7.- (10 Points) You place a point charge 9' = +480x10'9 C at the center of a ring. A particle (electron) is projected from innity to the ring along the axis. The ring has charge Q = -61{}x10'9 C which is uniformly distributed and a radius of 240cm. The particle momentarin stops at 500cm from the center of the ring. Find the initial speed of the particle when it was projected