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Electromagnetic Induction
(A) Electricity from Magnetism
(1) Induced Voltage and Induced Current
A voltage is induced if
•
a conductor cuts magnetic field,
•
magnetic field through a coil changes.
For a closed circuit, induced voltage drives a current to flow round the circuit.
Induced voltage can be increased by
•
moving a magnet/coils faster,
•
using a stronger magnet,
•
using a coil of more turns/increasing the length of wire in magnetic field.
Faraday's law of electromagnetic induction states that the induced voltage in a conductor is
directly proportional to the rate at which the conductor cuts through the magnetic field lines, or
at which the magnetic field changes.
Direction of induced current can be found by:

Fleming's right-hand rule (for straight conductors):

.Lenz's law (for any conductors): Induced current flows in the direction to oppose the
change which started it in a magnetic field.
(2) Applications
(a) Moving-coil microphones
Sound waves cause the coil in the microphone to vibrate between the poles of the magnet.
An a.c. is induced then in the coil, which is later amplified and converted into sound.
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(b) Hard disk
In the writing process, information is firstly converted into an a.c. signals. The a.c.
magnetize and rearrange the grains on the platter in as certain pattern as the patter
passes over the head.
In the reading process, the magnetic field of the grains induces a certain a.c. in the head
as the platter passes over the head. The a.c. are then converted back to the information.
(c) Induction cooker
In an induction cooker, high frequency a.c is used to set up a fast changing magnetic field to
induce large currents in the metal cooking pot. The pots then gets hot and cooks the food.
(B) Generators
In a generator, a coil is made to rotate in a magnetic field. Field lines are then cut by the coil
and a current is induced. A generator can be a a.c. generator or a d.c. generator.
In a simple a.c. generator, slip rings pass the induced current in the rotating coil to the outside
circuit.
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In a simple d.c. generator, commutator reverses the connections of the coil to the outside
circuit every time the coil makes a half turn.
Current generated can be increased by

using a stronger magnet,

increasing the number of turns in the coil,

increasing the area of the coil,

winding the coil on a soft-iron core,

rotating the coil at a higher speed (i.e. higher frequency).
In some generators (e.g. bicycle dynamo), it is the magnets that rotate.
(C) Transformers and high-voltage transmission
(1) Mutual Inductance
Mutual inductance is the effect that a changing current in a coil producing an induced voltage
in a nearby coil. The following diagrams illustrate the effect of mutual inductance using Lenz’s
Law
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(2) Transformer
In a simple transformer, an a.c. flows through the primary coil, the magnetic field lines going
round the core change continuously. This induces an alternating current in the secondary coil.
The frequency of the induced a.c in the secondary coil is the same as that of the a.c. in the
primary coil.
The primary voltage and the secondary voltage are related by the following equation:
V
N
Primary Vo ltage
Number of turns in the primary coil

or P  P
Secondary Voltage Number of turns in the secondary coil
VS
NS
The input power of a transformer is given by Pin  V p I P
The input power of a transformer is given by Pout  Vs I s
The efficiency of a transformer is given by:
Efficiency =
Pout
VI
 100%  s s  100%
Pin
Vp I p
The efficiency of a transformer depends on its design and it is unchanged.
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If no power is lost (100% efficiency),
Vp
Np
Vs I s
I
 100%  100% . Then, we have s 

Vp I p
I p Vs
Ns
Transformers can step up/down voltages easily without much loss of power.
If Ns > Np, then Vs > Vp. The transformer is called a step-up transformer.
If Ns < Np, then Vs < Vp. The transformer is called a step-down transformer.
Circuit symbol for a step-up transformer
Circuit symbol for a step-up transformer
Power is lost in practical transformers in the following ways:

power loss in resistance of the coils (which can be decreased by using thick wires in the
coil)

power used in the magnetization and demagnetization of the core (which can be reduced
by making the core from soft iron)

power loss due to eddy current in the core (which can be reduced by using laminated
cores)
(3) High-voltage power transmission
Power generated at the power station is stepped up to a high voltage for transmission over a
great distance. Then it is stepped down for consumers.
Stepping up voltage steps down the current in cables and power loss in cables (I 2R) can be
reduced.
The following figure shows the power generation, transmission and distribution in Hong Kong.
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