# Physics iGCSE Section F Magnetism and electromagnetism

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## Magnets

Hard magnetic materials: Stay magnetised
Magnetic materials: Magnetise and demagnetise easily.
The arrows on the lines always point away from the north pole and towards the south pole.
Magnetic field lines do not cross
The closer together the lines are, the stronger the magnetic field.

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## Electromagnets

current in a wire produces a magnetic field, which can be made stronger by:

• increasing the current
• increasing the number of turns on the coil
• winding the coil around a magnetic material

The motor effect
When a current is passed through a wire placed in a magnetic field a force is produced which acts on the wire.

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## The generator

When a conductor is in a changing magnetic field, a voltage will be induced in the conductor.
The magnetic field can change if:

• the conductor is moving in/out of a magnetic field
• a magnet is moving towards or away from the conductor
• a varying magnetic field

The size of the induced voltage in a coil can be increased by increasing the rate of change of the strength of the magnetic field, by having more turns on the coil and by having a coil of a greater area.

bicycle dynamo: changing magnetic field induces an alternating voltage in coil
generator: inducing a voltage using brushes sliding over slip rings.

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## The transformer

The function of a transformer is to change the size of an alternating voltage. This is done by having two seperate coils with different numbers of turns.

input voltage / output voltage = primary turns / secondary turns
Vp / Vs = n / n

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