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Re: pinwheel (was Re: electrostatic motor)



At 07:24 AM 5/22/00 -0400, Steve Wonnell wrote:

If I understand this correctly, the "heart" of
this effect is a non-electrostatic process occuring in a non-uniform
electric field.

Non-electrostatic? Why not electrostatic? What else is there? Surely
magnetic interactions are negligible.

The non-uniform electric field exerts not only a torque,

The torque is negligible. You can show by Fermi's golden rule or otherwise
that to cause a transition between molecular rotational states you need a
high-frequency (infrared or higher) electric field.

but also a net force on nearby molecules, drawing them closer to the
point, into an increasingly strong electric field.

OK.

At some point in the
journey of the molecule or atom from the atmosphere toward the tip, the
field becomes strong enough to break down the molecule or atom, with one
part continuing on toward the tip and the other being ejected in the
opposite direction.

That is irrelevant or nearly so under normal conditions.

Of course, in addition to the process just described, there
should be secondary processes due to the collision of the ejected charged
particles with air molecules, charging the atmosphere near the point.

For practical purposes that's the primary process, not the secondary process.

The electric field accelerates the ions. They then smash into the
surrounding atoms/molecules thereby creating new ions.

This leads to a good estimate of the field strength required to cause
breakdown:

ionization energy per elementary charge
critical field = ---------------------------------------
mean free path

= something like volts per micron
= something like Megavolts per meter

If you underappreciate the ions and consider only the effect of the field
on uncharged atoms/molecules, the corresponding estimate of the critical
field is off by orders of magnitude.

Do you think Geiger-Müller tubes contain gas at high pressure or low pressure?

For a fixed, high voltage supply, the only difference
one should expect for different atmospheres is the precise location of the
point where the molecule or atom breaks down. This is not easy to measure
or see.

You could study the onset of breakdown
as a function of voltage,
as a function of the curvature of the pin-point, and
as a function of gas density.

There is plenty of literature on this subject. I recall finding the papers
of Prof. Geiger to be quite interesting, although I don't recall the details.