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

[Per Oni]

Claude et all, it’s been a long day at work. If I find some interesting relevant material on the net in the weekend I will post. God bless you all.

 

[cabraham]

Claude et all, it’s been a long day at work. If I find some interesting relevant material on the net in the weekend I will post. God bless you all.

Likewise.

Claude

 

[cabraham]

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)

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.

Well I agree with your tethering example re the blocks & elastic band. But I stated emphatically that the E & SN forces which are internal, simply bond the atoms/e-/p+/n0 w/o doing any work. But the transfer of energy is still the sticking point.

We seem to have overwhelming consensus that the B field exerts torque on rotor. Regarding energy transfer you seem to be conveying that the rotor gets its energy from E.J. I maintain that E.J does not directly transfer energy to rotor, which I will call Iω²/2. I've been told by more than one person that B cannot do work because it points in the wrong direction, normal to motion instead of along the motion (wrt free charges), which I was in agreement with.

Please examine my sketch & you will see that B force acts on the rotor along the motion, whereas E & J do not. E & J do no work, but they transfer energy to B as well as LI²/2. Again refer to my sketches, 4 pages worth. E is not acting along the rotor motion path. Nor is J. But B acts in a manner such that it has 1 component normal to rotor motion, doing no work at all, & another component along rotor motion, doing work. Not to beat a dead horse, but critics tell me that a force normal to the motion cannot do work, hence when B acts on free charges, no work is done. Fair enough.

Then when a rotor is examined, B acts along the motion, yet E & J are normal or skewed in a different plane to the motion. Yet the same critics insist E is doing the work. The fact that E points in the wrong direction does not seem to bother them. So I say this. In the case of the rotor, E acts normal, B acts along the motion. If "along" does work, & "normal" does not do work, well then the issue is settled.

Of course when the poles, rotor & stator, are directly aligned, the normal force component is max, & this does no work, while the *along the motion* component is zero, hence no work is done. This is for the poles aligned. At the position where the poles are 90 degrees apart, we have the component of B along the motion at maximum value, doing work, with the normal B component at minimum value, virtually zero.

I will draw another sketch & post it later. The fact that E.J transfers energy which eventually transfers to the rotor as Iω²/2 is not being challenged. What I'm saying is that the E.J energy first transfers to B²/2mu, then transfers to Iω²/2. We seem to agree on all but that. Anyway, it deserved to be mentioned, & I thank all in this thread for a most interesting discussion. More to come later. I'm still at work. BR.

Claude

 

[chingel]

The B force doesn't just act on the rotor, it acts on the electrons moving inside it. No electrons moving, no force. The magnetic field doesn't simply apply torque to the rotor, it only affects the moving electrons directly. The electrons even accumulate on one side of the wire when they are moving in an magnetic field and an electric field is created across the wire, not just along the wire. Read about the hall effect: http://en.wikipedia.org/wiki/Hall_effect

You are not considering the electric forces that are present once the electrons paths are changed and they start "colliding" with and accumulating at the sides of the wire.

 

[Dale]

Well I agree with your tethering example re the blocks & elastic band. But I stated emphatically that the E & SN forces which are internal, simply bond the atoms/e-/p+/n0 w/o doing any work. But the transfer of energy is still the sticking point.

Work IS the transfer of energy, by definition. Internal forces don't do work and so they don't transfer energy, it is two ways of saying the same thing.

E & J do no work

Which of Maxwells equations do you think is violated by a motor? E.j does work. It is a general result derived from the EM laws, if you disagree then please specify which EM law you disagree with.

You are so focused on the details of your drawings that you are forgetting the laws of physics that govern the motor. The reason that these general derivations are done is so that you can apply them to all situations, regardless of the specifics.

Btw, did you ever find a reference regarding E.j outside of the wire?

 

[cabraham]

Work IS the transfer of energy, by definition. Internal forces don't do work and so they don't transfer energy, it is two ways of saying the same thing.

Which of Maxwells equations do you think is violated by a motor?

You are so focused on the details of your drawings that you are forgetting the laws of physics that govern the motor. The reason that these general derivations are done is so that you can apply them to all situations, regardless of the specifics. E.j does work. It is a general result derived from the EM laws, if you disagree then please specify which EM law you disagree with.

