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

DaleSpam

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.

EEH4

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

gabbagabbahey

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.

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

Likewise.

Claude

cabraham

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.

DaleSpam

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? 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

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

DaleSpam

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 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

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

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,

DaleSpam

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

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

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.

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.