Can a magnetic fields/forces do work on a current carrying wire?

[Miyz]

I am sorry, but this is very funny advice coming from you. You are very closed-minded, and have shown no indication of even considering alternative viewpoints.

I didn't really read the comment properly because I was to tired.(So I'll start over.)

Now, when you say I am closed minded because I'm not considering alternative viewpoints? Ok, first of that was a really disrespectful for you to say. I don't want to add any irrational comments because I respect this forums and I respect this tread and its members. I'd like to keep a professional & respectful discussion between us all(sofar I feel its going in a bad way).

Secondly, I have considered all you're viewpoints and stayed hours and hours studying this and trying to develop a reasonable explanation to relate... I even thanked all you're efforts even when I sometimes did not agree and did not understand why? Because you placed effort in answering my question. I appreciate it all. Thats my respect to you and everyone in this thread who have contributed to it.

Now, you all agreed that magnetic fields/force do no work? Ok,you even supplied multiple equations to support you're claims I didn't really understand them. So to be wise and logical I wen't to study about Maxwell's & Poynting's & Faraday's & Ampere's Laws and found that they bring nothing relevant to a current carrying loop and its cause of motion, and who is exactly! Doing work. Hell! Even watched Yale-universities lectures and still nothing provides a proper answer(If I'm closed minded I wouldn't even bother to look this up now would I?). Between all their equations... Honestly I found nothing(Please correct me if I'm wrong site you're references specifically ) relevant to a loop of wire which I believe some find to have no difference in effect compared to a freely charged particle.

Now THE ONLY ONE! who is supporting that effect/example is Lorentz. & yet the website still does not refer what exactly is doing the work in that system.Now, since none - of theses physicists stated by word: MAGNETIC FIELD'S CAN DO NO WORK ON CURRENT CARRYING LOOP. Or even stated a equation like F= qv x B , that states magnetic fields/force do NO WORK on a "FREELY CHARGED PARTICLE". So I would've agreed many many days ago that magnetic fields/forces do no work. If those physicists has stated and you Dale! Would have never put all this effort to back up you're semi-conclusion that magnetic fields/force do no work on a current loop of wire. Where honestly this topic "OP" is not popular nor answered properly as a known fact. And if some of you state that magnetic fields/forces do no work at all. How do you know? We haven't yet tried many amazing effect upon that mag fields/force to state that now can we? Our knowledge is basic and just stated to understand more complicated situations and questions like this draws and inspires someone to take years and years of his time to dedicated into this matter and maybe find's out something amazing and useful for out generation of future one.

Cabraham believed in his point supported it to the SUBATOMIC LEVEL! Described why and used you're equations and simply showed perfect illustration while you all read numbers and numbers and vector's scalers(Not saying their wrong their perfectly stated and used but... How about using those equations and built a scenario to support it? The human mind needs an image of this whole case to believe and understand... Imagination is key sir).

I do believe magnetic fields/force's do work INDIRECTLY on a loop of current-carrying wire and my argument to you all is: COMPARE between a loop of wire carrying charge, and a free charge. Because I feel you all are confusing between the two,... Because the only equation that should be used or mostly concentrated is F = IL x B.

Since magnetic forces/fields can do no WORK on a charged particle I believe most of you assume the same for a loop of current carrying-wire. Please stick to the OP question.

A loop of wire ONLY under a magnetic field/force: Is in motion due to the force,
I agree again that magnetic field and forces DO WORK only in the presence of electric charges in that loop.

Then I'll proceed with definitions and equations to back up my claims. So everyone could have a proper grasps of my opinion and why.

Please rebuttal/argue/ against me using a simple answer containing the equation you BELIEVE gives the proper answer and who's theory is supporting it.

Now I know Dale, would use the Lorentz, Maxwell's equations while Van will uses Poynting theories and Maxwells as well. So let the FINIAL CONCLUSION argument begin. (I know some already have stated them but I find most of them are lost in this previous argument, now simple answers will be given and simple response will be stated out.) I'm proud and glade to have asked this question because so far... It started to make people think about this mater further and further into complication far more greater of my only level of expertise. Well done everyone. Now let's begin to finalize our conclusions.

