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

[harrylin]

I said the same earlier. At the time up to the lower magnet ascending, the upward force can be only Fm, not Fe, because prior to motion, no E field has been induced yet. We seem to agree.

Yes I just wanted to add that I now noticed that you already said that in post #126 but you beat me to it. Hopefully my stronger post that emphasises physical principles will have more effect :tongue2:

One thing I would like to mention is that in addition to laws of physics, we should observe rules of logic as well. I can't understand how anybody can say that the moving magnet's induced E field is the source of its own lifting force. The cause is happening after the effect it seems to suggest. Thanks for your input. BR.
Claude

Yes, very right - also cause and effect doesn't allow it!
Thank you too.

Addendum: I did not find the derivation of the magnetic force in Wikipedia, but I found one here (unverified but the result is correct, so maybe the derivation is also correct!):
http://www.physicsinsights.org/force_on_dipole_1.html

 

[gabbagabbahey]

But I said that the force doing work is Fm, not B. B does indeed cancel, but vXB does not. It should be well understood that B acting in the direction of motion will indeed do no work since the Fm vector points normal to B. Once again, are we defining work as force integrated over path? If so, it is universally known that B itself is not acting along the path of motion, but rather Fm is. B will drop out of the above equation, but Fm will not.

This is just utter nonsense now. The cross-product $\mathbf{v} \times \mathbf{B}$ is not only perpendicular to $\mathbf{B}$, but also to $\mathbf{v}$, the direction of motion. This is why the magnetic force is always perpendicular to the direction of motion and does no work, according to the Lorentz Force Law

 

[cabraham]

This is just utter nonsense now. The cross-product $\mathbf{v} \times \mathbf{B}$ is not only perpendicular to $\mathbf{B}$, but also to $\mathbf{v}$, the direction of motion. This is why the magnetic force is always perpendicular to the direction of motion and does no work, according to the Lorentz Force Law

I'd advise that you think before you pronounce something to be "utter nonsense". I've already stated many times that the velocity of the **electron** is such that Lorentz magnetic force cannot do work on said e-. But we're talking about the force on a magnetic dipole, whose velocity is not in the same direction as that of the e-. The "v" in "vXB" must be stipulated. Up to this point we've been discussing the induced E field due to the motion of the lower magnet being lifted upwards. It so happens that the Lorentz magnetic force is in the direction of the velocity of the magnet, not the electrons comprising a current loop.

If this discussion was about "which force accounts for electrons circulating in a current loop, Fe or Fm, the answer would be unanimous. Everybody here that has stated they feel that Fm does work lifting a magnet, not Fe, has also stated that on a moving charge, only Fe can do work, not Fm.

But if the e- is part of a loop, the Fm force acting on it cannot do work on the electron, but it can do work on the loop because Fm aligns with the loop motion. Likewise for a magnetic dipole. We must differentiate between the two. BR.

Claude

 

[gabbagabbahey]

I'd advise that you think before you pronounce something to be "utter nonsense". I've already stated many times that the velocity of the **electron** is such that Lorentz magnetic force cannot do work on said e-. But we're talking about the force on a magnetic dipole, whose velocity is not in the same direction as that of the e-. The "v" in "vXB" must be stipulated. Up to this point we've been discussing the induced E field due to the motion of the lower magnet being lifted upwards. It so happens that the Lorentz magnetic force is in the direction of the velocity of the magnet, not the electrons comprising a current loop.

If this discussion was about "which force accounts for electrons circulating in a current loop, Fe or Fm, the answer would be unanimous. Everybody here that has stated they feel that Fm does work lifting a magnet, not Fe, has also stated that on a moving charge, only Fe can do work, not Fm.

But if the e- is part of a loop, the Fm force acting on it cannot do work on the electron, but it can do work on the loop because Fm aligns with the loop motion. Likewise for a magnetic dipole. We must differentiate between the two. BR.

Claude

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

 

[cabraham]

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

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

Claude

 

[Darwin123]

1. I find the induced current explanation suspect: a pair of magnets doesn't warm up noticeably and doesn't act like a shock damper.

