CHAPTER 8: ELECTROMAGNETISM 8.1 Analysing the magnetic effect of a current-carrying conductor. [ ………../ 33 x 100 = ………….% ] A student 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 astraight wire 1. Right-hand Grip Rule states that the thumb of the right hand points to the direction of current flow and the other four curled fingers points to direction of its magnetic field lines
2. Using the right hand grip rule, rule , draw the direction of current flow and pattern of magnetic fields lines formed.
1
8.1.2 8.1.2
Magnetic Magnetic Field Field due due to a Curren Currentt in a Circul Circular ar Coil 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 right hand grip rule. 8.1.3 8.1.3
Magnetic Magnetic Field Field due due to to a Current Current in a Solen Solenoid oid
1. A solenoid 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 current flow is anti-clockwise , the polarity at end P is north
ii. Look from end Q of the solenoid, the current flow is clockwise, clockwise , the polarity at end Q is south iii. Now draw draw the directi direction on of the compa compass ss needle needle in the the space space provided.
2
8.1.4 8.1.4
Factors Factors that that affec affects ts the strength strength of an electrom electromagne agnett 1. Based on the apparatus shown below, the higher the number of paper clips attracted to the solenoid, the greater the greater the strength of that electromagnet.
Factors Number of turns Electric current Use of normal iron-core Use of soft-iron
Condition
Number of paper
The strength of
clips attracted
electromagnetic electromagnetic field
increase
increase
increase
increase
decrease
decrease
increase
increase
increase increase
-nil-
core
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 magnetised . ii. The soft iron armature is pulled towards the electromagnet and the hammer hits the gong iii. At the same time, the contact is opened and the soft iron core loses its magnetism. iv. The spring brings the armature back to its original position v. The contact is restored and the process is repeated
3
8.2 Understanding the force on a current-carrying current-carr ying conductor in a magnetic field. [ ……/ 40 x 100 = ………% ] A student 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 8.2.1
Force Force Acting Acting on a Curren Current-ca t-carry rrying ing Conduct Conductor or in a Magnet Magnetic ic Field Field 1. If a current-carrying conductor is placed in a magnetic field as shown in the experiment, the conductor will experiences a force.
+
2. Draw the catapult field ( combination of the two magnetic fields) below and show the direction of force, F acting on the conductor.
F
4
3. Fleming’s left-hand rule can be used to determine the direction of the force acting on the conductor.
Thumb
(
Force
Magnetic field
First Finger (
)
)
Current Centre Finger (
)
4. The factors that affect the magnitude of the force on a current-carrying conductor in a magnetic field are: i. magnitude of current ii. strength of the magnetic field iii. length of the current-carrying current-carrying conductor
8.2.2 8.2.2
Turning Turning Effect Effect of of a Current Current-car -carryin rying g Coil in in a Magne Magnetic tic Field 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 upward force acts on the arm AB whereas a downward 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.
Direct Current Motor 3. Complete and draw the four stages of the motion of DC motor below
N
Coil
1 A
current
Degree : 0 0
4 3
B
2
Current flow through carbon brushes: Yes / S
No Arm AB : upwards / downwards
C
Arm CD : upwards upwards / downwards Rotation : clockwise direction
D
Degree : 90 0 Current flow through carbon brushes : Yes / No Arm AB : right / left Arm CD : right / left Rotation : clockwise due to inertia
Degree : 180 0
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Current flow through carbon brushes : Yes / No Arm AB : upwards upwards / downwards Arm CD : upwards / downwards Rotation : clockwise
Degree : 270 0 Current flow through carbon brushes : Yes / No Arm AB : right / left Arm CD : right / left Rotation : clockwise due to inertia
8.2.3 8.2.3
The The speed speed of rota rotatio tion n of an elec electri tric c motor motor can be increased by: 1. increasing the current 2. using a stronger magnet 3. increasing the number of turns in the coil 4. increasing the area of the coil
8. 3 Analysing electromagnetic electromagnet ic induction.
[ ………./ 39 x 100 = …………..% ]
A student student is able to :
7
•
•
• • •
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 8.3.1
Electr Electrom omagn agneti etic c Induc Inductio tion n 1. When a conductor is moved to cut through the magnetic flux, an induced current is produced. 2. It requires a relative 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 electromagnetic induction.
8.3.2 8.3.2
Induc Induced ed e.m. e.m.ff by a mov moving ing cond conduc uctor tor
Action
Observation
Inference
The wire is moved
Galvanometer deflect to
Current flows in wire
upwards The wire is moved
left Galvanometer deflect to
Current flows is reversed
downwards
right
The wire is move
No deflection
No current flows
horizontally Magn Magnet et is moved moved upw upward ards s
Galva Galvano nomet meter er defl deflec ectt to
Current flows in wire
right Thumb ( motion ) Fleming’s Right-hand rule can be used to determine the direction of the induced current produced First finger ( finger ( Field )
8 Center finger ( induced current )
8.3. 8.3.3 3
Indu Induce ced d e.m e.m.f .f by coil 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 . Current induced
No deflection on the galvanometer. No current is induced
Moving the coil towards a magnet also induces current
Current induced in opposite direction
Lenz’s Law states that the direction of the induced current is such that it always oppose .the change producing it.
