Re: magnetic north and south

[William Beaty <billb@eskimo.com>]

On Tue, 5 May 1998, LUDWIK KOWALSKI wrote:

> It would be great if somebody could describe the "self-exciting dynamo"
> model in a language suitable for an introductory physics course.

Here's my take on this:

Get a conventional DC PM (permanent magnet) generator, short it out, then
crank the shaft.  The flux linkages between the "stator" magnets and the
"rotor" coils are changing, and therefor a large current appears.  Now
connect some electromagnets in series with the shorted ouput, and while
cranking the generator, replace the permanent magnets with the
electromagnets.  If this is done suddenly, while the generator is already
creating a current, then the current will continue, but part of the output
energy is now going into the resistance of the new "stator"  electro-
magnets.  The generator is creating its own "stator" fields.  Not
perpetual motion, but more like the "bootstrapping" effect which also
occurrs with Kelvin's Thunderstorm devices which create their own voltage. 
If we should stop cranking our generator, the current decays away, and if
we start up again, there is no current!  But any tiny noise within the
coils will cause a tiny field, which will cause a tiny current, which
grows exponentially until thermal output of the coils equals mechanical
work done in cranking.  The polarity might be reversed though!

I believe that industrial AC generators work as above.  A temporary power
supply is connected to the field-coil windings, the generator is brought
up to speed, then the generator's own output is regulated and applied to
its field coils.

But where in the Earth-generator are the "commutators"?  I don't know, but
I suspect that the "stretching" motions of volumes of convecting liquid
iron act as the commutators.  Say we have a loop of current already
existing in a volume of liquid metal.  If we stretch and fold this blob of
metal I believe that the parts of the current loop will follow the moving
metal, and so the loop changes shape.  The path for current remains in a
particular part of the metal, and if that part of the metal swirls around,
it carrys the moving charges along as it goes, which changes the current
path.  Complicated convection patterns could weave the current paths into
a tangled moving shape.  If the moving shape tends to resemble two
single-turn loops having relative rotation, then a self-acting electric
generator is formed.  (Of course if something were to temporarily halt the
currents, then it might take a very long time before the "generator" would
ramp up its currents again. 

One thing to realize is that the Earth's generator behaves more like
roiling liquid superconductor than like moving copper coils.  The
inductance of the titanic current loops and the resistance of the cubic
miles of iron lead to inductive circuits which have very long time
constants (Tc=L/R). They might be measured in decades rather than the
mega-eons of real superconductors, but this does indicate that we should
think of the earth's core in terms of near-zero resistance.  A desktop
model of the earth's core would require superconductors if it was to
simulate the long-term persistant currents.

If I recall correctly, the Earth's field resembles a dipole superimposed
on a quadrupole, along with small components of higher-order pole
patterns.  Neither the Geographic poles nor the Magnetic poles have the
strongest fields on the Earth's surface.  In the north, there are two 
strong spots: one in Canada and one in Siberia.  In the south there is one
strong spot fairly close to the southern magnetic pole ("Southern" pole,
which of course acts as an N-type pole!)


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