The motor effect

When a wire carrying a current is placed in a magnetic field, the two fields interact and the wire feels a force: the motor effect. The force is greatest when the wire is at right angles to the field. Its size is F = B I l, where B is the magnetic flux density in tesla (T), I is the current in amps and l is the length of wire in the field in metres.

Fleming's left-hand rule gives the direction: the first finger points along the field (north to south), the second finger along the current (positive to negative), and the thumb shows the motion. In a d.c. motor a coil sits in a magnetic field; forces on opposite sides act in opposite directions, so it turns. A split-ring commutator reverses the current every half turn so it keeps spinning the same way. Loudspeakers also use the motor effect.