Re: Sparks (and ions)

[William Beaty <billb@eskimo.com>]

On Fri, 4 Sep 1998, Ed Schweber wrote:

>     For reasons too complicated to get into I am teaching an out-of
> normal-sequence H.S. class and I began today with the usual introduction to
> electrostatics: rubbing your feet along a carpet will cause an excess of
> electrons 

Hi Ed!

One point: it's not electrons.  Our bodies are electrolytic conductors, so
the "charges" upon our fingertips are sodium ions, chloride ions,
potassium, charged proteins, etc.  If we avoid saying "electrons" and
instead say "excess of charge" or "charge-imbalance", then we don't have
to discuss the details, such the ions which make up that charge.  Free
electrons are important in metals and in plasmas, but not where human
flesh is concerned.  E.g. when we receive a shock, no electrons flow
through us.  And when electrons are dumped onto our skin, they cancel an
equal number of positive ions, leaving behind some non-cancelled negative
ions, and so the electrons have been "converted into" ions.  Or I could
just say that "negative charge" was placed on my body, and leave electrons
and ions out of the description. 

> to accumulate on your body, to concentrate on thin objects like your
> fingers, and when you reach for a metal door knob the mutual repulsion 
> between the electrons causes some of them to jump across the gap forming
> the spark. 
>
>    In one of those obvious questions no one has asked me before until today,
> a student wanted to know why the door knob was necessary. Why don't the
> electrons just leave your finger due to their repulsion whether or not the
> doorknob is nearby. My intuitive response (which is just that, a plausible
> guess) was that the electrons on your finger also induce a positive charge
> on the doorknob and the force from this induced charge adds to the force
> from the electron repulsions. Is it that simple?  Would it also explain why
> lightening will more likely strike a metal pole?

You're leaning in the right direction.  Another way to say the same thing:
when you bring your fingertip near a neutral object, you "compress" the
surrounding e-field and make it much stronger, and when the field is
intense enough, a spark occurs.

But the increasing force upon the "CHARGES"  :)  never grows strong enough
to make them leap from your finger.  Instead the electrostatic force
applied to the air molecules causes the air to 'ignite' and turn into
plasma, which is a conductor.  It's as if the e-field caused a metal wire
to spontaneously appear in the space between the doorknob and your finger.
As others pointed out, if the pressure is low, air breaks down and becomes
plasma at lower field strength.  

Yet if all trace of air was eliminated, would charges still leap?  In a
hard vacuum, if there is an area of imbalanced surface charge on a
conductor, why wouldn't the charge simply leap off into space?  The charge
cannot escape because conductive objects attract charge.  If a positive
sodium ion were to depart from your fingertip, that ion would induce an
image charge in the conductive skin nearby, and so would be attracted back
to the skin.  Or another way to say the same thing: charge cannot leave a
conductor until the e-field at the surface of the conductor is strong
enough to defeat the "work-function" of that material.  At very high
values of e-field, charges do start spewing from a conductor.  This is
called "field emission", and is usually achived by using incredibly sharp
metal points in a vacuum.  If you placed your hand in a vacuum, and
cranked the voltage up (to megavolts? teravolts?), the charges would
finally start to leap from your skin because of their own repulsion.  But
the tens of kilovolts from rug-scuffing are too low to cause this, unless
you happen to have some very sharp dust-motes clinging to your fingertip.

>     What if your walking along the carpet created an electron deficit of the
> same magnitude on your finger. Your finger would then attract electrons from
> the door knob but it would seem that there wouldn't be any additional
> inductive effect and therefore it is less likely that a spark would jump. Is
> this also the case.

I think polarity has a small effect, but not because deficits cannot jump. 
In fact the word "jump" is a bad description.  I would say that the air is
electrically "fractured" by the intense e-field, and once the "crack" has
appeared, charges can flow because the crack itself is a conductor.  The
"fracture" is caused by strong e-fields in the air, and the air does not
much care in which direction the fields point.  A positive surface charge
can produce exactly the same e-field strength as a negative surface
charge, therefor either polarity would be expected to produce a spark at
the same voltage and spacing.

On closer examination I THINK that we do find a small difference caused by
polarity.  If high voltage is applied between a needle and a metal sphere,
I think the breakdown voltage is slightly different depending on the
polarity of the needle.  (I don't recall which polarity gives sparks at
lower voltage.)  Just before a spark ignites at the needle tip, there are
lots of little "avalanche breakdowns" going on in the air adjacent to the
metal tip.  In these micro-sparks there are free electrons being
accelerated and colliding with air molecules, releasing more free
electrons.  The polarity of the metal surface must have an effect on these
micro-avalanches occuring in the air, and so it must be easier for one of
the avalances to grow into a macro-size spark depending on the polarity of
the metal. 


  See "Which way does 'electricity' flow?" at
  http://www.eskimo.com/~billb/ele-edu.html

  Once place I recall seeing info on this is in LIGHTNING, the paperback
  book by Dr. Martin Uman at U. FL.


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