I find your implicit hypotheses incorrect. An induced current doesn't have to warm up noticeably or act like a shock absorber.
It seems that your picture of the induction of an electric field in a ferromagnetic substance is completely different from my picture of the induction of an electric field in a ferromagnetic process. The induction that I imagine is like precession. The induction that you imagine is like rolling.
You picture the magnetic field creating an electric current loop that didn't previously exist in the ferromagnetic material. In your picture, there is no electric field previous to the approach of another magnet. The magnetic field from the approaching magnet induces a nonzero electric field. The new electric field pushes electric charges in a circle, creating a "new" current loop that didn't previously exist. The electric charge carriers, which were no moving before, bump into atoms which generates heat. The "new" current loop generates heat which can be measured.
In your picture, there are no magnetic domains, so everything occurs in a homogenous bulk material. Therefore, there aren't any centripetal forces that make the electric current travel in a circle. When the electron bumps into an atom, the momentum can be in any direction. So the induced motion of atoms is random.
I picture the magnet field as rotating an electric current loop that previously existed in the magnetic field. There is a previously existing electric field on the surface of a magnetic domain that forces the electrons to move in a circle. The atoms and the electrons have equal and opposite momentum before the other magnetic comes near.
In my picture, the electric current loop had previously been there in the ferromagnetic material. The current loop concentrates near the surface of a magnetic domain, where there is no resistance. The approaching magnetic field rotates the current loop. The magnitude of the current is unchanged, but the axis of rotation is different. There is no extra heating because the electric current always has the same magnitude.
According to my picture, the component of electric force in the direction of the electric current is generated by rotation instead of creation. After the current loop is rotated, the electric charge carriers bump into atoms that had not been previously subjected to an electric current. So the total momentum of the atoms is changed. The atoms don't move, so the magnetic field doesn't apply force to the atoms. The atoms feel a force only because of the electric fields coming from the electrons.
In my picture, of the other magnetic there are many loops of electric current with their axial directions oriented in a certain direction. The magnetic field applies a total torque to the electric charges that rotates the axes of the loops. However, the loops never increase or diminish their intensity. The electric fields associated with these current loops are also rotated. The rotation induces an electric field in the material that is in the same direction as the current.
In my view, the torque that rotates the electrons is orthogonal to the torque on the atoms. Because they are orthogonal, they can not be the same. The torque on the electrons is caused directly by the magnetic field. However, the torque on the atoms is caused directly by the electric field. The work is done by the second torque, not the first torque.
A torque can do work only if the bodies it is acting upon are moving (i.e., velocity) perpendicular to that torque. A torque can't do work if it is moving in the same direction as the body it is acting upon.
Another way to look at it is picturing each magnetic domain as a bicycle wheel. The angular momentum of the bicycle wheel. The magnetic field is like a person trying to twist the bicycle wheel in a new direction. He applies a torque perpendicular to the angular momentum of the wheel. The work isn't done until the wheel starts to precess. The work is done because the axis of rotation is changing.
The person doesn't make the tire spin. The tire spins at the same rate all the time. However, the precession generates a component of force in the direction of the linear velocity of the tire.
The twisting doesn't cause the spin of the tire any more than the magnetic field causes the loop of current. The force on the tire is due to precession, not due to extra spin. A tire that isn't spinning is easy to twist. The precession of the tire doesn't cause heating.
I do not have the software to draw a diagram and post it in a reply. However, it should be easy for someone else to draw given that description. A magnetic field is applying a torque to a electric current loop. The axis rotates without changing the angular velocity of the electric current.
I know that there are such objects on the macroscopic scale that obey "classical" equations. For example, there is super fluid helium 4. A vortex of helium 4 can be rotated without heating because it has no viscosity. Therefore, I can't see what the difficulty is in picturing electrons in a ferromagnetic material acting in an analogous way. I picture the magnetic dipole of a single electron the same way.
An electron is just droplet of superfluid with a current loop on its equator. I know quantum mechanics is involved. I know about wave-particle duality. However, the electron can behave as a semiclassical particle.
In the semiclassical approximation, an electron is merely a droplet of electrically charged super fluid. The laws of Newton apply to this super fluid. The anomalous behavior can be modeled by letting the viscosity of the superfluid be zero. A droplet of super fluid can rotate without violating Newton's Laws.
So that is what I suggest. Imagine the charge carriers in a ferromagnetic material as being in droplet of an electrically charged super fluid. The droplets are spinning. The magnetic field rotates the droplets, but doesn't change the magnitude of their spin.
Again, I can't present a drawing. However, I can imagine those little loops of current precessing. It is precession that you should be thinking of.

