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Re: FUN: high-speed electrostatic air-threads



On Tue, 16 Jun 1998, Steven T. Ratliff wrote:

In the pinch effect you have a radially inward force due to the
interaction of the current the azimuthal magnetic field that it
creates. The force is in the direction J x B, where J is the current
density and B is the magnetic field. It doesn't always just slow the
outward expansion, but can squeeze the current also.

Then I'm confused about the origins of pinch effect. Take an electron
beam in a vacuum for example. You're essentially saying that there is
some value of current which, if exceeded, leads the electrons to ATTRACT?
I don't understand how this could occur. I don't see how a magnetic field
could ever cause contraction of a moving volume of *net* charge, since
"current" in this situation is observer dependant. Isn't the b-field
simply the same as e-field under special relativity? If my ion stream is
moving at 5kph and I should run alongside it, I will see zero current. My
uniform motion has eliminated the pinch force, but my motion does not
cause the ion stream to suddenly expand. Only if the moving charges
exceeded c would they be time reversed and might contract. :)

My mental model tells me that pinch effect cannot collapse an electron
beam or an ion cloud. It can "weaken" the electrostatic repulsion, but
can never cause a net radial attraction. However, it can collapse an
electric current composed of relative motion between equal population of +
and - charges, such as current in a metal conductor or a current in a
plasma. If these ion-streams contained equal and opposite flows of
oppositely charged ions, then I would suspect Pinch Effect. But if
negative air is leaving a negative needle, where is the opposite flow of
positive air?

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