Why Do Sparks and Lightning Emit Light?

[fliptomato]

Greetings--I have a few questions about lightning and sparks. =)

(1) When you rub a glass rod on a piece of fur, I understand that there is a flow of electrons from one to the other. What property of these materials causes electrons to be released--i.e. is it because there are weakly-bound valence electrons, or does the thermal energy of the friction cause dissociation of electrons?

(2) When you see a spark (or lightning) what causes the discharge to be visible? I imagine a spark to be a current between two points--but you can't usually *see* current. Is the illumination associated with some kind of radiation from electron acceleration? (Or perhaps excitation of orbital electrons causing them to radiate as they fall back down to their ground state?)

Thanks much,
Flip

 

[Mk]

(2) When you see a spark (or lightning) what causes the discharge to be visible? I imagine a spark to be a current between two points--but you can't usually *see* current. Is the illumination associated with some kind of radiation from electron acceleration? (Or perhaps excitation of orbital electrons causing them to radiate as they fall back down to their ground state?)

When you see the electricity going from one point to another point by means of a gas being a conductor (such as air), that is actually ionized gas, gas that has been so electricly charged, that it changes color and glows. Plasma globes use this for entertainment, inert gases (the noble gases) are in a sealed sphere, and a modified tesla coil ( http://voltnet.com/tesla/index.shtml go to the bottom for cool pictures) is in the middle.

And this may be an interesting and relative article to you:

LASER LIGHTNING ROD. Lightning on demand, drawing down a bolt of
lightning for performing scientific studies, is usually done by
firing a rocket into an overhead cloud. The rocket spools out a
long wire, providing a conducting path between the charged-up cloud
and the Earth below. Soon this might be done using laser pulses. A
team of French and German scientists has performed experiments in
the lab in which a laser beam ionizes air molecules between an
artificial thunderhead (a high voltage electrode) with another
electrode, the equivalent of "earth" (a grounded electrode), several
meters away. The experiment is unique in that it can trigger
megavolt discharges across self-guided plasma filaments in air
generated by laser pulses. (Here are the potent characteristics of
natural lightning: peak power of ten megawatts, peak voltage of 100
MV, peak currents of tens of kilo-amps.) One of the lab results is
the surprising discovery that rain does not much perturb the
triggering or guiding of the discharge process. Next the team will
perform open-air lightning experiments. The aim of this work will
be to obtain the ability to trigger lightning before it occurs
naturally at sensitive sites such as airports or electrical
substations. (Ackermann et al., Applied Physics Letters, 6 December
2004; contact Jerome Kasparian, Universite Lyon,
jkaspari@lasim.univ-lyon1.fr)

From: http://www.aip.org/pnu/2004/split/712-2.html