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8.3.4 8.3.4
Faraday Faraday’s ’s Law of electroma electromagnet gnetic ic inductio induction n 1. Faraday’s Law states that the magnitude of the induced e.m.f. is directly proportional 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. the wire is moved faster II. a stronger magnet is used III. the length of conductor is increased
3. The magnitude of the induced current in a coil increases when:
I. the relative motion between the magnet and the coil is increased II. the number of turns on coil is increased III. the strength of the magnetic fieldl is increased 8.3.5 8.3.5
Applica Applications tions of electroma electromagne gnetic tic induction 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 falling water , steam or wind turbine. 2. The d.c generator and a.c generator make use of electromagnetic induction to produce induced current
8.3.6 8.3.6
Alter Alternat nating ing and and direc directt curren currentt (a.c / d.c) d.c)
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Complete the table of comparison below . AC Current
DC Current Graphs
Current, I/A
Time, t/s
Direction
Variable
Constant Examples of sources 1. dc generator
1. ac generator 2. Dynamo
2. Dry cell
8. 4 Transformer Transf ormer
[ ………./ 33 x 100 = ………….%] …………. %]
A student 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. • •
•
• • • •
state that
Vp
Np
for an ideal transformer. Ns state that V pI p = V sI s for an ideal transformer. 11 describe the energy losses in a transformer. describe ways to improve the efficiency of a transformer. solve problems involving transformers
Vs
=
8.4.1
Operating principle of a transformer 1. Complete the diagram below
Laminated soft iron core
output
Input
Primary coil
Secondary coil
Symbol
1. When an alternating current flows in the primary coil, it produces magnetic field lines which link the primary coil and the secondary coil 2. The magnetic flux linkage to the secondary coil is cut. 3. The changing magnetic flux cut by the secondary coil induces a current in the secondary coil. 4. When the current in the primary coil decreases, the magnetic field will collapse 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 alternating e.m.f of e.m.f of the same frequency is induced in the secondary coil. 8.4.2
Step-up and step-down transformers transformers 1. The output voltage depends on the ratio of the number of turns of primary and secondary coils. 2. For an ideal transformer, the relationship between the voltages and the ratio of the number of turns in primary and secondary coils is given as follows:
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Input voltage,V p = Output voltage, V s
Number of turns in primary coil,N p Number of turns in secondary coil, N s
3 If Ns is greater than greater than Np, then V s is greater than Vp . This type of transformer is a step up transformer 4. If Ns is less than Np, then V s is less than V p. This type of transformer is a step down transformer 5. For example, if the turns ratio is 1:50, the output voltage is stepped up 50 times
Np
Ns
Np
Ns
Output voltage
Input voltage
Step down transformer
Step up transformer
If we consider an ideal transformer , there is no loss of energy.
=
Power supplied to the primary coil
VpIp
=
Power used in the secondary coil
VsIs
Is/Ip
Vp/Vs
Comparing with the transformer equation
Is/Ip = Np/Ns
8.4.3 8.4.3
=
Energ Energy y los losse ses s in a trans transfor forme mer r 1. An ideal transformer has transformer has 100 % efficiency. 2. But in practice, the efficiency of a tranformer is less than100%. 3. The efficiency of a transformer is expressed as follows:
P out P in 13
Efficiency =
x 100%
4. Factors that affect the efficiency of a transfomer and ways to improve it: Complete the table below:
Type of losses •
Eddy current
Causes Changing magnetic flux induces current in
•
Way to reduce Used laminated core
the soft iron core. Heat is produced. As the number number of turn increases, increases, the
• •
Heat loss
resistance of conductor also increases. • •
Hysterisis • •
Flux leakage
•
Use low resistance copper wire
Heat is produced. The core is magnetized and demagnetized
•
Used soft iron core
alternately due to a.c current in primary
•
It is able to be
coil
magnetized and
Energy lost as heat. Leakage of magnetic flux in the primary
demagnetized easily Secondary coil is wound
coil
•
over the primary coil
8.5 Understa Understandin nding g the generation generation and transmis transmission sion of electric electricity ity [ ………./ 40 x 100 = ……….% } A student 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. 14 • •
• • •
8.5.1: Generation and Transmission of Electricity 1. Sources of Energy The generation of electricity comes from many sources such as : i. hydro power ii . gases iii. nuclear iv. diesel v. coal vi. biomass vii. wind viii. solar
8.5.2 8.5.2
Trans Transmis missio sion n of Elec Electri trici city ty 1. The diagram below shows a model of the transmission of electricity from an ac source.
2. Complete the diagram below which shows the transmission of electricity to consumers.
transmission
132 kV
15
33 kV
11 kV
Heav y
Light indust
450
240 My house
Buildi
Step up transformer
Step down transformer
Step down transformers
33 kV Power plant 3. The National Grid Network 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 heat is reduced,and increases the efficiency of transmission -- efficient energy distribution according to requirements when demand is high/ low -- good energy management when there is a breakdown/ breakdown / 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 heat energy. Power loss can be calculated as follow:
Pheat =I2R 6.
I = current flows in the cable R = resistance of the cable
The The power loss can be reduced by:
i.
Reducing the resistance of the cables
ii. Reducing the current or increasing or increasing the voltage in the cable 7.
8.5. 8.5.3 3
Transformers play an important role in the transmission of electricity at a higher voltage.
Rene Renewa wabl ble e ene energ rgy y 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 renewable energy. Renewable energy sources are continually replenished naturally and they are sustainable.
3.
Give the example of renewable of renewable energy:
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i. Hydr Hydroe oele lect ctri ric c ii. Solar iii. Wind iv. iv. Geothe therm rma al v. Biomass vi. Wave vii. Tidal
4.
Give the example of non-renewable of non-renewable energy: i.
Fossil fuels a) Oil b) Gas c) coal
5.
Give the benefits of using renewable energy in our nation:
i. Avoid depletion of fossil fuels ii. Cleaner sources for little pollution iii. Avoid harming harming flora and fauna iv. Avoid the disruption of ecological of ecological balance
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