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Home > Archive > Home Repair forum > December 2006 > Lightening characteristics (was Re: Solder?)
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Lightening characteristics (was Re: Solder?)
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| Roger Grady 2006-12-25, 5:25 pm |
| "The Other Funk" <bobbie@moondoggie.com> wrote:
>I haen't see a great deal of lightning protected buildings but I know that I
>have seen heavy gauge soild wire used.
>Also, I think, not know, that lightning wol be considered DC.
Lightening is definitely AC, with frequency components up into the
hundreds of megahertz.
As someone else mentioned, it's static electricity, but it's also a
big electrical spark, with a very fast rise time. That fast rise time
is what generates the high frequency components. The high-frequency
nature is what calls for consideration of more than just the current
flow when designing lightning protection.
The voltage drop across a conductor carrying AC depends on its
inductance as well as its resistance. A #10 wire will carry the
current of a typical lightning bolt without fusing (note I said
typical - that means maybe 10,000 amps. for several microseconds. The
occasional superbolts with many times that current for a longer
duration would need a much bigger wire.) But a #10 wire has a fair
amount of inductance at RF frequencies where lightning has significant
energy, and thus there will be a higher voltage drop because of the
inductance. To get a lower inductance for a given cross-sectional
area, strap conductors are used. The way the conductors are routed
also is important. A sharp bend has more inductance than a gradual
one, a straight wire even less.
Lightning protection is an interesting subject. Good protection is
possible, but it requires attention to lots of details. If you forget
even one and suffer a hit, the lightning will find the weak spot,
guaranteed.
Roger Grady k9opo@sbcglobal.qlfit.net
To reply by email, remove "qlfit." from address
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| Roger Grady wrote:
> "The Other Funk" <bobbie@moondoggie.com> wrote:
>
>
> Lightening is definitely AC, with frequency components up into the
> hundreds of megahertz.
>
> As someone else mentioned, it's static electricity, but it's also a
> big electrical spark, with a very fast rise time. That fast rise time
> is what generates the high frequency components. The high-frequency
> nature is what calls for consideration of more than just the current
> flow when designing lightning protection.
>
> The voltage drop across a conductor carrying AC depends on its
> inductance as well as its resistance. A #10 wire will carry the
> current of a typical lightning bolt without fusing (note I said
> typical - that means maybe 10,000 amps. for several microseconds. The
> occasional superbolts with many times that current for a longer
> duration would need a much bigger wire.) But a #10 wire has a fair
> amount of inductance at RF frequencies where lightning has significant
> energy, and thus there will be a higher voltage drop because of the
> inductance. To get a lower inductance for a given cross-sectional
> area, strap conductors are used. The way the conductors are routed
> also is important. A sharp bend has more inductance than a gradual
> one, a straight wire even less.
>
> Lightning protection is an interesting subject. Good protection is
> possible, but it requires attention to lots of details. If you forget
> even one and suffer a hit, the lightning will find the weak spot,
> guaranteed.
>
>
> Roger Grady k9opo@sbcglobal.qlfit.net
> To reply by email, remove "qlfit." from address
'Static' electricity
Before 1839, physicists regarded "static electricity" as a substance
distinct from four other kinds of electricity: "current" or "Voltaic"
electricity, "Animal" or "bioelectricity," "thermoelectricity" from
thermocouples and "magnetoelectricity" from coils. In that year Michael
Faraday published the results of his experiments on the Identity of
Electricities. He demonstrated that the divisions between static, current,
etc., were illusions, that all five "kinds of electricity" were actually
collections of phenomena, while electricity itself was a single entity
appearing in negative and positive forms.
Today we regard static electricity as a subject heading also called
Electrostatics: a class of various phenomena associated with substances or
objects having a net electric charge. In everyday usage, "static
electricity" typically refers to charged objects with voltages of sufficient
magnitude to produce visible attraction, repulsion, and electrical sparks.
Static electricity can be a serious nuisance in the processing of analog
recording media, because it can attract dust to sensitive materials. In the
case of photography, dust accumulating on lenses and photographic plates
degrades the resulting picture. Dust also permanently damages vinyl records
because it can be embedded into the grooves as the stylus passes over. In
both cases, several approaches exist to combat such dust deposition. Some
brushes, particularly those with carbon fiber bristles, are advertised as
possessing anti-static properties. Also available are handheld static guns
which shoot streams of ions to discharge static on records and lenses.
