A grounded sphere of radius 'a' is placed between point charges q1 and q2 with its centre O lying on the straight line joining q1 and q2. The distances of q1 and q2 from O are 2a and 4a respectively. Find the condition that the charge q2 experiences a repelling force. My question: How do I formulate the electrical force? I tried calculating the force for individual charges taking into account their image charges. But that makes my solution very cumbersome and I don't seem to be arriving at any particular answer. Help needed.