Why is lightning jagged?

  • #26
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That's the other side of the story, conductors provide a stable "channel" for the electrons to organize into an efficient "stream" around a wire. The problem with wires is they don't always lead exactly where the current wants to go, so at some point the easiest path veers too far off course and it has to redirect.
 
  • #27
davenn
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The pinch effect seems to prevent that.
not something I have heard about before as relating to lightning (electrical discharges in general)

will have to read up more

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.)
yeah it does split ... often
a pic from my own collection about 500m to 1km away from me ....

070224(2579).jpg




the final one or 2 paths taken are really controlled by the stepped leader coming down from the cloud finally meeting the streamer coming up from the ground
there can be a number of streamers but only one may connect. As in this pic. you can see 2 streamers one from a power pole on the left and another from the
tree. Amazingly that streamer from the tree didn't connect with the stepped leader. There must have been another streamer from the tree that reached a bit higher
NOT MY PIC !!

Lightning_hits_tree.jpg





cheers
Dave
 
  • #28
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That's the other side of the story, conductors provide a stable "channel" for the electrons to organize into an efficient "stream" around a wire. The problem with wires is they don't always lead exactly where the current wants to go, so at some point the easiest path veers too far off course and it has to redirect.
Clearly the current finds the easiest path. But just as clearly the easiest path isn't always the lowest resistance path. So the question is, what causes the impedance to be so different from the resistance?
 
  • #29
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what causes the impedance to be so different from the resistance?
The impedance simplifies to resistance in direct current, from what I've read. I'm still trying to fit the whole puzzle together.
allowing a pinch to form and drive the current through the hull.
I think the pinch you were referring to is caused by the current organising so that the magnetic field helps to condense the stream, meaning the current precedes the pinch not the pinch precedes the current.

It is obviously a complicated phenomenon but I'm determined to figure it out!
 
  • #30
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Clearly the current finds the easiest path. But just as clearly the easiest path isn't always the lowest resistance path. So the question is, what causes the impedance to be so different from the resistance?
Path singular? Event singular? My thinking keeps leading back to the plurals of both.

As time progresses, more ions will be created, thus changing the path resistance greatly. Resistance and impedance are transient and distributed. To model this problem you need differential equations distributed in 3D space. Maybe you even need :nb) Q.E.D.That sounds impossibly difficult to me.

But there.might be hope if you narrowed the OP question. A lightning rod's wire carries the current to ground. The wire does not evaporate, nor does it have sharp bends. Think of the guy wires for 1000' TV antennas. Does the current jump out of (or in to) the wire sometimes? There may be photographic evidence. If so why?


In past PF threads, Davenn said that no matter what, the protected equipment inside get fried sometimes. That suggests that anaomlous outcomes do occur. Have a look at the remarkable picture of the Washington Monument that Dave posted in this thread.

6a0105371bb32c970b011570fa51df970c-400wi.jpg
 
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  • #31
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The impedance simplifies to resistance in direct current, from what I've read. I'm still trying to fit the whole puzzle together.

I think the pinch you were referring to is caused by the current organising so that the magnetic field helps to condense the stream, meaning the current precedes the pinch not the pinch precedes the current.

It is obviously a complicated phenomenon but I'm determined to figure it out!
Current can (temporarily at least) travel through a capacitor. Then, at least theoretically, the current could form a magnetic field which forced the current to continue after the capacitor filled. Think of it as a small inductor in series with a low voltage capacitor. The inductor could easily break down the capacitor with careless design.

Of course that assumes the hull acts as a high value capacitor, which it doesn't under normal conditions. It's an unlikely explanation; probably more BFM.
 
  • #32
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I really think I'm missing something right in front of my face, but I can't see it. I keep looking at the leaders coming out of the ground, wondering why current would be going the wrong way. I wonder if that could be + ions being drawn towards the - charged cloud?
 
  • #33
davenn
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wondering why current would be going the wrong way.
what do you mean by that ?

I wonder if that could be + ions being drawn towards the - charged cloud?
a good probability

and don't forget that the charge polarities can also be the other way around

Dave
 
  • #34
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what do you mean by that ?
I meant I was stuck in a rut only considering the electrons and not the ions. Electrons wouldn't be coming out of the ground going towards the cloud while the electrons in the cloud are going towards the ground, that wouldn't make sense.
and don't forget that the charge polarities can also be the other way around
That was when it dawned on me, it goes either way for either polarity. It does make much more sense now. The leaders from the ions would be less "mobile" than the electrons, wouldn't they? That would explain why the branches are much more prominent on the electron side. Now I'm curious if I can find slo-mo of lightning going from ground to cloud... I think it would look different if charges were reversed.
It appears I am correct.:wink: Now I just have to figure out how the connection is formed to start the "seek and destroy" (discharge)
 
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  • #35
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I meant I was stuck in a rut only considering the electrons and not the ions. Electrons wouldn't be coming out of the ground going towards the cloud while the electrons in the cloud are going towards the ground, that wouldn't make sense.