Please have respect to one and other and do NOT respond in a disrespectful manner please... We're here to learn aren't we?+ This discussion has turned into an amazing one. Thanks again everyone!
Miyz,

 

[Miyz]

Please have a look at #255 in this thread.

The best discussion on this issue concerning macroscopic electromagnetics can be found in Landau-Lifgarbages's textbook (vol. 8 of the textbook on theoretical physics). There the whole issue of work and electromagnetic fields in matter is treated using (relatively simple) thermodynamics, and there it's stressed that magnetic fields do not do work on charge and current distributions.

Can't find it could you link me a site?

 

[vanhees71]

https://www.physicsforums.com/showpost.php?p=4019484&postcount=255

 

[Miyz]

https://www.physicsforums.com/showpost.php?p=4019484&postcount=255

Ow no no, not the post the textbook.

 

[Miyz]

A good question and a good answer here.

That is fully related to our topic but here is a quote: "We know that you can have a magnetic moment from an ordinary current going around a loop, and it can get pulled into a magnetic field just the way some permanent magnet would. Work gets done on it. Isn't it done by the magnetic field? And didn't we just show that couldn't happen?

I should put some drawings in here, and will try to do so later, but meanwhile here's words. Say that the magnetic field (from whatever source) is pointing mostly in the z direction, but getting weaker with increasing z, i.e. spreading out radially in the xy plane. This is just the standard picture of the field from a solenoid or cylindrical bar magnet aligned with the z axis. You've got a ring of conductor symmetrically arranged round the z axis with electronic current running around the loop. Let's say that it's a very good conductor, so the current isn't just running down over the time we're interested in, but not a superconductor so we can temporarily not worry about quantum effects. Let's say that the direction of the current is such that the loop is pulled into the stronger part of the field. The reason that the field along z can get stronger near the source is precisely that the field is spreading out in the xy plane. So there's a little radial field. Take the cross product with the tangential electron velocity and you get a force in the negative z direction on all the electron current. That's at right angles to the current, so there's still no work done. But the electrons can't leave the wire. They bounce off the bottom (low-z) side, imparting momentum to the wire, i.e. exerting force on the wire. As soon as the wire starts to move, that force (in the -z direction) is along the motion of the wire, so it's doing work. The electrons are doing work on the wire, by whatever (non-magnetic) force causes them to bounce off the surface of the wire and stay inside.

What happens to the electrons' energy? They are now all moving, on average, in the -z direction, with the wire. That drives a magnetic force on them (again from the radial part of B) that slows down the tangential current. Energy is flowing from the moving electrons into the overall motion of the wire. The magnetic field causes that without actually doing any work."

Now if the magnetic field/force is not doing any work since the "electrons" are doing all the work. What caused the electron's to be re-directed? Isn't it the magnetic force/field? We all agree on that? Based on the premis that magnetic field re-direct the direction of the electron without changing its KE based on F = qv x B. Ok then, in our cause of a loop the force or forces(I only know of the EMF) causes the electrons to stay in the wire so, didn't the magnetic field do ANY WORK on the electron in a loop? Since its been moved and "it" changed the magnitude of the wire? In a sense a magnetic field causes e- to move and because its "trapped" within the wire its doing work based on the magnetic force. Its kinda similar to what Claude said about his "tethering" statement. Also that good example of an electromagnet carrying a car(The electromagnet lifted and applied its force on the metal body, because of that.the other parts of the car is attached on the body the electromagnet is doing work on all the other PARTS in-directly) as we agreed before(Claude & I don't really remeber anyone else maybe Darwin123), magnetic field/forces do work but INDIRECTLY.

I'll quote another source: " 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[18] because the work in those cases is performed by the electric forces of the charges deflected by the magnetic field.) Source: here.

Now as I agreed EARLIER that magnetic fields/forces can not do work without the presence of multiple forces/atributes.

