St Elmo's Fire (was Re: Lightning)
Matthew Moore
>>
>>Maybe one could argue that by bleeding off the charge like that, one is
>>less likely to take a hit (reducing the potential difference); but on the
> ^^^^^^^^^^^^^^^^^^^^^^^^^
>This part is probably true.
>
>>other hand, pointy vertical rods are a much better target than other
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>things (lightning rods, while generally saving a structure, do tend to
>>increase its liklihood of being hit), and the fact that there was all this
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>I know this is the commonly accepted idea behind lightning rods, but as I
>learned in my college physics class, lightning rods actually REPEL lightning,
>rather than ATTRACTING it. The physics behind it goes something like this:
> The lightning rod MUST be grounded (electrically connected to the earth),
> otherwise it's useless. The other important feature of a lightning rod is
> that it is very pointy, not merely the highest point on a structure.
> A few basic facts need to be introduced before getting to the actual
> explanation.
> - Lightning is a discharge of excess negative (-) charge toward a source
> of excess positive (+) charge.
> - Unlike charges (+ & -) attract; like charges (- & - , or + & +) repel.
> - The earth is a natural source of negative charge.
> Okay, with those things in mind, the lightning rod is therefore (by being
> connected to ground) a source of negative charge. Due to the shape of the
> tip of the lightning rod being pointy (more precisely, a VERY small radius
> curve), the excess negative charge is radiated out from the tip of the rod
> in all directions. (an explanation of that gets rather technical)
> Therefore, with this field of negative charge radiating
> out from the rod, a bolt of lightning (also negative charge) is REPELLED
> from the rod, NOT attracted.
>
Lightning conductors _tend_ to prevent lightning strikes.
A LC is an earthed rod, attached to the top of the protected
structure. The base of any nearby thundercloud will be negatively
charged wrt the earth. This charge induces a positive charge at the
top of the rod (ie like charges repel, so the free electrons at the
top of the rod are repelled down it, leaving net +ve charge at the
top).
Charge may be distributed irregurlarly on the surface of any charged
conductor. In particular, it builds up and extends much further away
from any pointed area. (This is known as point action). So the air
above the pointy bits carries a high net +ve charge, and may become
ionized, (causing the discharge seen). This ionized region is
described as a space charge, and is what _prevents_ lightning
discharge between ground and cloud.
The mechanism seems to be similar to capacitance, in that potential
between the two plates of a capacitor is reduced because one plate is
earthed. Similarly, earthing the LC allows the flow of charge between
earth and the pointy tips, causing a reduction in potential difference
between cloud and ground.
Cross-posted to physics and electronics, corrections very welcome via
posting or email, particularly on the inferred similarity with
capacitance. The convention wrt to current flow has been ignored as
confusing, and the mechanism of lightning strike (leader spark from
earth to cloud, followed by return discharge from cloud to ground)
omitted for brevity.
Ken Greer
+++ [other stuff about a corona discharge (St. Elmo's Fire)]
+++
++I know this is the commonly accepted idea behind lightning rods, but as I
++learned in my college physics class, lightning rods actually REPEL lightning,
++rather than ATTRACTING it. The physics behind it goes something like this:
---------- stuff deleted -----------
+Lightning conductors _tend_ to prevent lightning strikes.
Ok, so "knowing" all this, as I understand it to be said, one should be
safer standing on the roof next to a lightning rod, instead of somewhere
else? Or do I completely miss the point?
--
Kim L. Greer try: k...@orion.mc.duke.edu
Duke University Medical Center kgr...@mcnc.org
Div. Nuclear Medicine POB 3949 k...@dukeac.ac.duke.edu
Durham, NC 27710 919-660-2711x5223 fax: 919-681-5636
Scott Johnson
>In article <1990Jul18....@ioe.lon.ac.uk> tee...@ioe.lon.ac.uk (Matthew Moore) writes:
>+++ [other stuff about a corona discharge (St. Elmo's Fire)]
>+++
>++I know this is the commonly accepted idea behind lightning rods, but as I
>++learned in my college physics class, lightning rods actually REPEL lightning,
>+Lightning conductors _tend_ to prevent lightning strikes.
>Charge may be distributed irregurlarly on the surface of any charged
>conductor. In particular, it builds up and extends much further away
>from any pointed area. (This is known as point action). So the air
>above the pointy bits carries a high net +ve charge, and may become
>ionized, (causing the discharge seen). This ionized region is
>described as a space charge, and is what _prevents_ lightning
>discharge between ground and cloud.
OK,
The point will have a higher electric field and IF great enough it could
cause ionization of the air in the vicinity.
Ionization of gas creates a plasma (which will also be POSITIVELY
charged, like the rod, since the electrons lost by the nitrogen
molecules will be
drained off by the conductor around which the field is concentrated)
If the field is only strong enough to ionize the air in the immediate
area, we have a cloud of glowing plasma around the conductor, (St. Elmo's
fire I would assume); if we have a strong enough field to break down
the air between the negative charge source in the atmosphere and
the rod, then plasma is created along the entire length and a lightning strike
results.
