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

  • Thread starter Miyz
  • Start date
  • #1
200
0

Main Question or Discussion Point

Hallo everyone,

Can a magnet do work on another magnet? (I believe it can. Just wanted to make sure.)

What formula's do you use to support you're answer?
Finally,
I know that magnetic field can not do ANY work on a free charge based on Lorentz force so no need to reference that.
Regards,

Miyz.
 
Last edited:

Answers and Replies

  • #2
200
0


A good article to look at published by MIT.

Page 8-12.

Miyz,
 
  • #3
28,719
4,987


Here is my previous response, with two minor edits:
Well, I think that I am ready to post some final conclusions on my part:

1) a [STRIKE]motor[/STRIKE] magnet is governed by classical electromagnetism. I.e. It follows Maxwells equations and the Lorentz force law, the "EM laws".

2) from the EM laws the power density transferred from the fields to matter (the work on matter) is E.j

3) therefore, the B field does not directly do work under any situation governed by the EM laws, including [STRIKE]motors[/STRIKE] magnets.

4) however, the B field does store energy and Faradays law relates E to B and Amperes law relates j to B and E, so the B field does do work indirectly, through its impact on E and j.

5) tethering and other related concepts are irrelevant because they are internal forces and internal forces cannot do work on a system
 
  • #4
732
3


Hallo everyone,

Can a magnet do work on another magnet? (I believe it can. Just wanted to make sure.)

What formula's do you use to support you're answer?
Finally,
I know that magnetic field can not do ANY work on a free charge based on Lorentz force so no need to reference that.
Regards,

Miyz.
The magnetic field is not doing any work on the electric charge carriers. It is doing work on the matrix of nuclei that keep the electrons in the magnet.
A magnetic is not composed of electric currents alone. It contains the atoms which supplied the electrons that created the electric current. The work done on the magnets includes the work done on the atoms from which the electrons were supplied.
Even when the magnetic field of one magnet does work on the other magnetic, it does not do work on the current. The electric currents can be pictures as d-orbital electrons jumping from nucleus to nucleus. However, the nucleus and the jumping electrons are oppositely charged. There is an electric field from the nucleus that attracts the jumping electrons.
The result is that when the electric current changes direction, the jumping electrons change direction. The jumping electrons then pull the nuclei by its electric field. It is the electric field that does work on the nuclei.
The thing to remember is that "the magnet" is a system. It isn't just electric currents. If there weren't nonmagnetic forces holding the magnet together, the electrons would not stay in the magnet.
Work done on any component of a system is not necessarily the work done on the system. The electric currents are a component of the magnet, not the magnet itself.
A magnetic field can't do work on a current. However, a magnetic field can do work on a system that includes a current.
 
  • #5
1,115
3
If the almost consensus position in https://www.physicsforums.com/showthread.php?t=621018, from which this follow-on obviously derives were true (dW = E.j dv for any EM system), then uniformly magnetized permanent magnets should interact as though they were perfectly conducting surface current inductors. That is, the so-called surface magnetizing currents Im owing to bulk cancellation of the combo of orbital and spin electronic contributions to magnetization, should perfectly obey Lenz's law and the consequences of the classical Faraday's law curl E = -dB/dt from which Lenz's law derives. So a long straight magnetized rod enclosed within a similarly shaped solenoid should completely demagnetize when the solenoid generates a B field Bs equal in magnitude and of the same sign as that of the magnet's initial B field Bm. [Edit: not quite perfectly, as there is a finite but relatively tiny 'angular KE' contribution owing to the electronic gyromagnetic ratio μ/S ~e/me] This manifestly does not happen. The actual magnetic response is known to be quite complex and material dependent - particularly in the demagnetizing regime when Bs opposes Bm. Assuming the rod is fully magnetized, when Bs has the same sign as Bm, typically there is very little change in the latter regardless of how great Bs is made.

