Magnetic fields do no work? How come

AI Thread Summary
Magnetic forces are often stated to do no work, particularly in the context of moving charged particles, where the Lorentz force is perpendicular to motion. However, when two magnets attract, they appear to exert a force over a distance, leading to confusion about whether work is being done. The discussion reveals that while magnetic forces themselves do not do work, the energy transfer involved in moving magnets is actually facilitated by electric forces within the system. This interplay between electric and magnetic forces complicates the understanding of how energy is transferred and work is accomplished. Ultimately, any perceived work done in magnet interactions can be traced back to the underlying electric forces at play.
  • #51
If it is true that no work can be done on or by a magnetic field then can someone please explain what field features in the work done when say two magnets are used one of them being completely surrounded by a Faraday cage?As far as I understand it the cage blocks electric fields.I tried a quick experiment using two fridge magnets and aluminium foil for the cage. I had to do work to pull them apart against the attractive force and work was done when I released them and they moved together again.The results were what I expected.
 
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  • #52
Doc Al said:
Yes.

Of course!

Please tell me the magnetic force on the stationary lattice?

Straw man. I've clearly stated already that the force on the lattice is that of the electrons and E force. The H force acts on the electrons which are moving and not stationary. In attempting to make me look bad, you are introducing straw men.

:bugeye: Really? What about Newton's 3rd law?

I already acknowledged that just as the electrons attract the lattice, so does the lattice attract the electrons. You pulled 1 sentence out of context with the intent of making me look bad.
Of course. I've been saying that all along.

That's sounds very close to what I've been saying all along, but you miss the punch line. Again I ask: What force directly acts on the positive lattice?

Already been acknowledged as the E force. But my point was that you cannot simply declare E as the only entity responsible. They both are involved, E & H. I can respond with "What force acts on the electrons holding them in place so as to make lattice attraction possible?" When placed in close proximity, a ping pong ball and a bowling ball, each with 1 uC of charge will mutually attract (opposite polarity) or repel (like). If the pp ball remains stationary or moves relatively little vs. the bowling ball which moves a greater distance, what is going on?

Answer - there is another force, quite significant, acting on the pp ball, holding it in its position.

Of course. But that force does not directly act on the positive lattice.

They are equally strong. And only one directly acts on the lattice.


As far as I can see, this is the first time you've been exposed to the issue, so I can't imagine why you think others have "laid it to rest". Please cite a learned researcher who claims that the statement made by Griffiths in his introductory E&M book is false.

I can name countless that affirm the right hand rule. Every text. I never said Griffiths was wrong. Since page 1 of this thread, I have fully acknowledged the E force. I never took issue with Griffiths. But you and others keep putting forth isolated facts emphasizing the role of E while neglecting all facts pointing to H as having great influence. When I force the issue, you acknowledge the role of H, but you don't bring it up on your own. You are obsessed with presenting only 1 side of the issue.

You then break up my post into fragments, isolating single sentences, then attacking the fragments by asking questions I've already answered. You are clearly here to "win". I only want to point out that there are numerous things going on here. Then you pit me against Griffith, with whom I have no beef.
Because it works just fine. No need to go into the nitty gritty details all the time--which gives you the same answer of course, only with more effort. (That was Vanadium's point, back in post #13.)

It sure does work fine. Yet I'm wrong for believing in it.

Claude
 
  • #53
cabraham said:
I can name countless that affirm the right hand rule. Every text. I never said Griffiths was wrong. Since page 1 of this thread, I have fully acknowledged the E force. I never took issue with Griffiths. But you and others keep putting forth isolated facts emphasizing the role of E while neglecting all facts pointing to H as having great influence. When I force the issue, you acknowledge the role of H, but you don't bring it up on your own. You are obsessed with presenting only 1 side of the issue.
Huh? Where did I question the "right hand rule"? Strawman, indeed! You claim that I "acknowledge the role of H, but you don't bring it up on your own", but describing the role of the H field is the very first step I made (post #32) in responding to your request (post #28) for an explanation of the attraction between current-carrying wires using electric fields.

Enough already. We are wasting each other's time.
 
  • #54
Doc Al said:
Huh? Where did I question the "right hand rule"? Strawman, indeed! You claim that I "acknowledge the role of H, but you don't bring it up on your own", but describing the role of the H field is the very first step I made (post #32) in responding to your request (post #28) for an explanation of the attraction between current-carrying wires using electric fields.

Enough already. We are wasting each other's time.

