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"Faraday's Disk" which started it all



For those who haven't encountered it before, the "simple magnets question"
stems from a real-world experiment called "Faraday's Disk."

If a copper disk is spun on axis and immersed in a uniform b-field
perpendicular to the face of the copper disk, and if the leads to a
standard voltmeter are touched to the center of the spinning disk and also
slid along its rim, the meter will measure a real potential difference.
According to the voltmeter, the "radial e-field" is real. Try it. Chuck
a metal disk in a lathe, hold a big ceramic speaker-magnet near its
surface, then measure the radial voltage with a DVM.

Also, if the disk is held still, and if the voltmeter's leads are touched
to it as before, and if the voltmeter lead which touches the rim of the
disk is slid along at high speed, then again the meter will measure a
potential difference. In other words, the meter and its metal wires act
as the "stator" of this generator, while the copper disk acts as the
"rotor." To create a voltage-reading on the meter, we can either spin the
copper disk, or we can spin the meter and its contact-brushes around the
disk. (The disk can be any metal. I say "copper" just to simplify the
situation by using a nonferrous disk.)

If a pair of disk magnets are used to produce that uniform axial field
which penetrates the spinning copper disk, we will find that the generator
"ignores" any rotation of the magnets. This is sensible: if rotating
disk-magnets can create a radial e-field, then a voltmeter cannot measure
it, since the line-integral of the e-field around the loop formed
by the meter leads must be zero. That is, it's zero if there are no
*sliding contacts* as part of the voltmeter circuit. When the metal disk
spins while the voltmeter leads *slide* along its surface, then the
voltmeter *does* indicate that a potential-difference exists. And
finally, if the voltmeter's leads are soldered solidly to the disk, and if
the entire disk/meter assembly is spun, then the meter indicates no
potential difference. It's fairly sensible: if the "rotor" and the
"stator" move as one, then the generator doesn't work. Ya gotta spin the
rotor relative to the stator to get the electromagnetism to come out. :)

Behind all of the above experimental results lurks an apparent radial
e-field which arises because of the relative rotation between a conductor
and a magnet. The Faraday-Disk experiment seems to prove that whenever a
CONDUCTOR DISK rotates in a uniform b-field, the conductor experiences a
radial e-field. Is the reverse also true? If the conductor is held
still, and if the MAGNET instead is spun, will the conductor again see a
radial e-field? Can a spinning magnet-disk cause charges to move and
redistribute themselves on the disk, even though the b-field of that
spinning disk is completely uniform?

The answer must involve some experimentation with individual charges. The
"Faraday's Disk" generator employs a closed electric circuit, and therefor
its voltmeter cannot detect a radial e-field surrounding a
constantly-spinning magnet. But if we use individual charged objects,
electrons in a vacuum for example, then we should be able to directly
detect whether this radial e-field is real or not.

An alternate method: if we *suddenly* rotate the magnet in a transient
manner, then the radial e-field should *briefly* appear and then vanish
again. Or, if we get rid of our PM disks and instead use an AC
electromagnet with a rotating iron disk-core, then that radial e-field
should be able to drive transient currents in nearby conductors. The AC
radial e-field should be detectable via an electrometer's antenna. I
think.

If it is real, this "radial e-field" is awfully strange. But then, so is
the e-field surrounding a toroidial inductor driven by AC! If a measured
potential-difference becomes dependant upon the particular path of
integration we use, then "lines of flux" might not be such a good
description for this type of field. Don't reject "Faraday's Disk" as
being too weird to be true. Instead, perhaps think of it as a DC version
of the multiply-connected region which surrounds any inductor which is
driven by AC.


((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
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