If you see a traveling bolt of lightning, then you are seeing a ionization
process begining first at the area of highest electric field.
In this discussion we are talking about charges and fields
of VERY LARGE proportions (look at the breakdown field for nitrogen)
and I would not believe that the leakage current
in air could ever have an effect
on the eventual result (strike vs. no strike vs. other place struck).
Also, resistance of almost any object (lets leave out rubber, plastic
and such insulators, of course) is small when compared with the air, so whether we had an
excellent ground or a 10K ohm ground i think is also irrelevent
to the final result.
With all this handwaving, I think we all should hire out as
scarecrows! :)
Kim Greer writes:
> Ok, so "knowing" all this, as I understand it to be said, one should be
>safer standing on the roof next to a lightning rod, instead of somewhere
>else? Or do I completely miss the point?
>--
Kim, I think they would tell you that this is a bad idea,unless you have
a well conductive pointy head (which I've yet to see at Duke) in which case
the lightning will avoid you also and strike a tree down the road.
I, however, have heard enough of this nonsense! I didn't study E-mag and
solid state physics this long to still be able to swallow these lines!
(by the way there is also a car down at the Ford place that 3 people
committed suicide in. They can't sell it and its going cheap...)
I think maybe this 'lightning rods repel lightning' stuff was started as
a cruel textbook joke to baffle physics students by some screwed up prof.
CAN ANYONE COME UP WITH A REFERENCE?
SURELY IF THIS 'STRANGE BUT TRUE STORY' IS FACT, THEN IT'S DOCUMENTED.
i'D LIKE TO GET TO THE 'THEORY' BEHIND WHAT I CONSIDER VICIOUS
RUMOURS ABOUT MY FAVORITE 'FIELDS'. :)
The earth is round.
Rock n Roll is best loud.
Sailing is expensive,
and lightning STRIKES lightning rods!
Thats how they protect a house, they deflect the current!
Come on guys!
scott johnson:
sjoh...@gumbee.mlb.semi.harris.com
Melbourne, Fl
sjoh...@booboo.mcnc.org
Dept. of Electrical Engineering
North Carolina State University
Raleigh, NC
Richard M. Mathews
The main error is the claim that "the earth is a natural source of negative
charge." This means the end result about lightning rods repelling lightning
is also wrong.
>>I know this is the commonly accepted idea behind lightning rods, but as I
>>learned in my college physics class, lightning rods actually REPEL lightning,
>>rather than ATTRACTING it. The physics behind it goes something like this:
>> The lightning rod MUST be grounded (electrically connected to the earth),
>> otherwise it's useless. The other important feature of a lightning rod is
>> that it is very pointy, not merely the highest point on a structure.
>> A few basic facts need to be introduced before getting to the actual
>> explanation.
>> - Lightning is a discharge of excess negative (-) charge toward a source
>> of excess positive (+) charge.
>> - Unlike charges (+ & -) attract; like charges (- & - , or + & +) repel.
>> - The earth is a natural source of negative charge.
>> Okay, with those things in mind, the lightning rod is therefore (by being
>> connected to ground) a source of negative charge. Due to the shape of the
>> tip of the lightning rod being pointy (more precisely, a VERY small radius
>> curve), the excess negative charge is radiated out from the tip of the rod
>> in all directions. (an explanation of that gets rather technical)
>> Therefore, with this field of negative charge radiating
>> out from the rod, a bolt of lightning (also negative charge) is REPELLED
>> from the rod, NOT attracted.
Here's the real scoop (source: The Feynman Lectures on Physics, Vol 2,
chapter 9). Away from thunderclouds, the earth has a negative charge
relative to "the sky." The potential difference is quite incredible:
about 100 volts per meter. Why does this not electrocute you? Because
your skin is so much better at conducting than air, your skin can be
treated as a perfect conductor. Your head and your feet have the same
potential as the earth. A foot from your nose, the air is at a potential
of +200V.
The "battery" which keeps this enormous potential charged is all of the
thunderclouds in the world. Relative to the earth, most of the bottom of
a thunderhead has a negative charge (the top of the cloud and a section
in the middle of the bottom have positive charges). Over most of the
world it is fair to say that "the earth is a natural source of negative
charge." Near the cloud, however, the earth is positive relative to the
cloud. Lightning is a discharge of the supercharged negative portion of
the cloud to the positive ground.
A sharp lightning rod will concentrate the positive charge of the earth
and bring it up closer to the source of negative charge. The lightning
rods do attract lightning. A lightning strike begins with a dark leader
which moves in steps towards the ground. This leaves a trail of ionized,
negatively charged air which conducts well. When the leader gets close
to a sharp grounded point (perhaps a hundred meters), the electric field
near that point gets very large. The air ionizes there, and a trail of
positive charge moves up to meet the lightning leader (well, negative
charges progressively farther and farther from the lightning rod move
down into the rod). When the trail coming up from the rod and the longer
trail coming down from the cloud connect, the circuit is completed and
the bright stroke of lightning moves up to the cloud. The current is
about 10000 amps at its peak, and it carries down about 20 coulombs
of negative charge.
