As many of you, I am soon going to start teaching electricity in the
introductory physics course. And I have two questions.
1) Solid Earth is said to be negatively charged with Q=-400,000 C.
I suppose that an equal and opposite amount of the charge must
be present in the atmosphere. Otherwise the planet would not be
electrically neutral. When was this actually tested in space?
2) A flat sheet of metal will have charges distributed uniformly on
both surfaces (except near the edges). We bend the metal, to
make a ring, and the inner surface becomes neutral. Suppose
we bend it to start making a letter C. How much bending is
sufficient to make the inner surface practically neutral?
I suspect that a half circle will already be neutral inside (with
the density of charge less that 1% in comparison with what it
were for the flat plate). The same is probably true with very
little of bending. But I am not sure.