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

Miyz

I think that has a smal weak effect on the motor... It can't do work if it was it would have been added in this formula F = IL x B.

And if you think it does Darwin123,how is it so?

Miyz

Darwin123, You're really confusing me here... Could you give me a SIMPLE conclusion that you agree upon? In a sentence perhaps?(Makes it all clear.)

As I said before and will continue to stand upon this point magnets can do work under certain circumstances. And magnetic field will possess potential energy which depends upon its orientation with respect to the magnetic field.

It's all complicated business lol, however. Interesting as ever :)

DaleSpam

A magnetic field can certainly do work on a current carrying wire.

For example, consider a superconducting loop with current. Such a current carrying wire forms a magnetic dipole. A uniform external magnetic field can exert a torque, and a non-uniform field can exert a net force, both of which may be arranged to do work on the wire.

A magnetic field can not do work on a classical isolated point charge, but that doesn't prevent it from doing work on other things.

DaleSpam

I provided an example where it does. I am sorry, but nature disagrees with you.

chingel

But isn't it also correct to say that the magnetic field does no work on the electrons, it just changes their path, and then the changed paths of the electrons cause the torque or force on the wire? As the electrons paths are changed, electric forces keep them from escaping the wire and due to Newton's third law the wire experiences a force.

Miyz

hahahahahaha!! THAT JUST MADE MY DAY! Seriously.

Miyz

Good point. Now I'm starting to get confused here to

Miyz

Didn't really understand that point well... Could you elaborate more DaleSpam?

DaleSpam

I doubt that it is correct to say in a superconducting wire. In general, electrons are not little classical point particles, but in most normal situations it is probably an OK approximation.

However, superconduction electrons are not even approximately like that. They are in a very strange quantum state where an individual electron is literally not localizable to any location in the wire and all of the superconduction electron pairs share the same state.

I don't think that under those conditions the Lorentz force law for a point charge is correct.

Miyz

Then in a normal non-superconducting loop. Are magnet's doing work?

DaleSpam

Here is a good page to begin understanding the forces between different configurations of magnets:
http://en.wikipedia.org/wiki/Force_between_magnets

A loop of current forms a magnetic field which is called a magnetic dipole. It is called that because it has the same mathematical form as an electical dipole (two point charges of equal and opposite polarity).

When a magnetic dipole is placed in a uniform external magnetic field it tends to align with the external magnetic field, this is how a compass needle functions. In a uniform field it experiences this torque, but no net force. However, in a non-uniform field it also experiences a net force, as described in the page above.

Darwin123

An isolated magnetic dipole can't exist without nonmagnetic forces that keep the current going in circles. The carriers in you superconducting loop are carrying the electric current through the wire. However, carrier would not move in circles unless an electric field applied a centripetal force to the carriers.
An electric field exists at the border of the superconducting loop, In addition, the superconductivity itself depends on forces other than the magnetic force. The conduction electrons in the "typical" superconductor are coupled by phonons to form Cooper pairs. The phonons are vibrational modes caused by the electric field of the nuclei of the atoms.
The force on a magnetic dipole by a magnetic field also has contributions from "nonmagnetic" forces. In fact, the example with the wire loop is also a magnetic dipole. Nonmagnetic forces make the carriers move in a closed curve, which generates a magnetic dipole.

DaleSpam

You are thinking of the superconduction electrons as classical little balls with a well-defined position and velocity and acceleration, it is simply an incorrect idea. A superconduction electron pair is not localized around the loop, there is no centripetal force because it is not accelerating. I.e. its wavefunction is not changing over time.

In fact, the electric field that you are describing does not exist in a superconductor. It is one of the defining properties of superconduction that the material cannot support such an E-field.

Miyz

If the loops was not a superconductor... Would the magnetic fields still be able to do work? On a regular loop. Generally whats you conclusion? Can magnetic fields do work on a current carrying loop?(That's not superconducting)

Miyz

Surly work being done here is by the magnetic force's...

DaleSpam

I mention the superconductor because it gets rid of a lot of the "smokescreens" that people try to put up in asserting that a magnetic field cannot do work. It shows that it is not impossible for a magnetic field to do work. Given that it is not impossible then I have no qualms about saying that the magnetic field in a motor does work on the wire.

The only formula which justifies the contrary applies only for classical point particles and is not a general law of nature.

Miyz

So F = q(V x B) Is only applied on the particle scale of things?