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I very much appreciated John Denker's arguments placing an upper and lower
bound on the capacitance of a disk.
However, I do have a concern about the
<< At surfaces, the electric field strength is proportional to the charge
per unit area. For the uniformly-charged disk (case c) this is
sigma = Q / (2 pi R^2) (eq 4)
where the factor of 2 is because the disk has two sides. >>
That "the electric field strength is proportional to the charge per unit
area" is correct for a uniformly charged disk of infinite radius and for a
conducting disk of finite radius.
However, it seems to me it is not correct
for a uniformly charged disk of finite radius.
The electric field of such a
disk is not normal it's the surface, except on the axis.
The field is always normal at the surface of a conductor.