Form 5 Physics Chapter 3 - Teacher's Copy

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 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, draw the direction of current flow and pattern of

magnetic fields lines formed.

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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 right hand grip

rule.

8.1.3 Magnetic Field due to a Current in a Solenoid

1. A solenoid is a combination of coils of wire wound around on some surface or on an

iron core.

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, the polarity at end Q

is south

iii. Now draw the direction of the compass needle in the space

provided.

8.1.4 Factors that affects the strength of an electromagnet

2

Q

P

_+

1. Based on the apparatus shown below, the higher the number of paper clips attracted

to the solenoid, the greater the strength of that electromagnet.

Factors Condition Number of paper clips

attracted

The strength of

electromagnetic field

Number of turns increase increase increase

Electric current increase increase increase

Use of normal iron-core

-nil-

decrease decrease

Use of soft-iron core increase increase

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

8.2 Understanding the force on a current-carrying conductor in a magnetic field.

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[ ……/ 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 force.

2. Draw the catapult field ( combination of the two magnetic fields) below and show the

direction of force, F acting on the conductor.

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+

F

3. Fleming’s left-hand rule can be used to determine the direction of the force acting

on the conductor.

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 conductor

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 upward

force acts on the arm AB whereas a downward force acts on the arms CD according to

Fleming’s Left Hand rule.

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Thumb ( ) First Finger ( )

Centre Finger ( )

Force Magnetic field

Current

Carbon brush

commutator

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

Degree : 00

Current flow through carbon brushes: Yes /

No

Arm AB : upwards / downwards

Arm CD : upwards / downwards

Rotation : clockwise direction

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

CoilN

S

B

A 23

4

1

current

C

D

6

Current flow through carbon brushes : Yes /

No

Arm AB : 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 The speed of rotation of an electric 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 induction. [ ………./ 39 x 100 = …………..% ]

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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 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 Induced e.m.f by a moving conductor

Action Observation Inference

The wire is moved

upwards

Galvanometer deflect to

left

Current flows in wire

The wire is moved

downwards

Galvanometer deflect to

right

Current flows is reversed

The wire is move

horizontally

No deflection No current flows

Magnet is moved upwards Galvanometer deflect to

right

Current flows in wire

Fleming’s Right-hand rule can be used to determine the direction of the induced current produced

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Thumb ( motion )

Center finger ( induced current )

First finger ( Field )

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

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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Magnetic field lines are being cut .Current induced

No deflection on the galvanometer.No current is induced

Current induced in opposite direction

Moving the coil towards a magnet also induces current

8.3.4 Faraday’s Law of electromagnetic induction

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 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 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 Alternating and direct current (a.c / d.c)

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Complete the table of comparison below.

AC Current DC Current

Graphs

Direction

Variable Constant

Examples of sources

1. ac generator

2. Dynamo

1. dc generator

2. Dry cell

8. 4 Transformer [ ………./ 33 x 100 = ………….%]

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Time, t/s

Current, I/A

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.

state that = for an ideal transformer.

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 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 the same frequency is induced in the secondary coil.

8.4.2 Step-up and step-down transformers

1. The output voltage depends on the ratio of the number of turns of primary and

secondary coils.

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Input

Primary coil

Laminated soft iron core

output

Secondary coil

Symbol

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:

3 If Ns is greater than Np, then Vs is greater than Vp . This type of transformer is a step

up transformer

4. If Ns is less than Np, then Vs is less than Vp. 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

If we consider an ideal transformer, there is no loss of energy.

Is/Ip = Np/Ns

8.4.3 Energy losses in a transformer

1. An ideal transformer has 100 % efficiency.

2. But in practice, the efficiency of a tranformer is less than100%.

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Power supplied to the primary coil = Power used in the

secondary coil

Comparing with the transformer equation

Input voltage,V p = Number of turns in primary coil,Np

Output voltage, Vs Number of turns in secondary coil, Ns

Step up transformerStep down transformer

Input voltage Output voltage

Np Ns Np Ns

VpIp = VsIs Is/Ip Vp/Vs=

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

Eddy current

Changing magnetic flux induces current in

the soft iron core.

Heat is produced.

Used laminated core

Heat loss

As the number of turn increases, the

resistance of conductor also increases.

Heat is produced.

Use low resistance copper

wire

Hysterisis

The core is magnetized and demagnetized

alternately due to a.c current in primary

coil

Energy lost as heat.

Used soft iron core

It is able to be

magnetized and

demagnetized easily

Flux leakage

Leakage of magnetic flux in the primary

coil

Secondary coil is wound

over the primary coil

8.5 Understanding the generation and transmission of electricity [ ………./ 40 x 100 = ……….% }

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P out P in

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 :

i. hydro powerii . gasesiii. nucleariv. dieselv. coalvi. biomassvii. wind

viii. solar

8.5.2 Transmission of Electricity1. 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.

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132 kV

transmission

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/ 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:

6. The power loss can be reduced by:

i. Reducing the resistance of the cables

ii. Reducing the current or increasing the voltage in the cable

7. Transformers play an important role in the transmission of electricity at a higher voltage.

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 renewable energy.

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450 V

Step up transformer

Step down transformer

33 kV

Power plant

33 kV

Heavy industry

Light industry

11 kV

Buildings

240

My house

Pheat =I2R I = current flows in the cableR = resistance of the cable

Step down transformers

Renewable energy sources are continually replenished naturally and they are

sustainable.

3. Give the example of renewable energy:

i. Hydroelectric

ii. Solar

iii. Wind

iv. Geothermal

v. Biomass

vi. Wave

vii. Tidal

4. Give the example 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 flora and fauna

iv. Avoid the disruption of ecological balance

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