----- Original Message -----
From: "Tower (K8RI)" <tower@rogerhalstead.com>
>
> First, Lightening is a discharge of static electricity, just like the
spark
> from your finger to the door knob after walking across a carpet. They are
> pretty much developed by the same kind of process which is friction
removing
> electrons from atoms.
Not exactly.. shuffling your feet across the carpet is a somewhat different
charging mechanism than, say, rain drops and ice crystals being carried by
up and down drafts. The exact mechanism for thunderstorm electrification is
the subject of a fair amount of research (made more difficult by the fact
that thunderstorms tend to kill probes inserted into them).
>
> The lightening strike occurs when a low enough resistance path is
> established between two points be they cloud to ground or between two
> clouds.
Resistance has very little to do with spark/arc breakdown in air. It's a
(not very well understood, in the details) ionization phenomena, with
cascading leaders proceeding in short jumps, with charge moving through the
leaders to the head end of the leader.
>
> It can be, and has been shown using a Tesla coil, and Vandegraff generator
> that some pretty hefty sparks can be developed.
And, as well conventional capacitors... A Tesla coil has a v, ery different
spark propagation mechanism than lightning or a DC single shot spark (like a
Van deGraaff generator, or, more commonly in impulse research, something
like a Marx generator). However, like lightning, these are all
characterized by extremely non uniform fields, at least at some point in the
process. (Tesla coils can make quite long sparks with relatively low
voltages because they are repetitive impulses and have high peak power in
the impulses)
Add a couple Lenhide Jars
> (sp?) and they and give you a poke you won't soon forget. It has also been
> shown that a single *sharp* protrusion added to the static ball on top can
> completely eliminate the *big* spark. A coronal discharge will develop
> around the sharp point bleeding off the energy.
Yes, indeed, having sharp corners will create corona, which will bleed off
charge, but that's a small scale effect. On a Van deGraaff generator, where
you're basically charging a capacitor of a few tens of pF with a current of
microamps, a corona current of a microamp is significant. When you're
looking at transferring 10's of Coulombs at kiloAmps, as in lightning, it's
another story entirely. Corona is the bane of HV engineers everywhere
(except those making ionizers<grin>) for a bunch of reasons: Where there's
corona, there's ozone and nitrogen oxides and other reactive/corrosive
species, which tends to degrade insulation; Corona is often a precursor to
breakdown (by effectively reducing the total gap length)
> This is the premise behind lightening rods, static balls, and static
wicks.
> How well it works in real life is the question.
>
> It should be noted that "Static wicks" are a requirement on most aircraft
> certified in the last 20 or 30 years.
These wicks are designed to reduce P-static, not eliminate lightning though.
What they're really designed to do is to reduce the corona
inception/breakdown voltage, so that as the airplane body gets charged, it
discharges in lots of little zaps as opposed to a few big ones. Essentially
the same as putting a needle point on your Van deGraaff generator collector.
Charge still accumulates (from the precipitation particles (impact
charging)) at the same rate, but it dissipates more quickly. They also
provide a "preferred location" for the discharge to occur (i.e. the static
wick, as opposed to your Com or Nav antennas).
>
> OK, so we know lightening is nothing more than the release of a very
> powerful "spark" of static electricity between two charged points
Ooops.. the big problem with lightning is that it isn't a simple gap between
two points. At least one of the sides is a fairly large (km) scale diffuse
charged cloud, and the other is the earth. Sure, there are rough and pointy
spots on the earth side, but in the context of the overall scale (several
thousand meters), a few tens of meter isn't going to make a lot of
difference. And, you'd have a hard time using the "shortest distance"
argument to explain why golfers standing in open fields get hit. The 2
meters or so isn't going to make a lot of difference in a gap of 1000
meters. Likewise, the field concentration from that golfer isn't all that
big, in the context of the 10kV/meter kinds of fields you see before a
strike.
>
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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