Force on a Current-Carrying Conductor: Fleming's Left-Hand Rule

We've seen that a current produces a magnetic field. The reverse also happens: a magnetic field exerts a force on a current-carrying conductor placed inside it. Suspend a small rod between the poles of a horseshoe magnet, pass a current through it, and the rod visibly jumps sideways, proof that a real force acts on it.

Reverse the current's direction, and the rod jumps the opposite way. Flip the magnet's poles instead (reversing the field), and the rod again jumps the opposite way. The force clearly depends on both the current's direction and the field's direction.

The force is largest when the current and the magnetic field are at right angles to each other, and in that case, the force acts in a third direction, perpendicular to *both* of them. This is captured by Fleming's left-hand rule: stretch the thumb, first finger, and middle finger of your left hand so all three are mutually perpendicular. Point the first finger along the field, the middle finger along the current, and the thumb then points along the force.

This 'motor effect', force on a current in a magnetic field, is the working principle behind electric motors, loudspeakers, and microphones, and it's the mirror image of a fact you already know: current-carrying conductors also produce magnetic fields.