All of the negative charge from all of the lightning strikes across the
world spreads out across the planet and produce the 100 volts per meter
potential difference observed under clear skies. Since some regions of
the world have frequent morning thunderstorms and other regions have
evening thunderstorms, the potential varies through the day. On the
average it stays around 90 V/m from 0h to 12h GMT with a minimum at 4h
GMT. It rises steadily to about 120 V/m at around 19h GMT and then
drops steadily back to 90 V/m at 0h GMT the next day. Note that the
potential varies over the whole world at the same time -- the peak is
at a specific Greenwich time, not local time.
Richard M. Mathews
Locus Computing Corporation
ric...@locus.com
lcc!ric...@ucla-se.ucla.edu
Dave Tiller N2KAU
-
-
-I, however, have heard enough of this nonsense! I didn't study E-mag and
-solid state physics this long to still be able to swallow these lines!
-
-I think maybe this 'lightning rods repel lightning' stuff was started as
-a cruel textbook joke to baffle physics students by some screwed up prof.
-
-CAN ANYONE COME UP WITH A REFERENCE?
-SURELY IF THIS 'STRANGE BUT TRUE STORY' IS FACT, THEN IT'S DOCUMENTED.
-i'D LIKE TO GET TO THE 'THEORY' BEHIND WHAT I CONSIDER VICIOUS
-RUMOURS ABOUT MY FAVORITE 'FIELDS'. :)
Popular Mechanics had an article on lightning protection for mountaintop
antenna installations. It used giant mushroom shaped wire mesh structures
to avoid the point charge buildup so the lightning would hit the mushroom,
_not_ be deflected toward the antenna tower. Unfortunately I can't provide
the chapter and verse, since it's been a few years, and my Pop Mechs are in
Va. Anyone else read this article?
--
David E. Tiller da...@tsdiag.ccur.com | Concurrent Computer Corp.
FAX: 201-870-5952 Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117
UUCP: ucbvax!rutgers!petsd!tsdiag!davet | Oceanport NJ, 07757
ICBM: 40 16' 52" N 73 59' 00" W | N2KAU @ NN2Z
Ken Greer
>>In article <1990Jul18....@ioe.lon.ac.uk> tee...@ioe.lon.ac.uk (Matthew Moore) writes:
>>+++ [other stuff about a corona discharge (St. Elmo's Fire)]
>>+++
>>++I know this is the commonly accepted idea behind lightning rods, but as I
>>++learned in my college physics class, lightning rods actually REPEL lightning,
>>---------- stuff deleted -----------
>
>>safer standing on the roof next to a lightning rod, instead of somewhere
>>else? Or do I completely miss the point?
>>--
>
My (not quite sarcastic enough) point also. (See question below).
> The earth is round.
> Rock n Roll is best loud.
> Sailing is expensive,
> and lightning STRIKES lightning rods!
> Thats how they protect a house, they deflect the current!
> Come on guys!
Agreed - (except for the part about R&R).
My only question now to those who *seem* to be saying it deflects
lightning (assuming "deflecting" means away from the lightning rod):
Would you hold an umbrella (which is a lightning rod connected to
ground by the person holding it) up during a lightning storm? I
think not.
I seem to remember some time back hearing (radio/tv?) that "protection" by
lighting rods was just a hoax to con people into buying them - sort of like
a vigilant pet rock - in years past. "Well, you mean you had lightning hit
your barn and you *had* lightning rods??? Well, you must not have had enough
of them up there". Yeah, right. I wish I had a reference for this.
USENET News Administration
Path: janus.Berkeley.EDU!bwood
>Ionization of gas creates a plasma (which will also be POSITIVELY
>charged, like the rod, since the electrons lost by the nitrogen
>molecules will be drained off by the conductor around which the
>field is concentrated)
Not so. The body of the plasma must be electrically neutral, or else it
would be destroyed by it's electrostatic repulsion. However, there
will be a sheath region around the conductor which is depleted of
electrons. The lighter electrons will leave the sheath region until
a field is built up which retards the flow of electrons and accelerates
the ions. This process will balance out to the point where equal fluxes
of electrons and ions are flowing out - zero net current. This is
exactly the same process which occurs when you create a pn junction.
Blake P. Wood - bw...@janus.Berkeley.EDU
Plasmas and Non-Linear Dynamics, U.C. Berkeley, EECS
Geoff Steckel - Sun BOS Software
Several articles (including one in Wireless World about 4 years ago)
noted that the necessity and efficacy of the `sharp point' of a traditional
lightning rod is disputed. The USA style lightning rod has a sharp point,
but in Europe, a (small, 1-5" or 2-12 cm.) round ball is considered more
effective. I can hypothesize a number of reasons for each one being effective:
Points give higher field strength (but at >> 1 MV, does this matter?!)
