Re: Earth's Magnetic Field

[William Beaty <billb@ESKIMO.COM>]

On Sun, 11 Jul 1999, paul o johnson wrote:

> William
>
> Your "pseudo perpetual motion" explanation of the Earth's magnetic field
> makes sense to me, but I'm still hung up on the question someone posed
> about positive and negative charges in the liquid core. Unless the
> Earth's core has a net charge or unless opposite charges are somehow
> made to move in opposite directions, it seems to me there can be no
> current.

Simple solution: during electric currents, the potential difference
applies opposite forces to opposite charges in the conductor, and if the
charges are free to move, then they do flow in opposite directions.  This
actually occurs in all everyday electrolytic conduction.  The opposite
currents do not cancel, instead they add together.  Positive ions create
positive current.  Negative ions can only create positive current if they
flow backwards relative to the flow of the positive ions.  Also see:

  Which way does 'current' really flow?
  http://www.amasci.com/amateur/elecdir.html

  All 'electricity' articles
  http://www.amasci.com/ele-edu.html


> It also seems to me that relative motion between liquid and
> solid cores is immaterial. Since the B field is fixed with respect to
> the Earth's surface, there must be relative motion between liquid core
> and crust.

If I understood the article, the relative motion takes place between
different parts of the liquid core itself, and the solid part of the core
only plays a secondary role.  The convection stirs the iron and the
Earth's rotation distorts it into complicated patterns of flow.  We have
rotating regions of iron adjacent to non-rotating or opposite-rotating
regions.  All the regions in the liquid metal are in electrical contact
with each other, so we have a system of rotating disks, sliding brushes,
and shorting bars.  (Neighboring regions affect each other, so we have
something that resembles "Conway's Life" algorithm, but in three
dimensions and with EM physics.  Expect emergent phenomena to appear.)

The magnetic field is not fixed with respect to the Earth's surface,
instead it simply changes very slowly.  The magnetic pole wanders around
over the centuries (although I think it is supposed to generally follow
the axis of rotation, and not just wander anywhere at all.)

One thing I don't understand is how the "self-acting" part of the Earth's
generator works.  We can cut spiral slots in a disk in an HPG, and when
that disk turns, it drags the moving charge in a spiral fashion.  I don't
see how a rotating volume of iron of uniform conductivity could do
something similar.  Maybe there are regions of high and low conductivity
which take the shape of spirals?  Or maybe the vortex motion forces the
currents to spiral, and therefor to create the fields that sustain the
generator action?

In the real-world generators, perhaps the spiral slots would not be needed
if the metal disk had a vortex-like rotation (imagine a liquid-metal disk
in which the RPMs of the metal near the axis are much higher than at the
rim of the disk)  If we made an HPG by creating a fast vortex in a round,
shallow tray of mercury, perhaps the "self-acting generator" phenomenon
would arise spontaneously, and a huge electric current would appear.  I've
never heard of a "liquid rotor" homopolar generator.  I mean, besides the
one that's supposedly in the Earth's core.  Unfortunately the mercury
wouldn't be like the liquid iron in supplying closed loops of iron which
would support much stronger b-fields.  Anybody have many gallons of
mercury, a cylindrical plastic tank, and a big pump?  Liquid indium/tin
alloy might be a safer working fluid: it's like Wood's Metal and is liquid
at around 150C.  Put powdered iron in the mix to make the stuff act a bit
more like liquid iron?



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William J. Beaty                                  SCIENCE HOBBYIST website
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