Secondary 4 Pure Physics
Theme: Magnetism
Electromagnetic Induction
Notes 2
Alternating Current (a.c.) Generator
•
Generators are devices that convert mechanical energy to electrical energy where the
source of mechanical energy may be a steam turbine, wind turbine, hand crank, etc.
In addition to comprehensive
note, illustrations are
included for students to
create mental association
such that it is easier for them
to remember information.
P
A
D
N
B
S
C
Slip rings
Q
External load
Carbon
brushes
Figure 1: Simple a.c. generator
•
•
An a.c. generator consists of the following parts:
−
A rectangular coil of wire mounted on an axle and placed between the N-pole and
S-pole of a magnet.
−
A pair of copper slip rings connected to the coil to ensure that the induced current in the
coil is transferred to the external circuit.
−
Carbon brushes providing sliding contact with the rotating slip rings.
As the coil rotates in the magnetic field, the change in magnetic flux creates an induced
e.m.f., hence an induced current flows in the coil.
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•
When the coil is at different positions, the rate at which it cuts the magnetic field lines differs.
•
Coil ABCD is rotated manually on axle PQ between poles of magnet from a vertical position.
•
As the coil is rotated, sides AB and CD move parallel to the magnetic field of the magnet.
•
The arms AB and CD do not cut across the magnetic field lines.
•
Since the rate of change of magnetic flux is zero, the induced e.m.f. is zero.
P
A
e.m.f. / V
B
D
N
Main points of the sentences
are bolded to allow students
to focus on the important
concepts during revision.
S
0
C
Angle rotated
by coil
Q
external load
Figure 2(a): Coil ABCD at the initial vertical position and the induced e.m.f. is zero
•
When the coil is rotated 90o clockwise from the initial position, sides AB and CD cuts across
the magnetic field of the magnet at the greatest rate.
•
Since the rate of change of magnetic flux is the maximum, the induced e.m.f. is at a
maximum.
•
Using Fleming’s right-hand rule, the induced current flows from A  B  C  D.
e.m.f. / V
D
A
N
S
C
B
0
90
o
Angle
rotated
by coil
external load
Figure 2(b): Coil ABCD rotated 90o clockwise from the initial position and
the induced e.m.f. is at the maximum
•
When the coil is rotated 180o clockwise from the initial position, sides AB and CD are once
again moving parallel to the magnetic field of the magnet.
•
The coil does not cut across the magnetic field and no e.m.f. is induced in the coil.
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•
The cycle continues as the coil rotates in the magnetic field to produce alternating e.m.f.
which in turn produces an alternating current. Hence, the name alternating current generator.
•
The period, T, is the time taken for the coil to make one complete rotation. The frequency of
the rotation, f, is given by:
f=
1
T
Formulas are placed in
boxes to draw students’
attention to it quickly.
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Examples are provided
throughout the notes to check
Example 1
For students’ understanding
of important concepts before
The graph of induced e.m.f proceeding
against time
simple
onfor
toathe
next a.c. generator is as shown below.
Draw on the same diagram,concept.
the output voltage when both the speed of rotation and the number
of turn on the coil is doubled.
e.m.f. / V
When the speed of rotation is doubled,
the frequency and the induced e.m.f.
are
.
4Eo
3Eo
When the number of turn on the coil is
doubled, the induced e.m.f. is
.
2Eo
Eo
0
T
/4
T
/2
3T
/4
T
Time / s
Thus when both the speed of rotation
and the number of turn on the coil is
doubled:
– Eo
- the induced e.m.f. will increase by
times
– 2Eo
– 3Eo
- the frequency of the alternating
current will increase by
times.
– 4Eo
Application of A.C. Generator
Bicycle Dynamo
•
In addition to the simple a.c. generator whereby the coil is being rotated, the coil is fixed in
some generators (fixed coil generator).
•
In this case, the coil do not rotates and it is the magnet that rotates with respect to the fixed
coil.
•
This changes the magnetic flux in the coil, generating the induced e.m.f. which in turns
creates the induced current.
•
As the coil is fixed, carbon brushes and slip rings are not required.
•
The absence of slip rings gives the following advantages:
−
There is no need to replace the carbon brushes when they wear and tear.
−
Energy will not be wasted, since eroded connection between carbon brushes and slip
rings has increased resistance, resulting in unnecessary heating.
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•
One example of fixed coil generator is the bicycle dynamo.
•
A permanent magnet is rotated inside an iron core with wire wound around it.
•
As the magnet rotates, the magnetic field of the magnet rotates with respect to the fixed coil.
This changes the magnetic flux in the coil and an e.m.f. is induced in the coil.
axle
permanent magnet
fixed coil of wire
output wires
Wheel of the dynamo
is attached to the
wheel of the bicycle
Fig 7: Bicycle Dynamo
List of key words are
provided in every set of
notes to help students
answer their examination
questions accurately and
precisely.

Describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use
of slip rings (where needed).

Sketch a graph of voltage output against time for a simple a.c. generator.
KEYWORDS
Generator
Dynamo
Slip rings
Carbon brushes
Induced e.m.f.
Induced current
Frequency
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Secondary 4 Pure Physics
Theme: Magnetism
Electromagnetic Induction
Exercise 2
A typical exercise will start
with 5 to 15 multiple-choice
questions. The first few
questions will be more
fundamental. The level of
difficulty of questions will
increase towards the end of
each exercise.
This is to allow students to
grasp the basic concepts
fully before applying the
concepts
to
solve
challenging questions.
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10.
The following graphs show the change of e.m.f. with time.
Which one shows an a.c. generator turning at a constant rate?
(A)
(B)
e.m.f. / V
e.m.f. / V
time / s
(C)
time / s
(D)
e.m.f. / V
e.m.f. / V
time / s
time / s
(
)
Section B
Structured Questions
Write the answers in the spaces provided.
1.
The figure below shows a set up in which wind power may be used to generate electricity.
When the wind blows, the blades attached to the spindle would turn, causing the
permanent magnet to turn as well.
Spindle
N
Towards output
Blades
S
Permanent
magnet
Iron Rod
(a)
Explain how electricity may be generated when the blades turn.
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(b)
State the purpose of the iron rod.
(c)
Suggest two ways to increase the current flowing toward the output without
increasing wind speed.
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4.
The graph below shows how the current changes with time for a coil turned in a
magnetic field to produce an induced current.
The position of the coil is shown for point A.
Current / A
A
0
B
0.01
0.02
Time / s
C
F
I
I
(a)
F
There is no current induced in the coil at point B.
(i)
Draw in the space below to show the position of the coil at point B and
indicate on your diagram, the direction of the magnetic field.
(ii)
Explain why there is no induced current at point B.
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(b)
The coil takes 0.01 s to rotate from point A to point C.
(i)
What is the period of rotation?
(ii)
What is the frequency of alternating current generated in the coil?
(c)
Describe and explain what happens when the number of turns in the coil of the
generator is doubled.
(d)
Describe and explain what happens when the permanent magnet used is twice
as strong.
(e)
Describe and explain what happens when the speed of the coil rotation is
doubled.
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S4OP | Electromagnetic Induction | Ex 2
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