Two pith balls carrying equal charges are suspended from a common point by strings of equal length, the equilibrium separation between them is r. Now the strings are rigidly clamped at half the height. The equilibrium separation between the balls now become

Two metallic spheres of radii 1 cm and 3 cm are given charges of $$-$$1 $$ \times $$ 10^$$-$$2 C and 5 $$ \times $$ 10^$$-$$2 C, respectively. If these are connected by a conducting wire, the final charge on the bigger sphere is

Four point charges $$-$$Q, $$-$$q, 2q and 2Q are placed, one at each corner of the square. The relation between Q and q for which the potential at the centre of the square is zero is

What is the flux through a cube of side $$a$$ if a point charge of q is at one of its corner?