Balls give more surface area, and up to the point that field strength
goes below some value, therefore give more local ionization.
Does anyone have any more information or references?
geoff steckel (gw...@wjh12.harvard.EDU)
(...!husc6!wjh12!omnivore!gws)
Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line.
This posting is entirely the author's responsibility.
Bill White
>In article <1990Jul19....@mlb.semi.harris.com> sjoh...@gumbee.mlb.semi.harris.com (Scott Johnson) writes:
>>In article <24...@speedy.mcnc.org> kgr...@mcnc.org.UUCP (Ken Greer) writes:
>>>In article <1990Jul18....@ioe.lon.ac.uk> tee...@ioe.lon.ac.uk (Matthew Moore) writes:
>>>++I know this is the commonly accepted idea behind lightning rods, but as I
>>>++learned in my college physics class, lightning rods actually REPEL lightning,
>>>safer standing on the roof next to a lightning rod, instead of somewhere
>>>else? Or do I completely miss the point?
> My (not quite sarcastic enough) point also. (See question below).
>
>> Thats how they protect a house, they deflect the current!
>> Come on guys!
>
>lightning (assuming "deflecting" means away from the lightning rod):
>
> Would you hold an umbrella (which is a lightning rod connected to
> ground by the person holding it) up during a lightning storm? I
> think not.
Well, I remember an article in some electronics magazine about
this subject. It seems that yes, lightning rods can repel lightning, and
yes, lightning rods can also attract lightning. It all depends on how
well the thing is grounded! The big problem is, many lightning rods are
not really well grounded, and as such, what they really do is just pick up
the charge that's in the surrounding environment (read: the air). The
article then stated that one of the best methods for lightning protection
was to erect a grid of metal pipes above a structure, and leave it
ungrounded. It seems that the lightning would then avoid the structure
completely, as a like charge would be induced in the grid as the first
(invisible) stroke occurred.
On the other hand, a well-grounded rod (for a really good ground,
you should have a bunch of "roots" coming off the ground rod in a star
pattern, and salt the whole thing with copper sulfate or equivalent) will
tend to attract lightning.
Keep in mind that, yes, the difference in resistance can be minimal
when the voltages of lightning are considered. But then again, so is
the difference in resistance between a flat open plain, and a person standing
on that plain. But lightning _will_ find the path of least resistance, no
matter how little difference there is from the surrounding terrain.
Of course, the best thing to do is just not build in open, high
terrain :-)
--
| Bill White Internet: bwh...@oucsace.cs.ohiou.edu |
| PAUL'S LAW: |
| You can't fall off the floor. |
John Whitmore
> The USA style lightning rod has a sharp point,
>but in Europe, a (small, 1-5" or 2-12 cm.) round ball is considered more
>effective.
>Does anyone have any more information or references?
>
I was once told by an architect that the local building
code required lightning protection, but almost NEVER a lightning rod
per se. Instead, it is found that an iron pipe (like the sewer gas
vent pipe required in all houses anyway) projecting within a foot
or two of the maximum height of a peaked roof, meets the requirement.
Accordingly, one might say most lightning rods are neither
pointed nor knobbed. They're more sawed-to-length.
So, by the way of references... I have searched in vain my
copy of the Uniform Building Code (UBC '82) for any reference to this.
Please, net.good.samaritan, peruse the (related) works:
Uniform Mechanical Code
Uniform Plumbing Code
Uniform Fire Code
National Electrical Code
and get back to us with a report?
With plastic sewer pipe taking over from cast iron, this
sort of lightning protection may be only applicable to older houses.
I am known for my brilliance, John Whitmore
by those who do not know me well.
Nico Garcia
lightning bolts]
Gentle folk, perhaps we could settle the question of repulsion/attraction by
examining a more basic question: what is a lightning bolt? Is it the massive
electrical current between thunderclouds and the earth carried by plasma,
accompanied by a lot of noise? Or is it any current carried between
thunderclouds and the earth large enough to create visible effects or carry
measurable current?
If I remember correctly from *my* old physics instruction, the lightning rod
acts to dissipate the electrical potential before the strike happens. Much
like doing your work on an anti-static pad, the slight conductive path it
provides dissipates local concentration of electrical potential energy and
prevents sparks between electrically isolated systems when brought into
contact (such as a thundercloud and the earth connected by a column of
rain-filled air.) So if there is a strike nearby, since the damn things
tend to be at the top of buildings in the open and at ground potential,
it'll happen there. But the chance of a nearby strike is much less, so they
are effective protection.
The effect is much like putting a needle into a water balloon: left there,
it drains the pressure before the system ruptures. Yes, that is where the
system is likely to break down. But frankly, if I lived in a wooden building
(which I do), I'd much rather the lightning hit a big conductor tied to the
pipes and drain to ground than let it bust open my roof and light my attic
on fire.
