Re: [Phys-l] [chemed-l] Electron Transfer Demo

[John Denker <jsd@av8n.com>]

On 04/12/2012 08:51 PM, Jeff Bigler wrote:
> I was demonstrating an induction coil in one of my physics classes
> today.  One of my students asked whether the arcing current could light
> a match.  I didn't know the answer, so i taped a match to a meter stick
> and tried it.  Nothing happened.
>
> At that point, because I now had a match on the end of a meter stick, I
> thought it might be fun to see whether the current affected the flame.
> So I lit the match and held the flame (on the end of the meter stick)
> near the arc.  When I did this, the arc jumped into the flame.
>
> In retrospect, this makes sense.  I believe this must occur because the
> flame is a redox reaction, and the reaction components were acting as a
> source and/or sink for the electrons arcing between the electrodes of
> the induction coil.

I would have explained it differently.

I see both experiments mostly in terms of the "path of least 
resistance" principle ... which is a consequence of Ohm's law.

1a) It is possible to light a match with an ordinary spark coil,
 but it takes more preparation than simply taping match to a
 meter stick.  That's because the match-head compound is
 electrically non-conducting, so the spark's path of least
 resistance avoids the match, other things being equal.

 You can fix this using the classic "spark plug" geometry,
 with the match-head filling the gap between two electrodes.

1b) There are other issues involved (such as the mean free 
 path in air) that affect how easily the spark coil can create 
 a spark ... but these do not conflict with the "path of least
 resistance" argument.  Mostly they serve to explain the resistance.

2) Conversely, the plasma in the flame is electrically conducting
 which tends to make it the path of least resistance for the spark
 (compared to comparable geometries where the spark passes near
 to the side of the flame).

See also next message.
 
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