Re: Electrostatics problem

["Miguel A. Santos" <msantos@ETSE.URV.ES>]

Ludwig Kowalski wrote:
> As many of you, I am soon going to start teaching electricity in the
> introductory physics course. And I have two questions.
>
> 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.
 I have seen this statement elsewhere too, stating that for an OPEN
cylinder the inner (mathematically, this has no sense) surface
becomes neutral. But how can this be justified? I guess this is
just a kind of a rule of thumb,isn't?
 Then I'd like to see some references, just to compare some numbers,
like your 1% above.

 Regards,
        Miguel A. Santos