Force on a current in a magnetic field

The book's activity: hang a short aluminium rod between the poles of a horseshoe magnet and pass a current through it. The rod moves. A current-carrying wire is a magnet itself, so in another magnet's field it feels a force.

Reverse the current, or swap the magnet's poles, and the rod moves the opposite way. The force is largest when the current is at right angles to the field, and zero when the wire lies along the field.

Fleming's left-hand rule: stretch the thumb, first finger and middle finger of your left hand so they are all at right angles to each other. Think Father, Mother, Child:

Point the first finger along the field and the middle finger along the current; the thumb gives the force.

Moving charges feel this force too. Treat a moving positive charge as a current in the direction it moves. For an electron, the current is in the opposite direction to its motion.

The force on a moving charge is always at right angles to its motion, so it changes the direction in which the charge moves (its velocity and momentum).

This force does useful work in electric motors, loudspeakers, microphones and measuring instruments.