Note that the charges associated with static electricity need not be still
or "static." The presence of charge motions and electric current does not
detract from the net charge, the electrostatic forces, nor from the sparking
and corona discharge, or other phenomena. Electric current and electrostatic
phenomena can exist simultaneously in the same system.
Static electricity is an important element in the biological process of
pollination by bees, since the charge on a bee's body helps to attract and
hold pollen.
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| Roger Grady wrote:
> "The Other Funk" <bobbie@moondoggie.com> wrote:
>
>
>
>
> Lightening is definitely AC, with frequency components up into the
> hundreds of megahertz.
>
<snip>
It's a pulse. It only exhibits one polarity during its duration.
Whether you characterize that as DC or AC is largely irrelevant.
If you call it AC, then you can easily get AC from a dry cell, and
there is probably no such thing as DC -- everything we normally
consider DC varies to some degree, if only off/on.
If you call it DC, then you'll have trouble when you find you need to
apply AC analysis tools to its propagation.
So it's probably best to just call it a pulse and let it go at that.
--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.
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| David Nebenzahl 2006-12-25, 8:25 pm |
| CJT spake thus:
> Roger Grady wrote:
>
> <snip>
>
> It's a pulse. It only exhibits one polarity during its duration.
> Whether you characterize that as DC or AC is largely irrelevant.
>
> If you call it AC, then you can easily get AC from a dry cell, and
> there is probably no such thing as DC -- everything we normally
> consider DC varies to some degree, if only off/on.
>
> If you call it DC, then you'll have trouble when you find you need to
> apply AC analysis tools to its propagation.
>
> So it's probably best to just call it a pulse and let it go at that.
It's pulsed *DC*. As you point out (and as I suspected), the pulse is of
a single polarity. AC, by definition, is bipolar, so it's not AC (or
it's a degenerate case of AC).
Which makes sense, knowing how lightning is produced: the charges in the
clouds which produce it all have a single polarity. If they didn't,
their charges would cancel each other out.
Now I wonder what polarity it is: time to fly a kite, with a recording
DVM attached to the string?
--
Just as McDonald's is where you go when you're hungry but don't really
care about the quality of your food, Wikipedia is where you go when
you're curious but don't really care about the quality of your knowledge.
- Matthew White's WikiWatch (http://users.erols.com/mwhite28/wikiwoo.htm)
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| The Other Funk 2006-12-25, 9:25 pm |
| Finding the keyboard operational
David Nebenzahl entered:
[color=darkred]
> CJT spake thus:
>
I can see that I have a lot to learn about lightning. I gaurentee that I
learned about it from someone who was teaching before 1839. Those nuns
seemed to be that old anyway.
Bob
--
--
Coffee worth staying up for - NY Times
www.moondoggiecoffee.com
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| David Nebenzahl wrote:
> Pop` spake thus:
>
>
> Isn't "lightening" that stuff they put into cakes to make them lighter
> and fluffier?
That's what I thought! But then maybe it's the "spilled" vs "spilt"
differences in language, who knows? Or, (gasp), ignorance? Dunno; thought
it was curious though. I'm kind of a light(en)/(n)ing buff<g>.
Pop`
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David Nebenzahl wrote:
>
> Which makes sense, knowing how lightning is produced: the charges in the
> clouds which produce it all have a single polarity. If they didn't,
> their charges would cancel each other out.
> Now I wonder what polarity it is:
Time to start a whole new debate!
Does polarity change between cloud-to-ground and ground-to-cloud
lightning strikes?
Is current flow negative to positive electron flow, or positive to
negative hole flow???
(For any of you who are not familiar with hole flow, see:
http://www.mste.uiuc.edu/murphy/HoleFlow/HoleFlow.html )
Red
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| Red wrote:
> David Nebenzahl wrote:
>
>
>
>
> Time to start a whole new debate!
>
> Does polarity change between cloud-to-ground and ground-to-cloud
> lightning strikes?
>
> Is current flow negative to positive electron flow, or positive to
> negative hole flow???
> (For any of you who are not familiar with hole flow, see:
> http://www.mste.uiuc.edu/murphy/HoleFlow/HoleFlow.html )
>
> Red
>
Air is not a semiconductor, AFAIK.
--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.
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