated by the battery) keeps the charge carriers moving around the loop and results in the loop as a whole moving (possibly) in the presence of the external magnetic field. This motion gives an additional component to the motion of the charge carriers in the loop.

I've already stated that E forces keep the current in the loop going as well as generate the magnetic dipole. But E force is not in the right direction to produce torque, where Fm is. Fm = qvXB spins the loop. You pretty much admit that Fm is what spins the loop. If you're telling me that Fm is zero unless an E force keeps charge carriers moving around the loop, please reread my posts & you will find that I've already stated that eons ago. Without E to maintain loop current, Fm is indeed zero. I've said that since day one. You seem to arguing my case. BR.

Claude

 

[K^2]

Yea, I think when two magnets are close to each other attracting/repelling, the magnetic field is changing and E is induced.(Not sure correct me if I'm wrong with supporting fact + equations please.)

Miyz,

Your question was phrased generally, so I gave a general answer. If you are talking specifically about a case where there is no external E field, then yes, the only thing that can do work is the induced E field.

I've already stated that E forces keep the current in the loop going as well as generate the magnetic dipole. But E force is not in the right direction to produce torque, where Fm is. Fm = qvXB spins the loop.

A current loop placed in magnetic field precesses work done is precisely zero. Same for a dipole placed in a magnetic field. The torque is applied perpendicular to the angular momentum already carried by the loop/dipole. Therefore, the direction the loop will turn is going to be perpendicular to torque. There is no work being done on the current loop.

Think of it as a gyro in a gravitational field. As gyro precesses, the work gravity does on gyro is zero. In order for there to be a net work on the gyro, a dissipative force, such as friction, needs to be introduced. Same deal with spins precessing in magnetic field.

 

[cabraham]

Your question was phrased generally, so I gave a general answer. If you are talking specifically about a case where there is no external E field, then yes, the only thing that can do work is the induced E field.


A current loop placed in magnetic field precesses work done is precisely zero. Same for a dipole placed in a magnetic field. The torque is applied perpendicular to the angular momentum already carried by the loop/dipole. Therefore, the direction the loop will turn is going to be perpendicular to torque. There is no work being done on the current loop.

Think of it as a gyro in a gravitational field. As gyro precesses, the work gravity does on gyro is zero. In order for there to be a net work on the gyro, a dissipative force, such as friction, needs to be introduced. Same deal with spins precessing in magnetic field.

Sorry, but you make no sense at all. The torque spins the loop. How can the loop turn in a direction normal to torque. We're talking very basic physics here, per Halliday & Resnick. The induced E field does not spin the loop. Refer to my diagrams. Fm is clearly what spins the loop, Fe is what clearly established loop current.

We're dealing with 3 dimensional forces, loops, rotations, moments, etc. Visualizing such is all but easy. That is why I keep nagging everyone to draw sketches. I don't think anybody is too smart to need to rely on sketches. A sketch shows us in detail what exactly is going on. Those who refuse to provide sketches are merely guessing, assuming, & arm waving. Draw a picture. Please draw a picture.

Draw a picture!

Claude

 

[harrylin]

A lot of articles treat the electrons in atoms as current loops that rotate in a magnetic field. Magnetic domains are a little more complicated since they can change shape, grow and rotate. However, the point is always that the current loops at the macroscopic scale rotate. [..]

Yes, sorry that my question wasn't clear. What makes you believe that explaining magnetic force needs a discussion of all those things such as magnetization etc, so that there would be no magnetic attraction between two electromagnets? I gave the example of two magnetic current loops, which explains the basics of magnetic attraction.

At your request, I gave you some references. Now I have three questions. Please give references and links, if possible.
1) How can a current loop exist without a centripetal force?
2) What centripetal force keeps the electrons in an atom moving in a circle?
3) Is there always an electric field between an electron and the nucleus of an atom?

1. Sorry, there is no mention of centripetal force in any such discussion in any textbook that I know - it's assumed to be irrelevant for calculating magnetic field strength. I already gave a link to Wikipedia which cites references; in addition I can mention Fundamental University Physics, Electromagnetism, by Alonso&Finn which corroborates the Wikipedia article on the force between magnets. For electromagnets the magnetic moment is calculated from the Lorentz force F=IBL, which drives as example a galvanometer.
- http://en.wikipedia.org/wiki/Galvanometer#Operation
2. Electrical force. According to the textbooks, it does not drive the galvanometer action.
3. Presumably yes.

