Assume that a ball of charged particles has a uniformly distributed
negative charge density except for a narrow radial tunnel through
its center, from the surface on one side to the surface on the
opposite side. Also assume that we can position a proton anywhere
along the tunnel or outside the ball. Let FR be the magnitude of
the electrostatic force on the proton when it is located at the
ball's surface, at radius R. As a multiple of R, how far from the
surface is there a point where the force magnitude is 0.30FR if we
move the proton (a) away from the ball and (b) into the
tunnel?