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Re:IONS/dielectrics



This evening another teacher, Bob Dorner, and myself started
an experiment. Results are interesting, as described below. We
hope some phys-L-ers will be sufficiently interested to start
other (or similar) experiments devoted to space charges. We
can learn faster together than in isolation. And it is more fun.

First I was thinking that the IONS thread can be used for
sharing data, ideas and critical remarks among those who are
actively involved. But now I think that many phys-L-ers
would object of being inondated by long messages in which
they are not interested. The solution is to form a dedicated list.

If the second option is chosen then anybody who wants to
participate would send me a private message and I will create
a list on our university server. Naturally this makes sense only
if at least three people are involved; two people can communicate
without a list. I am still not sure which approach is better, a
phys-L thread or a dedicated list. Naturally we can begin one way
and switch later. Please express your opinion and I will decide
accordingly.
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Here is what we did today. Two identical pieces of very flat mica
(each area=2.7 by 3.8 cm, each mass 0.22 g) were squeezed between
two copper plates and exposed to a d.c. source of 5000 V for 25
minutes. To avoid an air gap, copper plates were smaller than
mica (about 2 by 3 cm). A thin silk thread was glued (epoxy) to
two corners of each mica (to prevent direct handling).

Pieces of mica were found attracting each other strongly after
the exposure. Without separating them we inserted the pair
into a Faraday cup mounted on a gold leaf electrometer. The
response of the electrometer was rather weak, the angle of
the leaf was near 15 degrees; it would be zero if the pair were
neutral.

Then electrometer was discharged. Pieces of mica were
separated from each other and each piece inserted into the
Faraday cup. The response of the electrometer to each mica
was very strong (the leaf angle was near 85 degrees). This
shows that the net charge on mica #1 had a different sign
than the net charge on mica #2.

The opposite signs of net charges on two pieces was also
verified in another experiment. After removing the first
mica from the Faraday cup some charge remained on the
electrometer. Without discharging the electrometer we
inserted the second mica into the cup and saw that the
angle of the leaf was reduced, not increased.

How else can these data be interpreted than in terms of what
Faraday called "penetration of charges" from the metallic plates
into the dielectric? There is so much more to investigate but
our time is limited. It would be great if somebody could confirm
these observations. Apriori any insulating material is worth
trying. I wish we had a d.c. source of higher voltage. A longer
exposure to a higher voltage may result in deeper penetrations
of charges.

Our data are very crude. The main purpose was to see if the
presence of space charges can be detected with our primitive
equipment. This subject is never mentioned in the introductory
textbooks and for that reason it is ideal for student projects. Now
we know that space charges are real and we will try to attract
a student or two. The only references on the subject, as far as I
know, are Faraday (1837) and Walker/Jefimenko (1973); both
were already posted here. I did not have time to look for
references which are given by Walker and Jefimenko. Please
share what you know.

Regards, Ludwik Kowalski