Re: Electrostatics

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

REPOSTING TWO LOST (?) MESSAGES.     Ludwik Kowalski

MESSAGE 1
*********
Another way to convince people about the intimate connection between
the Q and d.o.p. is use the electrophorus to electrify an insulated
pail and then draw a spark. But, as you say, nothing is better than
a voltmeter to demonstrate the d.o.p. (Just an idea I never tried).
The elstat machines are much more convenient.

Pasco (and Vernier) probes do measure the d.o.p. without presenting a
significant load (R>10^12 ohms ?) but only up to 6 volts. The d.o.p.
range of a Pasco electrometer is higher but still limited. Having a
commercial instrument whose range exceeds 300 or 500 V would be very
useful to me too. Any suggestions?

William Beaty wrote:

> My situation is abnormal: the purpose of the voltmeter is to shatter the
> misconceptions of a person who believes that "static electricity" involves
> only surface charge, and has nothing to do with high voltage.  (This
> misconception is not rare.  It is caused by the teachings of K-12
> textbooks where qualitative attraction/repulsion forces are repeatedly
> emphasized, but the actual magnitude of the potential between the objects
> involved is never mentioned.)

MESSAGE 2
**********

The term "leaking Q" is often used instead of "neutralizing it with ions
which come from air". No matter how dry air is an electrostatic demo will
often not work near a burning candle. An electrified comb will be
discharged in a second near a flame (for example 2 or 3 cm away,
sidewise). Try this demo in front of a class and ask students to guess
what happens.

Is there any reason to think that humid air normally contains more ions
than dry air? If not then Karl's mechanism must dominate.
      Ludwik Kowalski

      Karl Trappe wrote:
> If I understand you, my VDG discharges along its column, not through the
> air.  That is a plausable explanation to me.  Does this mean that the
> *humidity* supplies the "solution" to allow the junk on the column to start
> an ionic conducting path?
>
> Certainly, I believe that oil and grime (usually from hands which picked
> up the VDG by the column) provides a multi-megohm resistance path from the
> sphere to the base (via the small cracks in the column which result from
> static produced ozone interacting with the plastic of the column).  I have
> been able to dramatically improve performance by cleaning the column with
> alcohol, and allowing the alcohol to evaporate.
>
> I also know that on a high humidity day, that static electricity demos
> don't work.  Some "crisp" weather parts of the country never have this
> problem.  I even keep all electrostatic demos in a dry cabinet with light
> bulbs burning 24 hours a day to insure their dryness.
>
> I realize that water, as a polar molecule, is not generally a good
> conductor (unless "dirty", and thus ionic), although it is an excellent
> dielectric when clean.  Clearly, suggesting that warm, moist air ("clean
> water carrying" air) is a good conductor makes little sense to me, but I
> don't have a better explanation.
>
> My confusion is that if I run a (cleaned column) VDG, or run a comb through
> my hair, I get good results when the air is dry.  If I open the door on a
> humid day, both of these "clean" sources cease to produce static charge
> separation.
>
> So...I'm all ears for a "clean" explanation.  Karlq