(a) (i) Explain the following statements giving reason : (I) An equipotential surface through a point is normal to the electric field at that point. (II) When a dielectric is placed in an external electric field, the electric field inside the dielectric is less than that outside it. (III) The potential difference between the plates of a charged parallel plate capacitor decreases when its plates are brought closer. (ii) Obtain an expression for the work done to dissociate the system of three charges $q$, $-4q$ and $2q$ placed at the vertices $A$, $B$ and $C$ respectively of an equilateral triangle of side '$a$'.