Btw, did you ever find a reference regarding E.j outside of the wire?

Motors violate none of Maxwell's equations. E.J indeed does work. I agree with that. But where you & I disagree is as follows. E.J does work by energizing the inductance & associated magnetic field, per B²/2mu. Then the energy in B²/2mu is transferred into rotor mechanical energy per Iω²/2.

Maxwell is upheld. E.J is work, but I can't say that E.J "does" work on the rotor directly, rather it energizes inductance, which then moves the rotor. I don't think what I'm saying goes against Maxwell, or conservation of energy. My scenario has 1 step in between E.J & Iω²/2. That step is B²/2mu. Cheers.

Claude

 

[Dale]

But where you & I disagree is as follows. E.J does work by energizing the inductance & associated magnetic field, per B²/2mu. Then the energy in B²/2mu is transferred into rotor mechanical energy per Iω²/2.

This is not correct. E.j is the transfer of energy between matter and the EM field. E.j can indeed energize the magnetic field, this is what a generator does, but in a motor the energy goes the other way. You cannot both have E.j energizing the EM fields and the fields doing work on the matter at the same time and place.

 

[Dadface]

The B force doesn't just act on the rotor, it acts on the electrons moving inside it. No electrons moving, no force. The magnetic field doesn't simply apply torque to the rotor, it only affects the moving electrons directly. The electrons even accumulate on one side of the wire when they are moving in an magnetic field and an electric field is created across the wire, not just along the wire. Read about the hall effect: http://en.wikipedia.org/wiki/Hall_effect

You are not considering the electric forces that are present once the electrons paths are changed and they start "colliding" with and accumulating at the sides of the wire.

The Hall Effect has been mentioned before including by myself.I feel that many things have been overlooked here and that the Hall effect should feature in a full analysis.

 

[Dadface]

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.



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.

Thanks for your comments.I didn't link the Gokul post but there are plenty of comments I could link just after a search by googling.
I think you agree that BIl is a force but if someone asked you "what type of force is it" what would your answer be? At the moment I would describe it as a "force which is electromagnetic in nature".If asked to choose a single word prefix I would say that it's a magnetic force but with some doubt and the knowledge that I need to look at in some more detail.

 

[Miyz]

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

Could you possibly post a copy of that statement? Because, it supports you and I both.
& I'm waiting for you're sketches!

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).

Music to my ears! Most definitions and most physicists would say that magnetic fields/forces do work on this system but "indirect" but it still does work. Just the same idea as the car being lifted by an electromagnet it also does work on the non-metal items.

Again I'd like to remind you that all of this is a "net total" of all the forces "interacting" with each other. Its like one big system where each relies on the other. We can't say who specifically did the work but each influenced the other.

Miyz,

 

[Dale]

Most definitions and most physicists would say that magnetic fields/forces do work on this system but "indirect" but it still does work.

I wouldn't make this claim unless you have recently conducted a survey of physicists and the results support the claim. It is hard to know what most physicists would say otherwise.

 

[cabraham]

Could you possibly post a copy of that statement? Because, it supports you and I both.
& I'm waiting for you're sketches!



Music to my ears! Most definitions and most physicists would say that magnetic fields/forces do work on this system but "indirect" but it still does work. Just the same idea as the car being lifted by an electromagnet it also does work on the non-metal items.

Again I'd like to remind you that all of this is a "net total" of all the forces "interacting" with each other. Its like one big system where each relies on the other. We can't say who specifically did the work but each influenced the other.

Miyz,

Sorry, I'll get those sketches tonight. Hate to make excuses but this is the season for Olympics. I'll get the sketches posted. Thanks for your valuable input & to others well.

Claude

 

[Miyz]

I wouldn't make this claim unless you have recently conducted a survey of physicists and the results support the claim. It is hard to know what most physicists would say otherwise.

Well, unfortunately they haven't worded it out with a "YES they do work". But eventually it shows. I think its a thing left for us to solve.

Although... I wish I could make this survey for them to give a simple answer : yes/no. simple as that. If they did I guess no one would have asked this question.

But I'd like to add you're conclusion is very well putted Dale.

Sorry, I'll get those sketches tonight. Hate to make excuses but this is the season for Olympics. I'll get the sketches posted. Thanks for your valuable input & to others well.

