1. The problem statement, all variables and given/known data A charge Q = - 710 nC is uniformly distributed on a ring of 2.4 m radius. A charge q = + 350 nC is placed at the center of the ring. Points A and B are located on the axis of the ring, as shown. In Figure below, an electron is projected along the axis of the ring from infinity. The electron reaches and comes to a momentary halt at a point on the axis which is 5.0 m from the center of the ring. The initial speed of the electron at infinity is closest to: http://img31.imageshack.us/img31/3900/q6g1.jpg [Broken] a) 1.4*10^7 m/s b) 9.1*10^6 m/s c) 6.9*10^6 m/s d) 4.6*10^6 m/s e)2.3*10^6 m/s 2. Relevant equations U=ke*q1q2/r1r2 V=ke*Q/sqrt(x^2+a^2) http://img38.imageshack.us/img38/7096/picture1oxd.jpg [Broken] 3. The attempt at a solution 1/2 mv^2=ke*q1q2/r12+ke*Q*qe/sqrt(x^2+a^2) plug in number, solve for v and i don't find any answer similar with my result, is there something wrong with my solution ?