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Q & A: pointed lightning rods

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Most recent answer: 11/18/2011
Q:
Hi, could you please help explain to me how lightning rods work? And why a lightning rod with several sharp points is more likely to be struck by lightning than one with a rounded top? I understand that sharp pointed ends accumulate more charge, so is it that there would hence be a greater force of attraction between the charged thundercloud and this sharp pointed rod than with the rounded rod or with a building? Or is it that the sharp points of the rod concentrate the charge of the rod when it is charged by a thunder cloud and so the air around it is neutralised and thus reduces the risk of the building being struck by lightning? I'm pretty confused at the moment and my exam is just around the corner. Thank-you for your kind explanation!
- anonymous (age 15)
New Zealand
A:
The key idea is that air does not obey ohm's law. The current is not simply proportional to the electric field. At low electric fields, there's almost nothing in the air that can carry current. At high electric fields, the air starts to break down, forming current-carrying ions.

The points are important because the electric field near the tip builds up to large values. The air is therefore more likely to ionize near the tip than somewhere else with a smaller field. That means that current can start flowing there, rather than somewhere else, so it flows directly to the conducting rod.

Mike W.



(published on 11/18/2011)

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