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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).

You therefore have two magnetic feilds: 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, depending on its orientation.

If the force the wire experiences 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 as movement.

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 will be reversed if either the direction of the current or the direction of the magnetic field is reversed.

The direction of the force 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 LeFt has an F in it).

Catapult effect

When a current is passed through a loose wire in a magnetic field the jerk-like movement of the wire is called the catapult effect because the loose wire is then catapulted horizontally away from the magnetic field because of the interaction of the magnetic fields.

The term 'catapult effect' is only used for the 'motor effect' acting on a loose wire...

See Also:

Electric Motor

Loudspeaker

Revision of KS3 work on magnets and electromagnets

Force on a moving charge (A level)

Force on a current carrying wire (GCSE)

An experiment to illustrate the Force on a Wire in a Magnetic Field:

At GCSE level 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, may not be examined - look carefully at the syllabus to see what is required - boards change their minds so often!

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