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CHAPTER 8: ELECTROMAGNETISM
8.1 Analysing the magnetic effect of a current-carrying conductor.[ … /33 x 100 = ……% ]
A student is able to :
 State what an electromagnet is.
 Draw the magnetic field pattern due to a current in a :
i)
straight wire,
ii)
coil,
iii)
solenoid.
 State the factors that affect the strength of the magnetic field of an electromagnet.
 Describe the application of electromagnet in an electric bell.
8.1.1
Magnetic field due to a current in a straight wire
1. Right-hand Grip Rule states that the thumb of the right hand points to the direction of
__________________ and the other four curled fingers points to direction of its
__________________
2. Using the Right hand grip rule, draw the direction of current flow and pattern of
magnetic fields lines formed.
-
+
1
8.1.2
Magnetic Field due to a Current in a Circular Coil
1. Draw the direction of current flows and pattern of magnetic fields lines formed.
-
+
2. The direction of magnetic field lines can also be determined using the ____________
8.1.3
Magnetic Field due to a Current in a Solenoid
1. A _______________is a combination of coils of wire wound around on some surface or
on an iron core.
Q
P
-
+
2. Draw the direction of current flow. To check the poles formed:
i.
Look from end P of the solenoid, the direction of current flow is _______________,
the polarity at end P is _____________
ii. Look from end Q of the solenoid, the current flow is ___________________, the
polarity at end Q is ______________
iii. Now draw the direction of the compass needle in the space
2
provided
8.1.4
Factors that affects the strength of an electromagnet
1. Based on the apparatus shown below, the higher the number of paper clips attracted
to the solenoid, the ___________the strength of that electromagnet.
Factors
Condition
Number of turns
increase
Electric current
increase
Number of paper clips
The strength of
attracted
electromagnetic field
Use of normal iron-core
Use of soft-iron core
-nil-
8.1.5 Application of Electromagnet in an electric bell.
1. Label the diagram of an electric bell as shown below.
i. When the switch is closed and current flows in the solenoid, the soft iron core
is__________________.
ii. The soft iron armature is________ towards the electromagnet and the hammer hits
the gong
iii. At the same time, the contact is ______ and the soft iron core ______ its magnetism.
iv. The __________ brings the armature back to its original position
v. The contact is restored and the process is repeated.
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8.2 Understanding the force on a current-carrying conductor in a magnetic field.
[ ……/ 40 x 100 = ………% ]
A student is able to :
 Describe how a current-carrying conductor in a magnetic field experiences a force.
 Draw the pattern of the combined magnetic field due to a current-carrying conductor
in a magnetic field.
 Explain the factors that affect the magnitude of the force on a current-carrying
conductor in a magnetic field.
 Describe how a current-carrying coil in a magnetic field experiences a turning force.
 Describe how a direct current motor works.
 State factors that affect the speed of rotation of an electric motor.
8.2.1
Force Acting on a Current-carrying Conductor in a Magnetic Field
1. If a current-carrying conductor is placed in a magnetic field as shown in the
experiment, the conductor will experiences a ________.
+
2. Draw the catapult field ( combination of the two magnetic fields) below and show the
direction of force, F acting on the conductor.
N
N
+
=
S
S
4
3. _______________________can be used to determine the direction of the force acting
on the conductor.
Thumb
(
First Finger (
)
)
Centre Finger (
)
4. The factors that affect the magnitude of the force on a current-carrying conductor in a
magnetic field are:
I.____________________________________
ii.____________________________________
iii.____________________________________
8.2.2
Turning Effect of a Current-carrying Coil in a Magnetic Field
1. Consider a current-carrying coil ABCD placed between the poles of a magnet as
shown in the figure below. As the current flows through the coil from A to B, an _______
force acts on the arm AB whereas a __________ force acts on the arms CD according
to Fleming’s Left Hand rule.
commutator
Carbon brush
5
2. Draw the catapult field formed and show with arrows the direction of the forces acting
on arms AB and CD.