Claude

Thanks! I've been working on this for a while and I hope you're sketches could help me more...

 

[Miyz]

So the OP question is finally answered and most of us agree each other.

 

[Dale]

I wish I could make this survey for them to give a simple answer : yes/no. simple as that.

I would answer a "simple as that" question: "no". It doesn't directly do any work, it just affects the things that do.

"Everything should be made as simple as possible, but no simpler" - Albert Einstein

 

[Miyz]

I would answer a "simple as that" question: "no". It doesn't directly do any work, it just affects the things that do.

Agreed, but let's add one point: I would answer a "simple as that" question: "no". It doesn't directly do any work, it just affects the things that do. So in a way, it does work but! Indirectly.

I feel its more of a perfected answer now.

 

[vanhees71]

The answer is an unanimous "no". Period!

 

[Per Oni]

Weekend has come and I want to add some concluding remarks.

If a science author writes:……….the force on a car causes it to move uphill against gravitational pull………, does the author imply that this force also provides energy? Or does he merely use everyday language to indicate forces involved, without really feeling if necessary to point out that a force doesn’t provide the energy required. To point out things like that each time, only spoils the flow of thought and does nothing to clarify an explanation.

However, in a discussion such as this it should be pointed out that force and energy are not the same thing. If you believe otherwise then give me a rough calculation of how many Joules there are to the Newton.

 

[cabraham]

Here it is, as promised, albeit 2 days late. Finally got around to it. I learned something interesting, never discussed in the thread, but came out when drawing a picture. Like I say, drawing the pic, examining forces, etc. sure does help. I recommend to all to carefully examine this paper before responding. I hope you like it. Cheers.

Claude

 

[Miyz]

Here it is, as promised, albeit 2 days late. Finally got around to it. I learned something interesting, never discussed in the thread, but came out when drawing a picture. Like I say, drawing the pic, examining forces, etc. sure does help. I recommend to all to carefully examine this paper before responding. I hope you like it. Cheers.

Claude

Thanks Claude! I'll hit you back when I'm done studying this.

 

[harrylin]

[..] 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!
[..]
it's stressed that magnetic fields do not do work on charge and current distributions [..]

That's a very useful clarification, and this topic sounds more and more like a matter of words to me... You talk about a magnetic dipole and "current distributions", while as I read it, this topic is about permanent magnets and electromagnets. Do you claim that when two permanent magnets push each other away, they do perform work on each other, but by means of their induced electric fields? Then, do you claim that the source of this electric field energy is not their magnetic fields? And if so, where was that energy stored before the electric field was induced, if not in their magnetic fields?
Or do you actually agree with Miyz on this point, with your saying that "Of course the origin of the force/torque is the magnetic field. I've never denied this"?

[..] 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, [..]

Exactly, that a magnet can store energy in its B field was your correction to me, and it was gladly taken.

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!

You forgot me and several others but I notice that you did find the same as I did. And yes, amazing discussion!

the E.J energy first transfers to B²/2mu, then transfers to Iω²/2

Claude thanks for the detailed analysis! I just came back from vacation and see that you now uploaded a new one, which I did not yet study. Do you maintain the above conclusion or do you now agree with Dalespam?

 

[vanhees71]

Again, from a classical em.-point of view the magnetization is described by a current distribution either,
\vec{j}_{\text{max}}=c \vec{\nabla} \times \vec{M}.

 

[Miyz]

Hey everyone!

How about joining this thread here!(Talk's about magnets doing work on another magnet)

Glade to open another fantastic discussion over there! Please do join!

 

[harrylin]

Hey everyone!

How about joining this thread here!(Talk's about magnets doing work on another magnet)

Glade to open another fantastic discussion over there! Please do join!

On hindsight, very good threads that refreshed my lost memory.

Surely you realize that a current loop is a magnet; the answers that you get there should be consistent with the answers here. So, as several others already concluded, I now reach the same answer here as there. The answer is YES:

Magnetic force can do work on a current loop by means of magnetic attraction, as a current loop can be pulled in by the inhomogeneous field of a permanent magnet. In detail: if oriented properly then there is a net Lorentz force by the magnet on the current loop towards the magnet.