--
Nico Garcia
Designs by Geniuses for use by Idiots
eplunix!cirl!ra...@eddie.mit.edu
Russ Cage
>> chapter 9). Away from thunderclouds, the earth has a negative charge
>> relative to "the sky." The potential difference is quite incredible:
>> about 100 volts per meter.
>
Because the source impedance is incredibly high; a 100K ohm/V DVM
won't see enough current to move it.
Using better contacts (radioactives, which ionize the air slightly
and make it conduct, bringing the contact to the same potential as
the air for small but reasonable currents), you can measure this
gradient quite simply. Wing-leveler autopilots for model airplanes
have been made with a couple of such contacts and some op amps.
--
Russ Cage Ford Powertrain Engineering Development Department
Work: itivax.iti.org!cfctech!fmeed1!cage (Business only)
Home: ru...@m-net.ann-arbor.mi.us (Everything else)
I speak for the companies I own, not for the ones I don't.
Michael Yoshinaga
> chapter 9). Away from thunderclouds, the earth has a negative charge
> relative to "the sky." The potential difference is quite incredible:
> about 100 volts per meter. Why does this not electrocute you? Because
> your skin is so much better at conducting than air, your skin can be
> treated as a perfect conductor. Your head and your feet have the same
> potential as the earth. A foot from your nose, the air is at a potential
> of +200V.
>
Just curious...
Why can't this be measured with a DVM?
Thanks, Mike
--
AM: michael yoshinaga EM: yosh...@cerritos.edu VM: (h)714-761-4322
SM: 11110 e. alondra blvd; norwalk, ca. 90650
Disclaimer ?: Admit nothing, deny everything, demand proof
Tom Gray
>In article <1990Jul19....@mlb.semi.harris.com> sjoh...@gumbee.mlb.semi.harris.com (Scott Johnson) writes:
>-
>-
>-I, however, have heard enough of this nonsense! I didn't study E-mag and
>-solid state physics this long to still be able to swallow these lines!
>-
>-I think maybe this 'lightning rods repel lightning' stuff was started as
>-a cruel textbook joke to baffle physics students by some screwed up prof.
>-
>-CAN ANYONE COME UP WITH A REFERENCE?
>-SURELY IF THIS 'STRANGE BUT TRUE STORY' IS FACT, THEN IT'S DOCUMENTED.
>-i'D LIKE TO GET TO THE 'THEORY' BEHIND WHAT I CONSIDER VICIOUS
>-RUMOURS ABOUT MY FAVORITE 'FIELDS'. :)
>
>Popular Mechanics had an article on lightning protection for mountaintop
>antenna installations. It used giant mushroom shaped wire mesh structures
>to avoid the point charge buildup so the lightning would hit the mushroom,
>_not_ be deflected toward the antenna tower. Unfortunately I can't provide
>the chapter and verse, since it's been a few years, and my Pop Mechs are in
>Va. Anyone else read this article?
>--
The July 1990 issue of TE&M magazine contains an article describing the latest
thinking in lightning protection.
It confirms that a space charge does build up around a lighning rod which
reduces its efficency. A lightning leader must make a closr approach to
the rod to trigger the upward intercepting leader to complete the ground
cloud path for the bolt.
Another contributer mentioned the use of heavy guage wire to conduct
the down current from the rod. As you might expect for currents
with such fast rise times and magnitude things are not that
simple. There is massive amounts of capacitive coupling from the
down conductor to the buildoing structure. This will cause the
structure to rise in potential with the down conductor. This may
not be bad since it causes electrical equipment to rise in in
potenetial with their surrounding and cabling which can prevent
destructive breakdowns.
Lightning protection is a matter for specialists and I am certainly
not one.
Kim Dorian Flowers
>Here's the real scoop (source: The Feynman Lectures on Physics, Vol 2,
>chapter 9). Away from thunderclouds, the earth has a negative charge
>relative to "the sky." The potential difference is quite incredible:
>about 100 volts per meter. Why does this not electrocute you? Because
>your skin is so much better at conducting than air, your skin can be
>treated as a perfect conductor. Your head and your feet have the same
>potential as the earth. A foot from your nose, the air is at a potential
>of +200V.
Anyway of hooking up to this potential difference and drawing some
energy from it? :)
Kim Flowers
Jeff Winslow
|> chapter 9). Away from thunderclouds, the earth has a negative charge
|> relative to "the sky." The potential difference is quite incredible:
|> about 100 volts per meter. Why does this not electrocute you? Because
|> your skin is so much better at conducting than air, your skin can be
|> treated as a perfect conductor. Your head and your feet have the same
|> potential as the earth. A foot from your nose, the air is at a potential
|> of +200V.
|
|Why can't this be measured with a DVM?
Because that 10 megohm input DVM is also a much better conductor than air.
Maybe with something like an electrometer... ??
Jeff Winslow
Russ Cage
>ric...@locus.com (Richard M. Mathews) writes:
>>chapter 9). Away from thunderclouds, the earth has a negative charge
>>relative to "the sky." The potential difference is quite incredible:
>>about 100 volts per meter.
