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Re: Saturday Morning Puzzle, Part 2



At 10:04 PM 10/6/01 -0700, Bernard Cleyet wrote:
The field E, g, e-m source, acoustical, etc. due to an effectively
infinite plane (near field) is constant (that's why g is considered a
constant "on earth."). Therefore, two of them will be mutually attracted
with considerably greater force than the equivalent spheres and, near
field, constant w/ separation

It depends on what you mean by "equivalent" sphere.
-- Sphere with same surface area as the large sheet?
-- Sphere with same mass, when a large thin sheet is wadded up into a
small sphere? I (belatedly :-) believe this is the case called for by the
original question. (There's no zealot like a convert.)


The capacitance of a very large parallel-plate capacitor is very
large. Its energy (at constant charge, not constant voltage) is
infinitesimal and proportional to the gap. Therefore the force is the
derivative w.r.t gap of this energy, namely infinitesimal and independent
of gap.

The aforementioned force is much smaller than what you get when the same
total charge is wadded up into compact spheres.

(In the gravitational analogy, mass plays the role of charge. Constant
mass <==> constant charge.)