Consider a round electromagnetically featureless axle, i.e. an axle that possesses circular symmetry electromagnetically. This could be either an arbitrarily shaped axle, that has similar magnetic permeability and electric conductivity as the medium around it. Or - if its conductivity and/or permeability differ from that of the surrounding medium - it has to have a circularly symmetric crosssection.

This axle is exposed to a radial magnetic field and the magnetic field direction is made to rotate around the axle axis.

What mechanism makes the axle spin?

After a recent argument about another answer here, I have tried to follow that poster's arguments but failed. I couldn't find an actual arrangement with a circularly symmetric rotor in the links provided by the user, so I tried to compare this to an induction motor. In this logic, the axis sees a \$\Delta B(t)\$ that is also spinning around the axis, but as it is spatially homogeneous, no net force would develop on the axle in my understanding.


1 Answer 1


If the axle is conductive but non magnetic the rotating magnetic field will produce eddy currents. These eddy currents together with the magnet produce a force which tends to keep their relative motion small. This effect is illustrated by the experiment of dropping a strong magnet through a copper or aluminum tube. The force between the eddy current in the tube and the magnets tends to oppose the falling of the magnet. Since the force tends to make the relative motion small, the axle will rotate in the same direction as the magnetic field. However, like an induction motor, the rotation of the axle will not be synchronous with the rotation of the magnetic field. Otherwise, there will be no eddy current. There must be "slip" to create the eddy currents.

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    \$\begingroup\$ Thanks, nice and clear. I think another good analogy is the eddy current disk brake. \$\endgroup\$
    – tobalt
    Commented May 21 at 15:29

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