In short, permanent magnets do not obey classical EM in this important respect, and it cannot be maintained that dW = E.j dv covers the situation. I therefore disagree with #3, while #4's picture of magnetization as "d-orbital electrons jumping from nucleus to nucleus" is at best only partially true (orbital contributions are an important contribution in ferrites but not otherwise) and imo misses the real point here. QM 'exchange interactions' stemming from Pauli exclusion principle are intimately tied up with any detailed energy exchanges (includes magnetic domain growth and reorientation), an observation I admit to not being at all qualified to expand upon in any detail. Beside that, there is the electron's intrinsic magnetic moment which clearly cannot be modeled as a tiny classical loop current. If it could, then particularly when Bs has the same sign as Bm, Lenz's law would continue to hold as for a classical perfectly conducting solenoid but does not. I followed only a tiny fraction of the postings in the above linked thread, so pardon please if I am repeating other's arguments already made there.
 
Last edited:
  • #6
28,719
4,987
permanent magnets do not obey classical EM in this important respect,
Please provide a mainstream reference supporting this or a rigorous derivation from Maxwells equations which supports your point.

Classical EM does not explain how a material is magnetized, but given that it is magnetized, it correctly describes the forces and energy.
 
  • #7
1,115
3
Please provide a mainstream reference supporting this or a rigorous derivation from Maxwells equations which supports your point.
The rigorous derivation you require of me and no one else here has been adequately summarized by use of my example in #5 imo. Instead of a terse challenge, how about furnishing your own 'rigorous', or just rational, reconciliation of your viewpoint in #3 with the long-magnetized-rod-in-solenoid scenario I have furnished.
Classical EM does not explain how a material is magnetized, but given that it is magnetized, it correctly describes the forces and energy.
Easy to just say. Then provide your own explanation of just where the total conventionally calculated magnetic energy density comes from when, say, Bs = 100*Bm (hint here: (a+b)2 = a2+b2+2ab, and, even allowing for non-linearity in getting to the b2 bit (non-linear magnetization), it's that 2ab bit that could be a bit worrying for your viewpoint - imho). [Forgot to explicitly add that we assume magnetic saturation early on here, say when Bm = 0.01 Bs (final)] That is, how do you equate net solenoid input energy with the notional net stored energy using only ME's? No problem computing solenoid input energy via integrating over a non-linear rod magnetic susceptibility. But how to explain how those tiny magnetic dipoles somehow resist Lenz's law to give that ~ 2ab part. Moral - don't try putting me on the back-foot pal! Looks like our relationship is back to the usual situation. :tongue:
 
Last edited:
  • #8
vanhees71
Science Advisor
Insights Author
Gold Member
2019 Award
13,781
5,602
I don't know, how often I posted this trivial thing now, but you can describe permanent magnets within usual classical Maxwell electrodynamics by using the effective magnetization current in the Ampere-Maxwell Law, [itex]\vec{j}_{\text{mag}}=c \vec{\nabla} \times \vec{M}[/itex], where [itex]\vec{M}[/itex] is the magnetization density of the magnet, in addition to the usual convection currents of moving charges, [itex]\vec{j}_{\text{convection}}=\rho \vec{v}[/itex]. The conduction current is usually described in linear-response approximation to the electric field by Ohm's law, [itex]\vec{j}_{\text{cond}}=\sigma \vec{E}.[/itex] These are the usual simplified equations assuming bodies at rest and the velocity of all charges much smaller than [itex]c[/itex], i.e., the non-relativistic approximation for the description of matter. Note that this approximation can be misleading. E.g., the description of the homopolor generator always needs a fully relativistic treatment. It's one of the most prolific direct application of relativistic effects in engineering :-).

I do not know of any example where Maxwell electromagnetics is proven wrong within the range of applicability of the classical approximation to full quantum electrodynamics, which is of course the most comprehensive theory we have about electromagnetics. This holds for both, "vacuum QED", where one considers scattering events of a few particles (usually two particles) mediated by the electromagnetic interaction, and "in-medium QED" like plasma physics and condensed-matter physics.
 
