Fleming's left-hand rule

The force on a given length of wire in a magnetic field increases when:

  • the current in the wire increases
  • the strength of the magnetic field increases

For any given combination of current and magnetic field strength, the force is greatest when the direction of the current is 90° to the direction of the magnetic field. There is no motor effect force if the current and magnetic field are parallel to each other.

The direction of the force can be found using Fleming's left-hand rule.

Left hand, thumb, forefinger, second finger at 90 degrees with arrows. Thumb shows direction of movement. Forefinger shows field (north to south). Second finger shows current positive to negative.

Hold your thumb, forefinger and second finger at right angles to each other:

  • the forefinger is lined up with magnetic field lines pointing from north to south
  • the second finger is lined up with the current pointing from positive to negative
  • the thumb shows the direction of the motor effect force on the conductor carrying the current

In which direction will this wire feel a force?

A bar magnet in an angular 'U' sort of shape. The N and S poles are marked. There is a wire running through and round the magnet, with arrows indicating direction of current.

With forefinger (magnetic field) pointing left to right, and second finger (current) pointing down, your left thumb (force) will point towards you. This is the direction in which the force acts.