1. The problem statement, all variables and given/known data A cavity of radius r is present inside a solid dielectric sphere of radius R, having a volume charge density of ρ. The distance between the centers of the sphere and the cavity is a. An electron e is kept inside the cavity at an angle θ = 45° as shown. How long will it take to touch the sphere again? 2. Relevant equations 3. The attempt at a solution The electric field inside the cavity is ρa/3 directed parallel to the line joining the centers of the sphere and the cavity ,i.e parallel to OC . The electron is at E .So it should reach point P .After that it will enter the sphere .But the electric field will be variable inside the sphere .I am unable to proceed from here. Not sure whether my approach is correct . I would be grateful if somebody could help me with the problem.