So let's summarize. Two wires are parallel and carrying current. What determines the magnitude & direction of the force incurred? The direction of the currents determines the polarity of the H fields. The polarity of the H fields determines whether the free electrons in the wire shift to the interior vs. exterior. Then, the positive charged lattice follows the free electrons due to E force.

That pretty much sums it up. If the current increases, so does the H field, and the electrons move further inward or outward. Then the lattices follow the electrons further in or out.

Thus the H field determines where the electrons move and how far. The lattice tags along like an obedient shadow due to E force between lattice and electrons.

That is prima facie evidence that the H field is primarily what determines if the wires attract or repel, and the magnitude of the force. The E field definitely participates, but is not what determines the above.

H force moves the electrons. Lattice tags along due to E force. It's that simple. H is primary, with E secondary. Case closed.

Claude
 
  • #55
cabraham said:
H force moves the electrons. Lattice tags along due to E force. It's that simple. H is primary, with E secondary. Case closed.
The issue is not which field, E or H, is "primary"; they come together--it's a package deal. The issue is, per the title of this thread: Does the magnetic field do work? The answer to that is technically no; it's the electric field that pulls the wire. This explanation is one that you objected to at first (recall your response in post #18 to diazona's rather clear statement in post #16).

The reason for this seemingly nitpicking discussion is one of understanding the Lorentz force law, which is the source of all the derived "right-hand rules".
 
  • #57
Doc Al said:
The issue is not which field, E or H, is "primary"; they come together--it's a package deal. The issue is, per the title of this thread: Does the magnetic field do work? The answer to that is technically no; it's the electric field that pulls the wire. This explanation is one that you objected to at first (recall your response in post #18 to diazona's rather clear statement in post #16).

The reason for this seemingly nitpicking discussion is one of understanding the Lorentz force law, which is the source of all the derived "right-hand rules".

Maybe an analogy would help. A steel ball is tethered to a rubber ball via a short cord, or even glued together. A powerful electromagnet is held above the tethered ball pair. The em is turned on and the steel/rubber ball pair is lifted into the magnet.

I certainly do not believe that a magnet is doing work on the rubber ball. But the rubber ball does not ascend if not for the mag force. So it is really splitting hairs to argue which force is responsible for the rubber ball ascending.

The mag force acting on the steel ball is what ultimately lifted both balls. The steel ball was lifted by the magnet directly. The rubber ball was lifted indirectly. The tether provided the means for the rubber ball to tag along with the steel ball.

With 2 parallel wires, the E force between the lattice and free electrons is the tether. The H force dictates where the electrons go, then the E force tethers the lattice yanking it in the direction of the electrons. To say that H is NOT responsible for the lattice moving is like saying that the magnet is NOT responsible for the rubber ball ascending. The electrons and the lattice are tethered via E force. But the H force is what moves the electrons, and is ultimately responsible for moving the lattice. The E force does indeed move the lattice, but the E force magnitude and direction is dictated by the location of the electrons which is dictated by the magnitude and direction of H.

It's difficult to separate the 2 forces. But it is clear as day that H is what dictates the magnitude and direction of the displacement of the wires. E follows H. I know that E & H are inclusive, and neither is the cause of the other. But under these narrow conditions, H is ultimately in control, with E tagging along.

H, however, is not more fundamental than E, nor less. They are inclusive.

Does this make sense? BR.

Claude
 
  • #58
cabraham said:
Does this make sense?
Yes! Sounds good to me.
 
  • #59
Doc Al said:
No, since the magnetic moment of the electron is considered to be an intrinsic magnetic moment.

So then the magnetic field can do work. Since the electron spin is what gives rise to any ferromagnetic material, the magnetic field is does work whenever a permanent magnet is involved.
 
  • #60
I may have to revise my answer to the Stern-Gerlach question in the light of the interesting paper that Academic linked to in post #56. (It might take me a while to find the time to digest it--hopefully someone more knowledgeable will chime in sooner.)
 
  • #61
Doc Al said:
Yes! Sounds good to me.

Very good. I'm glad we agree. This is an interesting thought problem. It gives us all a chance to review the theory and I feel I've gained a better understanding. Thanks for your input. BR.

Claude
 
  • #62
cabraham said:
Very good. I'm glad we agree.
Me too! :smile:

This is an interesting thought problem. It gives us all a chance to review the theory and I feel I've gained a better understanding. Thanks for your input. BR.
Yes, it's interesting--and subtle--stuff. It helps me to review it every now and then. Good discussion!
 
