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There is something I don't really understand about flashes of lightning.

When a flash occurs, how come electricity be transferred at the speed of light since electricity's displacement is very slow ?

It is not that I don't understand anything about that. I've read wikipedia and the others. But on that particular point there's something I really don't get ...

But the question could be for lighting as for any charge strike between two points of course.

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You say:

since electricity's displacement is very slow ?

I suspect you mean the electron drift velocity in metallic conductors is low, but that's completely different to the electron transport in a lightning bolt.

I'm not sure the mechanism of conduction in a lightning bolt is fully understood, but consider this. The dielectric breakdown of air occurs at a field strength of around three million volts per metre, so an unobstructed electron would gain an energy of 3MeV for every metre it travelled. As the electron rest mass is only 0.5MeV that means even after just one metre the electron would be moving at relativistic speeds.

I don't know what the free path of an electron travelling in a lightning bolt is, but it wouldn't have to be very long for the electron to build up a very high speed.

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