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Conductor rotate in a magnetic field generate voltage but magnetic field rotates while conductor remains stationary does not generate voltage. Hm...
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A magnetic field never rotates. No referential frame can be attached to a field. A magnetic field is just vectors giving B at points in space. They can dynamically change from location to location giving the illusion of a movement, as a light spot moving along a wall when nothing is really moving on the wall.
Moreover when a magnetic field source is rotating around its axis of magnetic symmetry, as a disk magnet about its axle, the field remains strictly identical to itself, i.e. the field remains the same in all space points, so the effect is the same as if the magnetic source was at rest. This is why there is no difference in the Faraday disk when the permanent magnet is rotating or at rest.
Now the question of relativity and reaction. The Lorentz magnetic force is:
F = q*VxB.
The most important point is that V is the charge speed relative to the observer, neither relative to the field source nor to the field (the latter being an absurdity). This point is generally not understood and the cause of the urban legend of the absence of reaction.
It means that the action/reaction applies between the observer and the charge, not between the charge and the field or the field source (the magnetic field acts just like a catalyst).
When this is understood, the functioning of the Faraday generator or motor becomes clear and without paradox.
For the case of the Faraday disk generator, we have a conductor disk rotating in a constant magnetic field which is perpendicular to the surface of the disk.
From the viewpoint of an observer at rest, e.g. linked to the sliding contacts connecting the axle to the rim of the disk, the electrons of the disk have a tangential linear speed V and so, the observer sees the electrons deviated by a radial Lorentz force which is not counterbalanced by the electrons of the sliding contacts at rest, therefore a current flows.
From the viewpoint of electrons on the disk, there is an electric field E=BxV which is the Lorentz transform of the magnetic field, thanks to the rules of Einstein's relativity. So the electrons feel an electric force F=q*E which is the same as the Lorentz force viewed by the observer at rest.
Now about the reaction:
From the viewpoint of our observer at rest, when a current flows, the electrons of the disk are seen going radially through the magnetic field. So the observer sees that a Lorentz force is acting on them transversally, which means here tangentially, opposing the mechanical force that rotates the disk.
From the viewpoint of the electrons on the disk, they are always submitted to an electric field E'=BxV' but now V' is the resultant speed of the electrons moving under the tangential mechanical force of the disk (as previously) AND the radial electric field. Therefore there is a restoring force opposing the mechanical force, which is the tangential component of F'=q*E'.
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Topic: Faraday's homopolar motor revisited (Read 71874 times)
