Answer
a) If the electric field is reduced, the magnetic force will be greater than the electric force. Thus, the positive charges in the velocity selector will be deflected upward and not enter $B_{2}$.
b) If $B_{1}$ is decreased, then the electric force will be greater than the magnetic force. The positive charges will be deflected downward and will not enter $B_{2}$.
Work Step by Step
a) If the electric field is reduced, the magnetic force will be greater than the electric force. Thus, the positive charges in the velocity selector will be deflected upward and not enter $B_{2}$.
b) If $B_{1}$ is decreased, then the electric force will be greater than the magnetic force. The positive charges will be deflected downward and will not enter $B_{2}$.
