Excellent explanation Roger. Thanks.
BTW, the surface of the sun is something like 10,000 deg K. I think the air
next to lightning plasma is about 30,000 deg K.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of K8RI on Tower talk
Sent: Tuesday, July 11, 2006 3:45 PM
To: TOWERTALK@contesting.com
Subject: Re: [TowerTalk] CATV & Phone grounds
> ORIGINAL MESSAGE:
>
> On Tue, 11 Jul 2006 14:29:33 -0500, you wrote:
>
>
>>Rather than thinking of lightning as an electrical current flowing
>>from ground to cloud or vice versa, I find it more interesting to
>>think about what it is composed of: plasma. The free electrons and
>>ions created by the plasma must be neutralized by some method. This
>>is where current flow is involved. Pick a path and connect via some
>>conductor (tower, copper wire, tel wire, power line, water pipe, etc)
>>to facilitate the neutralization.
>>It
>>is fairly easy to assume current will travel only one way on the
>>conductor.
>
> ------------ REPLY SEPARATOR ------------
>
> As I understand it, plasma (in the general sense) can exist with or
> without current flow.
Plasma can be created with high temperature, voltage, or both as you do have
a high temperature once the electrons are stripped off the atoms and
molecular bonds broken.
> In the absence of external heat, plasma will
The only difference I'd use the word "energy" instead of heat.
> quickly recombine into normal atoms, but while it is in the plasma
> state, it will conduct much like a wire would.
This is one of those "Kinda, sorta" things and it does help to think of the
plasma like a conductor, or conduit to support the initial step leader and
provide a low enough resistance for the voltage to "flash over" to create
the strike. The "step leader" forms a plasma conduit albeit neither the
straightest of most continuous. Note how the lightning flashes off to the
sides of the main strike or how some consisting of many lines spread out
over large areas.
Nor is the step leader a low resistance like we'd expect to find in a wire
or most conductors. At these voltages and environment every thing becomes
relative to a point. Initially the step leader is a relatively high
resistance, weak plasma (Ionized air). However the resistance is low enough
that the initial strike can "arc over". This produces a much hotter plasma
and much higher density (very high pressure for a short duration) and much
lower resistance which provides a path that requires much less voltage for
an arc to be established.
That in turn allows more, but weaker charge from the clouds to be drained in
repeated strikes.
I don't have the figures at hand for the pressure inside the initial bolt
but it is quite high. Temperatures are phenomenal.
If you've ever used a "plasma torch" for cutting metal you have at least
the beginnings of an idea as to the temperature inside a lightning bolt.
The typical "torch" uses low pressure air and a relatively low power arc to
generate a plasma of roughly 30,000 degrees. This is hot enough to vaporize
thin sheet steel so quickly I have cut painted "barn metal" without
scorching the paint either side of the cut. Even with a smaller torch and
using a guide I've cut 1/4" steel plate more smoothly than I could have done
in a band saw and I'm a rank amateur when it comes to the torch. However
IIRC lightning can produce temperatures many times this. I believe some
strikes are capable of producing something on the order of one to two orders
of magnitued more which is hotter than the surface of the Sun.
>
> In the case of lightning, the enormous voltage present rips the air
> molecules and atoms apart, creating the plasma, which then provides a
> current path for the lightning strike.
>
> If my understanding is wrong, please enlighten.
Close enough for government work <:-)) The short version (ignoring the so
called super strokes) is the step leader forms a weak plasma allowing the
initial stroke to arc over, which produces a much stronger plasma to support
the following strokes. Most of the HF component is due to the rise and fall
times of the strokes.
Roger Halstead (K8RI and ARRL 40 year Life Member) N833R - World's oldest
Debonair CD-2 www.rogerhalstead.com
>
> Bill, W6WRT
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