Re: explanation of sparks into air needed

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmdq-at-uol-dot-com.br> 

Tesla list wrote:
 >
 > Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net>
 >
 > I'm looking for a good short explanation of how sparks can go into air from
 > a charged object. The problem I'm having is trying to explain that not all
 > sparks have to go from one place to another place, partly because my
 > audience has a hard time conceptualizing the idea of a high field that can
 > cause a breakdown.

DC sparks always go from one place to another, although below a certain
energy level they disperse and become invisible before forming a spark.
This makes DC corona.
In the case of high-frequency AC corona, the charges try to do the same,
but the field reverses and pushes them back. The current (with the same
charges) moving back and forth through the ionized air produces a lot of
heat and light. The result is something that looks as sparks that don't
go anywhere: streamers.
This effect can be seen even in lightning. Initially the negative
charges spread down from the clouds forming branches in many directions,
until one of them makes contact with the ground. All the charges are
then quickly drained from the branches to the main channel, producing
what looks as streamers attached to the main channel, that flash briefly
at the beginning of the lightning stroke. Long DC sparks also show this.
(Although in DC sparks the branches appear to go in the direction of
the negative terminal, and in lightning they appear to go to the
positive
terminal (the ground), I don't know why. Different geometry of the
terminals, maybe)

Antonio Carlos M. de Queiroz