Electromagnetic Induction (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note
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Induced e.m.f.
Extended tier only
An electromotive force (e.m.f.) is induced in a conductor whenever there is relative movement between the conductor and a magnetic field
This could be when
the conductor moves in a stationary magnetic field
the conductor is stationary in a changing magnetic field
Induced e.m.f. due to a moving conductor
For an electrical conductor moving in a fixed magnetic field:
the conductor (e.g. a wire) cuts the field lines
an e.m.f. is induced in the wire
When an electrical conductor moves in a magnetic field an e.m.f. is induced
Induced e.m.f. due to a moving field
For a fixed conductor in a changing magnetic field:
as the magnet moved through the conductor (e.g. a coil), the field lines cut through the turns on the conductor (each individual wire)
an e.m.f. is induced in the coil
When a magnet is moved towards a wire, the changing magnetic field induces a current in the coil of wire
A sensitive voltmeter can be used to measure the size of the induced e.m.f.
If the conductor is part of a complete circuit then a current is induced in the conductor
This can be detected by an ammeter
Worked Example
A coil of wire is connected to a sensitive voltmeter.
When a magnet is pushed into the coil the needle on the voltmeter will deflect to the right as shown in the diagram below.
What will happen to the pointer on the voltmeter when the magnet is stationary in the centre of the coil?
A. The needle will deflect to the left
B. The needle will deflect to the right
C. There will be no deflection of the needle
D. The needle will deflect to the left and then to the right
Answer: C
There is no relative movement between the coil and the magnetic field when both the magnet and coil are stationary
Since no magnetic field lines are being cut, no e.m.f. will be induced
Therefore, the needle will not deflect
A, B & D are incorrect because a deflection on the voltmeter would indicate that an e.m.f. has been induced
This would only happen if there was relative movement between the coil and the magnetic field
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Factors affecting e.m.f. induction
Extended tier only
Factors affecting the magnitude of the induced e.m.f.
1. The speed at which the wire, coil or magnet is moved:
Increasing the speed will increase the rate at which the magnetic field lines are cut
This will increase the size of the induced e.m.f.
2. The number of turns on the coils in the wire:
Increasing the number of turns on the coils in the wire will increase the size of the induced emf
This is because each turn (loop) of wire in the coil cuts the magnetic field lines
Therefore, the total induced e.m.f. increases with each additional turn (loop)
3. The size of the coils:
Increasing the area of the coils will increase the size of the induced e.m.f.
This is because there will be more wire to cut through the magnetic field lines
4. The strength of the magnetic field:
Increasing the strength of the magnetic field will increase the size of the induced e.m.f.
This is because there will be more magnetic field lines in a given area
Factors affecting the direction of the induced e.m.f.
1. The orientation of the poles of the magnet:
Switching the poles of the magnet induces an e.m.f. in the opposite direction
2. The direction in which the wire, coil or magnet is moved:
Reversing the direction in which the wire, coil or magnet is moved induces an e.m.f. in the opposite direction
Examiner Tips and Tricks
When discussing factors affecting the size of an induced e.m.f., make sure to use the correct terminology:
say "add more turns to the coil" instead of “add more coils”. This is because these statements do not mean the same thing
say "a stronger magnet" instead of "a bigger magnet". This is because larger magnets are not necessarily stronger
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