(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) The electrostatic potential at
E is clearly zero since potential due to A and C is cancelled by that
due to B and D Hence, no work is required to bring any charge to
point E