The problem explaining how homopolar motors work

[Low-Q]

Since I discovered the homopolar motor, it has fascinated me a lot. Not because it has any practical use, but it works in spite of a non gradient magnetic field.

Normally, an electric motor works because the rotor is allways "chasing" a sticky magnetic spot. The magnetic gradient between weak and strong field is the very reason why the rotor is put in motion.

With the homopolar motor, I'm not longer so sure if it is the magnetic field in the single winding that cause rotation of the permanent magnet. Even if the right hand rule should explane it all.
So I did a test to compare a "traditional" homopolar motor, with one wire connected between minus on a AA-cell, and the magnets circumference.
The magnet rotates in a direction that is expected from the right hand rule.

In the next experiment I made a coil with some 10 windings. Connected the wire ends just as in the first experiment.
Now the magnet appearently wants to align itself with the coils electromagnetic field, as it swings back and forth, loosing contact periodically.
The rotational performance went down. Possibly due to higher resistance in the long wire. And only one of the windings, the one which goes through the magnet, cause the magnet to rotate.

If it isn't the magnetic field around the wire, what cause rotation of the magnet?
If it IS the magnetic field causing it, why doesn't it work to replace the electromagnetic field with another permanentmagnet?
Since permanentmagnets can't provide energy, I also assume there is another reason than magnetic fields that force the magnet to spin.

Any thoughts?

Here is a video I made yesterday of the two experiments.


Vidar

 

[jartsa]

A current that is going through the magnet does not cause any net force on the magnet. That is the special thing that makes the motor work. I mean that part of the circuit that is the magnet does not exert a force on the magnet.

So therefore those extra turns of wire are not increasing the torque of the motor.

When current does go through magnet there is a net torque on the magnet, and opposite net torque on the wires.
When current does not go through magnet net torque on the magnet is zero and net torque on the wires is zero.

(Current through a conductor that is glued on the magnet counts as a current through a magnet)Edit: Now I read the question carefully. Seems I just repeated the question.

 

[jartsa]

So the problem maybe is the reaction force of the Lorentz-force. And particularly why does the magnet rotate.

Umm well, magnet rotates when it is not centered relative to the thing that is experiencing the Lorentz-force? The reaction force of the Lorentz-force does not push the center of mass of the magnet?