Re: Sparks

["Robert W. Harris" <rwharris@cath-mem.org>]

-----Original Message-----
From: Ed Schweber <edschweb@ix.netcom.com>
To: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
Date: Friday, September 04, 1998 5:17 PM
Subject: Sparks


> Ed Schweber (edschweb@ix.netcom.com)
> Physics Teacher at The Solomon Schechter Day School, West Orange, NJ
> To obtain free resources for creative physics teachers visit:
> http://www.physicsweb.com
>
>
> Hi:
>
>    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 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?

Sounds good to me.  The negative charge on your finger causes the electrons
in the conducting lockset to redistribute themselves.  Assuming the door is
an insulator, the lockset remains electrically neutral but is polarized with
the side near you having a positive charge and the opposite side having a
negative charge.   Now the difference in potential (voltage) between your
finger and the knob is greater than would be the difference in potential
between your finger and a neutral piece of metal.  When the voltage is large
enough (and the distance small enough), a spark jumps.

> Would it also explain why
> lightening will more likely strike a metal pole?

Precisely.  Particularly if the pole were attached to the ground.  In that
case the pole would become charged, not simply polarized.

>    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.

No.  Unless I am missing something, this case appears to be precisely the
same as the first with all the signs changed.  Your positive finger causes
the electrons to redistribute themselves making the knob near you negative.
The potential difference is greater than it would be between your finger and
a neutral piece of metal, just as in the first case.
>

I hope this helps.

> Thanks for any input.
>
> Ed Schweber


Br. Robert W. Harris
Catholic Memorial High School
235 Baker Street
West Roxbury, MA, 02132
rwharris@cath-mem.org