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29. The disk is uniformly charged. This means that when the full disk is present each
quadrant contributes equally to the electric potential at
P
, so the potential at
P
due to a
single quadrant is one-fourth the potential due to the entire disk. First find an expression
for the potential at
P
due to the entire disk. We consider a ring of charge with radius
r
and
(infinitesimal) width
dr
. Its area is 2
π
r dr
and it contains charge
dq
= 2
π
σ
r dr
. All the
charge in it is a distance
22
rD
+
from

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