I say again that Claude stated: That the magnetic field do NO work directly but rather indirect work on the charged particles within a wire and then the electric forces are another key role for work being done. BY THAT DEFINITION without the presence of the magnetic field the charges can not be deflected and NO WORK would be done NO motion, nothing.

But isn't the "deflection" of the charged particle considered work? Since its not curving much as it would if it was freely/ unrestrained? Now the definition I used stated: "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"

Their (charges) motion is not constrained true, but their direction of motion is changed the causes them to change the magnitude of the wire. So that is caused by the magnetic force/field?

Author of that statement: ROBERT J. DEISSLER(For review)
Pretty sure someone. Stated his work before...But I think its Van.

Anyway. His main point why magnetic fields/forces do NO WORK. Its because the B field's are perpendicular to a charge(NOT LOOP , CHARGE!) amazing work of his I have to admit. But again He states of the magnetic field doning no work on the charge because of its perpendicular state. However, maybe on a loop things differ? Since in his own statement in Wikipedia that magnetic field do work INDIRECTLY that contradicts his points that magnetic fields DO NO WORK. Dosen't it? I do applaude all his work and mathematical equations but still.Starting off with clarified definition that we can all gradually build our points base on them and agree upon something.

(DaleSpam,I'm not being closed minded by this post. I am reminding you of the "maybes/chances" once more before we decide on a agreement, Being closed minded is COMPLETELY DIFFERANT.)
(Correct me if I'm wrong here. We always learn form our mistakes )

Thank you all for you're efforts again!

Miyz.

 

[vanhees71]

Ow no no, not the post the textbook.

Ehm, you find oldfashioned media like books in your library rather than online ;-).

 

[Miyz]

Ehm, you find oldfashioned media like books in your library rather than online ;-).

Ow darn! I'd go to the Library now but I am out of town...

 

[Dale]

Now, when you say I am closed minded because I'm not considering alternative viewpoints? Ok, first of that was a really disrespectful for you to say.

I agree, which is why I was upset to see you say that to me. I was only pointing out that the disrespectful comments you levied at me were applicable to you also. I was careful to not escalate to other insults beyond what you had started.

 

[Miyz]

I agree, which is why I was upset to see you say that to me. I was only pointing out that the disrespectful comments you levied at me were applicable to you also. I was careful to not escalate to other insults beyond what you had started.

I apologize for anything I said to offend you know that I did not intend that at all.
Now let's start solving this puzzel one more huh? How about that?

Because we need to look at countless point before we conclude this thread.

 

[cabraham]

My take on the original op question is that a magnetic field can exert torque but do no work.

If a put a weight on a frictionless horizontal bar I can increase or decrease at will the torque or moment around each support without input of any effort/work/joules. So torque and work done are different concepts.

If I hold 2 equal poles of a permanent magnet close together and let go the 2 magnets will push each other away and therefore it looks like the magnetic fields are doing work. However in the process of flying apart there are always electrical fields involved, since there are traveling magnetic fields. The Poynting vector shows the flow of energy.

The question of HOW energy reaches the rotor of a motor is an entirely different problem.

From: http://arxiv.org/abs/1207.2173
The above reference is not the only one I have come across in the past. As to how reliable the above source is I really don’t know.

Just to make again another point: LI2/2 does exist but has nothing to do with the output power of a motor. Asked for references are in short supply.

"Electric Machinery", Fitzgerald, Kingsley Jr., Umans; 6th edition, c. 2003, ISBN 0-07-112193-5; ch.3, sec 1, page 116: "Thus in a motor, the stator magnetic field rotates ahead of that of the rotor, pulling on it and performing work. The opposite is true for a generator, in which the rotor does work on the stator."

Can you be more specific? Which system in particular are you talking about here? I have seen multiple systems mentioned in this thread, so I am unsure which one you are referring to in this instance.

But magnetic fields don't do work on dipoles. The net force on a (ideal) dipole in an external magnetic field is given by \mathbf{F} = \mathbf{ \nabla } ( \mathbf{m} \cdot \mathbf{B} ), and this net force certainly does work (and depends on the magnetic field), but it is not truly a magnetic force.

