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Why is lightning jagged?

  1. Jun 14, 2015 #1
    In a discussion about lightning destroying electronics on sailboats the issue of controlling lightning came up. One member (Baluncore) proposed that lightning is completely predictable. I've been taught that lightning is somewhat random.

    To me, random implies non-linear equations with sensitive dependence on initial conditions (the butterfly effect). Yet the equations lightning seems to follow are linear as far as I can tell. A puzzling note is the tendency of lightning to shoot halfway down a conductor, then jump off it and through the hull of a fiberglass boats rather than following the wire to water on the side of a boat.

    So am I missing some non-linearity, or is it possible to model lightning well enough to prevent deaths analytically rather than just statistically? (The way lightning protection works now seems to be to throw enough lightning rods up to statistically catch most of it. I'd like to think there's a better way.)

    Do lightning charge carriers gain enough momentum to keep going despite the fairly strong forces bending them? In air, it would seem they might gain relativistic speeds (with millions of volts potential), but in copper? It doesn't seem likely. And if lightning wants to go straight so badly, why does it zig zag across the sky?
     
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  3. Jun 14, 2015 #2

    Vanadium 50

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    The conductivity of air is not uniform, and the lightning takes the path of least resistance. I don't think the exact reason is settled - one possibility is that these are paths of air ionized by cosmic rays.

    You can get relativistic electrons from lightning, although usually only the very largest flashes. More typically, electrons gain energy from the electric field, and then lose it to the atmosphere by ionization etc. You end up with a lot of electrons with a little energy rather than a few electrons with a lot. The rare cases with a substantial component of fast electrons is believed to be responsible for x-ray emission observed from some strokes.
     
  4. Jun 14, 2015 #3

    Drakkith

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    I believe the path lightning takes depends mostly on the ionization path through the air, called the leader. The formation of the leader is not well understood, but I several ideas have been proposed, including slight variations in the makeup of the air, cosmic rays, or the enhancement of the electric field by ice crystals.
     
  5. Jun 14, 2015 #4

    davenn

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    have a look at this video, particularly from around 2:45 to end



    it clearly shows the stepped non-direct path taken
     
  6. Jun 14, 2015 #5

    dlgoff

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    Abstract from http://ieeexplore.ieee.org/xpl/logi...ore.ieee.org/xpls/abs_all.jsp?arnumber=902309

     
  7. Jun 14, 2015 #6
    So does the fractal path cause the complex EM field, or does the complex EM field cause the fractal path?
     
  8. Jun 14, 2015 #7
    Lightning happens when electrical potential difference between two points becomes sufficient that a discharge is going to occur.
    The exact form (shape) of the discharge is not predictable because there are numerous factors which affect the "easiest path" for the electron flow,
    and the discharge is not instantaneous, it can take a few seconds and in few seconds, the easiest path can change quite a lot.
     
  9. Jun 14, 2015 #8

    anorlunda

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    I don't think it ads clarity to the answer, but rocket triggered lightning experiments make it graphic. Below is a still picture and a video. The still picture shows that what appears to be a single strike is actually multiple strikes all occurring in a fraction of a second. Wind moves the ionized air molecules between the strikes. The video shows what they called an anomaly in the path.

    02-vari-triggered.jpg

     
  10. Jun 15, 2015 #9
    That would explain the path above the lightning rod, but it wouldn't explain why lightning frequently leaves a heavy copper conductor to blast through a highly insulated fiberglass hull.
     
  11. Jun 15, 2015 #10

    A.T.

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    Does the current leave the conductor completely, or does it merely find an additional parallel route?
     
  12. Jun 15, 2015 #11

    When a trillion billion electrons are trying to go somewhere in a hurry, what conductor is going to be sufficient?
     
  13. Jun 15, 2015 #12

    davenn

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    usually the second part ... I have seen damage done further down the cable by the significant currents still travelling within the cable

    there was another thread recently on lightning where I stated experience with lightning strikes on phone cables etc
    the high voltage and current of lightning strikes does not like to go around corners. Any significant bends in the cable or lightning rod grounding strap and the lightning would jump from the corner of that bend out to anything else remotely earthed that it could find other cables, metal brackets etc

    I saw this effect many times in my years in the telecoms industry both on multi core cables or on radio comms poles with mast top lightning rod and large copper strap down the mast to the earth mat

    The #1 installation rule was ... NEVER do sharp bends. If you have to do bends, make sure they are
    rounded and as wide angle as possible


    regards
    Dave
     
    Last edited: Jun 15, 2015
  14. Jun 15, 2015 #13
    If the charge carriers are ions or something, the momentum might be a factor. But for electrons, I don't see it.

    The effect would almost need to be an electromagnetic effect to be as strong as it is at its distance scale. Perhaps the self inductance in a straight leg of conductor is strong enough to generate enough voltage at the turn to break down air? But that still doesn't explain why it goes straight instead of some random angle. Hmm...

    I make a prediction. I foresee math in my future.
     
  15. Jun 15, 2015 #14
    The video shows that the lightning starts with the rocket's wire, but by the end, it finds its own path. This seems strange to me. Clearly there are better electrical paths than a nice, straight line of copper ions (which seems counter-intuitive). It makes me wonder it the spectral content causes the fractal pattern rather than the other way around? But what would be the mechanism?
     
  16. Jun 15, 2015 #15

    anorlunda

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    The wire is vaporized on the first stroke. Then the wind moves the ions horizontally. Later strokes follow the displaced ionized path. I don't think you can think of it as a single event.