 

[harrylin]

[..]Those who refuse to provide sketches are merely guessing, assuming, & arm waving. Draw a picture. Please draw a picture.

Draw a picture!

Claude

This morning when I awoke I drew a picture of two current loops in my head, complete with Lorentz force vectors. That picture is of course only qualitative, but it agrees with you, Wikipedia and my textbooks.

 

[cabraham]

This morning when I awoke I drew a picture of two current loops in my head, complete with Lorentz force vectors. That picture is of course only qualitative, but it agrees with you, Wikipedia and my textbooks.

Thanks for your feedback. I wouldn't insist on drawing pics unless I knew first hand how helpful they are. Like I said, mentally visualizing all this stuff is not easy. Those who claim that they don't need a pic have abilities beyond mine. BR.

Claude

 

[PhilDSP]

What force is the cause of work? Electric Force/field.
What caused the electric force? The MAGNETIC FIELDS.
Miyz,

Hi Miyz,

I don't want to get embroiled in this interesting debate, but while you could *very loosely* state what you said above, to be rigorous, that statement is not true: magnetic fields are most definitely NOT the cause of electric fields as Jefimenko's analysis has shown:

http://en.wikipedia.org/wiki/Jefimenko's_equations

 

[vanhees71]

There is one and only one electromagnetic field, and thus some of its components are not the cause for other components. That doesn't make sense to begin with since it's a reference-frame dependent statement anyway, and there is no physical meaning in something that's frame dependent.

Further, according to Maxwell's equations, the sources of electromagnetic fields are charge and current distributions (including magnetization described effectively of a contribution to the current given by \vec{j}_{\text{max}}=\vec{\nabla} \times \vec{M}. Admittedly, I've to think about the relativistic covariant formulation of this ;-)).

 

[Darwin123]

Then what did the magnetic field/force do? Well, it created the electrical field that eventually does the work, did it do work or not? Well that depends on how people accept it... It created the force that does work...
Miyz,

The magnetic field doesn't have to make the force that does the work on the electric current. It can change the direction of an electric current so that it is moving in the direction of the electric field.
The magnetic field doesn't have to cause an electric field that does the work on the electric current. The magnetic field can change the direction of the electric current so that it is in the direction of a previously existing electric field that is unchanged.
The ferromagnetic material already has loops of electric current even without a externa magnetic field applied to it. The loops of electrical current are randomly oriented at different points. The ferromagnetic material already has electric fields even without an external magnetic field applied to it. The electric fields are randomly oriented at different points.
If the electric fields and the electric currents are randomly oriented with respect to each other, the net work done by the electric fields is zero. This is the case before the external magnetic field is applied to the magnet.
When a magnetic field is applied, then there is a torque applied to the loops of electric current so that the axis of the loop is rotated. This torque does not work since the velocity of the electric charges is orthogonal to the electric fields.
I believe this is what happens in the case of two permanent magnets that are acting on each other.
The electric currents are oriented in the direction of the electric fields. When the electric currents are moving in the direction of the electric fields, the electric fields do work on the electric current.
No electric field has to be induced by the magnetic fields for the electric fields to do work. All that has to happen is that the axes of the current loops rotate. The electric fields can remain oriented precisely as they were before.
Electric current is not created. Electric field is not created. The electric current is rotated into the direction of the electric field.

 

[gabbagabbahey]

I've already stated that E forces keep the current in the loop going as well as generate the magnetic dipole. But E force is not in the right direction to produce torque, where Fm is. Fm = qvXB spins the loop. You pretty much admit that Fm is what spins the loop. If you're telling me that Fm is zero unless an E force keeps charge carriers moving around the loop, please reread my posts & you will find that I've already stated that eons ago. Without E to maintain loop current, Fm is indeed zero. I've said that since day one. You seem to arguing my case. BR.

Claude

No, what I said was:

The Lorentz force doesn't act on the loop. Rather, it acts on the charge carriers in the loop. Its direction is perpendicular to each of the charge carrier's motion, so it does no work.

An you then claimed to have proved otherwise with some sketches you drew:

See the thread about work on a current carrying loop. I drew sketches proving otherwise. Then feel free to comment. BR.

