The Motor Effect

When a wire carrying an electric current is placed in a magnetic field, it may experience a force. Why should this be? It is 'simples' really...

When a wire carries a current it has a magnetic field around it. (You did this when you studied electromagnetism). The field from the fixed magnet interacts with the field due to the current. This can cause the wire to be attracted or repelled from the fixed magnet. If this force is strong enough (meaning that the wire has a small enough mass so that gravity doesn't stop it moving) you will see the effect of the attraction/repulsion in movement of the wire.

The force experienced by the wire is at a maximum when the current is flowing at right angles to the field lines of the fixed magnet and is zero when it is parallel to them.


The size of the force on the wire can be increased by:

  • increasing the strength of the magnetic field it experiences;
  • increasing the size of the current flowing through it.
  • increasing the number of turns of wire experiencing the magnetic field.

The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed. It can be found using Fleming's Left Hand Motor Rule. (You can remember that it is the left had by remembering that in Britain we drive motor cars on the left hand side of the road - or by remembering that the motor rule predicts the direction of a resulting Force and LEFhas an F in it).


See Also:


For the AQA GCSE you should be able to use diagrams and/or other appropriate information to explain how electromagnetic effects are used in simple d.c. motors and circuit breakers.

NB Details of the split ring commutator, for reversing the current to a d.c. motor each half turn, will not be examined.

Here is an interesting demonstration that you could try at home!