Magnetic fields do no work? How come

[Doc Al]

Sure. But does that change anything? With 2 currents in the same direction, the electrons shift inward creating a repulsive force between the 2 wires. But we observe that the force is attractive. How?

The shifted electrons exert an electrostatic pull on the positive wire lattice, thus creating the attraction between the wires.

Answer - there are both electric and magnetic components of force. The mag is much stronger. In machines class this was repeated often. What you're saying is not what is being disputed. What lies at the heart of this is that the H field attraction exists in addition to the E field attraction. Both are present, but the force due to H is stronger than that due to E.

The electrostatic force on the electrons is equal to the magnetic force.

 

[qsa]

Question

Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?

Asked by: Steven Leduc

Answer

The original assumption that a magnetic field can do no work is incorrect. A magnetic field has an energy density that is equal to the magnetic induction (B) squared divided by twice the permeability (mu sub zero). If you were to sum (integrate) this energy of the magnet over all of its field before it picked up the paper clip and compared it to the same sum after you picked up the paper clip, you would discover that there was a loss of field energy. The paper clip has in effect 'shorted out some lines of magnetic flux'.

How much energy was lost? If you took hold of the paper clip and pulled it out to such a distance that the magnetic pull was insignificant, the work you did in this process would exactly equal the amount of energy lost when the clip was on the face of the magnet. When you picked up the clip with the magnet the clip was accelerated toward the magnet acquiring kinetic energy. This kinetic energy will equal, ignoring air drag, the loss of magnetic energy in the field. This kinetic energy will be dissipated in the form of heat on impact of the clip with the magnet.

For further understanding of the energy in a magnetic field, you may want to study magnetic fields in solenoids. See the Reference below.

Physics, Volume 2 by Halliday and Resnick

Answered by: Robert Gardner, M.S., Retired Physicist

 

[Doc Al]

Question

Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?

Asked by: Steven Leduc

....
Answered by: Robert Gardner, M.S., Retired Physicist

Note that this "answer" doesn't really address the question of what force does the work.

 

[qsa]

Note that this "answer" doesn't really address the question of what force does the work.

I thought I should just respond to the original question since it was side stepped into another
detail. As to exactly what force, it seems more complicated and have to be analysed with QED and quantum vs classical (since magnets are classical objects) and other detail configurations.

 

[cabraham]

The shifted electrons exert an electrostatic pull on the positive wire lattice, thus creating the attraction between the wires.


The electrostatic force on the electrons is equal to the magnetic force.

But these same electrons repulse each other. The electrons are on the interior hence the 2 wires are more influenced by the repulsion as the electrons are closer together than the positive lattice charge. The electrons are not anchored to anything. What you describe makes no sense if you draw the diagram. Although the electrons attract the latice in their own wire, this surface density of electrons in each wire push the wires away. Also, just as the electrons attract the lattice, so does the lattice attract said electrons. Which is more of an "anchor", the lattice, or electrons? It has to be the lattice. The lattice is much more massive and less mobile than the free electrons which are very low in mass. The electrons in each wire are drawn towards their own lattice indicating repulsion, and the electrons in each wire repulse each other as well. But observation shows attraction!

Also, if the currents are opposite, the electrons are on the exterior. The lattice positive charges repulse. Of course the electrons on the exterior attract the lattice charge, but again these electrons are not anchored. The lattice charges also attract the electrons inciting an attractive force. We know that the wires repulse with opposite currents.

It can only be magnetic forces accounting for observed phenomena, not electric. Your scenario makes no sense at all. Have you taken a course on motors/generators? This issue is old. Any M/G course, aka "energy conversion" explains these questions in detail.

Case closed.

Claude

 

[cabraham]

Question

Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?

Asked by: Steven Leduc

Answer

The original assumption that a magnetic field can do no work is incorrect. A magnetic field has an energy density that is equal to the magnetic induction (B) squared divided by twice the permeability (mu sub zero). If you were to sum (integrate) this energy of the magnet over all of its field before it picked up the paper clip and compared it to the same sum after you picked up the paper clip, you would discover that there was a loss of field energy. The paper clip has in effect 'shorted out some lines of magnetic flux'.