1st underline: Yep.
2nd underline: ?! "A magnetic moment dot product with B is not truly a magnetic force"? I cannot debate a person who refuses to observe logic. If you can deny the work done by magnetic fields with a statement like that, one can deny anything. You think you can just make a blanket declaration that this magnetic entity doing the work is not truly a magnetic force, and it sticks because you say so.

Science is about searching, verifying, trial & error, disappointment, revelation, etc. It's been well acknowledged that the B force does the torque. But torque times angular displacement in radians is WORK. But if the force doing the work is not "truly magnetic", then what is it "truly"? Just asking, nothing personal, I'm not questioning your ability, just perplexed. BR.

Claude

 

[vanhees71]

I repeat myself: Please have a look at #255 in this thread on precisely this point!

 

[Miyz]

Ok, things haven't changed. Sadly...

 

[cabraham]

Ok, things haven't changed. Sadly...

Oh well, no harm done. Maybe we've reached the point where the facts are on the table, but we just don't agree on how they apply. Anyway, I've learned much from this discussion. To summarize I will say this.

B force differs from E force in that while E force can change a charged particles momentum as well as KE, B force can only change its momentum, not its KE. In the case of e- in the current loop, E provides momentum & KE change. E energizes these loop electrons. E, however does not exert force on the loop.

B provides momentum change to the e- in the loop, but cannot change their KE. B exerts force on the e- as well as the loop itself. Depending on the rotor poles position wrt the stator poles, the moment of said force is non-zero except when poles are directly aligned. This moment, F X R, is the torque on the loop.

This torque integrated with the angular displacement is the work done spinning the rotor. But did B force do the work? Well, can B force do work on the loop e-? No, it cannot. Can B force do work on the stationary lattice protons? No, it cannot. Can B force do work on the neutrons? No it cannot. So how the heck can B force do work on the loop?

Can internal E & SN forces do work spinning the loop? No they cannot. So what does the work? All I can surmise from all this is that B cannot do work on anything when acting alone. But when B interacts with E, & with SN forces, the combination results in torque, spin & work. To those who insist E is doing the work, any diagram detailing fields & forces affirms that B force is the torque producing agent.

But w/o other forces forget it. One thing we can all hopefully agree on is that this motor, a 19th century invention, is amazing. Not trivial to say the least. Anyway, that is how I see it. One more comment needs to be made.

Someone, I forget who, mentioned that LI²/2 exists but plays no role in energy transfer. This is false, as it is obvious that energy can couple from rotor to stator only via the air gap & magnetic flux. If it is E field, not B that links this energy, why are stator and/or rotor windings wrapped around magnetic steel laminations? The direction of said fields affirms that B is the linking field, not E. But B depends on current I (or current density J), & E is important in producing & maintaining this I/J.

E is indispensable, there is no denying that. Nobody ever said otherwise. BR.

Claude

 

[Per Oni]

"Electric Machinery", Fitzgerald, Kingsley Jr., Umans; 6th edition, c. 2003, ISBN 0-07-112193-5; ch.3, sec 1, page 116: "Thus in a motor, the stator magnetic field rotates ahead of that of the rotor, pulling on it and performing work. The opposite is true for a generator, in which the rotor does work on the stator."
Claude

I have not got a copy of that book but the text you quoted is in general about 3-phase squirrel cage motors. The stator magnetic field is rotating and therefore changing in time and space. Per Maxwell changing magnetic fields generate electrical fields hence the combination of fields will do work on the rotor. That has nothing to do with LI2/2. You really should do the power calculations involved for the simple 2-pole or 1-pole to see my point.

 

[cabraham]

I have not got a copy of that book but the text you quoted is in general about 3-phase squirrel cage motors. The stator magnetic field is rotating and therefore changing in time and space. Per Maxwell changing magnetic fields generate electrical fields hence the combination of fields will do work on the rotor. That has nothing to do with LI2/2. You really should do the power calculations involved for the simple 2-pole or 1-pole to see my point.

