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Motor effect of electric current: 11.1 The motor effect - the interaction of a electric current carrying wire and a magnet

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INDEX physics notes: motor effect of an electric current and applications


11.1 Explaining the motor effect - the interaction of a current carrying wire and a magnetic field

This page will help you answer questions such as ...  Why does a current carrying wire experience a force when placed in a magnetic field?   What is Fleming's left-hand rule?

When a conductor e.g. a wire, carrying an electric current, is placed between the poles of a magnet the magnetic field around the conducting wire interacts with the magnetic field it is placed in.

We are dealing with the interaction of two magnetic fields, each with their own north and south poles because a magnetic field is always produced around a wire carrying an electric current..

This will cause the magnet and the conductor to exert a non-contact force on each other.

The force will cause the wire to move and this phenomena is called the motor effect.

Here you are dealing with two magnetic fields (from the wire and magnet) each with its north and south pole, hence the interaction (as you get with any two magnetic poles)

The resulting magnetic field is stronger in one area and weaker in another, so there is a resultant force.

To get the maximum full force effect the wire should be at 90o to the direction of the magnetic field flux.

If the wire is parallel to the magnetic field, it won't experience a force at all.

So from 0o to 90o you get a steady increase in the force exerted on the wire.

diagram explaining Fleming's left-hand rule motor effect of an electric current relating direction of force direction of movement direction of current direction of magnetic field

The diagram above illustrates the 'kicking wire' experiment and Fleming's left-hand rule which allows you to predict the direction of wire's motion - the direction of the resultant force or thrust.

The force always acts at right angles to the magnetic field of the magnet AND the direction of the current in the wire see more on Fleming's left-hand rule below.

The magnitude of the force increases with ..

(i) increase in current flow, which increases the strength of the magnetic field around the wire,

(this could be increased by increasing p.d. (V) or a thicker wire creating a smaller resistance (R in Ω) for the same p.d.

(ii) the strength of the magnetic field of the permanent magnet - a 'stronger 'magnetic field around the magnet,

(iii) the length of wire exposed to the magnetic filed, greater length or area where the two magnetic fields interact.

INDEX physics notes: motor effect of an electric current and applications


Keywords, phrases and learning objectives on the motor effect of an electric current

Be able to explain the motor effect from the interaction of electric current carrying wire and an external magnetic field

Be able to explain the cause of movement in demonstrations like the kicking magnet experiment.


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