 

[gabbagabbahey]

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

Claude

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

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

 

[harrylin]

I find your implicit hypotheses incorrect. An induced current doesn't have to warm up noticeably or act like a shock absorber. [..]

I know; and that was mere intuition - it's nothing compared to the disproof of the electric field hypothesis that I presented next (and I wasn't the first).

You picture the magnetic field creating an electric current loop that didn't previously exist in the ferromagnetic material. [...]

That was the model that I distrusted by intuition. However, I finally rejected the possibility of any electrical induction as the driving force on other grounds, which I presented next. Moreover, Ampere's model is that we explain the magnetic field by the existence of electric current loops inside the material; I agree that that's the model that we should use - at least in this classical physics forum. That picture leads to the derivation of the magnetic force to which I finally referred with links. I find that description in Wikipedia satisfying and consistent with my textbooks. In that description, the driving force between magnets is basically magnetic, as it is due to the Lorentz force gradient on current loops (more precisely: the Lorentz force that is due to the gradient of the magnetic field at the current loops).

[...] In my picture, the electric current loop had previously been there in the ferromagnetic material. The current loop concentrates near the surface of a magnetic domain, where there is no resistance. The approaching magnetic field rotates the current loop. The magnitude of the current is unchanged, but the axis of rotation is different. There is no extra heating because the electric current always has the same magnitude. [...] the torque on the atoms is caused directly by the electric field. The work is done by the second torque, not the first torque. [...] So that is what I suggest. Imagine the charge carriers in a ferromagnetic material as being in droplet of an electrically charged super fluid. The droplets are spinning. The magnetic field rotates the droplets, but doesn't change the magnitude of their spin.
Again, I can't present a drawing. However, I can imagine those little loops of current precessing. It is precession that you should be thinking of.

That's interesting; can you likewise give a reference to show that your picture is established physics? I made a reference to standard physics; if there is doubt about it, we could try to do an order of magnitude calculation to verify the classical magnetic force equation (and likely we can save that trouble and find an example in a textbook).

Here again implicitly the answer on the OP's question (it should hardly surprise that the simple answer is the correct one: the force between the magnets and which pulls them together... is magnetic, as it results from the Lorentz forces in an inhomogeneous magnetic field):
https://en.wikipedia.org/wiki/Force_between_magnets

and once more a derivation:
http://www.physicsinsights.org/force_on_dipole_1.html

 

[Miyz]

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

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

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

Now does a magnet do any kind of work on another magnet? Yes.
What force is the cause of work? Electric Force/field.
What caused the electric force? The MAGNETIC FIELDS.
Then what did the magnetic field/force do? Well, it created the electrical field that eventually does the work, did it do work or not? Well that depends on how people accept it... It created the force that does work...

Miyz,

 

[Darwin123]

That's interesting; can you likewise give a reference to show that your picture is established physics? I made a reference to standard physics; if there is doubt about it, we could try to do an order of magnitude calculation to verify the classical magnetic force equation (and likely we can save that trouble and find an example in a textbook).

