[redsniper]

Okay so, if you have a loop of current in a uniform magnetic field, there's no net force on the loop only torque. But in a magnetized object, which conatains many small current loops from the electrons, there can be a net force. How can this be?

[perihelion]

Could you clarify the question?  What are these small current loops of which you speak?  Are you talking about ferromagnetism?

[redsniper]

This is assuming that the orbits of the electrons can be treated as current loops. I don't know if this is ferromagnetism specifically, or just any kind of magnetism.

[Bobboau]

there 'would' be a force on the wire loop.
unless I misunderstand the situation, keep in mind the orientation of the loop and magnetic feilds will have a significant effect on what happens.

[redsniper]

Right, as long as the loop isn't perpendicular to the magnetic field the forces will exert a torque on it and make it spin. But the net force would be zero, so the loop wouldn't translate anywhere. Plus, I believe the "loops" from the electrons would all line up in the presence of a magnetic field so that they would be parallel to the field.

[Descenterace]

Basically, lines of magnetic flux always try to shorten.

A lump of unmagnetised iron in a magnetic field gets attracted to a pole because it distorts the flux, and the process of 'shortening' exerts a force on it. There's also the effect of eddy currents; as the iron moves, a current will be induced which will also affect the process.

A current in a magnetic field is a different thing altogether. The current generates its own magnetic field, and this interacts with the standing field. Again, the process of flux shortening exerts a force on the conductor.

I forget the exact mechanism behind flux shortening. The 'shortening' is an abstract representation only, but it's good enough for determining what's going to happen when magnetic fields interact. It's been too long since studied Physics.

[Herra Tohtori]

Also note that attraction to poles does not apply in uniform magnetic field because there is no poles in it.

If you put a loop with current going through it, the current creates a magnetic field. If you put this loop into uniform magnetic field, the two magnetic fields will try to align with each other, which tries to turn the loop into an attitude where the field lines of the uniform magnetic field and the magnetic field generated by current are aligned, ie. perpendicular to the loop.

Exact same thing would happen to a ferromagnetic piece of metal in an uniform magnetic field. It wouldn't start moving, it would just turn so that the magnetic fields are aligned. For example that's what happens with compass needle in Earth's magnetic field, which can locally be viewed as close to uniform as is within most practical limits. The needle isn't pulled anywhere, it just aligns to the field lines of the Earth's magnetic field at the location. In other words, net force is practically zero when it comes to magnetism, but torque isn't.

That's just uniform magnetic fields, though. When you have two magnets pulling and pushing each other, that's because the magnetic fields are definitely not uniform, which causes all kinds of interesting things to happen as the magnets try to set themselves up into a position where they have the least energy potential possible.

[Dysko]

We just made it at school 1 hours ago.

But our teacher explains very badly, so I'm going to check it on the book

[Col. Fishguts]

Natch, Herr Doktor always beats me to those fun questions.