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

[Miyz]

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

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

 

[Dale]

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

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

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

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

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

 

[Miyz]

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

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

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

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

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

 

[Q-reeus]

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

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

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

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

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

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

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

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

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

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

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

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

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

 

[Dale]

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

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

 

[OmCheeto]

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

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

[ref]

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

[ref]

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

[ref]

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

F=(3μ₀/2∏)μ²/r²

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

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

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

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

 

[Miyz]

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

[ref]

[ref]

[ref]

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

F=(3μ₀/2∏)μ²/r²

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

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

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

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

Reasonable & Interesting point.

 

[Dale]

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

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

 

[OmCheeto]

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

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

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

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

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

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

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

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

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

[ref]

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

(15 minutes of Lorentz Force wiki study)

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

[ref]

This is going to take longer than 16 hours.

hmmm...

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

 

[gabbagabbahey]

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

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

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

 

[vanhees71]

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

 

[OmCheeto]

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

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

His equation would reduce to F = qE

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

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

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

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

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

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

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

 

[OmCheeto]

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

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

[ref]

I believe we are talking about magnetostatics.

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

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

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

?

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

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

 

[Miyz]

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

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

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

 

[gabbagabbahey]

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

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

That would negate Lorentz's B-field factor.

His equation would reduce to F = qE

I'm not sure why you think this.

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

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

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

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

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

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

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

 

[gabbagabbahey]

Other then Om's points I'd like to say that the magnetic field/foce does work but INDIRECLTY.

You keep saying this as though you are under the impression that "does work indirectly" is well defined. I think most physicists will scratch their heads at that statement.

A force \mathbf{F} does work on an object as it moves along a path C if \int_{C} \mathbf{F}( \mathbf{r} ) \cdot d\mathbf{r} is non-zero. Equivalently, a force does work on an object if it changes the object's kinetic energy. These are pretty much universally accepted definitions of what it means for a force to do work on an object, and I think the vast majority of physicists would agree.

I've never seen an analogous definition for what it means for a force to do work indirectly.

 

[Miyz]

You keep saying this as though you are under the impression that "does work indirectly" is well defined. I think most physicists will scratch their heads at that statement.

A force \mathbf{F} does work on an object as it moves along a path C if \int_{C} \mathbf{F}( \mathbf{r} ) \cdot d\mathbf{r} is non-zero. Equivalently, a force does work on an object if it changes the object's kinetic energy. These are pretty much universally accepted definitions of what it means for a force to do work on an object, and I think the vast majority of physicists would agree.

I've never seen an analogous definition for what it means for a force to do work indirectly.

See here's the problem. How can I prove to you exactly magnet field/forces does work would be very diffecult but simple words could at least set you're in motion to think about this matter.

Based on Maxwell-Faraday's law "A changing magnetic field creates an electric field". Now that show's to you that a magnetic field/force DOES something alteast if we can't say that it does work. Without B , E = 0. B creates E so in a sense B as a force creates another force that does work... Pretty interestings stuff. Now about work being done indirectly that seems to confuse you. Read this point from Wikipedia:

"Because the magnetic force is always perpendicular to the motion, the magnetic field can do no work on an isolated charge. It can only do work indirectly, via the electric field generated by a changing magnetic field. It is often claimed that the magnetic force can do work to a non-elementary magnetic dipole, or to charged particles whose motion is constrained by other forces, but this is incorrect[19] because the work in those cases is performed by the electric forces of the charges deflected by the magnetic field."

The link.[/PLAIN]

Miyz,

 

[gabbagabbahey]

Based on Maxwell-Faraday's law "A changing magnetic field creates an electric field". Now that show's to you that a magnetic field/force DOES something alteast if we can't say that it does work. Without B , E = 0. B creates E so in a sense B as a force creates another force that does work

If I press a button which causes a rocket to launch into space am I (or the force I apply to the button) indirectly doing work on the rocket? Without me pushing that button, the rocket doesn't get launched.

 

[Miyz]

If I press a button which causes a rocket to launch into space am I (or the force I apply to the button) indirectly doing work on the rocket? Without me pushing that button, the rocket doesn't get launched.

Yes. That does support my point.

 

[cabraham]

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

You're misinterpreting the equation. The power described in the equation is indeed what results in work done. But the E force is not directed upwards lifting the lower magnet, B is that force, more strictly Fm, where Fm = q*(uXB). The magnetic component of Lorentz force lifts the magnet. But said B field gets energy from E field under dynamic conditions, so E interacts.