How much energy was lost? If you took hold of the paper clip and pulled it out to such a distance that the magnetic pull was insignificant, the work you did in this process would exactly equal the amount of energy lost when the clip was on the face of the magnet. When you picked up the clip with the magnet the clip was accelerated toward the magnet acquiring kinetic energy. This kinetic energy will equal, ignoring air drag, the loss of magnetic energy in the field. This kinetic energy will be dissipated in the form of heat on impact of the clip with the magnet.

For further understanding of the energy in a magnetic field, you may want to study magnetic fields in solenoids. See the Reference below.

Physics, Volume 2 by Halliday and Resnick

Answered by: Robert Gardner, M.S., Retired Physicist

Very well stated. This is not even a legitimate debate. Any close examination of all forces reveals as plain as day that the E fields exert much less force than the H fields. The observed forces are consistent with the H field influence, and counter that of the E field. It's clear that E has the small influence and H is what plays the much larger role.

This is so open and shut, there is no reason to continue.

Good job and thanks for your input.

Claude

 

[Phrak]

Very well stated. This is not even a legitimate debate. Any close examination of all forces reveals as plain as day that the E fields exert much less force than the H fields. The observed forces are consistent with the H field influence, and counter that of the E field. It's clear that E has the small influence and H is what plays the much larger role.

This is so open and shut, there is no reason to continue.

Good job and thanks for your input.

Claude

The given explanation is a lot of mush that says nothing. I could take this reasoning and say that a magnet equally attracts aluminum.

Secondly, the OP question does not belong in the classical physics folder where magnetic dipoles are made of charge, and where magnetic fields do no work on charge.

 

[kanato]

I could take this reasoning and say that a magnet equally attracts aluminum.

No you couldn't.. that explanation requires the assumption that the permeability of the material that makes up the paperclip be different than the permeability of vacuum, otherwise the magnetic energy is the same regardless of where the paperclip is located. The permeability of aluminum is the same as that of vacuum for 4 significant figures or so.

 

[Phrak]

No you couldn't.. that explanation requires the assumption that the permeability of the material that makes up the paperclip be different than the permeability of vacuum, otherwise the magnetic energy is the same regardless of where the paperclip is located. The permeability of aluminum is the same as that of vacuum for 4 significant figures or so.

Yah. You're right. I'm batting 0 for 3 in this thread. My excuse is ...well, nevermind, I haven't got one.

Where does permiability come from?

 

[kanato]

Where does permiability come from?

It depends on the details of the material, but for a nonmagnetic metal (like aluminum), the first approximation of the susceptibility $$\chi_m = \mu / \mu_0$$ of the material would be Pauli paramagnetism.

That's an easy one. The magnetic field from one wire deflects the charge carriers (the electrons) in the other wire to one side, creating a static electric field which exerts a force on the positive lattice of the wire. That force pulls the wires together.

Is there a similar explanation for the Stern-Gerlach experiment? In my mind this is a strong indicator that the magnetic field can do work, since it is the force applied to the permanent magnetic dipole of the electron that is responsible for this effect, and the two different beams show up only due to the two different directions of this dipole.

 

[Doc Al]

But these same electrons repulse each other. The electrons are on the interior hence the 2 wires are more influenced by the repulsion as the electrons are closer together than the positive lattice charge. The electrons are not anchored to anything. What you describe makes no sense if you draw the diagram. Although the electrons attract the latice in their own wire, this surface density of electrons in each wire push the wires away. Also, just as the electrons attract the lattice, so does the lattice attract said electrons. Which is more of an "anchor", the lattice, or electrons? It has to be the lattice. The lattice is much more massive and less mobile than the free electrons which are very low in mass. The electrons in each wire are drawn towards their own lattice indicating repulsion, and the electrons in each wire repulse each other as well. But observation shows attraction!

