Naked Science Forum
General Science => General Science => Topic started by: Reuben on 05/04/2011 01:30:09
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Reuben asked the Naked Scientists:
Dear Chris,
Now to lightning. When it strikes, does it go at the speed of light, and if it does, then why does it, since it isn't light?
Kind regards
Reuben
What do you think?
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"Lightning" is electrical current, which heats the air, the heated air molecules produce EM radiation (http://en.wikipedia.org/wiki/Electromagnetic_radiation) which include light waves , (and radio waves and UV and X-rays and Gamma-rays).
Question: What is the speed of lightning?
Answer: According to the book, "It's Raining Frogs and Fishes,"by Jerry Dennis, lightning bolts travel at speeds up to 93,000 miles per second.
http://www.newton.dep.anl.gov/askasci/gen99/gen99096.htm
The light generated by the lightning-bolt current will travel at the same speed as other light travelling through air: at 0.997c (http://en.wikipedia.org/wiki/Speed_of_light), (i.e. almost the speed of light in a vacuum, c (http://en.wikipedia.org/wiki/Speed_of_light)).
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A lightning bolt propagates by a having sufficient voltage to "breakdown" the normally insulating barrier of air between the charged clouds or between a charged cloud and the ground. The breakdown is not a simple effect but it essentially a cascade of ionisation of the gases that make up the intervening air. As cited by RD, this does not occur at lightspeed. I have to admit to being surprised that it was even half lightspeed. In fact this wikipedia page ( http://en.wikipedia.org/wiki/Lightning ) says 140,000 mph which is much, much slower and, to my mind, much more believable.
The actual breakdown mechanism is not well understood, as can be gleaned from the wiki page.
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Agree with Graham - disagree with Jerry Dennis; I cannot believe the 93000 miles per second (thats half the speed of light!)
Here is a cool video shot at 9000 fps
http://technorati.com/videos/article/lightning-strike-recorded-in-slow-motion/
You can see the bolts moving (not merely the strikes growing) but the actual progression. At half the speed of light this would of course be impossible to notice even slowed down.
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I suspect the half speed of light is the speed of the electron current. Presumably electrons flow between zones of different potential, but while this might initiate lightning, I agree that the lightning bolt travels much more slowly.