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

[Dale]

I'll find the textbook. But here is a flaw in your reasoning.

In this case it isn't reasoning, that is just what the equations say. E.j is 0 anywhere E or j is 0. That is simply what E.j means. If you have a problem with it you need to take it up with Maxwell and Lorentz, they aren't my equations.

Remember that the energy in a motor that transfers from stator to rotor is chiefly in the air gap. If the energy was all confined to the wire, how does it transfer? The stator L has a flux which is in the stator iron core, links to the rotor iron core via the air gap. This flux is energy, LI²/2. This energy does not appear in the E.J product inside the wire.

If you believe that the energy can transfer from a point in the air gap directly to the wire without passing through the intervening space then do you also believe that a field a light year away can transfer energy to the wire now?

I will search for the text & post.

Please do so.

 

[andrien]

But E & B are from interacting loops. Maybe E¹, E², B¹, & B² are more appropriate.

E.J is no problem, & is consistent w/ what I've presented. You acknowledge B as producing torque, yet you deny its work contribution. For the 4thtime please draw a sketch showing the fields doing work. All work is ultimately done by input power source. We have 2 loops. Each loop has E & B. It is the interaction that makes motors run. An e/m have not ionizing an H atom is too simplistic dealing w/ motors. Please draw us a pic. Thanks.

Claude

you did not really answer that.but apart from that I am saying it once again that magnetic field can not do work on electric charges because of lorentz force law so F.v is zero.but on a magnetic dipole the force is ∇(m.B) and it is not perpendicular to any velocity so it can do work.electric charges no work but magnetic charges there can be.

 

[gabbagabbahey]

but on a magnetic dipole the force is ∇(m.B) and it is not perpendicular to any velocity so it can do work.

Classically, magnetic dipoles are treated on the same footing as any other current distribution.

Specifically, the force on an ideal dipole $\mathbf{m}$ is derived (typically as an example in a textbook or as an exercise for the student - c.f. Problem 6.4 from Griffith's Introduction to Electrodynamics 3rd edition ) from the Lorentz force law by treating the dipole as a limiting case of a current loop (with current $I$ running through it), as the vector area $\mathbf{a} \equiv \oint \mathbf{ \hat{n} } da$ scribed/enclosed (subtended?) by the loop is shrunk to zero in such a way that the product $\mathbf{m}=I \mathbf{a}$ remains unchanged. Thus $\mathbf{F}=\nabla( \mathbf{m} \cdot \mathbf{B} )$ is regarded as a composite force law in the sense that it can be derived from the Lorentz Force Law and Maxwell's equations, like every other EM force law, and not a pure magnetic force. The work on a dipole in an external magnetic field must once again come directly from the electric field that maintains the current, which is produced by an unknown (classically) energy source (Of course, permanent dipoles like those associated with an electron's spin are really a quantum phenomenon, but classically they are treated as described)

electric charges no work but magnetic charges there can be.

True Magnetic charges (monopoles) have never been observed, despite the repeated efforts of many experimentalists, so whether or not a magnetic field does work on them is purely speculation.

 

[andrien]

of course,but the result is of more general character then for any current loop.there is a mechanical energy associated with it amount to -m.B.the potential energy of the dipole from where the force formula comes.

 

[vanhees71]

you did not really answer that.but apart from that I am saying it once again that magnetic field can not do work on electric charges because of lorentz force law so F.v is zero.but on a magnetic dipole the force is ∇(m.B) and it is not perpendicular to any velocity so it can do work.electric charges no work but magnetic charges there can be.

Somewhere in this huge thread, I've shown that this is no exception to Maxwell's theory. The power density is again given by
$$\vec{E} \cdot \vec{j}=\vec{E} \cdot (c \vec{\nabla} \times \vec{M}).$$

In fact you can derive the coupled set of equations of motion for fields and matter by using the fundamental conservation laws from space-time symmetry, i.e., for total energy, momentum, and angular momentum.

 

[Dadface]

Force is a field.

( FROM DADFACE-No,work done is force times distance not field times distance.Perhaps I am being nit-picky here.)

