http://www.amasci.com/tesla/spark.html
TOP | MISCONCEPTIONS | ELECTRONICS | TESLA | NEW STUFF
Sparks and Lightning
(c)1996 William J. Beaty
------------------------------------------------------------------------
While attempting to explain sparks and lightning to some friends, I realized that I didn't have a good gut-level understanding of them myself. As usual, my lack of understanding was an attractive irritant, like a pimple that one can't help picking at. And so over many months I kept noticing concepts that could be applied to explanations of sparks. Here's what I've come up with.
To get a good understanding of sparks, you need to encounter their behavior in detail. One way to do this would be to magnify a small spark, but sparks happen so quickly that interesting behavior can't be seen, so in addition to magnifying it, we'd have to slow it down somehow. Here's a better idea: speed yourself up instead. Imagine that you've been exposed to Tholosian water from 'Old Trek.' This is the substance which causes you to live many times faster than normal. (TV-show science fiction trivia experts will recall the appearance of a similar drug on The Wild Wild West as well!) And then, instead of magnifying a tiny spark, let's go outside during a storm and look at the behavior of an already-large spark. Except for its size, the strange behavior of lightning is very similar to the behavior of tiny sparks.
So, we're standing outside in the time-frozen world of a raging thunderstorm viewed from our 1000X perceptual acceleration. The trees and bushes around us are thrashing frozenly in the stopped wind, and a few torn shingles flying from the nearby roof hang in the air nearby. Higher up we see a tangled, branching network of dimly glowing wiggly purple lines which look something like a tree root. And like a root, all the tips of the branches are lengthening. But this can't be lightning, it's dim and purple, not bright blue-white.
One branch-tip is about a hundred feet up from where we're standing. We can see that the wiggly line isn't moving, it's only growing at its tip. It takes a tortuous, kinky path as it lengthens, and occasionally a new branch starts growing from the side of the main one at a spot where there is a particularly sharp bend. Then we notice something else: everything on the ground is starting to glow. Bits of dim purple fire are popping into existence on the tops of bushes, the edges of the roof of the nearby house, the tips of the rooftop TV antenna, on the ends of all the tree branches, and even on the flying pieces of roof shingles. As the exploring finger of dim purple lightning comes downward, the purple "fire" on all the objects becomes more and more intense. If you hold your hand in front of you, the tips of all your fingers spout dim purple fire as well.
Now the dim purple lightning from above is about thirty feet away, and the downward growth of its tip seems to be speeding up. Then something really disturbing happens. One of the purple flames coming from your fingers has suddenly started growing upward as a narrow wiggly violet line! You pull your hand down, but it's too late, the streamer of purple stays attached and grows upward fast, it's two feet long by now. You notice that this purple streamer from your hand isn't the only one, there are now jagged purple lines growing upwards from many places which formerly had the little "St. Elmo's Fire" flames. There's a ten foot streamer coming from the tree, another from the bush, and a couple from the roof of the house and the TV antenna. They appear to be moving towards the incoming lightning strike. There are even several coming from the wind-blown shingles, but some of these are extending downwards towards the ground while others grow upwards. The one from you're hand isn't winning, it apparently had a late start, and the streamers coming from the tree and the shingles are really shooting upwards now ahead of all the others. And the downwards-growing streamer from the shingles has touched the ground and is spreading out into a small disk of purple rootlets on the surface of the ground.
Finally the upward-growing streamer from the shingles approaches the lightning streamer coming from above. The two growing branch-tips race together, and just before they meet they split into several separate branches which all connect. And NOW it suddenly looks like lightning, because the entire streamer from the shingles is glowing brighter and brighter. The little disk of purple filaments where it touches the ground is now several feet across and looks like blazing blue-white tree roots. The whole thing is far too bright to look at, and it's getting brighter still. And something is happening to you. Your fingers hurt, the muscles in your arm are tensing by themselves, and you feel yourself blacking out. As you lose consciousness, you note that the short, dead-end streamer from your hand is still jutting upwards into the air, glowing bright blue, though nowhere near as brightly as the streamer from the shingles.
-----
What the heck was all that?! Lightning struck an object hanging in the air?! Well, sort of, since the shingles somehow launched their own lightning. And how could lightning be coming from objects on the ground, and from your hand? Why were you knocked unconscious even though you didn't get struck directly by the main bolt? And isn't lightning supposed to travel at the speed of light? 1000 times speedup is nothing compared to lightspeed, so why did we see the lightning as a bunch of slowly-growing filaments?
There are some mental tricks you can use to understand some of what went on above. Number one: realize that lightning is not made of electricity. "Lightning is electricity" is a false concept which stands in your way of understanding, and you need to get rid of it before you can figure out what's going on. The long purple filaments which extended through the air are not electricity, they are actually made of air. They are nitrogen and oxygen which has been converted into plasma. Plasma is vaguely like fire, but it is not necessarily hot. When air is converted to plasma, the electrons of the gas atoms are knocked off the atoms and become able to flow along through the air. Plasma is a conductor, so it's not too wrong to think of purple plasma filaments as being like wires made of conductive air.