AB
CD
N
S
3. Complete and draw the four stages of the motion of a direct current motor in
operation
N
Coil
1 B
A
current
Degree : 00
4
3
2
Current flow through carbon brushes: Yes /
S
No
Arm AB : upwards / downwards
C
Arm CD : upwards / downwards
Rotation : clockwise direction
D
Degree :
0
Current flow through carbon brushes : Yes /
No
Arm AB : right / left
Arm CD : right / left
Rotation : ____________due to inertia
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Degree :
0
Current flow through carbon brushes : Yes /
No
Arm AB : upwards / downwards
Arm CD : upwards / downwards
Rotation : _____________
Degree :
0
Current flow through carbon brushes : Yes /
No
Arm AB : right / left
Arm CD : right / left
Rotation : _____________due to inertia
8.2.3
The speed of rotation of an electric motor can be increased by:
1. ___________________________________
2. ___________________________________
3. ___________________________________
4. ___________________________________
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[ ……./ 39 x 100 = …………..% ]
8. 3 Analysing electromagnetic induction.
A student is able to :
 Describe electromagnetic induction.
 Indicate the direction of the induced current in a:
i) straight wire,
ii) solenoid
 Explain the factors that affect the magnitude of the induced current.
 Describe applications of electromagnetic induction.
 Compare direct current and alternating current.
8.3.1
Electromagnetic Induction
1. When a conductor is moved to cut through the magnetic flux, an _______________ is
produced.
2. It requires a _____________motion between the magnet and the coil to produce an
induced current.
3. The production of electric current by a changing magnetic field is called
____________________________________.
8.3.2
Induced e.m.f by a moving conductor
Action
Observation
The wire is moved
Galvanometer deflect to
upwards
left
The wire is moved
downwards
The wire is move
horizontally
Magnet is moved upwards
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Inference
___________________rule can be used to determine the direction of the ___________
induced current produced
Thumb (
First finger (
)
Center finger (
8.3.3
)
)
Induced e.m.f by coil
Fill in the blanks to explain what happens when the bar magnet is moved in and out of the
solenoid
Magnetic field lines
are being cut .
_________ induced
No deflection on the
galvanometer.
____________ is
induced
Moving the coil towards
a magnet also induces
_____________
___________ induced in opposite
direction
____________law states that the direction of the induced current is such that it always
_______ ___ the change producing it.
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8.3.4
Faraday’s Law of electromagnetic induction
1. Faraday’s Law states that the magnitude of the induced e.m.f. is _________________
to the rate of change of magnetic flux linkage with the solenoid
2. The magnitude of the induced current in a conductor increases when:
I. ____________________________________________________
II. ____________________________________________________
III. ____________________________________________________
3. The magnitude of the induced current in a coil increases when:
I. _________________________________________________
II. _________________________________________________
III. _________________________________________________
8.3.5
Applications of electromagnetic induction
1. A generator is basically the inverse of a motor. It consists of a rectangular coil rotating
in a magnetic field. The axle is turned by some mechanical force from _____________ ,
___________ or wind turbine.
2. The d.c generator and a.c generator make use of electromagnetic induction to
produce _______________________
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8.3.6
Alternating and direct current ( a.c / d.c )
Complete the table of comparison below.
AC Current
DC Current
Graphs
Current, I/A
Current, I/A
Time, t/s
Time, t/s
Direction
Examples of sources
1.
1.
2.
2.
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[ ……/ 33 x 100 = …………..% ]
8. 4 Transformer
A student is able to :
 describe the structure and the operating principle of a simple transformer.
 compare and contrast a step-up transformer and a step-down transformer.
Vp
Np
=
for an ideal transformer.
Vs
Ns

state that




state that VpIp = VsIs for an ideal transformer.
describe the energy losses in a transformer.
describe ways to improve the efficiency of a transformer.
solve problems involving transformers
8.4.1
Operating principle of a transformer
1. Complete the diagram below
1. When an alternating current flows in the primary coil, it produces _______________
which link the primary coil and the secondary coil
2. The magnetic flux linkage to the ___________________ coil is cut.
3. The changing magnetic flux cut by the secondary coil ___________ a current in the
secondary coil.
4.
When the current in the primary coil decreases, the magnetic field will __________
and again be cut by the secondary coil. An e.m.f. acting in the opposite direction is
induced in the secondary coil. Hence, an ___________________ of the same
frequency is induced in the secondary coil.
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8.4.2
Step-up and step-down transformers
1. The output voltage depends on the ___________ of the number of turns of primary
and secondary coils.