Yes, this is due to the fact that there are other internal forces at play inside the wire. In this case, the only forces that directly contribute to the work on the wire are the internal electric forces that maintain the current (these of course are created by a battery or other power source providing a potential difference along the wire. cf. Example 5.3 from Griffith's Introduction to Electrodynamics 3rd edition)

(FROM DADFACE_Extending this, the B field of the magnets is one of the factors that determines how much input power there is and how much is converted to mechanical power and how much is converted to electrical heating power losses.As I have stated before the mechanical power is given by
EbI {Eb=back(counter) emf,I= current})

The general idea is that since all of Classical Electrodynamics (including all composite force laws such as F=IBL) can be derived from The Lorentz Force Law, Maxwell's equations and some assumptions about the composition of certain macroscopic bodies (like a bar magnet, etc), one can treat $\mathbf{F}_{ \text{mag} } = q \mathbf{v} \times \mathbf{B}$ (the magnetic part of the Lorentz force law) as being the fundamental magnetic force and $\mathbf{F}_{ \text{e} } = q \mathbf{E}$ as being the fundamental electric force. All other EM forces are treated as composites. So, when one says that magnetic forces do no work, one is unambiguosly referring to $\mathbf{F}_{ \text{mag, net } } = \int dq \mathbf{v} \times \mathbf{B}$ as the magnetic force on an object (where the integral is over the object's volume/charge distribution)

THE NOTES ABOVE IN BRACKETS ARE FROM MYSELF(Dadface).WHAT IS A BETTER WAY TO HIGHLIGHT THEM? THANKS.


The section starting with "the general idea" is the point that interests me most.It might be considered as trivial because it is to do with definitions.It all boils down to the question:

Is the force F as given by F=BIL a magnetic force?

I find your notes above confusing.Reference to a "composite force" seems to suggest that it is not a magnetic force,so what,if anything,should it be described as? In a previous post I suggested(if I remember correctly)that it could be described as an electromagnetic force.

The notes go on to express an integral in terms of Fmag.net and then describe this as as "the magnetic force on an object".So is it a magnetic force or not?(A quick google lead me to the hyperphysics textbook which described it as a magnetic force)

If the wire contained just one moving electron BIl would equal Bqv, so if BIl is not regarded as the fundamental magnetic force it could be argued that nor is Bqv.For N electrons F=NBqv=BIl.The electrons may be in a wire which restricts their movements but the force on each one is still Bqv.

 

[Dadface]

Somewhere in this huge thread, I've shown that this is no exception to Maxwell's theory. The power density is again given by
$$\vec{E} \cdot \vec{j}=\vec{E} \cdot (c \vec{\nabla} \times \vec{M}).$$

In fact you can derive the coupled set of equations of motion for fields and matter by using the fundamental conservation laws from space-time symmetry, i.e., for total energy, momentum, and angular momentum.

Hi vanhees.I would be interested to hear your answers to the following:

1.Can the force BIl do work?
2.Would you describe BIL as a magnetic force or otherwise?

Thanks if you can find the time to answer.

 

[Per Oni]

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

Earlier studies of the Poynting vector and the rate of flow of energy considered only idealized geometries in which the Poynting vector was confined to the space within the circuit. But in more realistic cases the Poynting vector is nonzero outside as well as inside the circuit. An expression is obtained for the Poynting vector in terms of products of integrals, which are evaluated numerically to show the energy flow.

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.

 

[Miyz]

The inner product between two vector fields is a scalar field; its value, just like the two vector fields, depends on position. If either of the two vector fields is zero at a specific location, their inner product will also be zero there.

Maxwell's equations, and the Lorentz Force Law both deal with vector fields. If you don't understand how to take the inner product of two vector fields, you don't fully understand Maxwell's equations.

Now, as for whether Magnetic field/forces do work on any classical system, the answer is fundamentally no. This assumes only that you take Maxwell's equations and the Lorentz force law as the basis for classical electrodynamics (which 99.9% of the current physics community likely does).

In many cases, the work done on an object depends on the value of the magnetic field applied to it, but this does not mean that the magnetic field is directly doing the work (and it isn't!).

