There charges q, 3q and 12q are to be placed on a straight line AB having 12 cm length. Two of the charges must be placed at end points A and B and the third charge can be placed anywhere between A and B. Find the position of each charge if the potential energy of the system is to be minimum. In the position of minimum potential energy what is the force on the smallest charge?

Answer

12q and 3q at the two ends and q at a distance 8 cm from 2q. Force on q is zero.