  • #63
Doc Al said:
I may have to revise my answer to the Stern-Gerlach question in the light of the interesting paper that Academic linked to in post #56. (It might take me a while to find the time to digest it--hopefully someone more knowledgeable will chime in sooner.)

I missed that one. I have skimmed through that article, and it looks like the author doesn't actually answer the question, but gives some conditions under which the answer could be understood. I will have to read it more carefully however. It is interesting that this is a topic of current research though, so the question of whether or not a magnetic field can do work at a fundamental level really isn't settled.
 
  • #64
"The usual explanation
is that there is a change in the “potential energy” by
an amount −2s ·B=−eB/mc, which implies that the magnetic
field did work on the electron’s magnetic moment.
However, if the electron has rotational kinetic energy,"

This is a quote from the paper. He states the conventional explanation, but he puts forward his own conjecture " rotational kinetic energy".
 
  • #65
The forces of electromagnetism do work. Acceleration can occur along electric field lines and acceleration and also along magnetic field lines. However, because a cyclical process of doing work requires a changing magnetic field which in turns produces an electric field, electric fields are seen as crucial in order for work to be done. A changing displacement is sufficient for an electric field to do work, but not so for a magnetic field, which requires a change in magnitude in place (implying a change in electric field). Therefore when work is done using electricity or magnetism, an electric field ALWAYS comes into play, but same is not true for magnetic fields (because sometimes they are not used). It is a rare circumstance in the macroscopic world to have a system with truly constant magnetic fields (no induction) when electric fields are being moved relatively to each other...
 
  • #66
Hi
It might be out of date but I show here a interesting case, a charged particle attached on a elastic body, with velocity v in perpendicular direction, under magnetic field B in another perpendicular direction.
|
|wwwwwwwwwww○ ↑ v  x B
|
Magnetic or Lorentz force pushes or pulls elastic body. It does work thus elastic energy would be stored.

I state from this example that magnetic force does not work on FREE charge, but it can work on charge UNDER CONSTRAINT.

The elastic body consists of multiple charged particles under electromagnetic interaction so we can say another way that magnetic force does not work on a system of SINGLE charge, but it can work on a system of MULTIPLE charges.

Regards
 
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  • #67
sweet springs said:
Hi
It might be out of date but I show here a interesting case, a charged particle attached on a elastic body, with velocity v in perpendicular direction, under magnetic field B in another perpendicular direction.
|
|wwwwwwwwwww○ ↑ v  x B
|
Magnetic or Lorentz force pushes or pulls elastic body. It does work thus elastic energy would be stored.
Can you please describe the case you have in mind in more detail and explain why you think it illustrates a magnetic force doing work on a charged particle.
 
  • #68
OK, I will.
|
|wwwwwwwwwww○ ↑ v  x B
|

the direction of Lorentz force is ← or → according to the sign of charge ○ and it pushes or pulls the elastic body or the spring.
Regards.
 
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  • #69
sweet springs said:
OK, I will.
|
|wwwwwwwwwww○ ↑ v  x B
|

the direction of Lorentz force is ← or → according to the sign of charge ○ and it pushes or pulls the elastic body.
While the Lorentz force (at the moment pictured) is ← or →, the motion of the charge is not. So the elastic material isn't being stretched yet.

An interesting case, but since the Lorentz force is always perpendicular to the velocity of the charge, I don't know why you'd say it's doing work on it. All it does is change the direction of motion of the charge, which may certainly end up stretching (or compressing) the elastic material due to the inertia of the charged mass. The only thing doing work on the charge is the elastic material.
 
  • #70
Hi.
Please teach me more.

A free charge motion draw a circle.

○xB○○○○○○○○○○
○○○○●●●○○○○○
○○○●○○○●○○○○
○○●○○○○○●○○○
○○●○○○○○●○○○
○○○●○○○●○○○○
○○○○●●●○○○○○
○○○○○○○○○○○○

In the case the spring with wheel moving up-down free in the figure, is tied to the charge, the spring is streched according to the motion of charge, isn't it?

|○xB○○○○○○○○○○
◎ww w●●●○○○○○
|○○○●○○○●○○○○
|○○●○○○○○●○○○
◎ww●○○○○○●○○○
|○○○●○○○●○○○○
|○○○○●●●○○○○○
|○○○○○○○○○○○○

In the case of No magnetic field

|○○●○○○○○○○
◎ww●○○○○○
|○○●○○○○○○○
|○○●○○○○○○
◎ww●○○○○○○
|○○●○○○○○○
|○○●○○○○○○
|○○●○○○○○○
|○○●○○○○○○

No stretch of course. Don't these mean that magnetic force can stretch the spring?
Regards.
 
