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Physics 8463 - 4.7 Magnetism and electromagnetism

4.7.2 The motor effect

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4.7.2.1 Electromagnetism

When a current flows through a conducting wire a magnetic field is produced around the wire.

The strength of the magnetic field depends on the strength of the current through the wire and the distance from the wire.

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.

The magnetic field inside a solenoid is strong and uniform.

The magnetic field around a solenoid has a similar shape to that of a bar magnet.

Adding an iron core increases the strength of the magnetic field of a solenoid.

An electromagnet is a solenoid with an iron core.

You should be able to:

describe how the magnetic effect of a current can be demonstrated

draw the magnetic field pattern for a straight wire carrying a current and for a solenoid (showing the direction of the field)

explain how a solenoid arrangement can increase the magnetic effect of the current.

You should be able to interpret diagrams of electromagnetic devices in order to explain how they work.

  
4.7.2.2 Fleming's left-hand rule

When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other.

This is called the motor effect.

You should be able to show that Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.

You should be able to recall the factors that affect the size of the force on the conductor.

For a conductor at right angles to a magnetic field and carrying a current:

force = magnetic flux density × current × length

F = B I l

This equation is given on the Physics equation sheet - but you need to know what the letters mean and are measured in.

F = force, in newtons, N

B = magnetic flux density, in tesla, T

I = current, in ampères, A (amp is acceptable for ampère)

l = length, in metres, m

 

  
4.7.2.3 Electric motors

A coil of wire carrying a current in a magnetic field tends to rotate.

This is the basis of an electric motor.

You should be able to explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.

  
4.7.2.4 Loudspeakers

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.


You should be able to explain how a moving-coil loudspeaker and headphones work.