Download Chapter 8: Electromagnetism

Chapter 8: Electromagnetism
Chapter Outline :
8.1
8.2
8.3
8.4
8.5
Analysing the magnetic effect of a current-carrying conductor
Understanding the force on a current-carrying conductor in a magnetic field
Analysing electromagnetic induction
Analysing transformers
Understanding the generation and transmission of electricity
Concept Map :
Electromagnetism
Current-carrying
cunductor
Induced e.m.f.
Magnetic field
Magnetic force
Fleming’s left-hand rule
Magnetic field pattern
Faraday’s law of
electromagnetic induction
Uses of magnetic force
Magnetic field strength
Lenz’s law
Uses of electromagnet
Applications of
electromagnetic induction
Transformers
Step-up & step-down
transformers
Power transmission
Fleming’s right-hand rule
Energy losses in
transformer
Generator
AC Generator &
DC Generator
7
8.1
Analysing the magnetic effect of a
current-carrying conductor
A magnetic field is formed when a large
current flows through a straight wire.
Electromagnets
1
A magnetic fields is a region around a
magnet.
2
It can be produced by permanent magnets
and current-carrying conductors.
3
4
5
An electromagnet can be produced by
conducting a current through a conductor.
8
The magnetism of a current-carrying
conductor occurs only when the current
flows through the conductor.
The cross-section of the field pattern is as
shown in Figure 8.3.
9
The current directions are represented by a
cross or a dot (see Figure 8.3).
The electromagnetism is very powerful and
more practical to use in many appliance.
10
(a) Current is switched off
(b) Current is switched on
Figure 8.1 [Pelagi e-masteri]
6
(a)
(b)
Figure 8.3 [Pelagi Illustrated Dictionary]
The properties of the magnetic field is as
follows:
(i)
The field lines are circles around the wire.
(ii)
The direction of the field lines can be
reversed by reversing the current direction.
(iii)
The field strength is stronger at the area
which is closer to the wire.
Field around a coil
The magnetism can be “turned off” by
switching off the current as shown in Figure
8.1 (b).
Magnetic field pattern
Field around a straight wire
Figure 8.6[Pelagi e-masteri]
11
The magnetic field pattern produced by a
current flowing through a circular flat coil
is as shown in Figure 8.6.
12
Figure 8.2 [Pelagi e-masteri]
The right-hand grip rule can be applied to
determine the direction of the magnetic field
around a circular flat coil.
Field around a solenoid
15
The magnetic field around a solenoid is
similar to that of a bar magnet.
16
The position of the poles depends on the
direction of the current.
Magnetic field strength
17
Figure 8.7 [Pelagi e-masteri]
13
A solenoid is a coil made up of a number of
turns of wire as shown in Figure 8.7.
The magnetic field strength can be increase
by:
increasing the current that flows through the
(i)
wire.
(ii)
increasing the number of turns in the coil for
the same length of solenoid.
(iii)
inserting a soft-iron core into the coil.
Electric bell
Figure 8.8 [Pelagi e-masteri]
14
Figure 8.8 shows that the direction of the
magnetic field around a solenoid can be
indicated using the right-hand grip rule.
Figure 8.11 [Pelangi : Illustrated Dictonary]
18
The electromagnet is switched on and off
rapidly by a contact breaker as shown in
Figure 8.11.
19
The electromagnet is created when the
current flows in the coil.
20
The hammer is attracted by the
electromagnet, causing it to hit the gong.
21
This will break the contact and switch off
the current.
22
The hammer is then returned to its original
position, causing the current to flow again.
23
The process will be repeated as long as the
switch is pressed on.
Figure 8.9 [Eastview : New Topical Mastery Vision Science]
Figure 8.10 [Pelagi e-masteri]
Electromagnet relay
31
Maglev is the abbreviation for magnetic
levitation.
32
The maglev train is a train with
electromagnets attached underneath that
provides magnetic fields.
33
The train can levitate just above the track as
the magnets repel each other.
34
The reduced friction enables the train to
move fast.
Understanding the force on a currentcarrying conductor in a magnetic field
Figure 8.12 [Pelangi : Illustrated Dictonary]
24
A magnetic relay is a device used to switch
on and off a circuit with large current safely.
8.2
25
Figure 8.12 shows that there are two major
circuits inside a relay.
Magnetic force
26
The arm will rotate and closes the switch in
circuit B when a small current is applied to
circuit A.
27
In this way, the circuit B will be switched on.
Earpiece
1
A magnetic field is formed when a current
flows through a wire.
2
When the current-carrying wire is brought
near to a permanent magnet, the two fields
will interact to produce a force.
Force produced by the combined magnetic field
“Catapult force”
(a)
(b)
Figure 8.13 [Pelangi : Illustrated Dictonary]
28
An earpiece is a device used to transform
electrical signals into sounds.
29
The strength of the magnetic field will
change as the varying currents flows through
the electromagnet.
30
The diaphragm in a microphone will
vibrate to create sounds.
Maglev train
(c)
Figure 8.17
3
Figure 8.17 (a) shows the magnetic field
around a current carrying wire.
4
Figure 8.17 (b) shows the magnetic field
between two slab-shaped magnets
5
Figure 8.17 (c) shows the combined field of
Figure 8.17 (a) and Figure 8.17 (b).
6
The combined field pattern appears as if the
wire is being “catapulted” from the strong
field region towards the weak field region.
7
Such a magnetic field is called a catapult
field.
8
The direction of the catapult force can be
identified by Fleming’s left-hand rule.
Highlight
Fleming’s left-hand rule
Figure 8.20
 Figure 8.20 shows that the direction
of the resultant force acting the short
