# Wire in a magnetic field

1. Jul 5, 2004

### pray_for_mojo

By passing a wire through a magnetic field and running a current through it I can get the wire to move perpendicular to the field. Why? Specifically, is it the reaction of the magnetic field it is passing through interacting with the magnetic field generated by the wire itself? ...The devil is in the details...

Any insight will be greatly rewarded.

2. Jul 6, 2004

### kuenmao

The particles that carry charge in the wire experience a electromagnetic force when they pass through a magnetic field. The particles are electrons of the wire, which is a conductor. Hence the wire moves.
Just believe it...it works that way, and there isn't really a way to explain "why the particles experience an electromagnetic force".

3. Jul 6, 2004

you can also generate a current by moving a wire through a magnetic field :)

4. Jul 6, 2004

### pray_for_mojo

Thanks to both...and kuenmao, that's as far as most people go..."that's just the way it is." I want the nitty gritty. Why? Comparing it to just a particle passing through a magnetic field we get the same result: it moves up or down, perp to the field, depending on charge. But why? Is it some quantum thing? Who makes up these rules anyway?

5. Jul 6, 2004

### kuenmao

First, I have to add something that I forgot to add: It has nothing to do with the magnetic field set up by the particle itself. The magnetic field mentioned here refers to an external magnetic field set up by another source.

I am not sure, but I think that the statement "a charged particle experiences a electromagnetic force perpendicular to the direction of a magnetic field which the particle is passing through" is more or less just describing what is happening. To me, it's more or less the same as saying "a mass accelerates under the action of a force". Is it really that meaningful to ask why?

Isn't quantum mechanics is also based on the assumption that charged particles experience electromagnetic forces when moving through a magnetic field?

6. Jul 6, 2004

### pray_for_mojo

Agreed to a point. But it's not a force and no work is done. Like gravity, another fundamental force, there is something lurking below the surface, the graviton. In the relationship I described the moving particles, either in the wire or out, move perpendicular to the external field...other than charge is there something going on? Some other interaction?

Perhaps I am asking the wrong question and should just accept it.

7. Jul 6, 2004

### kuenmao

It IS a force. It DOES do work. It's a electromagnetic force, one of the four fundamental forces in nature. It is explained by another elementary particle, whose name I have forgotten...perhaps you can read it up in books about elementary particles.
I am not sure that it has anything to do with why the charges move perpendicular to the field though...

8. Jul 6, 2004

### pray_for_mojo

Ooops. My mistake on the force part (was thinking of something else), but I'm going to stick to the work thing for now (given the same scenario).

Let me take this down the road a bit. All four of the fundamental forces involve the exchange of one or more particles. Even the underlying color force which is presumed to hold the quarks together to make up the range of observed particles involves an exchange of particles (gluons). Such exchange forces may be either attractive or repulsive, but are limited in range by the nature of the exchange force. Magnetic and electric forces are manifestations of an exchange force. That's what I'm wondering about and wanting to know things perhaps I cannot.

9. Jul 6, 2004

### pmb_phy

The magnetic force is a force. The magnetic force can be seen as a relativistic consequence of the electric force. That is to say, take the electric force, i.e. the Coulomb force. Transform to a frame which is moving relative to the rest frame of a charged particle, and in that frame there will be a magnetic field. The derivation etc. is rather complicated and can be found in Special Relativity, A.P. French, MIT Press, chapter 8 Relativity and Electricity.

There is an article called Is a magnetic field due to an electric current a relativistic effect? by Jefimenko (1996) which covers this topic. I haven't read it. I've just sent away for it in fact. You can read a comment on that paper at -- http://www.iop.org/EJ/abstract/0143-0807/18/2/013

However the magnetic force does not do work. For an example of why it does no work and where it appears to do work then see the article

Magnetic Forces Doing Work, Eugene P. Mosca, Am. J. Phys. April 1974 Vol. 2

I placed a copy online at - http://www.geocities.com/physics_world/mag.htm

Pete

10. Jul 6, 2004

### pray_for_mojo

Beautiful. That's what I'm looking for.

Thank you!

11. Jul 6, 2004

### pmb_phy

Be aware that what I explained is the subject of controversy. Some physicists(e.g. J.D. Jackson) disagree with the claim that magnetism is a relativistic effect. They hold that additional information must be known and that simply knowing Coulomb's law and relativity is not enough to deduce magnetism.

12. Jul 6, 2004

### Staff: Mentor

Hey Pete: Thanks for putting a copy of Mosca's paper online! I had lost my copy.