1. The problem statement, all variables and given/known data http://u.spig.in/forum/105320795.jpg [Broken] 2. Relevant equations general Electrostats formulas 3. The attempt at a solution I tried to solve the problem using Energy conservation ie., Change in PE = Work done. But that seem to yield wrong answer (1) while the correct answer is 2. I had read a similar sum in Irodov's ' Basic Laws of Electromagnetism ' in which it was written that this method fails in above case. Please help me solving this.