If you have a sheet of copper in a strong magnetic field and try to move it, ring currents will develop that will oppose the motion. Now I wonder, I you take a copper sphere and spin it with a rotation axis perpendicular to the magnetic field lines, will the field break the rotation? I can't really make up my mind if it should or not.

omnignost,

If you have a sheet of copper in a strong magnetic field and try to move it, ring currents will develop that will oppose the motion. Now I wonder, I you take a copper sphere and spin it with a rotation axis perpendicular to the magnetic field lines, will the field break the rotation? I can't really make up my mind if it should or not.

If you place a conductor in motion in a magnetic field such that it traverses flux lines it will induce eddy currents. Unless the conductor is truly a super conductor where such currents have no resistance, they will cause heating and that will slowly brake the rotation. As the speed of rotation decreases the eddy currents deminish and the braking action is reduced. So the slower it turns the less braking that will occur. Theoretically one can see that as meaning it will never completly stop except for an infinite amount of time.

I think MacM is right.

I have a vague memory of Lens' law... I think that the braking from the eddy currents is due not to heating, but to the force induced on the conductor.

Any conductor moving in a magnetic field has a current induced in it.
Any charge moving in a magnetic field experiences a force.

If I remember correctly, Lens's law combines the above effects and states that any conductor moving in a magnetic field will experience a force opposing its motion.

Another way of looking at it is that the eddy currents produce a magnetic field that opposes the field that induced the eddy currents in the first place.

Pete,

I agree. The link to heat of the current was incorrect. It is the counter magnetic field created by the current flow.

http://micro.magnet.fsu.edu/electromag/java/lenzlaw/

I'm pretty sure they use this effect to break large diesel locomotives now at high speeds as they only use the mechanical method for slow speeds and emergencies now.

It is true that the eddy currents take up the energy as a magnetic field, but just transiently, the energy is rapidly converted to heat by the ohmic resistance of the material. Nobody has given an answer to the sphere problem yet. Maybe I have to make one and see what happens.

You can find results for the torque on a sphere if you refer to the Griffiths Electrodynamics book, the section with the Maxwell stress tensor (my book is not in front of me otherwise I'd just refer you to the page). Although I don't like Griffiths' books for their lack of math, I think it does a pretty good job explaining the stress tensor.

Omnignost,

Nobody has given an answer to the sphere problem yet.

Just what do you think is lacking in the responses you have received. It appears to have been answered.

omnignost, actually I think it may be worth a try. The fact passed through in opposite directions simultaneously what would happen to the fields created in the sphere I wonder?

Then again it might show pretty boring outcomes too...oh well!

Omnigost,
A sphere rotating in a magnetic field will experience a braking torque.

The eddy currents will not be transient - they will be continually generated as long as the sphere is spinning. Some heat will be generated as a side effect due to ohmic resistance.

Yes, I actually did the experiment and it breaks. And yes I realize the current is generated as long as you move the sphere, but as soon as it stops the current dies away and the magnetic energy is dissipated as heat. End of thread I guess.