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

DaleSpam

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

Agreed, but lets 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

Attached Files

lorentz force components e b0001.pdf (201.0 KB, 39 views)

Miyz

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

harrylin

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"?

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

You forgot me and several others but I notice that you did find the same as I did. And yes, amazing discussion!
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,
[tex]\vec{j}_{\text{max}}=c \vec{\nabla} \times \vec{M}.[/tex]

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

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_%28physics%29:
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).

DaleSpam

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

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.

DaleSpam

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.

harrylin

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

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

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.

gabbagabbahey

The (magnetic part of the) Lorentz force doesn't need to be zero to not do any work, it only need to be perpendicular to the motion of the entities it is acting . In a current loop, it acts on little bits of moving charge/current, [itex]d\mathbf{F}_m = dq \mathbf{v} \times \mathbf{B}[/itex], and is always perpendicular to the motion [itex]\mathbf{v}[/itex] of each little bit moving charge. Therefor it does no work.