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  • #71
Does the magnetic force affect the motion of the charge and thus the stretch of the spring? Sure. Does the magnetic field do work on the charge? No. All it does is change the direction of motion of the charge. The energy of the system--elastic potential energy + kinetic energy--remains the same.
 
  • #72
Doc Al said:
Does the magnetic force affect the motion of the charge and thus the stretch of the spring? Sure. Does the magnetic field do work on the charge? No. All it does is change the direction of motion of the charge. The energy of the system--elastic potential energy + kinetic energy--remains the same.

Ok, so basically the energy is not added by this static magnetic field to the system (obviously). Instead, it just facilitates changes of potential energy into kinetic energy and vice versa.

Imagine a large magnetic field external to our solarsystem that changes only over the course of thousands of years. This "effectively static" magnetic field could give us an impression of anomalous effect on the movement of bodies in our solarsystem, but in such a way that the total energy, GPE + KE, is conserved. This would imply that magnetic fields can be used to facilitate the unleashing of potential energy, but that such potential energy does not come from the magnetic field itself.
 
  • #73
Expressing it a bit differently the charge has kinetic energy before being deflected and it has exactly the same kinetic energy after being deflected.Some of this kinetic energy is changed to elastic potential energy but the kinetic energy is not gained from the magnetic field,it is gained from the electric field that caused the charge to move in the first place.
 
  • #74
A magnetic dipole in an external magnetic field with a gradient will feel a force.

So, two ideal magnetic dipoles at rest can extert a force on each other.

That means two neutrons can exert a magnetic force on each other while they are at rest. If you don't like composite particles, then the same can be said with electrons (but there is also an electric force as well then, but you can set it up such that the forces are in orthoganol directions to make it clear which force did what).

I would consider that work legitimately done by a magnetic field.
... unless you want to claim an electron is not a point particle.
 
  • #75
Doc Al said:
Does the magnetic force affect the motion of the charge and thus the stretch of the spring? Sure. Does the magnetic field do work on the charge? No. All it does is change the direction of motion of the charge. The energy of the system--elastic potential energy + kinetic energy--remains the same.

Hm.. Let me pose one more question.

In the case of motion of charge in NO magnetic field

|○○●○○○○○○
◎ww●○○○○○○
|○○●○○○○○○
|○○●○○○○○○
◎ww●↑○○○○○
|○○●○○○○○○
|○○●○○○○○○
|○○●○○○○○○
|○○●○○○○○○

By magnetic flux x B, the Lorentz force → and the elastic force ← balance,

x B
|○○○○●○○○○
◎www ●○○○○
|○○○○●○○○○
|○○○○●○○○○
◎www ●↑○○○
|○○○○●○○○○
|○○○○●○○○○
|○○○○●○○○○
|○○○○●○○○○

So what caused the stretch of the spring therefore increase of elastic energy?

Regards.
 
  • #76
I'm not sure I understand your diagrams, but in any case.
sweet springs said:
By magnetic flux x B, the Lorentz force → and the elastic force ← balance,
No reason to think that those forces are balanced.

So what caused the stretch of the spring therefore increase of elastic energy?
The mass is moving.
 
  • #77
Here's my answer to elliotr's origininal question. (Please be kind if it doesn't make sense.)

First and formost, nobody knows what magnetism is. We just know how it behaves.

Second is how one looks at a problem. I think that all textbooks state that magnetism can't do real work directly same as gravity. What I mean by this is that people percieve the Earth as the center for gravity and do not do the same for magnets. Each magnet is its own gravity well. On Earth we can use a ground state like a sidewalk and base calculations from that level. A cannon ball falling from a building converts its potention energy to kinetic energy. This is where it gets tricky. The cannot ball got it's energy from somewhere other than gravity. The person who took the cannon ball to the top of the building is the source of that energy. So when the cannon ball falls and breaks the sidewalk nobody questions that it did work and had energy. The same can be said about a paperclip falling up to a magnet. It will do work on the surface of the magnet as it hits, but the energy would not have come from the magnet. The energy came from many processes that created and placed the paperclip just prior to being pull (or pushed) up by the magnet.

We don't even know if magnetism is a pulling or pushing force. Anyhow, if you think about a magnetic well that things can fall into and be pulled from, it starts to make more sense. Well, that's my two cents worth...
 
