electromagnetism
TRANSCRIPT
Chapter 8: Electromagnetism
Chapter Outline :
8.1 Analysing the magnetic effect of a current-carrying conductor8.2 Understanding the force on a current-carrying conductor in a magnetic field8.3 Analysing electromagnetic induction8.4 Analysing transformers8.5 Understanding the generation and transmission of electricity
Concept Map :
8.1 Analysing the magnetic effect of a current-carrying conductor
Electromagnets
1 A magnetic fields is a region around a magnet.
2 It can be produced by permanent magnets and current-carrying conductors.
3 An electromagnet can be produced by conducting a current through a conductor.
4 The magnetism of a current-carrying conductor occurs only when the current flows through the conductor.
5 The electromagnetism is very powerful and more practical to use in many appliance.
(a) Current is switched off (b) Current is switched onFigure 8.1 [Pelagi e-masteri]
6 The magnetism can be “turned off” by switching off the current as shown in Figure 8.1 (b).
Magnetic field patternField around a straight wire
Figure 8.2 [Pelagi e-masteri]
7 A magnetic field is formed when a large current flows through a straight wire.
(a) (b)Figure 8.3 [Pelagi Illustrated Dictionary]
8 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).
10 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
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 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
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.
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 8.9 [Eastview : New Topical Mastery - Vision Science]
Figure 8.10 [Pelagi e-masteri]
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 The magnetic field strength can be increase by:
(i) increasing the current that flows through the 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 8.11 [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.
Electromagnet relay
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.
25 Figure 8.12 shows that there are two major circuits inside a relay.
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
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
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.
8.2 Understanding the force on a current-carrying conductor in a magnetic field
Magnetic force
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)
(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.
HighlightFleming’s left-hand rule
Figure 8.18The direction of the catapult force is perpendicular to both the current and the magnetic field of the permanent magnet.Example :
Figure 8.19Figure 8.19 shows the setup of an apparatus to
investigate the force on a current-carrying conductor in a magnetic field.
Current flows from A to B The direction of the magnetic field is
from N to S
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
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.
Turning axis
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.
HighlightThe 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
Ammeter
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.
16 The higher the current flows through, the further the coil turns.
Direct current motor17 A direct current (DC) motor which
consists of a coil of many turns uses the magnetic turning effect.
Figure 8.25 [e-masteri page 330 Rajah 7.71]
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.
HighlightThe 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.
QuickcheckQuestion : Which of the following best represents the correct poles of both sides of the solenoid?
Answer : D
8.3 Analysing electromagnetic inductionHow 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:
(i) moving the wire faster.(ii) using a stronger magnet.(iii) increasing the length of wire in the magnetic field.
HighlightFaraday’s law of electromagnetic inductionThe voltage induced in a conductor is directly proportional to the rate at which the conductor cuts through the magnetic
field lines.
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.
HighlightLenz’s lawAn 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.
HighlightFleming’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 inductionAC and DC generators
8 A generator is a device that produces electrical energy by electromagnetic induction.
9 Generators are the inverse of motors.
Figure 8.30 [e-masteri page 337 Rajah 7.83]10 Figure 8.30 (a) shows a simple AC
generator.
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:
(i) rotating the coil faster.(ii) using a stronger magnet.(iii) increasing the number of turns in the coil.(iv) increasing the area of the coil.(v) winding the coil on a soft-iron core.
15 The DC generator is produced when the slip rings are replaced by a commutator.
Figure 8.31 [e-masteri page 337 Rajah 7.84]16 Figure 8.31 (a) shows a simple DC
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.
HighlightAC and DC
(a)
(b)
(a)
(b)
Figure 8.32 [e-masteri page 339 Table]
Alternating current (AC)A current which flows in two opposite direction alternately.
Figure 8.33 [e-masteri page 339 Table]
Direct current (DC)A current which flows in one direction only.
8.4 Analysing transformersOperating principle of a transformer
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:
(i) the primary coil(ii) the secondary coil
3 The electrical energy is transferred from the primary coil to the secondary coil even they are not connected directly to each other.
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:
Figure 8.38 [Form 5 text book Page 100]
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 = Power output
VPIP = VSIS
From,
We get,
Energy losses in transformer
16 Some of the energy supplied to the primary coil may be lost as heat in all practical transformers.
17 The energy losses are due to:(i) resistance of coils.(ii) magnetization and demagnetization of the core.(iii) eddy currents in the core.
18 The efficiency of a transformer can be determined using:
QuickcheckQuestion :
Figure 8.40Figure 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 10:1B 20:1C 30:1D 40:1
Answer : B
8.5 Understanding the generation and transmission of electricity
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) Renewable(i) Hydro power(ii) Wind(iii) Waves(iv) Solar(v) Biomass(vi) Geothermal
(b) Non-renewable(i) Fossil fuel (coal, petroleum, natural gas)(ii) Radioactive substances
4 Each of these source has its own advantages and disadvantages.
Transmission of electricity
5 Electricity is transmitted through wires from a distant.
6 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.
QuickcheckQuestion : 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 20WB 40WC 60WD 80W
Answer : D