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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi everyone,
I'm making a seminar about influence of lightning strikes to electrical power distribution system, and I winded up with some articles using complex terminology that is not well explained, and I wish to do a good intro on everything first. There are still some basic things I would like to finally learn properly, as I should probably a while ago considering all the arcs and sparks in my life.
First thing I want to clear is avalanche breakdown. So far, this is how I got it to work: It's usually erronously stated that electrical breakdown, as ionization of atoms in air or other dielectric is caused by electric field alone. This can't hold up when one considers the coulomb forces at ranges of size of an atom (100pm, between eletron and proton) are in range of 10^-8 newtons, and force impinged onto an electron by an electric field of 3MV/m is only in order of 10^-13N, way too small to be of any importance actually.
Hence atoms of a dielectric can only get ionized when hit by a fast moving electron, with energy sufficient to ionize the target atom - and only way for this to happen is to have the electron accelerated over a sufficient distance by the electric field, without bumping into atoms while it's energy is still too low to cause ionization and just lose it as heat. The average distance electrons can travel in air is called their mean free path, and this has to be the reason why it is easier to break down gasses at low pressure - electrons can on average accelerate further in dilute gas and gain more energy for same field strength.
Since there are always some losses, an electron always needs to free more than one electron on average before being recombined itself (it would be good to know how big this factor is in reality?). Only then can an sustained avalanche start. Note that this is all before closure of an arc; I suspect electrons avalanching the air actually act quite resistive, drawing constant current for a constant voltage source, and no negative resistance effect yet.
Also, after a path is ionized the electric field in it is going to drop and electrons are no longer going to get accelerated as much, and ionization is kept through heating only.
This all is the basis of formation of what they call a "stepped leader" of lightning. But there's one very important phenomenon that I need to consider, and that is Z-pinch. The electrons creating an avalanche would tend to spread away from each other unless there was large magnetic force holding them together, and this is what holds an arc thin and filamentous. Seems simple for small marx sparks but gets very weird on the scale of lightning.
Finally, there is that enigma of how lightning occurs with voltages that seem to be too low for breakdown on such a large distance. I remember that theory of "runaway breakdown" or whatever it was called, which postulates that at large scales breakdown distance is going to deviate very far from linear against voltage, due to relativistic effects on electrons. I think it's simple and fits perfectly into general avalanche breakdown idea: with sufficiently large potentials and energies in the source (cloud), some electrons might gain velocities approaching speed of light, and their mean free path will appear to dilate from their reference frame. In other words, they'll appear more and more massive and gain much more energy over short distances than they would at non-relativistic speeds. Then they can knock out large amounts of electrons at high speeds as well, and at some point the effect will become self sustaining as long as there is enough energy available - the voltage can remain nearly constant through the process.
I guess this is where my knowledge stops though. These are the main questions I have:
- I'm wondering why does Z pinch manifest much less at low gas pressures, such as in a fluorescent tube. In such conditions mean free path is much larger but also the atoms from which carriers can be liberated are much more scarce, so electric field is much larger in correspondence, and a high ionization rate can be achieved with current that is so low that no Z pinch occurs. Could one say that thermal ionization is predominant in high pressure gasses, while avalanche ionization is dominant in low pressures?
- Why are actually stepped leaders so steppy? I can get that electrons might follow relatively straight direction (confined by magnetic pinch) until some instability occurs, and the path suddenly changes direction or branches. But what really confuses me is the time discontinuity that seems to occur - the leader seems to pause briefly every few meters or so, which avoids explanation. In overall it also moves relatively slowly, 10^5m/s or so at most, and is easily captured by camera. I don't understand what causes all these delays since avalanche and especially relativistic avalanche effects should be super fast.
- One thing I don't understand yet are those "dart leaders". They are said to occur after the first stroke is over, but I'm not sure what exactly they are. Can anyone point on them on a slow-mo video for me?
- There are also those tiny leaders they call "recoil leaders". This video shows them exactly:
I'm puzzled how they actually occur, and whether they are the same thing as dart leaders or are those something else. They can be seen in Lichtenberg figures when they are discharged, and persist for several minutes. They apparently only occur when charges are trapped in insulators, and they seem to play major role in delivering energy to the main leader, and they seem to mostly follow pre-ionized channels.
I can't figure out why they occur in such a random, pulsed fashion with long time delays. Could this be because of slow attraction of charged particles to conductive channels, which are re-ignited only after enough charge has gathered around them?
I'd be thankful if you guys could continue this story so we have a full picture one day. I'd also be grateful of any articles that are not too long and explain all the basics, the internet seems to have a shortage on those.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
If I worked for an electricity supply company I would have a small interest in the physics of lightning, and a major interest in - what attracts lightning? - what repels lightning? - how do I asses the risk of lightning? - what do i need to know to be good at my job?
Someones personal theory probably will not help very much. Unless you can make it useful........? Common working practices are very useful Applicable regulations (if any) would be useful. Explaining/presenting 'rules' is often a clear way to explain a topic, depends on the audience that you expect.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Ash Small wrote ...
Is one of the questions you are asking 'What initiates the avalanche?'
Elsewhere I've suggested cosmic rays can initiate such discharges, but others have disagreed.
I was wondering about that and I don't think it's really necessary. A short mean free path in air does not mean that no electrons will ever be found to travel a much larger distance than mean free path. In other words, I suspect that there is much larger chance that some electrons might simply have luck to get accelerated to high energies and start an avalanche, than actually everything being dependent on cosmic rays (one might as well ask what starts a low energy avalanche process, there always ahve to be some free electrons around.)
It would be interesting to compare large marx discharges on surface underground with shielding to verify this, or perhaps with artificial radiation sources.
If I worked for an electricity supply company I would have a small interest in the physics of lightning, and a major interest in - what attracts lightning? - what repels lightning? - how do I asses the risk of lightning? - what do i need to know to be good at my job?
Someones personal theory probably will not help very much. Unless you can make it useful........? Common working practices are very useful Applicable regulations (if any) would be useful. Explaining/presenting 'rules' is often a clear way to explain a topic, depends on the audience that you expect.
Well, this describes pretty well the article I've got as my main literature, it's a bunch of engineering data with usage of terms and theory that's not well described. Since I need to present it to students, I think I should really do a good intro (based on newest findings) to make the text possible to follow.
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Questions on theory of Lightning.
