I have heard it before, that lightning strikes actually go from the ground up. I was just watching a YouTube video that shows it. Using HTML5 with the ability to slow down the video to frame by frame, you can see the first frame shows the bolt apparently coming from the lightning rod. It's the first time I've seen evidence of that claim.
It's not like hole flow instead of electron flow in a semiconductor. There has to be something there and the Earth has a hell of alot more electrons t han the air. When it rains, protons, neutrons and electrons all fall to the Earth. And they take some extra electrons usually. This causes ionization.
On a smaller scale, a helicopter pushes air down to fly, as such it is movi ng protonss, neutrons and electrons, but again a few more electrons. As suc h, you cannot step down a stairway or whatever out of a helicopter (or cert ain other aircraft) because you would be electrocuted. So you have to jump. /I don't mean necessarily with a parachute, but whatever the volage is, wh en it is dry it is 600volts per thousandth of inch I think. But don't be t hinking an inch is enough, with this, 600,000 volts is nothing. That thing disturbs alot more atoms than a thunderstorm. (at least until it starts lig htnin'').
The capacitance of an isolated sphere is equal to its radius in centimetres times 1.12, iirc. Thus even a huge helicopter would be only 100 pF at most when it's far from the ground.
Takes quite the voltage on 100 pF to electrocute somebody.
I think the atmospheric conditions prior to a thunderstorm (perhaps low pressure?) cause the lighter negative ions to rise into the clouds, resulting in a buildup of heavier positive ions near the ground. These positive ions may cause mood changes and feelings of apprehension and fear, which also cause animals to instinctively seek shelter. But such behavior may also be due to the lower air pressure, or sounds of distant thunder.
In any case, I think the reason the lightning starts near the ground, especially when it occurs on a lightning rod, is that the object has one or more sharp points, which cause a very high electrostatic field which eventually arcs and forms a conductive plasma. This plasma then extends from that arc upward to the more negative charge and current flows to neutralize the charge difference. But it may also oscillate back and forth for a while until it more fully neutralizes.
pressure?) cause the lighter negative ions to rise into the clouds, resulting in a buildup of heavier positive ions near the ground
It's triboelectric charging of water droplets in air that causes the charge separation. Then thunderstorms hoist the charged droplets high into the air, very much like a huge van de Graaf generator.
I have done many test flights in helicopters (not as a pilot) and never had to "jump" out and never felt any discharge. The charge, if any, will be gone when it's touching the ground which is the same ground I will be touching when I go off board.
There have been instances of ground crew electrocuted when grabbing the bow hawser of an airship after a long flight. The rule was to always let it drag on the ground before touching it.
It is possible that the cause of the Hindenburg fire was electrostatic discharge when the frame was grounded and a spark discharge occurred to metalwork from some of the aluminium-painted panels, which were not properly bonded to the frame. A good example of a 'lossy' capacitor.
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An airship is a wee big bigger than a helicopter, of course.
Well, the root cause was having all that hydrogen in a reducing environment. :( Watching the film, it's hard to believe there were so many survivors.
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Phil Hobbs
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Environmental ESD: Part 1 ? The Atmospheric Electric Circuit
Other articles by Professor Jonassen may be of interest also so I posted the link to a list of his articles in Compliance magazine rather than directly to the article.
The ship came down quite gently, with the crew correctly dumping water ballast to reduce the speed of descent until the last possible moment when all the hydrogen (and buoyancy) had gone.
It is interesting to note two things from the film:
1) A streamer of flame burst from the centre of the nose when the hull was at its steepest angle. This was probably because this ship had a central catwalk through the middle of the gasbags which acted like a chimney when it was tilted.
2) The port-side canvas panels facing the camera charred and burnt through simultaneously in large areas, there is no sign of the flames spreading gradually downwards on those panels. My guess is that the fire was already very intense on the starboard side of the ship away from the camera (the side which had dried out in the sunshine after the storm she had flown through). As the gasbags collapsed, they exposed the inside of the port panels to the radiant heat from the burning starboard panels, causing the strange patchy burning behaviour.
There is a theory that the aluminised sun-reflecting paint had been applied on top of iron oxide primer and the two had mixed to produce a type of Thermite compound, which would have propagated the fire rapidly. The speed with which the fire seems to have spread along the dry side of the ship at a level much lower than the burning hydrogen would support that theory.
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~ Adrian Tuddenham ~
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100pF seems small for a helicopter. I think I measured my capacitance as something around 200pF. (JL's discharge yourself through your x10 probe and 'scope trick.)
When helicopters transfer persons by winch, a grounding hook and cable is often used, (Military requires it) and for good reason. Especially in dust storms, where the dust acts as a charge transfer medium like the belt in a Van De Graf Generator.
The body capacitance of the Helo might be small, but the rotor is continually producing more charge.
If you search Youtube, you can find videos taken thru NVG goggles showing static discharge at the rotor tips.
Discharges in such a case are ALWAYS from the charged element TO Earth or the nearest receptive accumulator/attractor.
It grabs ions "(free electrons) out of the air, so it will always be considered "negative" with respect to any would be attractor/dump site.
Flow is from negative to positive.
Not many "positive pool" "hole clouds" up there.
Regardless of how the ladders get propagated when a lightning event occurs, the actual plasma/stroke event will be a cloud to Earth discharge. The visible illumination of said plasma could "travel" in any direction, giving the illusion that the stroke went upward.
I say not EVER is that the case. The stroke fires AT the Earth. Period.
In other words... DOWNWARD.
There are "sprites" which fire "bolts" upward into space from above the cumulonimbus formations. In the strato and meso sphere layers.
What is their attractor? Or is it like "back blast"?
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