An electron gun fires electrons into a magnetic field directed straight downward. Find the direction of the force exerted by the field on an electron for each of the following directions of the electron’s velocity:
(a) Horizontal and due north;
(b) Horizontal and 30° west of north;
(c) Due north, but at 30° below the horizontal;
(d) Straight upward. (Remember that an electron has a negative charge.)