It also applies to synchronous motors as well. One field chases the other. The leading field does the work. So do you acknowledge that the magnetic field of 1 winding can do work on another, yes or no? As far as your statement "the combination of fields will do work on the rotor", I have acknowledged that w/o E, B cannot do work, since E does work maintaining loop current, as well as bonding e- to lattice.

But the force which immediately produces torque on the loop, angular motion, & their product which is work, is indeed the B force.

Finally you conclude with "You really should do the power calculations involved for the simple 2-pole or 1-pole to see my point". I should? I've drawn pics, given references, & you have not. Would you mind if I asked you to do these power calculations? Just to make a point, what would power calculations prove? They demonstrate that power inputted electrically gets converted into mechanical, plus thermal (losses), plus stored field energy (reactive power in VArs). We are examining the force which does work on the rotor. If power calculations answer that question, please grace us by producing such calcs. Thanks in advance for your help & input. BR.

Claude

 

[Miyz]

Oh well, no harm done. Maybe we've reached the point where the facts are on the table, but we just don't agree on how they apply. Anyway, I've learned much from this discussion. To summarize I will say this.

B force differs from E force in that while E force can change a charged particles momentum as well as KE, B force can only change its momentum, not its KE. In the case of e- in the current loop, E provides momentum & KE change. E energizes these loop electrons. E, however does not exert force on the loop.

B provides momentum change to the e- in the loop, but cannot change their KE. B exerts force on the e- as well as the loop itself. Depending on the rotor poles position wrt the stator poles, the moment of said force is non-zero except when poles are directly aligned. This moment, F X R, is the torque on the loop.

This torque integrated with the angular displacement is the work done spinning the rotor. But did B force do the work? Well, can B force do work on the loop e-? No, it cannot. Can B force do work on the stationary lattice protons? No, it cannot. Can B force do work on the neutrons? No it cannot. So how the heck can B force do work on the loop?

Can internal E & SN forces do work spinning the loop? No they cannot. So what does the work? All I can surmise from all this is that B cannot do work on anything when acting alone. But when B interacts with E, & with SN forces, the combination results in torque, spin & work. To those who insist E is doing the work, any diagram detailing fields & forces affirms that B force is the torque producing agent.

But w/o other forces forget it. One thing we can all hopefully agree on is that this motor, a 19th century invention, is amazing. Not trivial to say the least. Anyway, that is how I see it. One more comment needs to be made.

Someone, I forget who, mentioned that LI²/2 exists but plays no role in energy transfer. This is false, as it is obvious that energy can couple from rotor to stator only via the air gap & magnetic flux. If it is E field, not B that links this energy, why are stator and/or rotor windings wrapped around magnetic steel laminations? The direction of said fields affirms that B is the linking field, not E. But B depends on current I (or current density J), & E is important in producing & maintaining this I/J.

E is indispensable, there is no denying that. Nobody ever said otherwise. BR.

Claude

I'm with you on that point just wanted to make sure by stating some definitions. However, many physicists are against the idea magnetic fields/forces do work in ANY CIRCUMSTANCE and I find that... Weird.

 

[Per Oni]

So do you acknowledge that the magnetic field of 1 winding can do work on another, yes or no?

Is this a static magnetic field? Is the other winding carrying a current?

We are examining the force which does work on the rotor.

Since when does a force do work?

I am examining your statement of 232:

Remember there is mutual inductance between rotor & stator. Each receive energy from the other. Although I is constant, LI2/2 still changes, since inductance changes.

I still would like to know how this is relevant to the DC motor as described in the OP.

 

[Dale]

Because we need to look at countless point before we conclude this thread.

Well, I think that I am ready to post some final conclusions on my part:

1) a motor 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 motors.

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

6) the B field does provide torque in a motor, but work is a transfer of energy, and it does not transfer energy directly, only through E and j

 

[Dale]

I will search for the text & post.

Did you ever find the textbook? Do you now agree that E.j is zero outside the wire?

 

[cabraham]

Did you ever find the textbook? Do you now agree that E.j is zero outside the wire?

I found the text & posted. B does work, period. Today at 9:33 a.m.