Another mental trick: when you take a conductive object, a metal bar for example, and hold it in a strong electric field, flame-like "St. Elmo's Fire" sprouts from the ends of the bar. The "fire" is nitrogen/oxygen plasma. And plasma itself IS a conductive object. So, if an electric field is strong enough, and if a tiny bit of air is somehow converted into plasma, it's as if your conductive rod has grown little conductive pieces on its ends. And next, the "sharp" parts of the plasma globs will themselves sprout extra bits of plasma. And so your metal rod has started "lengthening itself" via fingers of air-plasma. The air can "catch fire" with an outbreak of plasma which grows and grows, with more air turning to plasma as the rods of plasma grow more plasma on their tips.
The plasma takes a particular form: long thin twisty rods. This occurs because "St. Elmo's Fire" always starts on the sharpest part of an object, and the sharpest part of a rod is the end of the rod. And so a pre- existing rod of plasma will grow more plasma on its tips and lengthen itself. This self-forming plasma conductor is vaguely like a motorized metal antenna on a car which extends upwards. But the plasma-antenna can lengthen itself continuously as long as it's tip is still in a strong electrostatic field.
If the twisted plasma rod should make a sharp bend as it grows, the bend can behave as a sharp point and more plasma fingers can take off from the bend. In this way a lengthening plasma streamer develops branches as it goes. Growing plasma doesn't just form twisted rods, it often forms trees, it forms entire complicated systems of rootlets which advance and spread. Whether it forms trees or straight unbent paths depends on the shape of the e-field. in the space around it. A parallel e-field will allow tree-shapes to grow. A spreading, radial-shaped field will tend to force one plasma finger to grow faster than all the others, resulting in a needle-straight spark.
Since plasma is a conductor, what do you think would happen if a piece of air-plasma were to connect itself between two highly-charged objects having opposite charge? ZAP! The opposite charges would be shorted out. An enormous electric current would exist for a moment. This is what happens during a lightning strike, or during the tinyest spark. Long filaments of air-plasma within the clouds extend and explore downwards towards the ground and upwards into the charged raindrops. A system of fine plasma-rootlets develops which connects most of the raindrops to the main conductive plasma tree structure. When the conductive plasma touches the ground, it discharges both the charge on itself and the charge on the the huge number of electrically charged raindrops. The large momentary electric current makes the dim purple plasma explode with light and sound.
So, what about lightning and the speed of light? Why can we see lightning "strike" across the clouds, yet light itself moves so fast that we never see moving light beams? Why can we sometimes see sparks jump from object to object? This is because the growing motion of lightning and sparks is actually the growth of plasma filaments. It is not a movement of light. Lightning can "strike" slow or fast depending on how fast the plasma filament tips are extending themselves. In very large Tesla Coil systems, the giant sparks can lengthen VERY slowly, a human can sometimes outrun them.
In the speed-up story at the top of this page, how come there were plasma filaments appearing on the ground and growing upwards? And why did the wind-blown shingles send plasma filaments both up AND down? This is hard to explain without going into detail about electric fields and atoms. But here's a similar question: suppose you squeeze a clod of dirt between your thumb and forefinger until it cracks. Would you expect the crack to start at your thumb, or at your finger? Or might it start from a small spot in the dirt and grow outwards in two directions at once? In truth, applying force to the dirtball can cause a crack to start ANYWHERE within the dirt.
Cracks tend to start at defects, and a similar thing is true with lightning and sparks. An invisible field of electric force, if applied to air, can cause plasma filaments to burst into existence anywhere in the part of the air where the field exists. When lightning is advancing towards the ground, there is a strong electric field all through the air around the plasma branch and in the space above the surface of the earth. This strong field can trigger new plasma filaments to grow anywhere. Of course its main effect is to make the main lightning filament lengthen and grow downwards. But those blowing shingles represented a "defect" in the air, they distort the invisible electrostatic field in the air and strengthened it near the shingles, just as a bubble in stressed glass can distort the mechanical forces and initiate a crack in the glass. The electric field present throughout the air caused two plasma dendrites to take off from the shingle and "strike" simultaneously upwards and downwards. The defect in the air caused the air to "crack" electrically, the crack being made of 3D plasma filaments.
The same thing happens when aircraft fly between oppositely charged parts of a thunderstorm: the plane acts as a triggering defect in the air, and plasma fingers launch themselves from two spots on the airplane. Flying a plane near a thunderstorm is like poking a highly-stressed windowpane with a nail: the cracks start where the nail touches. Yes, that's right, research has shown that aircraft rarely are struck by lightning, instead the aircraft themselves do the striking, since the plasma starts on the wingtips and zips outwards, striking the clouds.
-----------------------------------------
REFERATY KE STAHNUTI -DOC