  • #9
28,719
4,987
Instead of a terse challenge, how about furnishing your own 'rigorous', or just rational, reconciliation of your viewpoint in #3 with the long-magnetized-rod-in-solenoid scenario I have furnished.
Fair enough. See here for a rigorous proof based on established mainstream science demonstrating that the power density delivered by an EM field is given by E.j in all cases: http://farside.ph.utexas.edu/teaching/em/lectures/node89.html

You are the one attempting to overthrow established mainstream science with nothing more than a handwaving assertion of some problem with no evidence to support it, either theoretical or experimental. It isn't up to me to prove you wrong, it is up to you to prove yourself right.

Moral - don't try putting me on the back-foot pal! Looks like our relationship is back to the usual situation. :tongue:
I don't know what you mean by "the back-foot", but if you cannot justify your assertion here then there is nothing left to do, the default position (particularly on PF) is that the mainstream scientific theory stands in the absence of compelling evidence to the contrary (which you certainly have not provided).
 
Last edited:
  • #10
1,115
3
Flux quantization in superconducting circuits, as experimentally verified in 1961 by Fairbanks & Dever, Doll & Nabauer, means that a closed circuit supercurrent can only respond to a time-changing flux - and thus an E = -dA/dt, in discrete jumps. In between, no change occurs and in that interval Lenz's law fails totally, and holds only as an average over a periodic interval. An electron can in a way be thought of as the ultimate in supercurrent miniaturization - with the added restriction there are no flux jumps at all. Hard then to see how any time-changing E fields involving relative motion of fully magnetized magnets can be made to 'do work' on each other - save for the usual resistive eddy currents which are not the main concern here. As we are merely talking about a redistribution of energy within a system, seems a whole lot more sensible to me to treat the situation in terms of magnetic energy only, at least for slow motions in the frame of interest.
 
Last edited:
  • #11
1,115
3
Fair enough. See here for a rigorous proof based on established mainstream science demonstrating that the power density delivered by an EM field is given by E.j in all cases: http://farside.ph.utexas.edu/teachin...es/node89.html [Broken]
I will give it a look over. [Edit: Now had a look; this is just a pretty standard derivation of Poynting's theorem. Not at all in conflict with #5 and #10 imo.]

As far as this bit goes:
You are the one attempting to overthrow established mainstream science with nothing more than a handwaving assertion of some problem with no evidence to support it, either theoretical or experimental. It isn't up to me to prove you wrong, it is up to you to prove yourself right.
Overthrowing mainstream science? Putting things a little dramatically there. Well as I say, why not just give your own explanation. Still, if it comes down to a case of quoting esteemed authority figures, I just managed to find the following, which might give pause for thought: http://physics.stackexchange.com/questions/10565/does-a-magnetic-field-do-work-on-an-intrinsic-magnetic-dipole A range of opinions, but I side with Lubos Motl there.
 
Last edited by a moderator:
  • #12
Dotini
Gold Member
621
229
To a non-physicist, it seems intuitively obvious that a magnet does work, as when picking up a paperclip. I was easily able to find the following link to confirm that is true.

http://www.physlink.com/education/askexperts/ae354.cfm

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


Respectfully submitted,
Steve
 
  • #13
28,719
4,987
Overthrowing mainstream science? Putting things a little dramatically there.
You are specifically claiming that Maxwell's equations predict that putting a bar magnet in a current carrying solenoid will demagnetize it, are you not?

That is clearly contrary to experiment, so if Maxwell's equations actually made such a prediction then they would be overthrown. Indeed, that was your point in suggesting that they make such a prediction. So I think the description is accurate, not dramatic.

However, your claim is also unsubstantiated (either via reference or derivation) and it appears that you have no plans to substantiate it, so it can be safely dismissed.