We're not talking about the electrostatic force between the wires--that's negligible. We're talking about the electrostatic force within each wire.

The magnetic force deflects the moving electrons. Do they go flying off of the wire? No. What stops them? Their deflection due to the magnetic force is balanced by the attraction of the positive lattice--an electrostatic force. That electrostatic force is what literally pulls the more massive wire.

Ask yourself: What pulls the massive positive lattice of the wire? It obviously can't be a magnetic force, since there is no magnetic force on stationary charges.

Also, if the currents are opposite, the electrons are on the exterior. The lattice positive charges repulse. Of course the electrons on the exterior attract the lattice charge, but again these electrons are not anchored. The lattice charges also attract the electrons inciting an attractive force. We know that the wires repulse with opposite currents.

It can only be magnetic forces accounting for observed phenomena, not electric. Your scenario makes no sense at all. Have you taken a course on motors/generators? This issue is old. Any M/G course, aka "energy conversion" explains these questions in detail.

I could just as well ask if you've ever taken a physics course. Yes, this issue is old.

Very well stated. This is not even a legitimate debate. Any close examination of all forces reveals as plain as day that the E fields exert much less force than the H fields. The observed forces are consistent with the H field influence, and counter that of the E field. It's clear that E has the small influence and H is what plays the much larger role.

This is so open and shut, there is no reason to continue.

:rofl: When in doubt, simply declare victory. Good job!

Kidding aside, this is not something I would expect you'd learn in an engineering class. It's only something of interest to physics pedagogues. (Who need to be prepared when students read statements such as the one by Griffiths quoted in the first post.) Looking back at this thread, I see that I essentially agree with what diazona stated in post #16. But I also agree with the point that Vanadium 50 makes in post #13--overemphasizing these details is not particularly helpful.

 

[Doc Al]

Is there a similar explanation for the Stern-Gerlach experiment?

No, since the magnetic moment of the electron is considered to be an intrinsic magnetic moment.

 

[arithmetix]

Hi
If work=force times distance, then my guess is that whoever supplies the force does the work.

 

[cabraham]

We're not talking about the electrostatic force between the wires--that's negligible. We're talking about the electrostatic force within each wire.

The magnetic force deflects the moving electrons. Do they go flying off of the wire? No. What stops them? Their deflection due to the magnetic force is balanced by the attraction of the positive lattice--an electrostatic force. That electrostatic force is what literally pulls the more massive wire.

Ask yourself: What pulls the massive positive lattice of the wire? It obviously can't be a magnetic force, since there is no magnetic force on stationary charges.

I could just as well ask if you've ever taken a physics course. Yes, this issue is old.



:rofl: When in doubt, simply declare victory. Good job!

Kidding aside, this is not something I would expect you'd learn in an engineering class. It's only something of interest to physics pedagogues. (Who need to be prepared when students read statements such as the one by Griffiths quoted in the first post.) Looking back at this thread, I see that I essentially agree with what diazona stated in post #16. But I also agree with the point that Vanadium 50 makes in post #13--overemphasizing these details is not particularly helpful.

So you're saying that the H force pulls the electrons to the interior, but the E force from the lattice provides an opposing force. But as I stated, just as the lattice is attracted to the electrons, so are the electrons attracted to the lattice. If the lattice incurs a force of attraction to the other wire, is it the E or H force that is accountable? The E force alone won't do it. The electrons would be attracted to the lattice, not vice versa. The mass of the lattice is enormous vs. the electrons. Of course, the electrons do not go jumping off the wire. But their attractive force on the lattice cannot happen without the H force holding them. The strong H force yanks the electrons towards the interior. The lattice and electrons exert a mutual E force on each other. Without the H force, the electrons would be drawn to the lattice. But the strong H force holds the electrons so that the lattice in addition to the electrons are moved in the direction of attraction.

The strong H force is ultimately responsible. I've already stated that E plays a role, but the main contribution is H force. I regard the electrons as an array of ping pong balls, and the lattice as an array of bowling balls. PP balls do not attract bowling balls forcing them to move unless a very strong force is holding the PP balls in place. That would be the H force. Otherwise, the PP balls would be forced to move towards the bowling balls.

