Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 20/11/2006 01:00:41

Title: how does lightning work?
Post by: thebrain13 on 20/11/2006 01:00:41

I understand that lightning is a discharge of negative charge up in the clouds to a positively charged ground. But why does all the electric charge in a cloud strike in one place?

Title: Re: how does lightning work?
Post by: daveshorts on 20/11/2006 12:16:46

water droplets tend to charge up when air rushes past them, a bit like rubbing a balloon with your hair. if this way you can end up with different parts of clouds at different potentials (voltages) and clouds at different potentials to the ground.

These voltages can discharge slowly in a distributed manner, however if the electric field (change in voltage  per metre) is large enough it can rip some of the electrons of the air molecules, you then get some electrons and positive ions - a plasma. These will get accelerated by the electric field, in opposite directions, if it is strong enough they will hit other air molecules and knock more electons off. This forms a chain reaction and you get a line of ionised air, there is lots of energy in this and it will give off light - you know it as a spark.
 As there are free electrons and positive ions, this plasma conducts electricity very well unlike air which is a good insulator, and electricity will follow the easiest path, so all the charge from a significant portion of a cloud will follow this path, creating a huge spark - a lightning bolt.

  Because there is so much energy being dumped into a narrow piece of air, it gets extreemly hot, hot air expands and this explosive expansion is what you hear as thunder.

Title: Re: how does lightning work?
Post by: Dr. Praetoria on 21/11/2006 21:37:16

"The development of a bolt of lightning is very interesting. I won't go into
>all the details here, but a cloud-to-ground lightning bolt begins from the
>cloud in a series of "stepped leaders." As these leaders near the ground, a
>powerful "return stroke" of positive charge rushes up from the ground (at a
>speed up to 60,000 mph, or about 1/3 the speed of light) to meet the stepped
>leader. These static charges travel back and forth between the cloud and
>ground until the electrical potential is neutralized."

see--http://www.newton.dep.anl.gov/askasci/wea00/wea00030.htm

Doc