Why does E do work on an e- but not B? It has to do with direction of force vectors. In a loop where a time changing B induces an E & J in the loop, charges move around the loop (closed for this example). E acts tangentially to the loop so E force moves the e- around the loop. B force acts normal. Although E does the work, w/o B, there is no E at all. B²/2μ is energy density. If this quantity varies w/ time, we have non-zero power density. Hence it can be argued that the energy in the B field accounts for the work, which is true. But E is what does the work since the E force is oriented in a direction to do work on the e- whereas B is not.

No we look at the magnet being lifted. In this case E certainly can be equated with work per the integral above. To say that the equation you posted accounts for the work done would be correct. But only magnetic component of Lorentz force is oriented in a direction to lift the magnet. E.J provides energy for B to do the work.

These questions are as much about logic as they are about fundamentals. Also, some core concepts need to be emphasized. One thing I don't agree with is the idea that E does work but B cannot in a general sense. E & B are not one & the samr entity of course, but neither are they totally divorced. They are 2 sides of the smae coin. In dynamic conditions, neither is prsent w/o the other. When 1 does work, the other is exchanging energy with the 1 doing work. They are a pair working in tandem.

For the magnet, Fm points the right way, so it does the work, not Fe. Thanks, & BR,

Claude

 

[OmCheeto]

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

Ok.

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

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

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

This seems to make sense.
But what causes the torque? Should I trust the answers I've read from people I respect the most?

There are lots of these "Does the magnetic field do work" threads here at the forum, and many of the mentors seem to have implied that it is in fact the magnetic fields that interact and do work.

2004, 2006-8, etc.

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

It may have been me adding together the comments of several people:

Another example is the electron by itself. It has never exhibited any behaviour suggesting it is a composite particle. We can investigate a model where its magnetic moment arises from an extended distribution of charge, spinning at the appropriate rate. But we find that we end up with no coherent picture.

[ref]

The electron magnetic moment, which is responsible for ferromagnetism, is a relativistic QM effect that is not related to any current or "current".

[ref]

As I understand, it was Ampere's "model" that developed the virtual current as the equivalent needed to generate the magnetic dipoles. (ie. Nobody quote me on this!)

 

[gabbagabbahey]

Yes. That does support my point.

You don't find that (indirect work) to be an absurd concept? The energy it takes to launch a rocket into orbit is typically many many orders of magnitude higher than the energy it takes to push a button. Not only that, but the energy expent pushing the button does not get transferred to the rocket, it just temporarily completes an electrical circuit allowing a signal to propagate through.

 

[gabbagabbahey]

Ok.

This seems to make sense.
But what causes the torque? Should I trust the answers I've read from people I respect the most?

There are lots of these "Does the magnetic field do work" threads here at the forum, and many of the mentors seem to have implied that it is in fact the magnetic fields that interact and do work.

2004, 2006-8, etc.

I disagree with those viewpoints (classically, anyways) as they require you to treat magnetic dipoles as fundamentally different from the other two types of sources in classical electrodynamics - charges and currents.

It may have been me adding together the comments of several people:

[ref]

[ref]

As I understand, it was Ampere's "model" that developed the virtual current as the equivalent needed to generate the magnetic dipoles. (ie. Nobody quote me on this!)

So basically, the problem is that classical electrodynamics doesn't correctly predict/explain the quantum behaviour of permanent magnetic dipoles?

That certainly is a problem, and its one that requires quantum electrodynamics to solve. Treating magnetic dipoles as current loops is not the source of the problem, and treating them as a different type of source/sink does not solve the problem classically. I see no reason why these problems should be used as an argument for treating magnetic dipoles, classically, as being fundamentally different from current loops.

 

[OmCheeto]

I disagree with those viewpoints (classically, anyways) as they require you to treat magnetic dipoles as fundamentally different from the other two types of sources in classical electrodynamics - charges and currents.

Given that dipoles are dipoles, and charges are charges, I don't understand why you wouldn't treat them differently. That's like saying a stick and a ball are the same thing.

So basically, the problem is that classical electrodynamics doesn't correctly predict/explain the quantum behaviour of permanent magnetic dipoles?

That certainly is a problem, and its one that requires quantum electrodynamics to solve. Treating magnetic dipoles as current loops is not the source of the problem, and treating them as a different type of source/sink does not solve the problem classically. I see no reason why these problems should be used as an argument for treating magnetic dipoles, classically, as being fundamentally different from current loops.

So we have nothing more to discuss, as this is all opinion now?
Ok.
Thank you everyone. This has been most interesting.
I'm off to study Quantum Electrodynamics.

But I'll be watching.

 

[gabbagabbahey]

Given that dipoles are dipoles, and charges are charges, I don't understand why you wouldn't treat them differently. That's like saying a stick and a ball are the same thing.