That was when it dawned on me, it goes either way for either polarity. It does make much more sense now. The leaders from the ions would be less "mobile" than the electrons, wouldn't they? That would explain why the branches are much more prominent on the electron side. Now I'm curious if I can find slo-mo of lightning going from ground to cloud... I think it would look different if charges were reversed.

It appears I am correct.:wink: Now I just have to figure out how the connection is formed to start the "seek and destroy" (discharge)
Charge mobility is different for different charges. The most mobile seem to be electrons. The next most mobile are holes I think with about ⅓ the electron's mobility. Holes exist in semiconductor material in what I assume is a quantum effect. They are unfilled electron shells which act as positive charge carriers even though the constituent atoms are locked in a matrix. I mention them to show there are other possible charge carriers than electrons or ions. (I don't know which work in plasma though.)

Ions are several thousand times more massive than electrons and should be much slower for that reason. But I suspect Anorlunda's :nb) Quantum Electrodynamics would be needed to figure out which charges are moving where.

Also, cloud charges are sometimes reversed. I seem to recall reading this is common in monsoon storms for some reason. It is infrequent (I've read 10%) in temperate storms. Anyway, I tend to discount which charge is which for this reason, though perhaps that's a mistake.
 
  • #36
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In the ground. The melted sand. Fulgurites also exhibit jaggedy branchings. different looking but oddly frozen in motion. maybe measurable consistencies in the distribution of branch endings. Any body know where this potential study is?
 
  • #37
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Hi and welcome to PF! Thanks for joining the discussion.
The melted sand. Fulgurites
I am sure I've seen these at some point, but never remembered until you mentioned them, very much appreciated!

I'm definite I have the system down in my mind but I don't know how to relate all the variables just perfectly. It has quantum roots so prediction is probable but certainly not classic.
 
  • #38
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  • #39
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http://www-solar.mcs.st-andrews.ac.uk/~alan/sun_course/Chapter2/node11.html
The "frozen-in" theorem, is just saying the magnetic field stays fixed to the plasma pinch, correct? So as the "electron gas" as if it were a Bernoulli relation drags the field with it across the counteracting ions... you get the "z" pinch?
I looked at the link you provided, and I also read about Alfvén's frozen in theorem on Wikipedia. Sorry, but I don't understand what they are saying, or how those relate to the question in this thread. Can you elaborate please?
 
  • #40
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http://www-solar.mcs.st-andrews.ac.uk/~alan/sun_course/Chapter2/node11.html
The "frozen-in" theorem, is just saying the magnetic field stays fixed to the plasma pinch, correct? So as the "electron gas" as if it were a Bernoulli relation drags the field with it across the counteracting ions... you get the "z" pinch?
I'm a bit confused.

The frozen in theorem seems to apply in a PEC fluid. But it's not clear to me that it applies in an intense electric field (which is sort of the opposite of a PEC).

I can understand how a moving stream of charge carriers (perhaps an electron gas) could be modeled as having a fixed magnetic field around it (with the field moving at the same speed as the gas). But I am missing the insight this view brings.

The frozen in theorem seems to allow charges to move only along field lines. The field lines are circular around the current path. Are you claiming the charge carriers will spiral along the current path? If so, how is this relevant? Or are saying this is the mechanism that keeps the current in a single(ish) stream?
 
  • #41
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Or are saying this is the mechanism that keeps the current in a single(ish) stream?
Yeah pretty much what I'm getting at. The plasma pinch goes up in to the cloud normally. That's the + ions. The branching down from the clouds is the + ions going up.
 
  • #42
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I've been taught that lightning is somewhat random.
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?
The lightning spreads (branches) in ways, that have the least resistance, that is the most conductive paths. These ionized and short-lived paths, ion channels (anions and cations) are formed already during the initialization phase of the lightning.
 
  • #44
davenn
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And if lightning wants to go straight so badly, why does it zig zag across the sky?
I don't really think the hilited bit is a valid assumption, considering all lightning discharges I have ever seen invalidate that statement
 

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