Charges +q,–q,+q,–q are placed on the vertices of a square ABCD respectively. Let $\overline{\mathrm{E}}$ be the electric field and V the potential at the centre. If the charges on A and B are interchanged with those on D and C respectively, then :

In the first situation the electric field and the potential at the center is zero . On interchanging the charges that is A with D and B  with C , the charges orientation of teh charges still remain the same , i.e the field and the potential the center is still zero and remains the same .