Roger (K8RI) wrote:
>
> On 5/28/2010 1:39 AM, jimlux wrote:
>> Uhhh... I'd kind of like to see a physics explanation of not like to
>> travel in circles or bends and getting off and going somewhere else.
>>
>>
> Well, lightning likes to take the shortest path although that may not be
> a straight line
A bit anthropomorphic isn't it..
I think a fairer statement is that lightning is an electrical current,
and it distributes according to the impedances. "take shortest path"
leads folks to believe that it ALL goes on the shortest path, and
therein lies much trouble.
> Any conductor, straight, bent, circular, or crooked has inductance.
> The rise time of a pulse creates a reverse EMF. The steeper the rise
> time the greater the reverse EMF.
Sure
> A discontinuity in the coax such as a bend or coil increases the
> inductance at that point.
> In the case of a coil and a rapid rise time the shortest path may be
> from turn to turn, to another cable, or to the tower rather than to
> continue through the coil.
Nope.. I think you have a non-physics definition of "shortest path"..
What happens is this.. the voltage rises (because of the inductance) and
then, if and only if, that voltage is greater than the breakdown
strength of the insulation, you get a breakdown to the adjacent
conductor. If we immersed your coil of coax in a tank of insulating
oil, the "shortest path", mechanically, hasn't changed, but it won't
breakdown the same as it would in air.
Or, you could have coupling via the magnetic field (e.g. a transient on
the shield of one cable couples a transient to an adjacent cable)
> I've seen lightning strike the top of a tower and get off half or 3/4 of
> the way down without following the guy wires. This was explained to me
> that the rise time created enough reverse EMF to make it easier for the
> lightning to get off the tower and jump to ground rather than proceed
> straight ahead even though the tower provided a substantial conductor.
But what does "make it easier" mean here? It's not like there's some
resistance that changes as a function of rise time. It's a breakdown
phenomenon... if the voltage is above the breakdown voltage for the gap,
then it breaks down, an arc forms, and *now* you have a new path.
If you're talking about the stroke hitting the top and then coming out
halfway down, and then forming another arc connecting to the ground,
that's sort of a different matter. That's more the chaotic nature of
the breakdown in a very non-uniform field, and I doubt that small
changes in inductance have much effect. Lightning goes in jumps, with
the prevailing theory going something like this..
Just before a discrete jump, the spark channel is fairly conductive, and
current flows in the channel towards the end of the spark leader
(because the surrounding electric field is oriented that way). As charge
accumulates at the end of the leader, the voltage at the end starts to
rise, increasing the Efield at the tip.. when the E field exceeds the
breakdown strength of the air, a new section of channel starts to form,
in a pretty much random direction, although generally guided by the
surrounding large scale field (i.e. sparks twist and turn, but generally
go in one direction). That new channel grows as long as it can accept
charge from the existing channel behind it. The initial breakdown is
fairly low current, and just ionizes the air, but once the ionization is
done, the resistance drops, so the current flow into the newly ionized
area is high enough to heat the air, which reduces the resistance more,
etc. It's sort of like connecting a piece of transmission line to the
end of a longer piece, with the longer piece already having a high DC
voltage on it.
Same deal with hitting an object and jumping off.. current flows, charge
builds up, exceeds local breakdown strength, new spark leader forms, etc.
>> A loop has inductance. There will be some amount of voltage drop across
>> the inductance. But a 1 turn loop doesn't have a heck of a lot of
>> inductance (I think 6" diameter is like 0.5 uH)... about the same as a
>> half a meter of straight wire.
>>
>> Lightning has a rise time of about 1 microsecond, and peaks at say, 50kA
>> (higher than most strokes, but makes the math easy).. so the voltage
>> drop is Ldi/dt is 0.5E-6*50E3/1E-6... about 25kV... that's big, but not
>> huge.. not going to leap feet, in any case. Maybe an inch or so.
>> Certainly would punch through the jacket on most coax and arc to any
>> touching metal. (hence the electrical code requirements to space
>> conductors that might carry lightning away from other ones)
>>
>>
> Yet I've seen lightning bolts in both photos and in person, that jumped
> many feet, or rather 50 to 60 yards when they had only 50 to 100 foot of
> tower between them and ground.
Yes.. but that's not because of a few microhenries of inductance. The
inductance of the stroke itself is probably greater than that of the
tower. It's all in the
"stop/charge/jump/extend/stop/charge/jump/extend" stepwise extension
process.. that "jump" phase when it starts a new step is essentially random.
>
> The energy in lightning bolts is many times the stuff we were working
> with, so I can understand the induced and reverse EMF values being
> enough to jump quite a ways.
Sure, the currents are high, but even with fairly high inductance, it
can't get enough voltage to jump large distances. To get meter scale
jumps you need hundreds of kV AND fairly nonuniform electric fields.
>
> What do you get for a square steel tower, 2' on a side? Yet the
> inductance was enough to cause the bolt to get off the tower and jump
> sideways.
fairly low.. probably half uH per meter. The jumping off is likely due
to other factors than the inductance...
That is, you could hit that same tower 100 times with the same impulse,
and a few of them will jump off one direction, a few will jump off
another, and most will just go down the tower.
It's like a big tesla coil or Van deGraaff generator.. you know there
will be sparks, you know "generally" where they will go, but you can't
predict the exact path.
(the difference is that a Tesla Coil or Van deGraaff has very different
rise time properties: the energy source is much lower inductance than
lightning, the rise time is very much faster, so the steps/branches in
the spark development are at a finer scale.. current flows through the
stepped leader to the new tip much quicker.)
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