Re: Threading a moon
- From: Logan Kearsley <chronosurfer@xxxxxxxxx>
- Date: 31 Oct 2007 15:21:02 -0700
"IsaacKuo" <mechdan@xxxxxxxxx> wrote in message
news:1193806088.480812.262170@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Oct 30, 6:41 pm, Logan Kearsley <chronosur...@xxxxxxxxx> wrote:
On Oct 30, 7:07 am, IsaacKuo <mech...@xxxxxxxxx> wrote:
First off, whether the drag is inward or outward depends
on how fast the planet is spinning, not the magnetic field
(although obviously these may be related). The planet's
rotation rotates the surrounding plasma field along with
it (it's essentially really really upper atmosphere).
Wah? What matters is the relative velocity between the conductor and
the local magnetic field lines, which depends on how fast the magnetic
field is rotating.
No, in fact the relevant effect works just fine if the magnetic
field isn't rotating at all.
Essentially, you're using the exact same effect as an
MHD generator. There's a more or less constant
magnetic field. You have a couple electrodes as well
as plasma flowing past those electrodes. In this case,
the electrodes are the non-insulated ends of the tether.
The flowing plasma is the really thin exosphere of
the planet.
Lorentz force acts on the electrons in the plasma as
well as the electrons in the tether. Because they're
not moving at the same speed, the Lorentz force acts
differentially--and a current results.
The direction of this current depends on whether
the plasma is moving faster or the tether is moving
faster. In turn, this determines the direction of the
drag force. Essentially, the plasma will try to drag
the tether to its velocity and vice versa.
There are two relevant, and opposing (as far as being able to extract
energy), effects, then- voltage across the
conductor due to moving through the planet's magnetic field, and
voltage due
to the surrounding plasma moving through the field. I had not
considered
motion of the plasma through the planetary magnetic field before....
It seems to me that the net voltage, then, will depend on the
difference in
rotation rates between the planet's atmosphere and its magnetic field,
as
well as orbital speed. If they move in lock-step, you get no MHD
effect, and
all of the voltage is due to the motion of the conductor relative to
the
magnetic field.
-l.
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