1. The problem statement, all variables and given/known data Two point charges 2q and 8q are placed at a distance r apart. Where should a third charge -q be placed between them so that the electrical potential energy of the system is minimum. a). at a distance r/3 from 2q b). at a distance 2r/3 from 2q c). at a distance r/16 from 2q d). none of these 2. Relevant equations PE of system= PE between all the individual charges. EPE of two point charges can be given as: 1/4πε * q1*q2 / r 3. The attempt at a solution I simply derived the equation of potential energy of system, which is: http://i46.tinypic.com/nvt72s.jpg Then I differentiated the equation to find out the value of x where potential energy is minimum. I got x = 2r + 8 +- sqrt( 5r^2-8r+16) Now the problem is I dnt know whether my solution is correct. Since the question is MCQ, i ticked none of these. which, hopefully was correct. Need help.