copper wire can be determined using
Fleming’s left-hand rule.
Turning force on a current-carrying coil in a
magnetic field
Turning effect on a coil
Turning axis
Figure 8.18
The direction of the catapult force is
perpendicular to both the current and the
magnetic field of the permanent magnet.
Example :
Figure 8.19
Figure 8.19
 shows the setup of an apparatus to
investigate the force on a currentcarrying conductor in a magnetic
field.
 Current flows from A to B
 The direction of the magnetic field is
from N to S
Figure 8.21 [e-masteri page 332 Rajah 7.75]
9
There is a turning effect on a coil which lies
between the poles of a magnet as shown in
Figure 8.21.
Figure 8.22 [e-masteri page 331 Rajah 7.72]
10
Figure 8.22 shows the end view of the coil
in a magnetic field.
11
The current flows in opposite directions
along the two sides of the coil.
12
As a result, one side of the coil is pushed up
and the other side is pushed down, causing
the coil to turn clockwise as shown in Figure
8.22.
Highlight
The turning effect on the coil can be
increased by:
i)
increasing the current.
(ii) increasing the strength of the
magnetic field.
(iii) increasing the number of turns in
the coil.
(iv) increasing the area of the coil
16
The higher the current flows through, the
further the coil turns.
Direct current motor
17
A direct current (DC) motor which
consists of a coil of many turns uses the
magnetic turning effect.
Ammeter
Figure 8.25 [e-masteri page 330 Rajah 7.71]
Figure 8.23 [e-masteri page 341 Rajah 7.95a]
13
Figure 8.23 shows a moving-coil ammeter.
Figure 8.24 [e-masteri page 341 Rajah 7.96]
14
The moving-coil ammeter uses the magnetic
turning effect on a coil to show deflection on
a scale as shown in Figure 8.24.
15
A large current will produce a full scale
deflection on the scale.
18
Figure 8.25 shows the simplified model of a
DC motor.
19
The current flows into the coil through a pair
of carbon brushes. The carbon brushes
push against a commutator.
20
The commutator changes contact from one
brush to another when the coil is turning to
keep the coil rotating continuously.
Highlight
The speed of rotation of an electric motor
can be increased by:
(i)
increasing the current.
(ii) increasing the strength of the
magnetic field.
(iii) increasing the number of turns in
the coil.
Quickcheck
Question :
Which of the following best represents
the correct poles of both sides of the
solenoid?
field lines.
Answer : D
8.3
Analysing electromagnetic induction
How induced e.m.f. is produced?
Figure 8.26 [e-masteri page 333 Rajah 7.78]
1
Figure 8.26 shows that a voltage is induced
when a conductor cuts a magnetic field.
2
The movement of a conductor in a magnetic
field produces an electromotive force
(e.m.f.) in the conductor.
3
An induced current is produced when the
conductor is connected to a closed circuit.
4
This effect is called electromagnetic
induction.
5
The magnitude of the e.m.f. can be increased
by:
moving the wire faster.
using a stronger magnet.
increasing the length of wire in the magnetic
(i)
(ii)
(iii)
field.
Highlight
Faraday’s law of electromagnetic
induction
The voltage induced in a conductor is
directly proportional to the rate at which
the conductor cuts through the magnetic
6
The direction of induced current depends
on the direction of the motion.
7
The direction can be identified using Lenz’s
law or Fleming’s right-hand rule.
Highlight
Lenz’s law
An induced current always flows to
oppose the movement which started it.
Example :
Figure 8.27 [e-masteri page 334 Rajah
7.80]
 The south pole of the magnet is
moving into the coil.
 The induced current flows in such
direction to as to produce a south pole
to oppose the approaching of the
magnet
Figure 8.28 [e-masteri page 334 Rajah
7.80]
 The south pole of the magnet is
moving away from the coil.
 The induced current flows in such
direction so as to produce a north
pole to oppose the leaving of the
magnet.
Highlight
Fleming’s right-hand rule
Figure 8.29 [e-masteri page 334 Rajah
7.81]
The direction of the induced current
which flows in a wire cutting through a
magnetic field can be identified as shown
in Figure 8.29.
Application of electromagnetic induction
AC and DC generators
8
A generator is a device that produces
electrical energy by electromagnetic
induction.
9
Generators are the inverse of motors.
11
The output current varies during the rotation
of the coil as shown in Figure 8.30 (b).
12
The current is zero when the coil is vertical.
13
The current is greatest when the coil is
horizontal.
14
The magnitude of the induce voltage can be
increased by:
rotating the coil faster.
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.
(i)
(ii)
(iii)
(iv)
(v)
15
The DC generator is produced when the
slip rings are replaced by a commutator.
(a)
(a)
(b)
Figure 8.31 [e-masteri page 337 Rajah 7.84]
16
Figure 8.31 (a) shows a simple DC
generator.
(b)
Figure 8.30 [e-masteri page 337 Rajah 7.83]
10
Figure 8.30 (a) shows a simple AC
generator.
17
The output current varies but still flowing in
one direction as shown in Figure 8.31 (b).
18
The commutator reverses the contacts of the
coil when the coil passes through the
vertical position.
Highlight
AC and DC
Figure 8.32 [e-masteri page 339 Table]
 Alternating current (AC)
A current which flows in two opposite
direction alternately.
4
When an AC input flows through the
primary coil, the magnetic field changes
continuously.
5
This induces an AC output in the
secondary coil.
Step-up and step-down transformers
6
The voltages in the primary and the
secondary coils depend on the number of
turns in the coils.
7
The relationship between the voltages and
the number of turns in the coils is given as:
Primary vo ltage
Number of turns in the primary coil

