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Re: "Faraday's Disk" which started it all

On Sat, 3 Jul 1999, Bob Sciamanda wrote:

Hi William,
I think I need to emphasize a point: There is a measurable difference
between relative rotation and "absolute" rotation (take this latter phrase
to mean rotation relative to the fixed stars).

Right. I'm keeping this very much in mind. I'm also noting that the
converse sometimes applies: in a conventional electric generator, the same
current is produced regardless of whether we rotate the Rotor and keep the
Stator stationary, or instead if we lock down the Stator and spin the
Rotor around it. Absolute rotation is important in some situations, while
in others only the relative rotation is important.


Perhaps the charges within a rotating conductor near a stationary magnet
do experience a q(VxB) force, but the charges in a stationary conductor
near a rotating magnet do not feel a qE force. Such an asymmetry would
not suprise me. But is this asymmetry real?



The same must be said of a spinning magnet. Its electromagnetic properties
change, TO ALL OBSERVERS, when IT spins.

That's what I'm not sure about. Certainly when the non-electromagnetic
properties involve inertia, then absolute rotation changes them for all
observers. Some electromagnetic properties do involve inertia, and
therefor change for all observers. For example, the Tolman Effect
arises because of absolute rotation, because free carriers have mass and
are forced towards the edge of a rotating conductor-disk.


However, (regarding the spinning disk-magnet) if only the *tangential*
motion of the magnet's internal dipoles has electromagnetic consequences,
while the acceleration and axial rotation of each dipole does not, then I
must conclude that, at least in this particular limited situation, the
absolute rotation does not matter much.


Will the accelerated dipoles in a spinning disk-magnet act as radiators?
And, does the spin of the disk-magnet contribute an absolute spin to each
dipole and so produce some EM effect? If not, then how else would the
absolute spin of the disk-magnet have EM consequences?


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