Then if magnetic field/force do no work. Then what does it do in this system?
We all agreed that the main source of doing work is the current throughout the wire. Work is done due to multiple forces interacting with each other. Not only by the magnetic field/force.

By saying the magnetic field/force do no work that makes no sense. Although its a known fact that magnetic field's do work on dipoles.
A loop is technically is a dipole.

Im not familiar fully with the equations. However, in the process of studying them.
Only equations I do know: F= q(v x B) , F = IL x B
Both applied on different states.

 

[vanhees71]

Miyz, it is really a bit disturbing that you obviously do not read my answers. I have clearly demonstrated by using Maxwell's equations that not the magnetic field is doing work on a magnetic dipole but the induced electric field. If you don't agree with that simple calculation, tell me where you think I (or all physicists since Maxwell ;-))) made a mistake!

 

[Miyz]

Miyz, it is really a bit disturbing that you obviously do not read my answers. I have clearly demonstrated by using Maxwell's equations that not the magnetic field is doing work on a magnetic dipole but the induced electric field. If you don't agree with that simple calculation, tell me where you think I (or all physicists since Maxwell ;-))) made a mistake!

I do apologize.

Maxwell's equations to me personally is not fully understood. So I can't judge or say anything till I do understand them all.
Be back soon. Till then.

 

[gabbagabbahey]

THE NOTES ABOVE IN BRACKETS ARE FROM MYSELF(Dadface).WHAT IS A BETTER WAY TO HIGHLIGHT THEM? THANKS.

I think dividing the quote up into sections that you wish to respond to and responding outside the quote is the best way. Adding text to a quote is usually considered a no-no.

FROM DADFACE-No,work done is force times distance not field times distance.Perhaps I am being nit-picky here.

Force is still a vector field, for example the force on a point charge $q$ placed at location $\mathbf{r}$ (relative to some origin) in an external electric field $\mathbf{E} ( \mathbf{r} )$ is $\mathbf{F} ( \mathbf{r} ) = q \mathbf{E} ( \mathbf{r} )$. Its value depends on the position of the charge, since the electric field, in general, varies in space.

FROM DADFACE_Extending this, the B field of the magnets is one of the factors that determines how much input power there is and how much is converted to mechanical power and how much is converted to electrical heating power losses.As I have stated before the mechanical power is given by
EbI {Eb=back(counter) emf,I= current}

Is this just a general note, or are you trying to make some point about magnetic force and work here?

The section starting with "the general idea" is the point that interests me most.It might be considered as trivial because it is to do with definitions.It all boils down to the question:

Is the force F as given by F=BIL a magnetic force?

Certainly, the force on the current $I$ through a wire in an external magnetic field $\mathbf{B}$

$$\mathbf{F}_{ \text{mag} } = \int dq\mathbf{v} \times \mathbf{B} = \int I d \mathbf{l} \times \mathbf{B}$$

is a magnetic force, but it is incapable of doing work on the charges that make up the current (and hence on the wire) since it is everywhere perpendicular to the current (motion of the charges).

There are situations, where it appears as though this force is doing work (again, c.f. Example 5.3 from Griffith's Introduction to Electrodynamics 3rd Ed.) but these can all be shown to be due to whatever other agent which maintains the current.

 

[gabbagabbahey]

Then if magnetic field/force do no work. Then what does it do in this system?

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.

By saying the magnetic field/force do no work that makes no sense. Although its a known fact that magnetic field's do work on dipoles.
A loop is technically is a dipole.

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.

Im not familiar fully with the equations. However, in the process of studying them.
Only equations I do know: F= q(v x B) , F = IL x B
Both applied on different states.

Then this discussion is probably very difficult to follow for you, but will likely make more sense once you've continued your studies of Electrodynamics.

 

[Miyz]

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.

On a loop.

I believe you use this law F = qv x B to argue that magnetic field/force do no work at all? Since it can't do work on a charged particle and charged particles flowing through a conductor makes no difference?

So you agree that magnetic fields do no work in ANY CASE directly or indirectly?

Look at this.

 

[vanhees71]

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.

 

[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