Claude

So, to clarify, which part of my quoted statement are you claiming to have disproved and why?

 

[PhilDSP]

Admittedly, I've to think about the relativistic covariant formulation of this ;-)).

And I have to think about the apparent disconnect between classical EM and the relativistic variant that lies in the effective redefinition of very many values such as E and B ;-) Does the Minkowski EM tensor preclude you from having separate fields where one (the former B field) deals only with the rotational aspects of EM and the other only the linear aspects of EM? Does the stepping from the transformational matrices of the Lorentz and Poincare groups into the tensor formulation force a reinterpretation of the distinction between rotation, boost and translation operations? (But this is certainly off-topic for this thread)

 

[harrylin]

The magnetic field doesn't have to make the force that does the work on the electric current. It can change the direction of an electric current so that it is moving in the direction of the electric field.[..] I believe this is what happens in the case of two permanent magnets that are acting on each other.[..]

Please clarify how your explanation works with two electromagnets like this (current loops 1 and 2; x = current direction into screen, 0=current direction out of screen):

1 x---0

2 x---0

I have not yet tried it, but this looks like a good example to work through, complete with numbers to calculate the force between them according to the different explanations.

 

[K^2]

Sorry, but you make no sense at all. The torque spins the loop. How can the loop turn in a direction normal to torque. We're talking very basic physics here, per Halliday & Resnick.

I'm also talking about very basic physics. Do you understand how a gyroscope works? If not, look it up. The precession axis is perpendicular to applied torque. Gravity does zero work on a gyro.

In order to do work on an object with angular momentum (aligned with principal axis) torque vector must have a non-zero projection onto angular momentum vector. Torque due to magnetic field is always perpendicular to angular momentum of an ideal current loop or elementary dipole.

What happens to a physical loop of wire carrying a current in a magnetic field is a little different and involves electric field built up due to hall effect. It is that electric field that actually does work on the metal lattice of the wire carrying the current.Just to make sure you understand. If you place a piece of wire carrying a current in a magnetic field, the magnetic field does not apply force on a wire. Wire isn't moving, so vxB=0. It's the electrons in the wire that are moving, get deflected to one side of the wire, causing that side of the wire to become negatively charged. That results in a net electric field in the wire, which acts now on the positively charged nuclei within the metal itself and move the wire.

 

[harrylin]

[..] If you place a piece of wire carrying a current in a magnetic field, the magnetic field does not apply force on a wire. Wire isn't moving, so vxB=0. It's the electrons in the wire that are moving, get deflected to one side of the wire, causing that side of the wire to become negatively charged. That results in a net electric field in the wire, which acts now on the positively charged nuclei within the metal itself and move the wire.

We have gone through that discussion before, perhaps you missed it - it's a mere matter of definitions. According to your definition of "work": if you pull a cart by means of a rope, do you do work on the cart or not? And if something does work on electrons which are part of a wire, does it do work on the wire or not?

If such a discussion over words is all that there is left to discuss, then I will from here on abstain from the discussion.

 

[chingel]

It is not similar to the cart pulling, because the magnetic field doesn't affect the wire, only the electrons, and it does no work on the electrons. With the cart you could argue that you do work on the rope and the rope does work on the cart, because it pulls the cart through a distance along the direction of movement, or whatnot, but the magnetic field does zero work on the electrons, which is the only thing it affects.

 

[harrylin]

[..] With the cart you could argue that you do work on the rope and the rope does work on the cart, because it pulls the cart through a distance along the direction of movement, or whatnot, but the magnetic field does zero work on the electrons, which is the only thing it affects.

Consequently, your analysis of the basic example in post #166 will be interesting.

 

[vanhees71]

And I have to think about the apparent disconnect between classical EM and the relativistic variant that lies in the effective redefinition of very many values such as E and B ;-) Does the Minkowski EM tensor preclude you from having separate fields where one (the former B field) deals only with the rotational aspects of EM and the other only the linear aspects of EM? Does the stepping from the transformational matrices of the Lorentz and Poincare groups into the tensor formulation force a reinterpretation of the distinction between rotation, boost and translation operations? (But this is certainly off-topic for this thread)

These are interesting questions. I think, that there is no "non-relativistic" electromagnetics, except in the sense of certain approximations you call "non-relativistic". This has been worked out by LeBellac, Leblond and others, but from a principal point of view there is only relativistic electromagnetics, and one should consider the electromagnetic field as one object, described by the antisymmetric field-strength tensor (Faraday tensor).