Claude

 

[cabraham]

Is this a static magnetic field? Is the other winding carrying a current?


Since when does a force do work?

I am examining your statement of 232:
I still would like to know how this is relevant to the DC motor as described in the OP.

Static magnetic field would be in the rotor in a ac synchronous machine, or in a permag dc machine. Of course when the rotor spins the mag field has spatial variation. In an ac induction motor, rotor mag field varies spatially & with time. Yes the other winding is carrying a current.

W = integral F.dl. But I am caught off guard when you ask "Since when does a force do work?" If, as you seem to indicate, that a force does not do work, then E force cannot do work either. So what does the work? Ultimately it has to be the input power supply, but the energy is coupled through E & B fields.

As far as relevance of inductance variation w/ rotor position, I was told that if motor was driven by constant current source that energy would be unchanging based on W = LI²/2. I pointed out that although I is constant, L varies as rotor turns due to variation in flux path, i.e. differing air path for flux. I did not offer this as an answer to the OP question. Along the line I was asked a hypothetical question about driving a motor with a current source instead of a voltage source & I gave that as the answer. It is off the path of the thread, but it was asked so I answered.

Anyway, maybe I now see the cause of the dispute. If a "force cannot do work" then we have varying concepts of work. If a B force produces torque on the rotor, resulting in spin, what did the work, the B force, or the source of the B force? The source of B is the inductance & stored energy. Its source is E field & current. Its source is input power supply. Its source is the power plant generator. Its source is coal burning, or fission.

So you are saying that if I raise a ball 3 feet off the ground I did work on it. The force I exerted did not do the work, rather I did the work? Is that your view? If I release the ball, it acquires KE from its PE. What did the work, the gravitational force, or the earth? I think I see the dispute. You seem to indicate that a force cannot do work, but the source of said force does the work. Please clarify. Thanks.

Claude

 

[Dale]

I found the text & posted. B does work, period. Today at 9:33 a.m.

But did the text say that E.j was nonzero outside the wire?

 

[Dadface]

In UK schools the motor effect is often introduced and demonstrated by having a straight conductor which is resting on parallel horizontal conducting rails in a uniform and vertical (approximately) B field.A current is passed through the conductor by means of the rails and the conductor moves along the rails as in accordance with the left hand rule.(Those familiar with this know that it's fiddly to set up and that sparking between the conductor and rails is one of the problems encountered)

When this is dealt with mathematically we can write that if the conductor moves in the direction of the force by a distance dx in time dt then the mechanical output power is given by:

Power=work done/time taken from which:

Power=BIldx/dt=BIlv (v=velocity)...(1.)

With this example it is easy to get an expression for the back(counter) emf generated.From Faraday's law the back emf is given by

Eb=dphi/dt=BdA/dt (dA/dt= area sliced out per second=lv)from this we can write

Eb=Blv substituting thids into equation (1.) we can write:

P=EbI

In other words the mechanical power output is given by the product of the back emf and the current.I mentioned this in my first post on this thread and have repeated it a couple of times since.I think it further clarifies the part that magnetism plays in the motor effect.
It must be remembered that the magnetic field is not just B,the field due to the magnet(and which features in Faraday's law).There is a resultant field(which earlier I referred to as the catapult field) which is due to B and the field created as a result of the current.The resultant field moves as the conductor moves(or coil turns)

Anyway I have been googling and have found several examples where it has been claimed that magnetic forces can do work.One deals with the system I described above,but more rigorously.
Try googling "Introduction to Magnetic Fields-MIT"

Go to 8.9.1 Rolling Rod and read on:

It states:

"Using the coordinate system on the right,the magnetic force acting on the rod is given by"

after presenting the BIl type equation it states

"The total work done by the rod as it moves through the region is"

It then continues with a mathematical analysis.

I should stress that the rolling rod example is just a simpler version of the rotating coil example,the analysis we can apply to both being similar.

 

[cabraham]

But did the text say that E.j was nonzero outside the wire?

Did not say either way, I'll search some more.

Claude

 

[cabraham]

Well, I think that I am ready to post some final conclusions on my part:

1) a motor 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 motors.

