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From: delt0r <greg.ew...@gmail.com>
Newsgroups: sci.space.science
Subject: Re: an electrostatic propulsion system concept
Date: Sat, 13 Mar 2010 08:58:31 EST
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On Mar 8, 3:13 pm, Alex <abelov0...@gmail.com> wrote:

> Here is an electrostatic propulsion system concept.http://knol.google.com/k/alex-belov/electrostatic-propulsion-system-c...

So a simple way to understand conservation of momentum, is to note
that forces are aways between things. So when i use an electric field
to accelerate an electric charge. The forces are between the mobile
charges and the charges bound in the electrodes. It does not matter if
the the charge gets more "massive" since then it takes longer to
accelerate and hence the force on the bound charges is also over a
longer time, and thus the same amount of momentum is transfered to the
acceleration structure as to the charge.

So in the first case (not spiraling) it is easy to work out the net
momentum on the electrons. But the same amount of momentum in the
opposite direction was applied via electric force to to the whole
craft. And then the electron hits the top plate and puts all its
momentum back. The net momentum change from the right side is zero.

On the spiraling side we have the same thing no matter how fast we
"spiral". First energy is conserved. So the max speed of electrons on
both sides is determined by the potential difference. We will ignore
loss from synchrotron/Cyclotron radiation (these in practice would be
large) and assume you use cross field to produce the spiral (to
produce a helix you would need to inject electrons with some sideways
velocity). In this case the electron go nowhere (magnetic
insulation!). We can come up with different spiral patterns, but with
no losses you get no momentum change. the electrons start with no
momentum. They finish with no momentum and the net change in momentum
is nill.

Remember that rest mass and relativistic mass are not the same thing.
The relativistic mass depends on the frame of reference. The rest mass
does not. And that momentum is a vector!

greg