I just managed to find the following, which might give pause for thought: http://physics.stackexchange.com/questions/10565/does-a-magnetic-field-do-work-on-an-intrinsic-magnetic-dipole A range of opinions, but I side with Lubos Motl there.
As I said in in my point 4 both E and j depend on B, so B does work indirectly. In particular, Lubos is talking about the magnetization which is directly linked to j via the equation that vanhees71 posted above.
 
Last edited:
  • #14
28,719
4,987
To a non-physicist, it seems intuitively obvious that a magnet does work, as when picking up a paperclip.
The work is entirely accounted for by E.j so the B field only does work via its influence on E and j. See the reference I posted above for a derivation.
 
  • #15
vanhees71
Science Advisor
Insights Author
Gold Member
2019 Award
13,781
5,602
Dotini, there is no contradiction between the quote you give and the fact that magnetic fields don't do work. Of course, if you compare the final state (paper clip attached to the magnet) with the initial state (paper clip separated from the magnet), you of course find that the change in magnetic-field energy is given by the work necessary to pick up the paper clip, attaching it to magnet.

This is precisely the content of Poynting's theorem, nicely explained at DaleSpam's link. This epxlains the the work is done by induced currents and electric fields during the transient (i.e., time-dependent!) situation when picking up the paper clip!
 
  • #16
Dotini
Gold Member
621
229
I can create a charged object by rubbing a balloon on my hair. And then do work with it by rolling an aluminum can around on the floor. No magnet is required in this instance, as the work is done by an electric field.

Is this agreeable? Or is the work done by me moving the balloon?

Respectfully,
Steve
 
  • #17
Dotini
Gold Member
621
229
  • #18
1,115
3
You are specifically claiming that Maxwell's equations predict that putting a bar magnet in a current carrying solenoid will demagnetize it, are you not?
Go back and read what I actually said and argued in #5, and then try and not twist it out of shape, as you proceed to do below.
That is clearly contrary to experiment, so if Maxwell's equations actually made such a prediction then they would be overthrown. Indeed, that was your point in suggesting that they make such a prediction. So I think the description is accurate, not dramatic.
Nonsense. The aim was to show that Faraday's law applied to a permanent magnet modeled as a classical charge/current distribution would indeed necessarily 'discharge' as claimed there. Do you dispute that simple observation? That this is not the case points to the radically different response of a real magnet - owing to QM. Stop twisting what I have been saying!
However, your claim is also unsubstantiated (either via reference or derivation) and it appears that you have no plans to substantiate it, so it can be safely dismissed.
Is this how the real DaleSpam defends his own position?
Q-reeus: "I just managed to find the following, which might give pause for thought: http://physics.stackexchange.com/que...agnetic-dipole" [Broken]
The derivation there is for force density, not power density. If you go through the next step then you get E.j. As I said in in my point 4 both E and j depend on B, so it does work indirectly.
Huh? As I said, there are a range of views expressed there, and I back those of Lubos, which is *not* just about force density! Try reading at least all of his entries there.
As Lubos rightly imo points out in that link I gave, electrons cannot be successfully modeled as the limit of a classical loop current. Electrical interactions of the E.j type simply do not and cannot apply in that wholly QM regime. And it carries over to a permanent magnet as a QM glued ensemble of such. You can dismiss this yet again as 'hand-waiving', but I notice you still haven't supplied any kind of coherent rebuttal to either #5 or #10. Just hope I don't get stuck in a useless circular dialogue with you here. Please, be prepared to shift position, however painful it seems at the time. Must go. :zzz:
 
Last edited by a moderator:
  • #19
28,719
4,987
I can create a charged object by rubbing a balloon on my hair. And then do work with it by rolling an aluminum can around on the floor. No magnet is required in this instance, as the work is done by an electric field.

Is this agreeable? Or is the work done by me moving the balloon?
I don't think that classical EM (Maxwell's equations and the Lorentz force law) accurately describes the work on the can. Classical EM does accurately describe the work on the paperclip. The two situations are not analogous.
 