In matter and going to the effective description, usually called "macroscopic electrodynamics", you better formulate all equations, including the "material equations" covariantly. You obtain this description from fundamental electromagnetics by calculating the linear response approximation of many-body systems in thermal equilbrium to a disturbation given by an external electromagnetic field and/or by adding charges and currents.

Concerning the special relativistic space-time model, of course nothing changes from switching from a three-dimensional non-covariant description (within a given inertial frame) to the manifest covariant formalism. Of course, what you call a rotation and what a boost depends on the choice of your reference frame, or more generally your choice of "time slicing" of the Minkowski manifold. I think the discussion of this would be well worth another thread.

 

[K^2]

We have gone through that discussion before, perhaps you missed it - it's a mere matter of definitions. According to your definition of "work": if you pull a cart by means of a rope, do you do work on the cart or not? And if something does work on electrons which are part of a wire, does it do work on the wire or not?

Ok, let's go with a rope analogy. The electric current does sort of work in that capacity here. Except, what's pushing it through the wire is the applied voltage, not the magnetic field. The magnetic field merely redirects the current. If you are pulling a cart by a rope via a fixed pulley, would you say the pulley is doing the work on the cart? Because that's the purpose the B field serves here.

 

[harrylin]

Ok, let's go with a rope analogy. The electric current does sort of work in that capacity here. Except, what's pushing it through the wire is the applied voltage, not the magnetic field. The magnetic field merely redirects the current. If you are pulling a cart by a rope via a fixed pulley, would you say the rope is doing the work on the cart? Because that's the purpose the B field serves here.

In the earlier thread, I first held that the rope is not doing work at all.
https://www.physicsforums.com/showthread.php?t=621018&page=6
However, it then turned out that my definition is not standard, and that according to other people's definitions the rope does work on the cart (see that whole page + post #97)).

Therefore I now repeatedly link (also in this thread) to the definition to which I refer, such as in Wikipedia, in order to avoid wasting time on useless discussions over words (from now on I'll either not reply or simply insert a link to this post).
Here again, http://en.wikipedia.org/wiki/Work_(physics):

"In physics, a force is said to do work when it acts on a body so that there is a displacement of the point of application, however small, in the direction of the force. Thus a force does work when there is movement under the action of the force."

From the other thread:

The magnetic force acts on currents, not wires. To see that simply measure the force on a current without a wire and the force on a wire without a current.

Also already discussed in that thread, and here - the electrons are part of the wire. If I accused you of pushing my car, would you reply that you didn't act on my car but merely on the paint?
Clearly if contact forces can act on a body then according to the definition here above which I now refer to for these discussions, you acted on my car, despite the fact that you only indirectly acted on most of the body. That's also what you stated on the page that I linked here above:
"If the rope is applying a force to the system along some distance then by the usual definition of work the rope's force is doing work on the system. "

So, I'm fed up with that and won't discuss it again!

 

[K^2]

harrylin, do you know what a pulley is? Explain to be how a fixed pulley does any work without undergoing any displacement.

 

[harrylin]

harrylin, do you know what a pulley is? Explain to be how a fixed pulley does any work without undergoing any displacement.

Ah right - I meant rope, corrected now.

 

[chingel]

The car paint scenario is not analogous. It would be, if for example you could prove that you only pushed the paint and that pushing the paint couldn't possibly do the things claimed (for example cigarette burns on the seats), then you have a case that you didn't cause the cigarette burns. Same with the current, the magnetic field can only push electrons and because it pushes them in such a way that the force is always perpendicular to motion, it cannot possibly do work on them or on anything else, because it doesn't affect anything else.

 

[cabraham]

No, what I said was:



An you then claimed to have proved otherwise with some sketches you drew:



So, to clarify, which part of my quoted statement are you claiming to have disproved and why?

Here is your quote from before:

"The Lorentz force doesn't act on the loop. Rather, it acts on the charge carriers in the loop. Its direction is perpendicular to each of the charge carrier's motion, so it does no work.""

