Magnetic fields do no work? How come...

cabraham

It sure does work fine. Yet I'm wrong for believing in it.

Claude

Doc Al

Huh? Where did I question the "right hand rule"? Strawman, indeed! You claim that I "acknowledge the role of H, but you don't bring it up on your own", but describing the role of the H field is the very first step I made (post #32) in responding to your request (post #28) for an explanation of the attraction between current-carrying wires using electric fields.

Enough already. We are wasting each other's time.

cabraham

So let's summarize. Two wires are parallel and carrying current. What determines the magnitude & direction of the force incurred? The direction of the currents determines the polarity of the H fields. The polarity of the H fields determines whether the free electrons in the wire shift to the interior vs. exterior. Then, the positive charged lattice follows the free electrons due to E force.

That pretty much sums it up. If the current increases, so does the H field, and the electrons move further inward or outward. Then the lattices follow the electrons further in or out.

Thus the H field determines where the electrons move and how far. The lattice tags along like an obedient shadow due to E force between lattice and electrons.

That is prima facie evidence that the H field is primarily what determines if the wires attract or repel, and the magnitude of the force. The E field definitely participates, but is not what determines the above.

H force moves the electrons. Lattice tags along due to E force. It's that simple. H is primary, with E secondary. Case closed.

Claude

Doc Al

The issue is not which field, E or H, is "primary"; they come together--it's a package deal. The issue is, per the title of this thread: Does the magnetic field do work? The answer to that is technically no; it's the electric field that pulls the wire. This explanation is one that you objected to at first (recall your response in post #18 to diazona's rather clear statement in post #16).

The reason for this seemingly nitpicking discussion is one of understanding the Lorentz force law, which is the source of all the derived "right-hand rules".

Academic

This paper helped me understand it. :
http://academic.csuohio.edu/deissler..._77_036609.pdf

cabraham

Maybe an analogy would help. A steel ball is tethered to a rubber ball via a short cord, or even glued together. A powerful electromagnet is held above the tethered ball pair. The em is turned on and the steel/rubber ball pair is lifted into the magnet.

I certainly do not believe that a magnet is doing work on the rubber ball. But the rubber ball does not ascend if not for the mag force. So it is really splitting hairs to argue which force is responsible for the rubber ball ascending.

The mag force acting on the steel ball is what ultimately lifted both balls. The steel ball was lifted by the magnet directly. The rubber ball was lifted indirectly. The tether provided the means for the rubber ball to tag along with the steel ball.

With 2 parallel wires, the E force between the lattice and free electrons is the tether. The H force dictates where the electrons go, then the E force tethers the lattice yanking it in the direction of the electrons. To say that H is NOT responsible for the lattice moving is like saying that the magnet is NOT responsible for the rubber ball ascending. The electrons and the lattice are tethered via E force. But the H force is what moves the electrons, and is ultimately responsible for moving the lattice. The E force does indeed move the lattice, but the E force magnitude and direction is dictated by the location of the electrons which is dictated by the magnitude and direction of H.

It's difficult to separate the 2 forces. But it is clear as day that H is what dictates the magnitude and direction of the displacement of the wires. E follows H. I know that E & H are inclusive, and neither is the cause of the other. But under these narrow conditions, H is ultimately in control, with E tagging along.

H, however, is not more fundamental than E, nor less. They are inclusive.

Does this make sense? BR.

Claude

Doc Al

Yes! Sounds good to me.

kanato

So then the magnetic field can do work. Since the electron spin is what gives rise to any ferromagnetic material, the magnetic field is does work whenever a permanent magnet is involved.

Doc Al

I may have to revise my answer to the Stern-Gerlach question in the light of the interesting paper that Academic linked to in post #56. (It might take me a while to find the time to digest it--hopefully someone more knowledgeable will chime in sooner.)

cabraham

Very good. I'm glad we agree. This is an interesting thought problem. It gives us all a chance to review the theory and I feel I've gained a better understanding. Thanks for your input. BR.

Claude

Doc Al

Me too!

Yes, it's interesting--and subtle--stuff. It helps me to review it every now and then. Good discussion!

kanato

I missed that one. I have skimmed through that article, and it looks like the author doesn't actually answer the question, but gives some conditions under which the answer could be understood. I will have to read it more carefully however. It is interesting that this is a topic of current research though, so the question of whether or not a magnetic field can do work at a fundamental level really isn't settled.

qsa

"The usual explanation
is that there is a change in the “potential energy” by
an amount −2
s ·B=−eB/mc, which implies that the magnetic
field did work on the electron’s magnetic moment.
However, if the electron has rotational kinetic energy,"

This is a quote from the paper. He states the conventional explaination, but he puts forward his own conjecture " rotational kinetic energy".

kmarinas86

The forces of electromagnetism do work. Acceleration can occur along electric field lines and acceleration and also along magnetic field lines. However, because a cyclical process of doing work requires a changing magnetic field which in turns produces an electric field, electric fields are seen as crucial in order for work to be done. A changing displacement is sufficient for an electric field to do work, but not so for a magnetic field, which requires a change in magnitude in place (implying a change in electric field). Therefore when work is done using electricity or magnetism, an electric field ALWAYS comes into play, but same is not true for magnetic fields (because sometimes they are not used). It is a rare circumstance in the macroscopic world to have a system with truly constant magnetic fields (no induction) when electric fields are being moved relatively to each other...

sweet springs

Hi
It might be out of date but I show here a interesting case, a charged particle attached on a elastic body, with velocity v in perpendicular direction, under magnetic field B in another perpendicular direction.
|
|wwwwwwwwwww○　↑ ｖ　 x B
|
Magnetic or Lorentz force pushes or pulls elastic body. It does work thus elastic energy would be stored.

I state from this example that magnetic force does not work on FREE charge, but it can work on charge UNDER CONSTRAINT.

The elastic body consists of multiple charged particles under electromagnetic interaction so we can say another way that magnetic force does not work on a system of SINGLE charge, but it can work on a system of MULTIPLE charges.

Regards

Doc Al

Can you please describe the case you have in mind in more detail and explain why you think it illustrates a magnetic force doing work on a charged particle.

sweet springs

OK, I will.
|
|wwwwwwwwwww○　↑ ｖ　 x B
|

the direction of Lorentz force is ←　or →　according to the sign of charge ○ and it pushes or pulls the elastic body or the spring.
Regards.