Here are strange results from an experiment in which Q1 and Q2
are not equal. All this started two weeks ago. Do you still remember
the crude experiment with two mica sheets done by Bob Dorner and
myself? It was described here. The experiment was repeated many
times with different dielectric materials and we can now be quite
confident that the following happens. An undergraduate physics
student, Gill Rifler, is working with us on this project.
In case you want to conduct this “scientific investigation” do the
following. From a plastic material cut two flat rectangular sheets,
for example 4 by 5 cm; each about 1 mm or so. Pieces cut from
a milk container will probably work as well as the polycarbonate
material “ lexan” we are now using. Place these sheets on top of a
metallic block and cover them with another metallic block. You
now have a disectable capacitor. Connect it to a d.c. power supply
of 1000 volts for several of minutes. The maximum d.o.p. of our
source is 5000 V. Then discharge the capacitor and dissect it. Do
not touch surfaces while doing this. Our dielectric pieces have silk
threads glued to them with epoxy; this facilitates handling. Also
make sure each piece is neutral before assembling the capacitor.
Measure the net charge on each piece. According to most textbooks
the net charge on each piece must be zero. But it is not. Pieces are
electrically charged, the positive charge on one piece is nearly always
much larger than the negative charge on the other. Our chargemeter
is an empty soup can placed on top of an old gold-leaf electrometer.
We use the can as a Faraday cup; pieces are lowered into the can
and the gold leaf deflection angle is measured. The instrument was
precalibrated in volts. Charges are obtained from Q=volts/C, where
C is about 20 pF (so far only estimated rather than measured). They
are not large, typically tens of nF.
We noticed that Q increases with the time of exposure (which is
strange considering the extremely small value of our R*C) but we
had no time to explore this. The same can be said about the effect
of the d.o.p. Please join us. You can do this experimentally and
share the results. Or you can help by commenting. There is some
kind of interesting physics here. Saying “you are rediscovering
electrets” is not very constructive; physics of electrets is not at
all clear. You can check this in the book “Electrets” by G.M.
Sessler, Springer-Verlag, 1980. (Also see O. D. Jefimenko and
D. K. Walker in The Physics Teacher, December 1980, p. 651).
Please share your wisdom, share your skills.
Ludwik Kowalski