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

The electric potential V at any point (x, y, z), all in metres in space is given by V = 4x² volt. The electric field at the point (1, 0, 2) in volt/meter, is

Three charges, each +q, are placed at the corners of an isosceles triangle ABC of sides BC and AC, 2$$a$$. D and E are the mid points of BC and CA. The work done in taking a charge Q from D to E is

Four electric charges +q, +q, $$-$$ q and $$-$$ q are placed at the corners of a square of side 2L (see figure). The electric potential at point A, midway between the two charges + q and +q, is