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

http://www.physics.sfsu.edu/~lea/courses/ugrad/360notes15.PDF
“In a ferromagnetic material, the atomic dipoles tend to align with each other,
even in the absence of applied fields. the dipoles align in regions called domains.
Normally the domains are oriented randomly. When an external field is applied,
the dipoles at the edge of a domain feel an additonal torque, and the net effect is
to cause the domains with magnetization parallel to the applied field to grow. If
the applied field is strong enough, the other domains actually rotate to align with
the applied field. “

Maybe this is your heating. The hysteresis losses cause heating in the ferromagnetic material.
http://ecee.colorado.edu/~mcleod/teaching/EandM3400/Lab%20Book/Chp_13.pdf
“In magnetic terms, atoms and molecules inside matter resemble tiny current loops. If a
piece of matter is situated in a magnetic field, the moment of magnetic forces partly aligns
these loops, and we say that the substance is magnetized. The magnetic field produced by
the substance is due to these aligned current loops, known as Ampere's currents. A substance in the magnetic field can therefore be visualized as a large set of oriented elementary current loops situated in a vacuum. These oriented loops can be replaced by equivalent macroscopic currents situated in a vacuum, known as the magnetization currents.
…
In ferromagnetic materials, groups of atoms (Weiss' domains) are formed as small saturated magnets. Magnetization of ferromagnetic materials is obtained by aligning these domains, which is accompanied by hysteresis losses.”http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magpr.html
“All atoms have inherent sources of magnetism because electron spin contributes a magnetic moment and electron orbits act as current loops which produce a magnetic field. In most materials the magnetic moments of the electrons cancel, but in materials which are classified as paramagnetic, the cancellation is incomplete.”http://www.cientificosaficionados.com/libros/aceleradores2.pdf
“On a macroscopic scale, when two fixed adjacent current loops have the same orientation, the magnetic forces act to rotate the loops to opposite polarity (Fig. 5.9a). This is a consequence of the fact that when the magnetic moments are aligned antiparallel, magnetic fields cancel so that the field energy is minimized. With no applied field, atomic currents are oriented randomly, and there is no macroscopic field.”

http://whites.sdsmt.edu/classes/ee692gwmm/notes/Lecture3.pdf
“Consequently, this loop will rotate if free to do so.”

Note: All the above references show that magnetic dipoles can be modeled as current loops which can rotate.

Just two more references dealing, not with permanent magnets, but with the concept of electrons as classical balls of charged fluid.
http://www.usd116.org/lbeuschlein/physics/PowerPoint/magnetism.pdf
“Note: Electrons are not actually little balls that rotate and revolve like planets, but imagining them this way is useful when explaining magnetism without quantum mechanics.”

This this not a link. I believe that there is a link to an online copy of the following book, but I forgot where it is. In this book, the writer really does model an electron as an ball of electrically charged fluid, held together by some nonmagnetic force. He did not include spin. However, there is nothing in his model that contradicts the idea of spinning electrons. One can give his electron a spin. The model used by Lorentz would work quite well. Further, his model is consistent with Maxwell's equations.
"The Theory of Electrons" by H. A. Lorentz (1915).

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

 

[K^2]

That's just wrong K...

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

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

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

 

[Miyz]

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

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

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

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

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

Miyz,

 

[Miyz]

But who said B changes?

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

Miyz,

 

[harrylin]

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

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

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

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

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

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

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

 

[cabraham]

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

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

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

Claude

 

[K^2]

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

Miyz,

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

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

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

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

 

[cabraham]

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


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

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

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

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

Draw a picture!

Claude

 

[harrylin]

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

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

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

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

 

[harrylin]

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

Draw a picture!

Claude

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

 

[cabraham]

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

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

Claude

 

[PhilDSP]

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

Hi Miyz,

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

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

 

[vanhees71]

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

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

 

[Darwin123]

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

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

 

[gabbagabbahey]

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

Claude

No, what I said was:

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

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

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

Claude

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

 

[PhilDSP]

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

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

 

[harrylin]

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

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

1 x---0

2 x---0

I have not yet tried it, but this looks like a good example to work through, complete with numbers to calculate the force between them according to the different explanations. :tongue2:

 

[K^2]

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

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

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

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

 

[harrylin]

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

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

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

 

[chingel]

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

 

[harrylin]

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

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

 

[vanhees71]

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

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

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

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

 

[K^2]

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

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

 

[harrylin]

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

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

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

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

From the other thread:

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

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

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

 

[K^2]

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

 

[harrylin]

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

Ah right - I meant rope, corrected now. :tongue2: