Sunday, 5 October 2014

Lightning Strikes Again

It is said that lightning never strikes twice in the same place. There is actually no special reason why it shouldn’t, and indeed there is a house near mine here in Greece that has been twice struck by lightning while I’ve been here; local people tell me there have been other strikes there before. Electric charges seek the best and quickest route to earth, so if something pokes up above its surroundings it’s likely to get struck by lightning. That’s why it’s not such a good idea to shelter from the storm under a lone tree in the middle of a field. So in fact lightning is really quite likely to strike in the same place twice.

Following my first post about lightning, my most faithful reader has written in with some questions, which I shall now try to answer:

Does that mean that electric lights make a tiny noise all the time? What causes the high-pitched noise that bulbs sometimes make, just before they stop working?

That’s two questions. To the first: Well no; normal or traditional light-bulbs, in which electricity passes through a thin wire (the ‘filament’), thus heating it up, don’t make a noise, or shouldn’t. It’s when electricity jumps across a gap that you get a spark, and as I said lightning is just a very big spark. To the second: If a bulb makes a high-pitched noise just before it burns out, it’s probably because the filament is already broken, but the electricity is managing for a while to jump across the break, thus making continual mini-lightning until it gives up.


Is it true that the number of seconds in between thunder and lightning is the distance in miles of how far away the centre of the storm is? (Because light travels faster than sound?)

Something like that. Light travels at 186,000 miles per second, so we can forget the time it takes to reach us. But sound travels much more slowly. I can’t remember its speed, but indeed the time between the lightning flash and the sound it makes — the thunder — does enable one to work out how far away the centre of the storm is.


I didn’t do the cat experiment (you probably knew I wouldn’t). Am I right in thinking that the electrons would jump from any knuckle that was near her nose, not just to one connected to the body doing the stroking? Why nose rather than, say, tail?

Well yes I suppose any knuckle, or better still the central heating radiator or other large metal object, should the cat be close enough to it, would do nicely. And the nose is just more fun for cat-surprisers.


Negative electrons jump from earth to the cloud (to restore the balance with the positive charge of the molecules’ nuclei, that I understand), so I would have thought that the flash of lightning went the same way (i. e. upwards), but you say the reverse in para 3, or that it doesn’t matter. I thought lightning conductors on tall buildings funnelled the lightning safely down to earth, but if the electrons are travelling up towards the clouds, surely the surge of power would go the same way? I don’t understand.

That’s two questions. When we see a lightning flash it’s not really the electrons we see but rather the air, which has been heated white-hot by the passage of the electrons. And indeed I suppose you can say that the lightning flash goes from earth to ground, though in fact I suspect that there’s a sort of very rapid to-and-fro movement. I did in fact make one or two corrections to my post in an after-thought called ‘Oops!’

The electric charge on a body tends to spread itself evenly over the surface, especially if the body is roundish. It tends however to concentrate on any pointy bits. So a big roundish thing could build up a huge charge, so that when it finally discharges it can cause quite a bit of damage. From a pointy thing it will discharge sooner, and so at a lower voltage, and so cause less damage. And the lightning conductor can be on the building rather than (somewhat inconveniently) on the cloud, because a positively charged cloud passing over, say, a church tower is, electrically, equivalent to the tower’s having a negative charge. If you look carefully at the picture in the previous post you will see that the lightning hitting the top of the Eiffel Tower is a lot ‘thinner’ than the fat distant strike which was presumably to some less pointed object. The Eiffel Tower must surely have a lightning conductor on it.

Readers will have gathered that science and technology are among my interests, though I have many others. Here is something I am working on just now that combines a number of my interests:


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