>Anyway of hooking up to this potential difference and drawing some
>energy from it? :)
Yes. An ion source (whether radioactive or merely a corona point)
will conduct micro-amp to milli-amp currents into the air, depending
on the configuration. You need to put it high up.
The problem is using this energy: it is extremely high-voltage and
low-current. You are dealing with a source impedance of megohms to
gigohms. I have seen propotypes of electrostatic motors to convert
this electricity directly to work. They are... different. The
supply is definitely dangerous, and can spark over a good part of
an inch.
Protecting the collection system from lightning is problematic.
This was pretty hot stuff in the alternate-energy school of
thought a while back, but it's pretty cool now. The technical
problems seem to be more than just slightly hard to solve.
Greg Ebert
>>potential as the earth. A foot from your nose, the air is at a potential
>>of +200V.
>
>Anyway of hooking up to this potential difference and drawing some
>energy from it? :)
>
Er, um, it depends upon how determined you are. I read, perhaps in Scientific
American , that there is a current density of 1000 amps/sq mile over the
surface of the earth, but I don't recall what the weather conditions were; I
would presume it would be during an impending electrical storm. That figures
out to ~40uA/sq foot, unless you happen to be 'lucky' enough to snare a
lightning bolt. The same article mentioned that NASA has measured the current
of several bolts which struck launch towers to be up to 30,000 amps (OY !!).
I had an insulator from a 16KV power line which had an unusual melted spot on
the mounting flange, like someone heated it with a torch and let the molten
metal 'run' a bit. I've always wondered if it was from a lightning strike.
Has anyone ever seen lightning strike the ocean ? Must be rather trippy...
Bob Tidrick
>> about 100 volts per meter. Why does this not electrocute you? Because
>> your skin is so much better at conducting than air, your skin can be
>> treated as a perfect conductor. Your head and your feet have the same
>> potential as the earth. A foot from your nose, the air is at a potential
>> of +200V.
>Why can't this be measured with a DVM?
This potential has an "extremely" high impedance. A static meter just might
measure it.
--
Bob Tidrick
GPID Engineering
Tektronix Inc.
Wilsonville OR.
Brian Hurley
Someone asked about lightning strikes over the ocean. My dad has this
one in a millon photograph of lighting striking a pillar of water that was
thrust up by a depth-charge. The sky was clear, it was in full daylight,
and there was not a cloud in the sky.
The top third of the pillar was incandesent where the bolt entred. The
film was WAY over exposed along the path of the bolt, but ther was no glare.
Makes the bolt look as if it was a solid object protruding out of the
column of water.
Some where in storage I have this TimeLife book on weather, containing
very general topics on weather with lot's of cool pictures. The book has
a large section on lightning, and pictures that will raise the fur on the
back of your neck!
- Brian
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Kevin William Ryan
Heard a nasty lightning story some time back, thought folks might
find it interesting.
A friend of mine had a bunch of Hewlett Packard (sp?) equipment set
up. The equipment was connected by a 'daisy chain' arrangement, which
made a loop connecting the various modules. Due to space requirements
they had the equipment set up on a table around a pillar, so that the
daisy chain circled the pillar itself.
What they didn't know was that the ground wire for a lightning rod
ran down inside the pillar.
There was, of course, a storm. When the lightning struck the rod the
equipment all but exploded. Smoke, cracked boards, warped containers,
melted wires, fused components, the works. Imagine the induced voltage
in a circular conductor roughly four feet in diameter around a conductor
carrying up to 30,000 amps...
Needless to say the equipment was a _total_ loss.
kwr
Internet: kr...@andrew.cmu.edu
Trevor Peacock
> energy from it? :)
>
> Kim Flowers
Well, I seem to remember reading how experimenters in the early days obtained
their high voltage supplies from insulated wires held aloft on poles and strung
across the countryside. The end of the wire was brought into the lab and
connected to a metal sphere to hold the charge. There was mention that the
charge increased significantly when it was snowing.
I haven't tried this - my garden is a bit too small :)
I think the book was called "the man who was frankenstein" or something like
that.
Trev..
Fred Brooks
>I had an insulator from a 16KV power line which had an unusual melted spot on
>the mounting flange, like someone heated it with a torch and let the molten
>metal 'run' a bit. I've always wondered if it was from a lightning strike.
I've read that dc voltage (maybe .0000115 HZ ?) is induced into power lines
at about 7 volts per kilometer with current in the amp range. I wonder
if anyone has tried to use this as a power source. A article in Spectrum
said that this was suspected as the cause of a major power backout in
Canada a few years ago. The induced DC voltage saturated the core of
the big step up transformers which then generated harmonics that overloaded
and tripped the breakers.
--
Defend your 2nd amendment rights.