2. For an ideal transformer, the relationship between the _______________ and the
_________ of the number of turns in primary and secondary coils is given as follows:
Input voltage,Vp
= Number of turns in primary coil, Np
Output voltage,Vs
Number of turns in secondary coil, Ns
3
If Ns is greater than Np, then Vs is ____________ than Vp This type of transformer is
a ________________ transformer
4. If Ns is less than Np, then Vs is ________ than Vp. This type of transformer is a
________________transformer
5. For example, if the turns ratio is 1:50, the output voltage is stepped up _____ times
.
Np
Ns
Input voltage
Np
Ns
Output voltage
_________ transformer
______ transformer
If we consider an ideal transformer, there is ____loss of energy.
Power supplied to the
primary coil
=
Power used in the
secondary coil
=
=
Comparing with the
transformer equation
=
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8.4.3
Energy losses in a transformer
1. An ideal transformer has _________ efficiency.
2. But in practice, the efficiency of a tranformer is ______ than100%.
3. The efficiency of a transformer is expressed as follows:
Efficiency =
---------
x 100%
4. Factors that affect the efficiency of a transfomer and ways to improve it:
Complete the table below:
Type of losses
Causes
Way to reduce
 Changing magnetic flux induces current in
the soft iron core.
 Heat is produced.
 As the number of turn increases, the
Heat loss
resistance of conductor also increases.
 Heat is produced.
 The core is magnetized and demagnetized
alternately due to a.c current in primary
coil
 Energy lost as heat.
 Leakage of magnetic flux in the primary
Flux leakage
coil
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8.5
Understanding the generation and transmission of electricity
[ …../ 40 x 100 = …………]
A student is able to :
 list sources of energy used to generate electricity.
 describe the transmission of electricity.
 describe the energy loss in electricity transmission cables and deduce the advantage of
high voltage transmission.
 state the importance of the National Grid Network.
 solve problems involving electricity transmission
 explain the importance of renewable energy
 explain the effects on the environment caused by the use of various sources to generate
electricity.
8.5.1: Generation and Transmission of Electricity
1. Sources of Energy
The generation of electricity comes from many sources such as :
a. ……………………
b. ……………………
c. ……………………
d. ……………………
e. ……………………
f.
……………………
g. ……………………
h. ……………………
8.5.2
Transmission of Electricity
1. The diagram below shows a model of the transmission of electricity from an ac
source.
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2. Complete the diagram below which shows the transmission of electricity to
consumers.
transmission
____ kV
__ kV
Heavy
industry
__ kV
__ V
Light
industry
___V
My house
Building
s
_________
transformer
r
_________
transformer
__________ transformers
__ kV
Power plant
3. The _________________ is a network of underground cables or pylon connecting all
the power stations and substations in the whole country to the consumers. This
network starts at electrical power plant and ends at our houses.
4. The advantages of the National Grid Network are:
-- energy loss as ______ is reduced,and increases the efficiency of transmission
-- efficient ________ distribution according to requirements when demand is high/ low
-- good energy management when there is a _____________/ interruption of supply
5. Electrical energy is transmitted from the power station to the consumer using long
transmission cables. This will bring to power loss as ______ energy. Power loss can
be calculated as follow
2
Pheat =I R
6.
I = current flows in the cable
R = resistance of the cable
The power loss can be reduced by:
i.
Reducing the ____________ of the cables
ii. Reducing the current or _____________ the voltage in the cable
7.
______________ play an important role in the transmission of electricity at a higher
voltage.
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8.5.3
Renewable energy
1.
Energy plays a very important role in economic development but the reserves of
fossil fuels such as oil and gas are very limited.
2.
Hence, there is modern trend of the nations harnessing the ______________
energy.
Renewable energy sources are continually replenished naturally and they are
sustainable.
3.
Give the example of renewable energy:
i ………………………
ii. ……………………..
iii. …………………….
iv. ……………………..
v. …………………….
vi. …………………….
vii. ……………………...
4.
Give the example of non-renewable energy:
i.
Fossil fuels
a) _____
b) _____
c) _____
5.
Give the benefits of using renewable energy in our nation:
i.
Avoid ____________ of fossil fuels
ii. Cleaner sources for little _____________
iii. Avoid harming _________ and fauna
iv. Avoid the disruption of _________________ balance
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