Actually, its direction is not perpendicular to the loop motion, but rather along the direction of loop motion. Therefore Fm is doing that work. Fe, OTOH, acts normal to loop motion, along the direction of charge motion. The only logical conclusion is that Fe does work on the charge carriers, but not on the loop. Fm does work on the loop, but not on the charge carriers.

What my sketches disproved was your claim that Fm being normal to the charge carriers, it does no work on them, which is correct, but you then concluded that Fm likewise does no work on the loop, which is not supported by physics. My sketches agree w/ classical physics that Fe does work on the charge carriers in the loop, but Fm does work on turning the loop. Since Fm provides torque, & torque produces rotary motion, the torque times the angular displacement is the work done by Fm.

Fe is perpendicular to torque & loop motion, so by your own definition Fe cannot be doing work on the loop. Classical motor/generator texts affirm the Fm does the work spinning the loop, Fe does the work on charge carriers in the loop. The dipole formed by a loop carrying the induced current is due to Fe. To summarize, E & B, like Fe & Fm, are not the same entity, but then again, nor are they divorced. They work together. Wherever you find one, the other is right there next to the first, like Siamese twins.

I hope I have stated my case clearly. I will answer any questions if desired. BR.

Claude

 

[harrylin]

[..] the magnetic field can only push electrons and because it pushes them in such a way that the force is always perpendicular to motion, it cannot possibly do work on them or on anything else, because it doesn't affect anything else.

https://www.physicsforums.com/showthread.php?p=4053762#post4053683

 

[Q-reeus]

harrylin, do you know what a pulley is? Explain to be how a fixed pulley does any work without undergoing any displacement.

And I have a similar question in turn for you K^2. Back in #129 you gave a somewhat partial reply to my #124. Admittedly I had never considered precession or non-localization as relevant factors when arguing for zero electrical work being done on an intrinsic moment. However in the end I don't see it mattering. I realize you acknowledge an E field cannot take advantage of precession to do work, but you seem to imply the non-localization allows E.j type work - somehow. Fact is, magnetic response in ferromagnets is very far from the diamagnetism always predicted if real electrical work acts on perfectly conducting circulating currents. So please just explain how you arrive at, to quote from #129:

All of the work is done via actual currents. Yes, these don't contribute much to the magnetic field of a ferromagnetic in a steady state, but that's where all of the work will be done. Induced currents.

What exactly are these induced currents? And how do you get them to equate to the mechanical power change when two fully magnetized magnets draw together? You are probably quite aware that the actual currents, as in translational motion of electrons - eddy currents, are exceedingly small in hard ferrites, and anyway even in reasonably good conductor ferromagnetic material will be a minor contribution to overall energy exchanges. Particularly if motions are slow, and we are free to make such motions arbitrarily slow. If these 'induced currents' are the fictitious Amperian currents arising from domain growth/reorientation, we are really back to arguing over whether E.j type work can even in principle be done on intrinsic moments. If you have some notion of 'smeared out' moment, please indicate how that squares with the fact of magnetic saturation in ferromagnetic material - especially sharp and dramatic in high susceptibility media like supermalloy.

 

[K^2]

Ah right - I meant rope, corrected now.

But when we are talking about B field acting on a segment of wire carrying the current, B field is playing role of pulley, not rope. It merely redirects the current, allowing the electric field applied from battery or whatever is pushing current through wire to apply a force perpendicular to the wire itself. The work here is done by applied electric field via the current with assistance of the Hall Effect. All B field does is bend the current. Absolutely no work is done by it in any sense of the word.

If you have some notion of 'smeared out' moment, please indicate how that squares with the fact of magnetic saturation in ferromagnetic material - especially sharp and dramatic in high susceptibility media like supermalloy.

Tell you what. Sit down and solve hydrogen atom in a magnetic field gradient. Now, compute expectation values for electric dipoles of each state. What happens to a net electric dipole of an atom placed in magnetic field if you have an un-paired electron in a d-orbital?

You don't need non-local eddie currents to form. You already have a nice little loop current around every single atom. And the B field gradient shifts the electron cloud. Naturally, the shift depends on orientation of electron spins. If average magnetic moment is zero, the average electric dipole will also be zero. If you have a ferromagnetic, then you'll end up with a rather strong electric dipole that will pull the magnet.