Secondary voltage Number of turns in the secondarycoil
VP N P

VS
NS
Figure 8.33 [e-masteri page 339 Table]
 Direct current (DC)
A current which flows in one direction
only.
8.4
Analysing transformers
Operating principle of a transformer
Figure 8.38 [Form 5 text book Page 100]
Figure 8.37 [e-masteri page 345 Figure 7.104]
1
Figure 8.37 shows the simple structure and
the symbol of a transformer.
2
A transformer consist of two coils of wires
which are known as:
the primary coil
the secondary coil
(i)
(ii)
3
The electrical energy is transferred from the
primary coil to the secondary coil even they
are not connected directly to each other.
8
Figure 8.38 shows the simple structure and
the circuit symbol of a step-up transformer.
9
A step-up transformer has more turns in the
secondary coil than in the primary coil (NP <
NS).
10
The secondary voltage is greater than the
primary voltage (VP < VS).
Figure 8.39 [Form 5 text book Page 100]
11
Figure 8.39 shows the simple structure and
the circuit symbol of a step-down
transformer.
12
A step-down transformer has more turns in
the primary coil than in the secondary coil
(NP > NS).
13
The primary voltage is greater than the
secondary voltage (VP > VS).
Primary and secondary current
14
In an ideal transformer, all the power
supplied to the primary coil will be
transferred to the secondary coil.
15
This is given as:
Power input =
VPIP =
VP

VS
VP

From,
VS
IS

We get,
IP
Power output
VSIS
IS
IP
NP
NS
NP
NS
Quickcheck
Question :
Figure 8.40
Figure 8.40 shows an ideal transformer
used to operate a 12V bulb from the ac
mains. What is the turns ratio of the
transformer?
A 10:1
B 20:1
C 30:1
D 40:1
Answer : B
8.5
Generation of electricity
1
Electricity is generated by electromagnetic
induction.
2
The electromagnetic induction is carried out
by a generator which has a huge dynamo
that is turned by a turbine.
3
Types of energy sources that can be used to
produce the electricity are:
(a)
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Renewable
Hydro power
Wind
Waves
Solar
Biomass
Geothermal
(b)
(i)
(ii)
Non-renewable
Fossil fuel (coal, petroleum, natural gas)
Radioactive substances
4
Each of these source has its own advantages
and disadvantages.
Energy losses in transformer
16
Some of the energy supplied to the primary
coil may be lost as heat in all practical
transformers.
17
(i)
(ii)
core.
(iii)
The energy losses are due to:
resistance of coils.
magnetization and demagnetization of the
18
The efficiency of a transformer can be
determined using:
eddy currents in the core.
Efiiciency 
Output power
100 %
Input power
Understanding the generation and
transmission of electricity
Transmission of electricity
5
6
Electricity is transmitted through wires from
a distant.
The magnitude of the current has to be
lowered before the current is transmitted to
consumers.
Figure 8.41 [e-masteri Page 350 Rajah 7.108]
7
Figure 8.41 shows a simple model of
electricity transmission system.
8
A step-up transformer is used to increase the
voltage and lowered the current at the power
plant.
9
The electricity is then transmitted through a
grid system.
10
A step-down transformer is used to decrease
the voltage before the current is being
delivered to consumers.
The National Grid Network
Figure 8.42 [e-masteri Page 350 Rajah 7.109]
11
Figure 8.42 shows a model of the National
Grid Network.
12
The National Grid Network is a system
which connects all the power plants, the
station and consumers to form a closed
network.
13
Some of the advantages of the system are:
(i)
The power station can be built away from
the populated area.
(ii)
The power supply is uninterrupted since the
breakdown in a power station can be supported by
another power station.
(iii)
The power supply is distributed
according to the demand to prevent
energy wastage.
Quickcheck
Question :
A 5 000 W of power is transmitted
through a cable of resistance 5. What is
the power loss in the cable if the current
is transmitted at 1 250V?
A 20W
B 40W
C 60W
D 80W
Answer : D