🚀 go-pugleaf

johncobb@ut-emx.cc.utexas.edu (John W. Cobb) writes:

>In article <51016@seismo.CSS.GOV>, black@seismo.CSS.GOV (Mike Black) writes:
>|>NASA claims they intend to generate electricity with the tether 
>|>experiment coming up Monday.  Seeing as how a magnetic field has
>|>no energy, wouldn't any energy gained come the movement of the
>|>shuttle & tether?  If so, you would lose orbital velocity which
>|>would have to be made up by additional thrust thereby negating
>|>any energy gained.  Or am I missing something?

>You're missing something. There is a potential drop across the distance.
>So you are connecting a junction at two points and extracting electrical
>energy from it. It is a free lunch scenario. The energy is extracted
>from the stored energy in the magnetic field structure of the earth.
>The idea is that magnetic field lines tend to run from pole to pole.
>electrostatic potential drops can exist perpendicular to the magnetic
>field since the magnetic field itself can be an electric insulator
>when the charge carriers are ionized particles. This is all a result of

I thought that the actual explanation is that for a conductor of length l
and moving at velocity v there is a emf e across the conductor of

e = - d(psi)/dt  where psi = B*A=flux, B=magnetic field and A = area 

setting B constant since the tether length is small compared to the earth's
radius, d(psi)/dt = B*dA/dt, and dA/dt = l*v, hence

e = -Blv, now if nothing is done to either end, you have a static condition
and the charge builds up to some value and stays there and no drag results
on the wire.  But if you remove charge from say the shuttle end by ejecting
it with an electron gun back into the earth's atmosphere, then you get a 
current in the conductor and a lorentz force on the wire OPPOSING the 
direction of the orbit.  Of course the positive end of the wire has to 
get new electrons from somewhere (the ionosphere/exosphere?) for it to
keep conducting.

Mark S.  
Schlegel@unomaha.edu      Physics Dept.

>a very fundamental property of plasma containment. The zero-th order
>orbit of a particle in a magnetic field is a helix with axis parallel to
>the magnetic field. Now there are 1st order corrections, etc, but this
>is the idea.

>If this is still counterintuitive, think back to the common sophomore
>physics lecture demonstration of a homopolar generator. That is, take
>a copper disk and spin it in a magnetic field. You will find a potential
>drop between  the center and the edge. Now the electric and magnetic field
>structure in the magnetosphere is much more complex, and to the best of
>my knowledge is not a rotation induced phenomenon, but you can get the
>idea of how you potentials can exist across field lines.
>john w. cobb
>jwc@fusion.ph.utexas.edu
>