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82. The work done is equal to the change in the (total) electric potential energy
U
of the
system, where
32
13
12
012
023
013
444
qq
U
rrr
πε
=++
and the notation
r
13
indicates the distance between
q
1
and
q
3
(similar definitions apply to
r
12
and
r
23
).
(a) We consider the difference in
U
where initially
r
12
=
b
and
r
23
=
a
, and finally
r
12
=
a
and
r
23
=
b
(
r
13
doesn’t change).
Converting the values given in the problem to SI units

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