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

6) the B field does provide torque in a motor, but work is a transfer of energy, and it does not transfer energy directly, only through E and j

1) Agreed.

2) I would say yes, but I still wish to resolve this inside/outside the wire issue.

3) Again, what is meant by "directly". I've already stated that w/o E & SN forces, B cannot move the rotor all by itself. If that is what you imply by "direct", then I would agree. Also, the energy in the B field comes from E & J. B is a link in the chain of energy transfer from input power source to the rotor.

4) Again, when you say B does work "indirectly", I don't want to assume I know what you're thinking, so I will ask you to define "indirectly". Otherwise I would agree.

5) Dead wrong. Tethering is relevant because it accounts for the motion of lattice protons & neutrons when subjected to B force. Tether forces do no work, I agree with that, but they make rotor motion possible. W/o tether forces, namely E & SN, the e- would fly off the wire. B does no work on e- so no work is done at all by B.

6) I agree that B provides torque & that work is a transfer of energy. Again, "directly" is a term you use, but I would like you to clarify it. You say "it does not transfer energy directly, but only through E & J." I thought it went the other way. E & J transfer energy to the rotor through B. B acts directly on the rotor producing torque & spinning the rotor. It transfers its energy as well. The energy comes from E.J no doubt, but B is 1 step closer than E.J when it comes to transferring energy to the rotor.

Anyway, I only ask for clarification on your use of the terms "directly & indirectly". Most of what has been stated seems to be agreeable, but for the work thing & its definition. BR.

Claude

 

[gabbagabbahey]

2nd underline: ?! "A magnetic moment dot product with B is not truly a magnetic force"? I cannot debate a person who refuses to observe logic. If you can deny the work done by magnetic fields with a statement like that, one can deny anything. You think you can just make a blanket declaration that this magnetic entity doing the work is not truly a magnetic force, and it sticks because you say so.

Is the force \nabla\left[\mathbf{m} \cdot \left( \frac{\mathbf{v}}{c^2} \times \mathbf{E} \right) \right] an electric force because it depends on \textbf{E}?

But if the force doing the work is not "truly magnetic", then what is it "truly"?

Answer: Not a magnetic force. The magnetic force on a charge/current/magnetic dipole is always perpendicular to the motion, so whatever force is doing the work, it is not magnetic plain and simple.

To quote Griffiths:

It may help to consider a mechanical analogy. Imagine you're pushing a trunk up a frictionless ramp, by pushing on it horizontally with a mop (Fig. 5.12). The normal force (\mathbf{N}) does no work because it is perpendicular to the displacement. But it does have a vertical component (which in fact is what lifts the trunk) and a (backward) horizontal component (which you have to overcome by pushing on the mop). Who is doing the work here? You are, obviously - and yet your force (which is purely horizontal) is not (at least, not directly) what lifts the box. The normal force plays the same passive (but crucial) role as the magnetic force in Ex 5.3: while doing no work itself, it redirects the efforts of the active agent (you, or the battery, as the case may), from horizontal to vertical.

In the case of a current loop, the electric force which maintains the current in the loop directly does the work (whether created by a battery, or tiny ants throwing electrons down the wire, it doesn't matter). The magnetic field simply redirects the electric force (which points parallel to the wire) to produce a net force (which points perpendicular to the wire) which does work. This net force obviously depends on the Magnetic field applied, but is not a magnetic force (it is the net result of a magnetic force and an electric force)

A magnetic dipole is ,classically, just a limiting case of a current loop, so the same argument applies. The only difference being that we can no longer say that a battery is maintaining the current/magnetic moment, but something obviously must be, and that whatever that something is, it is the agent that does work on the dipole.

 

[Dadface]

Answer: Not a magnetic force. The magnetic force on a charge/current/magnetic dipole is always perpendicular to the motion, so whatever force is doing the work, it is not magnetic plain and simple.

