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

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.

Thanks for any input.

Ed Schweber