The total work
required is
=
−
+
+
−

 +
−






q
d
2
0
4
0
1
1
1
2
2
1
2
πε
( )
( )
 =
−
(−
)
q
d
2
0
4
4
2
πε
The work done depends only on the arrangement of the charges, and
not how they are assembled By definition, this is the total
electrostatic energy of the charges (Students may try calculating same work/energy by taking charges
in any other order they desire and convince themselves that the energy
will remain the same )
(b) The extra work necessary to bring a charge q0 to the point E when
the four charges are at A, B, C and D is q0 × (electrostatic potential at
E due to the charges at A, B, C and D)