Fred Brooks
(503) 255-3990 Portland Oregon
Paul M. Koloc
>From: bw...@janus.Berkeley.EDU (Blake Philip Wood)
>Path: janus.Berkeley.EDU!bwood
>
>>Ionization of gas creates a plasma (which will also be POSITIVELY
>>charged, like the rod, since the electrons lost by the nitrogen
>>molecules will be drained off by the conductor around which the
>>field is concentrated)
>
>Not so. The body of the plasma must be electrically neutral, or else it
>would be destroyed by it's electrostatic repulsion.
Corona is a weakly ionized plasma, existing in a strongly divergent electric
field and it IS continuously "DESTROYING ITSELF". That is what produces
the spectacular "fire" of the Saint Elmo's Fire. The electric field
closest to the lightning rod tip is strongest. Overhead low flying rain
clouds or thunderheads can develop substantially high enough local fields
at an elevated grounded conductor to produce electron avalanches with
enough energy to ionize air molecules. The stronger the field the more
distant from the tip of the conductor the ionization can extend. Mobility
of the ions is clamped by the low diffusivity of ions and molecules, so
they do not reach velocities that ionize. Charge recombinations occur
which produce photon emissions making the process faintly visible. It must
be slightly charged due to the presence of the electric field. The weakly
ionized plasma distorts the local electric field topology and actually
concentrates force lines locally, thereby strengthening that field. Thus
Lightning is not repelled but quite the opposite IS ATTRACTED ! Weakly
ionized means that the plasma is conducting but still very resistive.
Diverting back to lightning -- it is interesting to compare Saint Elmos's
fire and the positive arc or return stroke. Before a return stroke can
be initiated a step leader approaches the ground from the cloud, and
stores charge in the local atmosphere as it descends. The cloud to ground
electric field converges radially upon the tip of the leader. As a result
electrons spray outward from the tip driven by this field. They are
pulled radially outward for twenty or thirty meters before the electric
field drops to the point where they can attach to oxygen molecules. They
then become "frozen in" the atmosphere suspended for an instant while the
leader continues to close the ground/channel tip gap. A large diameter
invisible Charge Cylinder surrounding the dart leader channel exists at
the instant the return stroke starts as well as surrounding many branches
of the step or dart leader, most of which do not reach the ground.
The cloud to ground electric field is essentially intensified inversely
to the distance of the ground gap. As that distance closes
to 300 meters, the field reaches breakdown levels for air of several
megavolts (typically 2.5 MV/m) per meter. Then a positive arc is formed
at the ground and it ascends to meet the downward coming step or dart
leader. After conjunction of the arc and dart channels, the sign of the
conjoined channel reverses from negative to positive, causing the radial
electric field surrounding the upper channel to reverse in direction.
The stored electrons are then ripped from their molecular moorings and
yanked into the channel with rise times of a few tens of nanoseconds.
That produces a magnetically driven shock wave we know as thunder.**
**Fortunately, we live below the 300 foot level so we don't get the really
super big blast that occurs further up (above 300 feet) the channel).
Now, back to our charged plasma. It only takes a difference of very very
teeny tiny fraction of the charge difference between electrons and ions
to produce a whopping strong field in plasmas of moderate or high density.
MOST plasmas are essentially equal in charge density. BUT .. .
Continuously charging an air bounded plasma (relative to some part of the
accessible surroundings) takes us from a static plasma to a dynamic one.
Saint Elmo's Fire (coronal discharge) results from a few hundred thousand
volts per meter level of applied electric field which in turn requires a
maintainance current of from few amperes maximum to even as little as a
few milliamperes.
If the electric field is brought up abruptly to surpass the breakdown
voltage and then sustained there for a few hundred microseconds to ten
milliseconds (connected to large charge reserves of the charge cylinder
and cloud), then conditions for lightning and in particular the return
stroke or "positive leader" portion of the lightning stroke exist. The
electrons located in the highest electric field region near the ground
(i. e. the lightning rod tip) are yanked toward the tip, ionizing and
heating ALL of the gas molecules in their paths.
Ultra violet radiation from this plasma then produces electron/ion pairs
within a radius of a centimeter to two beyond the plasma tip boundary.
Since this plasma is highly conducting it becomes an extension of the
lightning rod and is positively charged. Consequently, the next and larger
batch of photo ionized electrons are then accelerated to completely
ionize the air within their region as these electrons follow the electric
field into the previously electron generated plasma. That process
continues extending the return stroke upward at about 1/3 light speed.
Notice that the application of a few 100 megawatts per meter overcomes
the plasma losses and instead of the plasma self destructing it gains
considerable energy and extent. Ahhhh!! The power of sophisticated brute
force.
Once the return stroke passes a point along its potential trajectory, the
radial component of the electric field collapses into the channel and
becomes co-axial with the channel. Consequently, radial diffusion of
ions is not the problem that a few laboratory plasma physicists might
think it would be (as noted above). Another point, is that magnetic
confinement can also be employed, especially for "Super Lightning".
It would not aid radial ion containment in St. Elmo's, however.
>.. .. . The lighter electrons will leave the sheath region until
>a field is built up which retards the flow of electrons and accelerates
>the ions. This process will balance out to the point where equal fluxes
>of electrons and ions are flowing out - zero net current. This is
>exactly the same process which occurs when you create a pn junction.