The misconception that magnetic fields can do no work likely comes from particle physics (magnetic fields cannot do work on freely moving charges because the magnetic force is always perpendicular on the motion).

PS suddenly the picture of your first post is visible again: and yes, also for that case, following the definition of work in Wikipedia,
http://en.wikipedia.org/wiki/Work_(physics):
As the Lorentz force displaces the wire in the direction of the force, it "does work" according to that definition (and how that is possible has been discussed already).

 

[Dale]

The misconception that magnetic fields can do no work likely comes from particle physics (magnetic fields cannot do work on freely moving charges because the magnetic force is always perpendicular on the motion).

You are making the same mistake that I made also, thinking that the rules were different for free charges and more general charge and current distributions. It turns out that for arbitrary charge and current distributions the magnetic field does not do work either.

 

[harrylin]

You are making the same mistake that I made also, thinking that the rules were different for free charges and more general charge and current distributions. It turns out that for arbitrary charge and current distributions the magnetic field does not do work either.

Hi Dalespam, in the parallel thread I provided links to the full explanation which is also in my textbooks (I simply forgot it!). My mistake was that I did not immediately check my classical physics textbooks (did you?). Already the way the Ampere is defined relates to a magnetic force that acts on wires.

 

[Dale]

Hi Dalespam, in the parallel thread I provided links to the full explanation which is also in my textbooks (I simply forgot it!). My mistake was that I did not immediately check my classical physics textbooks (did you?)

I did. The thing is that all of the things like the force on a dipole due to an inhomogenous field are calculated from Maxwell's equations and the Lorentz force law. They are the fundamental equations of classical EM. The only way to get something other than E.j to perform work on matter is to violate one or more of those equations.

Already the way the Ampere is defined relates to a magnetic force that acts on wires.

Sure, but a force isn't work. As long as the wires are stationary no work is done and there is only a magnetic field. As soon as one of the wires begins to move there is an E field. So you cannot get work without an E field and the equations of classical mechanics dictate that the work is given by E.j.

 

[harrylin]

I did. The thing is that all of the things like the force on a dipole due to an inhomogenous field are calculated from Maxwell's equations and the Lorentz force law. [..]

That's correct of course (and it's exactly what I explained).

Sure, but a force isn't work. As long as the wires are stationary no work is done and there is only a magnetic field. As soon as one of the wires begins to move there is an E field. So you cannot get work without an E field and the equations of classical mechanics dictate that the work is given by E.j.

Already explained in the other thread: the equations of classical mechanics dictate that the Lorentz force drives the motion. Surely it doesn't go to zero when the wires start to move, there is no law of nature according to which that would happen. But if you really think so, please give a reference in which such magical disappearance is derived or where that magnetic force disappearance law* is given.

*such a weird law should prescribe complete and instant magnetic force disappearance for a current loop, but none at all for an electron in a cyclotron!

 

[cabraham]

I did. The thing is that all of the things like the force on a dipole due to an inhomogenous field are calculated from Maxwell's equations and the Lorentz force law. They are the fundamental equations of classical EM. The only way to get something other than E.j to perform work on matter is to violate one or more of those equations.

Sure, but a force isn't work. As long as the wires are stationary no work is done and there is only a magnetic field. As soon as one of the wires begins to move there is an E field. So you cannot get work without an E field and the equations of classical mechanics dictate that the work is given by E.j.

Chickens & eggs. You are saying that the work is done by E which is created by the loop's own motion. So we have a paradox. In order to spin the loop, torque is needed. Torque times angular displacement is work. Motion does generate E field, but refer to my diagrams. E field is not oriented so as to spin loop.

Your theory that the motion creates an E field which does the work cannot be right. The work needed to generate said E field comes from where? The torque on the loop is due to Lorentz magnetic force, Fm = qvXB. E does do work setting up the loop current. This current makes the torque possible. Without E, there would be no loop magnetic dipole & no motion.

The Fm vector spins the loop. E vector keeps the loop current & magnetic dipole going. It's too easy. The fact that this is controversial amazes me.

Claude

 

[harrylin]

Chickens & eggs. You are saying that the work is done by E which is created by the loop's own motion. So we have a paradox. [..]

Yes indeed, and this is worthy of emphasis.
It's a logical impossibility for an effect to be its own cause; a force that is induced by motion cannot be driving that motion.