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

A charge Q is enclosed by a Gaussian spherical surface of radius R. If the radius is doubled, then the outward electric flux will

The electric field at a distance $${{3R} \over 2}$$ from the centre of a charged conducting spherical shell of radius R is E. The electric field at a distance $${R \over 2}$$ from the centre of the sphere is