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89. (a) The charges are equal and are the same distance from
C
. We use the Pythagorean
theorem to find the distance
rd
d
d
=+
=
22
2
b
g
b
g
. The electric potential at
C
is
the sum of the potential due to the individual charges but since they produce the same
potential, it is twice that of either one:
()
92
2
6
6
8.99 10 N m C
2
2 2.0 10 C
2.5 10 V.
4
4
0.020 m
q
q
V
dd
εε
−
×⋅
×
==
=
=
×
00
ππ
(b) As you move the charge into position from far away the potential energy changes
from zero to
qV
, where
V
is the electric potential at the final location of the charge. The
change in the potential energy equals the work you must do to bring the charge in:
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 Spring '10
 Rebello,NobelS
 Charge, Electric Potential

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