# P3 Revision Notes:Electromagnetism

Revision Notes for P3

Electromagnetism

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• Created on: 30-03-12 14:32

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REVISION NOTES: PHYSICS 3
ELECTROMAGNETISM
The Motor Effect
When a current is passed along a wire in a magnetic field, a force may be exerted on the wire. This
is the motor effect.
There is always force acting on the wire unless the wire is parallel to the magnetic fields. The force
can be increased by increasing the current or using a stronger magnet.
The force depends on the angle between the wire and the magnetic field lines. The force is biggest
when the wire is perpendicular to the field lines, nil when parallel.
An electric motor is designed to use the motor effect. We can change its speed by alternating the
current, and reverse its speed by reversing the current.
When a current is passed through the coil, it spins because:
a force acts on either side due to the motor effect
the force on one side is in the opposite direction to the other side
The split ring commutator reverses the current every halfturn of the coil ­ and because the sides
swap over every half turn, the coil is pushed continually in the same direction over and over.

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REVISION NOTES: PHYSICS 3
ELECTROMAGNETISM
Electromagnetic Induction
Hospitals have emergency electricity generators in case the mains fail. A generator contains coils of
wire which spin in a magnetic field. A potential difference (or voltage) is created (or induced) in
the wire when it cuts across field lines. If the wire is part of a complete circuit, the induced potential
difference makes an electric current pass around the circuit.…read more

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REVISION NOTES: PHYSICS 3
ELECTROMAGNETISM
The coil is connected to an ammeter via the metal brushes attached to the commutator rings. When
the coil turns steadily in one direction, the meter pointer deflects one way first, then the opposite
way, and then back again. This carries on as long as the coil continues turning. The current through
the meter is alternating current (a.c.).
The faster the coil rotates, the larger the peak value of the alternating current and the greater the

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REVISION NOTES: PHYSICS 3
ELECTROMAGNETISM
Transformers
Electricity reaches our homes from a power station via a network of cables called the National
Grid. A transformer is used at each stage to change the alternating voltage. A transformer consists
of two coils of insulated wire, both wound round the same iron core. When an alternating current
passes through the primary coil, an alternating potential difference is induced in the secondary coil.…read more

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REVISION NOTES: PHYSICS 3
ELECTROMAGNETISM
The National Grid
Electricity is supplied to our homes via the National Grid. The higher the potential difference, the
greater the efficiency of transferring the electrical power through the grid. This is why we use:
stepup transformers to take the p.d. from a power station up to the Grid p.d.
stepdown transformers to step the p.d. down to mains voltage
The secondary p.d. of a transformer depends on the primary p.d.…read more

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REVISION NOTES: PHYSICS 3