  • #20
28,719
4,987
Go back and read what I actually said and argued in #5, and then try and not twist it out of shape, as you proceed to do below.
Tell me how I am twisting this out of shape:
So a long straight magnetized rod enclosed within a similarly shaped solenoid should completely demagnetize when the solenoid generates a B field Bs equal in magnitude and of the same sign as that of the magnet's initial B field Bm. ... This manifestly does not happen.
Huh? As I said, there are a range of views expressed there, and I back those of Lubos, which is *not* just about force density!
You must have started responding before my edit.
 
  • #21
Dotini
Gold Member
621
229
I don't think that classical EM (Maxwell's equations and the Lorentz force law) accurately describes the work on the can. Classical EM does accurately describe the work on the paperclip. The two situations are not analogous.
Franklin and Faraday may have begun the study of electricity and magnetism, but I must assume that Maxwell and Lorentz did not end it.

Respectfully,
Steve
 
  • #22
28,719
4,987
Electrical interactions of the E.j type simply do not and cannot apply in that wholly QM regime.
I agree.

And it carries over to a permanent magnet as a QM glued ensemble of such.
I do not agree. If this were correct then classical mechanics would never be valid as every classical object is a "QM glued ensemble". As long as the energies and masses in your system are all much larger than the Planck scale then you are in the classical limit of QM and classical physics applies.

I notice you still haven't supplied any kind of coherent rebuttal to either #5 or #10.
Correct, and I will not until you provide some solid supporting evidence. The burden of proof is on you, not me; I will not accept it simply because you cannot be bothered to support your own claims. I have supported mine.
 
Last edited:
  • #23
28,719
4,987
Franklin and Faraday may have begun the study of electricity and magnetism, but I must assume that Maxwell and Lorentz did not end it.
Agreed. So what? Do you think that means that classical EM does, in fact, accurately describe the work done on the aluminum can?
 
  • #24
Dotini
Gold Member
621
229
Agreed. So what? Do you think that means that classical EM does, in fact, accurately describe the work done on the aluminum can?
What else could it be? I'm only a retiree/hobbyist, reading only basic textbooks on electricity and magnetism at this time, so I really have no idea of what else it could be.

Respectfully yours,
Steve
 
  • #25
gabbagabbahey
Homework Helper
Gold Member
5,002
6


A good article to look at published by MIT.

Page 8-12.

Miyz,
MIT is certainly a respectable organization, but that does not mean that every detail of everything published by them is rigorous and accurate. First, it looks as though these are course notes for an introductory level course on electricity and magnetism, and so it is not unreasonable to expect that some important details may be glossed over in the interest of not making the course material too complex for a 1st or 2nd year student. Second, any author, whether they work for MIT or not, is capable of both making mistakes, and having fundamental misunderstandings.

That said, I would argue two things that are in disagreement with that article:

(1) The net force on a magnetic dipole placed in a magnetic field [itex] \mathbf{F}=\mathbf{ \mu } \cdot \mathbf{ B }[/itex] is not truly a magnetic force,. It is the net result of the magnetic force on the current elements in the loop and whatever (nonmagnetic!) agent/force is responsible for maintaining the current in the loop (there are of course other forces at play which make the loop go where the current elements go, by keeping the current confined to the loop, but these are irrelevant to this discussion, and incapable of doing work on the loop/dipole as they are internal forces).

(2) Since the magnetic force on each current element is always perpendicular to the motion of the current element, the work is not done by the magnetic field, but rather by whatever agent maintains the current in the loop/dipole.
 

Related Threads for: Can a magnet's magnetic field perform work on another magnet?

  • Last Post
2
Replies
48
Views
7K
Replies
7
Views
2K
Replies
2
Views
4K
Replies
4
Views
2K
Replies
32
Views
10K
Replies
5
Views
609
  • Last Post
Replies
4
Views
1K
Top