Re: Sparks (and ions)

["JACK L. URETSKY (C)1998; HEP DIVISION, ARGONNE NATIONAL LAB ARGONNE, IL 60439" <JLU@hep.anl.gov>]

Hi all-
	William Beaty writes:
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On Sun, 6 Sep 1998, Ed Schweber wrote:

> Fair enough. But wouldn't the ultimate flow of charge through our bodies be
> electrons migrating from atom to atom. Admittedly this is not a single
> electron making the entire run from foot to finger, but is anything lost by
> thinking of it as a single electron.

Hi again Ed!  Nope, yours is a description of a wire.  In electrolytic
conductors, charged atoms migrate and electrons do not.  I think it's OK
to imagine that negative ions are carriers of electrons.  But then I would
insist on describing the positive ions as CARRIERS OF PROTONS.  Whenever
positive ions flow, the electric current is made of flowing excess
protons, protons which are missing the electrons which normally act to
"shield" them.  This is very different than metal wires, where protons
(positive ions) do not participate in the electric current. 
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	Quite right, and easily demonstrated, as any 19th century educator
would have known.  In electroplating, one gets deposits of ions at each
terminal.  In electrolysis of water, there is an accumulation of Hydrogen
at one terminal, and of Oxygen at the other.  The physics of the previous
century (now known as "chemistry") is very relevant for us "educators".

	Also, William Beaty comments
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>    Also, what would account for the light from the spark in the plasma
> model? I had always assumed that the "jumping electrons" collided with the
> H2 and N2, knocking their interior electrons electrons to a higher energy
> level and that the light was emitted in the subsequent decay. In the plasma
> model, I imagine that the free electrons would also emit EM radiation when
> they reattach to the atoms molecules after the conditions to maintain the
> plasma no longer exist. But this I suspect would involve primarily the
> outermost electrons and I don't know if the resulting radiation would be
> visible light.

Yep, I'm pretty sure that much of the light comes from flourescence of the
gas because of electron bombardment.  Sparks give off a line spectrum, and
some of the lines are in the visual range.  I don't know if the emission
frequency from the outermost electrons cause visible light.  But because
the light is a line spectrum, there must be all sorts of energy levels
involved.  If the spark temperature is high, there must also be a
continuous spectrum from the white-hot gas.  I don't know if the major
portion of the light is from heating, or from the line spectrum.
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	Yeah!  Plasmas are tough - and still not completely understood.
Stuff in a plasma is ionized, and there is certainly electron flow ->
which implies ion flow.  The educational toy is the Crooke's tube, in which
one makes a plasma by lowering gas pressure.  One can do e/m experiments
with Crook's tubes, but I'm too much of a theorist (ham-handed, that is) to
get very good results.

						Regards,
							Jack

	"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow.  Both of us
had marveled over that mystery a long time.  We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
		Mark Twain, Extract from Eve's
		Autobiography