"Vince Morgan" vinharAtHereoptusnet.com.au wrote in message
u...
"Don Kelly" wrote in message
news:PcB5k.11995$Jx.5774@pd7urf1no...

If the field of the magnet is radially symmetric (i.e. B is the same at
any
given distance from the axis, in all directions), why should spinning
change
the field at any point in space? You don't actually have "flux lines"
attached to the magnet as shown for visualization. No change in B- then
what? If the magnet is not symmetric-then there could be a voltage
induced
(causing current). There is no need to consider various frames of
reference.

I see your point Don. The field would be homogeneous at any given point
from the radius, rotating or otherwise. However, that leaves the question
of why rotating the faraday disk 'will' induce current I would think?
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Not really- this can be approached through Coulomb's force law. Faraday's
law also will work. I said no need to consider frames of reference but it
would be best to use a path fixed on the rotating disc.
---------

I fail to see the problems with homopolar machines- sure it is clear
using
Coulomb's force law, that the induced voltage can be found- and the
result
agrees with the use of Faraday's Law (which appears to present problems
to
some).

I was thinking primarily of the brush problem in the generator now that I
think about it. If you could make a faraday disk where the disk was
stationary with respect to the lab frame current transfer becomes a lot
simpler.
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True -but attempts to do this result in the need for a multipole magnet and
then a little geometric reshaping results in a conventional machine.

A question I would like to know the answer to, and haven't seen asked
specifically, is whether it is possible to create a magnetic field that
rotates about it's axis. And if not, why not?

A field is a specific region of influence. If the source provides a field
with radial symmetry, then rotating the source doesn't change the field- The
customary visualisation of the field in terms of "lines of force" doesn't
indicate any more than the direction of the field at any point, and or, to
some extent the strength of the field. These "lines" don't actually exist.
-----------
It's also occurred to me recently that the earth rotating about it's own
axis might actually be a giant homopolar generator wherein a conducting
fluid core might undergo forces that contribute to tectonics. Just a
thought I might add.
-------------
You'll get Archmedes Plutonium excited. However, it appears that the
rotation and movement of fluids set up currents in the outer core- which in
turn produce the field. In that case it may be that the forces that produce
plate tectonics are also responsible for the field.

http://www.geomag.bgs.ac.uk/earthmag.html

http://en.wikipedia.org/wiki/Dynamo_theory

--

Don Kelly
remove the X to answer
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