Lightning - where is the power dissipated? - Page 1
Forum section: electronics / general

author: hamster_nz
author_rank: Super Contributor
timestamp: August 26, 2019, 08:40:52 am
content: Seen a few lightning strike videos recently on the news (I think one was at a golf tournament).It has me wondering - when lightening strikes, where is the bulk of the power dissipated? I see numbers like 30,000 A, and I am guessing that the bolt itself is plasma and will have low resistance.. so is the bulk of the energy is lost in the ground where the strike was? Or is it lost in the sky where the plasma ends because ground has relatively low resistance?

author: tautech
author_rank: Super Contributor
timestamp: August 26, 2019, 09:00:15 am
content: In what it strikes.A 30m pine tree 200m from my house was hit a few months back and the strike blew a strip of back from it from top to root level and scattered back some 10m away. IME this tree was 70-80 years old and now it's toast and will loose the last of its green in the next few weeks.Left as it is will present a danger to anything that passes near it so it has to come down this summer............just another job we didn't need. Good thing I've got big chainsaws to deal with it.

author: Brumby
author_rank: Unknown
timestamp: August 26, 2019, 09:01:36 am
content: Watch the 2002 movie called "Sweet Home Alabama".

author: soldar
author_rank: Super Contributor
timestamp: August 26, 2019, 10:59:49 am
content: Thunder is the explosion of air being heated up

author: Gyro
author_rank: Super Contributor
timestamp: August 26, 2019, 11:20:15 am
content: Yes, there's a lot of energy dissipated in the voltage drop of the 'arc' and the sonic boom that it creates. Ground / tree resistance dissipates more. A case from only yesterday:https://www.bbc.co.uk/news/uk-northern-ireland-49464557

author: duak
author_rank: Super Contributor
timestamp: August 26, 2019, 03:57:30 pm
content: A lot is emitted as electromagnetic radiation or waves in a spectrum from LF to UV.

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2019, 04:29:44 pm
content: Quote from: hamster_nz on August 26, 2019, 08:40:52 amAll of the electromagnetic radiation released including the visible light and the heating of the air to product a plasma resulting in the thunder are where some of the power goes. Restive heating in the ground is significant.It would be a fun physics calculation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2019, 11:29:11 pm
content: Anything at ground level (aside from ground itself) dissipates a hopeless fraction of the total -- even a 1MV drop is small compared to the, what, 100s MV, at cloud level.Not to say that's not a destructive amount of power, even for much smaller voltage drops.Typical loads are direct strikes to metal structures (local melting, burning or exploding of cables), distribution lines (typically, MOVs are placed every 3rd pole or so, clamping the surge to maybe 3-5x nominal line voltage plus ground return voltage), and induced voltage due to ground currents and EMP (nearby lightning, indirect).Note that the transient voltage drop is much higher than the resistance alone dictates, due to inductance, but this doesn't dissipate heat by itself, it just acts to slow things down. The plasma channel, is basically a wire in free space, so its equivalent inductance will be in the ballpark around 1.257 uH/m (the permeability of free space). For 100s m, that's 120s uH, and 100 MV / 120uH = 0.83 MA/us. Evidently for a peak current in the 100kA range, the rise time should be around 120ns (as far as I know, it tends to be slower than this), or if it's a resonant circuit with a peak current of 100kA, the resonant impedance is ~1000 ohms and so the cloud capacitance should be around 120pF. (Thinking it should be a lot higher than that though, more like 10nF?)Interesting. I'm thinking the erratic and extended sparking of the phenomenon amounts to considerable equivalent resistance, so the stroke has more of a lumpy damped-discharge response. This may be counter-intuitive and hard to appreciate, or explain?Consider: an individual plasma channel may have relatively low voltage drop, but it took voltage to get there, and power is dissipated in the transition from insulating to conducting. And the averaged response, over the cloud, has an increase in current flow causing more channels to form, and more voltage drop. This has significant positive resistance, even if each individual channel may exhibit negative resistance. If you looked at it with a fine enough magnifier, you'd see a very jagged resistance, alternating positive and negative all the time; but smoothed out, it's positive overall.Which is really to say that sparks dissipate power, but it can be hard to figure out just how much, and when, when you look at a simple example, like a neon tube or welding arc, which has such-and-such (low) voltage drop and negative resistance.On a related note, it's interesting to think about the plasma discharge of a Tesla coil, because the increased conductivity of the air effectively connects it to a larger volume -- the plasma itself acts as a top load, increasing the capacitance. But this doesn't come at no cost, it's a lossy capacitance. And that loss depends on the driving power. It has an overall diffusion sort of characteristic I think, which limits the Q factor to a fairly modest value (though I'm not sure how much; less than 100, probably more than 10?).Tim