The motion of what?The motion of the wire itself is in the direction of the force.The circular motion applies to free charges not necessarily those confined within the wire.
What is so confusing is the mixed messages coming in from different sources.Griffiths,for example,seems to be saying one thing and other sources such as the MIT publication I referred to in my previous post seem to be saying something else.I quote again from MIT

..."the magnetic force acting on the rod is given by" Fs=IL*B...

..."The total work done by the magnetic force on the rod as it moves through the region is" etc

 

[Dale]

3) Again, what is meant by "directly". I've already stated that w/o E & SN forces, B cannot move the rotor all by itself. If that is what you imply by "direct", then I would agree. Also, the energy in the B field comes from E & J. B is a link in the chain of energy transfer from input power source to the rotor.

4) Again, when you say B does work "indirectly", I don't want to assume I know what you're thinking, so I will ask you to define "indirectly". Otherwise I would agree.

By directly or indirectly I simply mean that E.j accounts for all of the work done. B does not do any additional work beyond what is already accounted for by E and j, but both E and j are functions if B, so B can be said to do work due to its effect on E and j. I.e. P=E.j=E(ρ,j,B).j(E,B)

5) Dead wrong. Tethering is relevant because it accounts for the motion of lattice protons & neutrons when subjected to B force. Tether forces do no work, I agree with that, but they make rotor motion possible. W/o tether forces, namely E & SN, the e- would fly off the wire. B does no work on e- so no work is done at all by B.

Despite your emphatic use of language, I think that we agree. In this case the internal forces serve to keep the wire intact, but do nothing to transfer energy in or out. They cannot do work so are irrelevant to the questions of how much work is done and which forces do the work. They are relevant to other questions like whether or not the rotor falls apart.

For example, consider a system consisting of two blocks initially at rest and an internal force consisting of a massless elastic band tethering the two blocks. The system is acted on by an external force which does a certain amount of work, W, on one of the blocks. In the limit of a very strong band the blocks stay together, their velocity is equal and the KE of the system is W. In the limit of a very weak band, one block stays in place, and the other block is accelerated to a higher velocity than in the previous example but the KE of the system is still W. Thus, the work done on the system is completely independent of the internal force. The tethering force is irrelevant to the work done on the system, it only determines the configuration of the system, not its energy.

 

[EEH4]

Darwin 123 should have his own science show.Very talented person.

 

[gabbagabbahey]

The motion of what?The motion of the wire itself is in the direction of the force.The circular motion applies to free charges not necessarily those confined within the wire.

The motion of each point charge, and/or as currents are not really made up of individual charges moving all the way around a circuit, the direction of each infinitesimal bit of current. The microscopic details of what goes on inside the wire are very complicated (and really a quantum phenomenon) but the current is confined to the wire, so where the infinitesimal bits of current go, the wire goes too.

What is so confusing is the mixed messages coming in from different sources.Griffiths,for example,seems to be saying one thing and other sources such as the MIT publication I referred to in my previous post seem to be saying something else.I quote again from MIT

..."the magnetic force acting on the rod is given by" Fs=IL*B...

..."The total work done by the magnetic force on the rod as it moves through the region is" etc

Indeed, there is also the post you linked to by Goku on this forum, which treats the force on a magnetic dipole as being fundamentally different than the lorentz force on a current, by claiming that the magnetic field/force does work on it.

You will of course have to make up your own mind on which sources to trust, but in favor of David J. Griffiths' Introduction to Electrodynamics (which claims explicitly that magnetic forces do no work) and J.D. Jackson's Classical Electrodynamics (which makes the equivalent claim that magnetic forces do not change an entity's kinetic energy), I can say that they are probably the two most used textbooks in North American universities for undergraduate (and in the case of Jackson's book, some graduate) Electrodynamics courses. In addition, a quick Google search of the two authors of these texts reveals that they have both published multiple peer-reviewed articles on Electrodynamics, which, along with their texbooks, have been cited in countless articles. Both have a Ph. D in physics from a prestigous university (Harvard for Griffiths, MIT for Jackson). Both have received prestigous awards from their peers. Jackson's cv ( http://www-theory.lbl.gov/jdj/CV2006_extended.pdf ) in particular is quite impressive.