    Even if the wire doesn't vaporize, it could ionize the air near the wire surface. Then wind moves the air while the rolling boat moves the wire before the next stroke.

    Also, there is often a lot of vertical wind shear near the ground.

    Complexities everywhere you look. Your question is very difficult to answer.
     
  17. Jun 16, 2015 #16
    I don't doubt that even weak lightning would vaporize the sort of wire an inexpensive rocket could carry.

    What you say about the wind is true, yet at the very end, the lightning seems to finds a wildly different path. Perhaps this is due to the wind destroying the path, but perhaps not. I suspect that is the most likely explanation. Though natural lightning seems to occur in stages, not nearly as many I think. The multiple events are possibly due to the rocket continuing into new cloud layers.

    I agree the complexities are immense. But if it were easy, someone would have done it already. Where's the fun in that? This is an important scientific question, at least from economic and human perspectives even if it's unlikely to lead to new insights on how the universe works on a grand scale. The standard model likely applies, but we don't know how yet. It's a pleasant puzzle to chew on with real world benefits.

    It's possible, even likely there are non-linear qualities that will make more than statistical prediction impossible, but we don't know that yet. The basic equations all look linear.
     
  18. Jun 21, 2015 #17
    Yes , yes I loved one of the points earlier , remember that copper itself has a resistance which of course will of course will increase greatly with an increase in temperature due to the increased energy from the lightning , hence , at some point , theoretically the air once more becomes a better conductor
     
  19. Jun 21, 2015 #18
    Maybe it really is not unpredictable but very difficult to predict , like the possibility that a man can fly is possible just not very possible
     
  20. Jun 21, 2015 #19

    davenn

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    I would strongly suggest to you that the vaporisation of the copper wire happens long before that

    Dave
     
  21. Jun 22, 2015 #20

    A.T.

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    I was rather thinking about the increase in charge density in the copper limiting the current.
     
  22. Jun 22, 2015 #21
    That's worse than it sounds because the skin effect will limit the charges (at least the moving ones) to the surface of the conductor. (The Skin Effect)

    However, since conductors regularly vaporize in lightning strikes, we can be confident that the resistance doesn't rise enough to kick the current out. Once the copper ionizes, it's at least as conductive as the ionized air is. (I think, could I be wrong on this?)

    The pinch effect should limit the ionized path. Perhaps it then pinches the ionization trail until it can no longer support the current do to the skin effect?

    But this doesn't explain the main case where the cable doesn't melt the copper, yet jumps off anyway. And it does so in the same gauge wire it's happy to follow in a sraight line.

    Lightning definitely doesn't like sharp changes in conductor direction, yet seems to like such changes while in the sky. It's a puzzle.
     
  23. Jun 22, 2015 #22

    davenn

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    you have to remember those zigzags seen in the lightning path in the sky are over many metres probably 10's of metres quite different to a sharp bend in a cable of less than a couple of metres

    no real puzzle there :smile:

    Did you watch the video I posted much earlier in the thread ( the slo mo one) showing how the zigzag path forms ?

    Dave
     
  24. Jun 23, 2015 #23
    I did watch the video. It was informative. It showed the mechanism of the direction change, but not a clear reason. The assumption that the current is choosing the low resistance path is belied by the currents choice to leave the low resistance copper to jump what should be a leaderless path through fiberglass.

    Nor does the change itself seem to be over a large volume of space. The turns look sharpish.

    The distance of each segment does seem largish. Still the distance from the cloud to the ground is only a few thousand feet. It's easy to misjudge vertical distances.

    Lupo G. examined the fractal nature of the current. It occurs to me the straight paths may act as antenna segments which discharge energy depending on spectral content. Thus there may be a limited set of possible segment lengths depending on what the spectrum of the lightning is at any particular point. But the mechanism isn't clear to me.

    Speculating:

    Accelerating charges emit photons. But due to the phase (gauge?) relationship with the length of the current path certain energy/frequency photons can't be emitted (or at least are much less probable). This would cause the charge density mobility (current) to clump into some speeds/frequencies. This might create some sort of back electromagnetic force which would cause a build up of charge at a sharp turn in the conductor (allowing flashover through the fiberglass). However I know of no mechanism for such a back EMF. It could be a quantum effect, or it could just naturally fall out of the equations in what to me is a counter intuitive manner. Or it could be bovine fecal material (BFM). My bet is on the BFM, but if I'm wrong, this could be an explanation.
     
  25. Jun 23, 2015 #24
    I simplify the jaggedness of lightning down to the repulsion of electrons in the drift. Imagine: +[ ]- you have 2 electrons at the plus that are heading in generally the same direction towards the minus. They repulse branching out from the drift, but then one path is always favorable, hypothetically. So now the next pair of electrons to follow tend towards the favorable path ahead of it, branching perpendicular to the guiding path, giving electricity it's 90 degree kick every step.
     
  26. Jun 23, 2015 #25
    The pinch effect seems to prevent that. It maintains current primarily in one path. Of course this works only up to a point, so lightning does occasionally split. But it clearly doesn't take all paths, only a few, and often only one. (At least for the vast majority of the current.)

    I do have another contender pre-hypothesis: It has been postulated that marine epoxy might contain copper dust as an anti-foulent. At high voltage levels this might make it behave as a supercapacitor. I have no idea if either of these conditions are valid. If they both are true, the hull could act as a short to the high frequency components, allowing a pinch to form and drive the current through the hull.
     
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