(a) Deduce the expression for the potential energy of a system of two charges q1 and q2 located →r 1 and →r 2 , respectively, in an external electric field.
(b) Three point charges, + Q + 2Q and – 3Q are placed at the vertices of an equilateral triangle ABC of side l. If these charges are displaced to the mid-point A1, B1 and C1 ,respectively, find the amount of the work done in shifting the charges to the new locations.
OR
Define electric flux. Write its S.I unit.
State and explain Gauss’s law. Find out the outward flux to a point charge +q placed at the centre of a cube of side ‘a’. Why is it found to be independent of the size and shape of the surface enclosing it? Explain.