The fact that there is attraction between the electrons & the lattice due to E force, one you've emphasized, has never been disputed. I'm just asking that you consider all forces involved. Heck, in cases 1) & 2), the gravitational force always provides attraction, but how strong in relation to H, or E for that matter?

Yes, I've taken physics courses. I had 2 quarters of basic phy (Halliday-Resnick text), 1 quarter of modern phy including relativity, QM, & kinetic theory of matter (Tipler text), and 1 quarter of solid state physics (Kittel text), at the undergraduate level. Of course, it was some time ago, in the 1970's.

As far as declaring victory goes, to acknowledge that many other learned researchers, all more capable than moi, have already laid this issue to rest, is hardly "declaring victory". Why do the universities teach us that to determine the direction of the force, that we must use the right hand rule?

I'm not out to show anybody up. I would rather be corrected than to continue to believe a false doctrine. If what you say was the whole truth, every physics and EE text would affirm the same. But to acknowledge what universities teach is not declaring victory on my part. If this is a contest to see who can outdo the other, count me out. I am not here to "win", just to learn, and contribute.

Maybe somebody else with a strong e/m fields background can chime in and offer their viewpoint. I've said enough. Good day to all.

Claude

 

[Doc Al]

So you're saying that the H force pulls the electrons to the interior, but the E force from the lattice provides an opposing force.

Yes.

But as I stated, just as the lattice is attracted to the electrons, so are the electrons attracted to the lattice.

Of course!

If the lattice incurs a force of attraction to the other wire, is it the E or H force that is accountable? The E force alone won't do it.

Please tell me the magnetic force on the stationary lattice?

The electrons would be attracted to the lattice, not vice versa.

Really? What about Newton's 3rd law?

The mass of the lattice is enormous vs. the electrons. Of course, the electrons do not go jumping off the wire. But their attractive force on the lattice cannot happen without the H force holding them.

Of course. I've been saying that all along.

The strong H force yanks the electrons towards the interior. The lattice and electrons exert a mutual E force on each other. Without the H force, the electrons would be drawn to the lattice. But the strong H force holds the electrons so that the lattice in addition to the electrons are moved in the direction of attraction.

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?

The strong H force is ultimately responsible.

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

I've already stated that E plays a role, but the main contribution is H force.

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

As far as declaring victory goes, to acknowledge that many other learned researchers, all more capable than moi, have already laid this issue to rest, is hardly "declaring victory".

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.

Why do the universities teach us that to determine the direction of the force, that we must use the right hand rule?

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.)

 

[Dadface]

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.

 

[cabraham]

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.

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

 

[Doc Al]

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.

 

[cabraham]

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

 

[Doc Al]

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".

 

[Academic]

This paper helped me understand it. :
http://academic.csuohio.edu/deissler/PhysRevE_77_036609.pdf

 

[cabraham]

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

 

[Doc Al]

Does this make sense?

Yes! Sounds good to me.

 

[kanato]

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.

 

[Doc Al]

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.)

 

[cabraham]

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

 

[Doc Al]

Very good. I'm glad we agree.

Me too!

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!

 

[kanato]

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.

 

[qsa]

"The usual explanation
is that there is a change in the “potential energy” by
an amount −2
s ·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".

 

[kmarinas86]

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...

 

[sweet springs]

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

 

[Doc Al]

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.

 

[sweet springs]

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.

 

[Doc Al]

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.

 

[sweet springs]

Hi.
Please teach me more.

A free charge motion draw a circle.

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○○●○○○○○●○○○
○○○●○○○●○○○○
○○○○●●●○○○○○
○○○○○○○○○○○○

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●●●○○○○○
｜○○○●○○○●○○○○
｜○○●○○○○○●○○○
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In the case of No magnetic field

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No stretch of course. Don't these mean that magnetic force can stretch the spring?
Regards.