The presence of a Saint Elmo's Fire implies STRONG electric fields,
WEAK but REAL NET currents, and potentially is a precursor to a
lightning strike.
PS. DO NOT STAND directly under lightning rods, trees, etc. AT A
LOCATION NEAR a GROUNDING POINT. Peak currents can be so high
that the even a SMALL FRACTION of the radial current flow through the
ground away from the grounding point may find a path into one foot
then up the leg and down and out the other leg and foot. Such current
levels could easily stun the victim into respiratory and cardiac arrest.
Best to put you feet tightly together, squat down in a shallow (not
standing water), put your arms around your shins with your head down,
elbows tucked and the broad of your rounded back the highest point.
Think --- ground hugging toad stool.
I had one close call as close as half a meter, even felt the heat and
blast and my vision was bleached white. But other than my legs turning
into rubber after a few steps right afterwards, I had no ill effects.
.. .. . .. . so far.
Lightning is fun
Ball Lightning
is Even
Funner
+---------------------------------------------------------+**********+
+ +Commercial*
| Paul M. Koloc, President (301) 445-1075 ***FUSION***
| Prometheus II, Ltd.; College Park, MD 20740-0222 ***in the***
| mimsy!prometheus!pmk; p...@prometheus.UUCP **Nineties**
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Paul Amaranth
> Anyway of hooking up to this potential difference and drawing some
> energy from it? :)
There was an article in popular science quite a few years ago (20?, alright
a LOT of years ago) about electrostatic motors that could be driven from
^^^^^^^^^^^^^
the potential difference.
The idea being you could use a helium balloon to get up to a potential
of a few KV. Note these are not ordinary motors. It was a neat article,
they had cars running on these things and everything. (Maybe they
really had an anti-gravity device in there 8-). I have never seen
one of these devices myself. I would think there would be a problem
getting sufficient current to do useful work. Probably could make
a toy motor run (Don't do it during a lightning storm!)
Not to get off the track too much, but I had a lighting strike close
(*VERY* close) to my house. I was lying in bed watching tv when the
windows went white immediately followed by the BOOOM. After peeling
myself off the ceiling (THATS the secret of anti-gravity ;-) I went
to look at the damage: All my computer equipment was slagged
(so much for surge protectors - they don't work at ground zero) as well
as some audio equipment and my answering machine. My $5 digital clock
survived unscathed. Despite intensive investigation, I never found an
indication of where, exactly, it hit.
--
Paul Amaranth office: (313) 370 4541 home: (313) 650 2428
(internet) amar...@vela.acs.oakland.edu | Always remember -- That
(bitnet) amaranth@oakland | way, when somebody asks
(uucp) ...!uunet!umich!vela!amaranth | you'll know - J. Carson
John Kerr
and I don't see how this discussion applies anymore.
Thanks in advance,
jk
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John Kerr Canada-France-Hawaii Telescope Corp.
INTERNET: jo...@cfht.hawaii.edu
BITNET: john@uhcfht
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John Boggs
>
>Not to get off the track too much, but I had a lighting strike close
>(*VERY* close) to my house. I was lying in bed watching tv when the
>windows went white immediately followed by the BOOOM. After peeling
>myself off the ceiling (THATS the secret of anti-gravity ;-) I went
A friend and I got caught out on the river in one of our recent afternoon
thunderstorms. We had holed up in the cabin waiting it out as the rain
and lightning and thunder came down all around us. I was sitting in the
cabin leaning against an ungrounded aluminum trim strip. My friend was lean-
ing against an ungrounded aluminum strip which holds the weather boards in
place. A particularly close lightning strike set up some kind of charge in
those ungrounded pieces of metal that was sufficient to shock both my friend
and I simultaneously. Neither of us received enough of a shock to cause any
damage but I can tell you we kept away from metal parts for the rest of the
storm. Any explanations of how this occurred? As far as we can tell, the
lightning did NOT strike any part of the boat directly.
--
John Boggs
McDonnell Douglas Electronic Systems Company
McLean, Virginia, USA
Dave Tiller N2KAU
-
- ki...@arrester.caltech.edu (Kim Dorian Flowers) writes:
->
-> Anyway of hooking up to this potential difference and drawing some
-> energy from it? :)
->
-> Kim Flowers
-
-Well, I seem to remember reading how experimenters in the early days obtained
-their high voltage supplies from insulated wires held aloft on poles and strung
-across the countryside. The end of the wire was brought into the lab and
-connected to a metal sphere to hold the charge. There was mention that the
-charge increased significantly when it was snowing.
-
Amen to the increase in static potential when snowing!! I got the BeJesus
shocked outta me when I reached over to tune my 10m Amateur radio whilst
traveling on a _metal_ suspension bridge (Del. Mem). I figured at the time
the snow added to the static buildup by imagining all of those snowflakes
hitting my antenna and losing electrons as they slipped past. Ouch!!!