  • #78
Dipoles have come up a couple of times in this discussion, but I still don't quite get it.

We can calculate the torque due to the magnetic force on a dipole, m, in a uniform magnetic field, B, and calculate a corresponding energy in rotating the dipole -m.B.

We are only considering the magnetic forces here, yet they have appeared to do work.

One way to construct this situation is using spinning spherical shells; a uniformly charged sphere rotating at constant angular velocity has a constant, uniform magnetic field inside and a dipolar magnetic field outside. So we can put one spinning shell inside another, using the outer shell to produce the uniform magnetic field and the inner shell to represent the dipole.

It turns out that the inner shell produces a torque on the outer shell, and so under a rotation of the dipole this produces a work +m.B.

Thus we find (at least in this case) the TOTAL work done by magnetic fields is +m.B - m.B = 0.

However the justification that the magnetic force does no work is done particle-by-particle. I would have thought this implied that there should be no magnetic work in rotating a dipole in a uniform magnetic field. Why is this wrong?
 
  • #79
Fantispug, I think I covered this in Post #13. Is there something unclear that I should elaborate?
 
  • #80
Hi.

Doc Al said:
The mass is moving.

Now I know my misunderstanding. Another resource to keep mass moving in constant velocity is necessary. Thank you for your teachings. I apologize the delay of my thanks. I was wondering of the following another situation.

On X-Y plane there exists around the origin a ring of radius a. This ring is made of charge, the line density of which is ρ, and is rotating with angular velocity ω. Let homogeneous magnetic field of B of direction x be applied. The parts of circle get force in Z direction of -ρaωB cosφ dl = -ρa^2 ωB cosφ dφ where φ is the direction angle in X-Y plane.

Does the ring start to rotate around the Y axis (stand up) therefore get rotation energy under the influence of magnetic force which keep perpendicular to the motion of charge i.e. rotating around Z and perhaps Y axes ?
If yes, does this Y rotation energy come from the work done by the magnetic field ,or just from the conversion of Z rotation energy ,therefore ω decrease, in conservation of energy ?

Regards.
 
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  • #81
Hi Vanadium 50,
I don't understand in the particular case of a dipole in a magnetic field where the electric forces come in; I'm interested in this case at looking at a careful analysis of the forces just to understand how it works.

The calculation that I eluded to, using the torque on a magnetic dipole by a magnetic field, seems to imply that the magnetic field does the work. The electric field plays no role in this argument; there are no external electric fields and we have not included the effects of the electric fields generated by the dipole.

(In fact the electric fields generated by the dipole give an additional energy to the system of +m.B)

So how are the electric forces responsible for the work in this specific case?
 
  • #82
What holds the dipole together?
 
  • #83
Vanadium 50 said:
What holds the dipole together?


We're going in circles again. All pertinent forces have been thorughly examined. Refer to pages 1 through 4 and you will find good info from several contributors.

We pretty much reached a consensus that H fields do no direct work on a charged particle, but H exerts force while E provides tethering force yanking the lattice towards the free electrons.

E does indeed hold the lattice and electrons together. H moves the electrons in a direction normal to its velocity, and the lattice is yanked along due to the tethering nature of the E force.

At a mIcroscopic scale, H does not literally "do work" on an electron. At the mAcro scale, H determines the magnitude and direction of the wire deflection. H moves electrons which yank the lattice along due to E.

It is well understood now. Every e/m fields text makes it a point to emphasize the mutually inclusive nature of E & H. They are strongly inter-related, and under time-varying conditions, cannot exist independently. Under static conditions, either can exist alone.

We should all agree at this point.

Claude
 
  • #84
Ok, I think I get what you're saying now. It doesn't matter what's holding the dipole together; whether its electric forces, gluons or Achillies. If we're thinking of a dipole as a spinning spherical shell, the important thing to notice is that if it is conducting when we put it in a magnetic field the charge will redistribute so that it is no longer a dipole at all.

Consequently when we put a dipole in a magnetic field, turn on the magnetic field and assume that the charge does NOT redistribute, we need to supply special angle-dependent forces to prevent this redistribution. The difference in electromagnetic energy between redistributing and not redistributing the charge is supplied by the work done by this "special force". We claim that this is then exactly +m.B.

I've been trying to verify this in a specific example, but the maths is a little tricky so I'll see how we go; but the explanation seems physically plausible. Thanks.
 
  • #85
cabraham said:
Under static conditions, either can exist alone.

Whether a condition is static is relative to the velocity of the obsever. I think a better term would be comoving.
 
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