Would you also treat a uniform sphere of charge as being fundamentally different from a uniform cube of charge? If so, your version of classical electrodynamics would have a whole lot of fundamental sources/sinks and force laws.

A magnetic dipole can be modeled (classically) as a specific type of current distribution, so why not treat it on the same footing as every other type of current distribution?

 

[harrylin]

If I press a button which causes a rocket to launch into space am I (or the force I apply to the button) indirectly doing work on the rocket? Without me pushing that button, the rocket doesn't get launched.

This has been discussed before - but perhaps not sufficiently explicit and elaborate?

The kinetic energy of the rocket is not supplied by you pushing the button.
In contrast, earlier we found that the kinetic energy of the magnets must be supplied by their magnetic fields; there is nowhere else that the energy could come from. Any induced currents are induced by those magnetic fields.

 

[Q-reeus]

There have been too many entries since my last one to start answering them all. Apart from OmCheeto, the rest of you are imho just not getting the real issue here. Despite what some maintain, intrinsic magnetic moments are very different in character to what otherwise might be an equivalent classical loop current (quite apart from the g factor issue). IF by some magic a classical loop current could resist responding to the solenoidal component of an -dA/dt E field, then sure it would act no differently to an intrinsic moment in terms of torque and force experienced in an external B field. But the majority here are insisting actual electrical work - integrating E.j over time and volume of magnetized media - is done on the constituent magnetic dipoles.

Yet no-one has come forth and met my challenge, in e.g. #45, #61, #66, #94, and even attempted to offer an explanation of where this so-called electrical work on a dipole appears. It does not. There is no E.j type work done on a real intrinsic dipole. If there were, the notional circular current in that dipole must respond to the solenoidal component of E by altering magnitude - SAVVY!? You know - loop current as inductor. And if the current alters magnitude, so also the dipole moment strength - SAVVY!? IT DOES NOT HAPPEN FOLKS - IT REALLY DOESN'T. There's this thing called quantization of magnetic moment, and it really does get in the way of what would otherwise be weak diamagnetism, as I have tried to make plain umpteen times now. The energy exchange between say two approaching bar magnets are drastically different to weak diamagnetic response precisely because no *actual* appreciable E.j type work is done.

By pretending those Amperian 'surface currents' via c∇×M are real, flowing currents, a purely formal equivalence between changed magnetic field energy and 'electrical work done' on those Amperian currents is found. An absurd, book-keeping-only balance given the induced E field never alters the magnitude of those Amperian currents, in say the case of two fully magnetized co-axial bar magnets that draw together - as per my unanswered challenge in #94. And the further absurdity is this 2-part balance conveniently ignores the additional player in the game - mechanical energy change. Since when do 3 equal amplitude scalar quantities balance out to a total of zero? It's only by recognizing that an intrinsic magnetic dipole moment totally ignores the solenoidal E in E.j, and thus has zero electrical work done on it, that one recovers the sane result there are only two real players in the energy balance: mechanical energy change + magnetic field energy change = 0. Well, as I have hinted at several times, there is a subtle internal energy issue, but not one germane to what is the issue here. So, a further round of madness ahead is it?

 

[Dale]

You are adopting a logically inconsistent position. You claim to agree with Maxwell's equations but disagree that E.j does work. The former implies the latter.

 

[Q-reeus]

You are adopting a logically inconsistent position. You claim to agree with Maxwell's equations but disagree that E.j does work. The former implies the latter.

That rubbish ignores the crucial role of quantization and you know it - otherwise you would have responded with the point-by-point rebuttal I asked of you much earlier but as expected that got conveniently ignored. Anyway, don't come back sniping at me with such silly claims, at least until you provide that retraction as per my first para #94. Clear that matter!

 

[Dale]

That rubbish ignores the crucial role of quantization and you know it

Yes, of course it ignores the role of quantization. Classical EM is a non-quantum theory. But it isn't crucial here, permanent magnets are accurately modeled by classical EM simply by using an empirically determined constituitive relationship between B and H, as described above and in Jackson and any other EM textbook.

If you believe that permanent magnets are not accurately modeled by the appropriate constituitive relationship then please provide good evidence to that effect. Meaning a rigorous derivation or a mainstream scientific reference.

Anyway, don't come back sniping at me with such silly claims, at least until you provide that retraction as per my first para #94. Clear that matter!

OK, I am glad to learn that I was in error with my mistaken belief that you did not accept the equations of classical EM.

Since you do accept classical EM and since classical EM logically implies that the work done is given by E.j then we must logically be in agreement.