CHAPTER 3: ELECTROMAGNETISM

3.1 Analysing the magnetic effect of a current-carrying conductor.

[ ………../ 36 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.

A. 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

(1) ……………………………and the other four curled fingers points to direction of its

(2) …………………………..

2. Using the right hand grip rule, draw (3) the direction of current flow and (4) pattern

of magnetic fields lines formed.

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B Magnetic Field due to a Current in a Circular Coil

1. Draw (5) the direction of current flows and (6) pattern of magnetic fields lines formed.

2. The direction of magnetic field lines can also be determined using the (7)

……………………………….

C Magnetic Field due to a Current in a Solenoid

1. A solenoid is a (8) ……………….of wire. When a current flows in a solenoid, a magnetic

field similar to that of a (9) ………………………….is produced.

2. Draw (10) the direction of current flow. To check the poles formed:

i. Look from end P of the solenoid, the current flow is (11)…………………., the polarity

at end P is (12) …………………………….

ii. Look from end Q of the solenoid, the current flow is (13)……………….., the polarity

at end Q is (14) …………………………….

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

provided. P (15) , Q (16)

2

Q

P

_

P

Q

+

D Factors that affecting the strength of an electromagnet

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

to the iron rod, the (17) ………………………………….the strength of the electromagnet.

Factors Condition Number of paper clips

attracted

The strength of

electromagnetic field

Number of turns of wire in a solenoid increases (18) (19)

Magnitude of electric current increases (20) (21)

Presence of a normal iron-core

-nil-

(22) (23)

Presence of a soft-iron core (24) (25)

E Application of Electromagnet in an electric bell.

1. Label the diagram of an electric bell as shown below. [ spring, contact adjusting

screw, electromagnet, switch, iron armature, hammer ]

i. When the switch is closed and current flows in the solenoid, the soft iron core of the

electromagnet is(32)………………….

ii. The iron armature is (33) …………….towards the electromagnet and the hammer hits

the gong.

iii. At the same time, the contact is (34)……………………….…. and the electromagnet

(35) ………………………its magnetism.

iv. The (36) ……………………………..brings the armature back to its original position.

v. The contact is restored and the process is repeated.

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

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[ ……/ 39 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.

A Force Acting on a Current-carrying Conductor in a Magnetic Field

1. If a current-carrying conductor AB is placed in a magnetic field as shown in the

experiment below, the conductor AB will experiences a (1) ………………………….

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

direction of force, F acting on the conductor.

3.

(2) (3) (4) (5) F

4. Fleming’s left-hand rule (16) can be used to determine the direction of the force

4

+

N

+ =

N

SS

acting on the conductor.

5. The factors that affect the magnitude of the force on a current-carrying conductor in a magnetic field are:

i. ………………………………………………….........(10)

ii. ……………………………………………………….(11)

iii. …………………………………………………………….(12)

B 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, a

(13) ……………………force acts on the arm AB whereas an (14) …… …………...

force acts on the arms CD according to Fleming’s Left Hand rule.

2. Draw the (15) catapult field formed and show with arrows the direction of the forces

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Thumb ( ) First Finger ( )

Centre Finger ( )

(7)………….

(8)……….

(9)………

Carbon brush

commutator

acting on arms (16) AB and (17) CD.

3 Direct Current Motor

Complete and draw the four stages of the motion of a DC motor in the table below

Angle of rotation : 00

Current flow through carbon brushes:

(18) Yes / No

Arm AB : (19) upwards / downwards

Arm CD : (20) upwards / downwards

Rotation : (21) ………………direction

Angle of rotation : (22) …………………

Current flow through carbon brushes :

(23) Yes / No

Arm AB : (24) right / left

Arm CD : (25) right / left

Rotation : (26) …………………..due to inertia

Angle of rotation : (27) ………………………..

CoilN

S

B

A 23

4

1

current

C

D

6

N S

AB CD

Current flow through carbon brushes :

(28) Yes / No

Arm AB : (29) upwards / downwards

Arm CD : (30) upwards / downwards

Rotation : (31) ………………………………..

Angle of rotation : (32) ………………………..

Current flow through carbon brushes :

(33) Yes / No

Arm AB : (34) right / left

Arm CD : (35) right / left

Rotation : (36) anticlockwise due to inertia

4 The speed of rotation of an electric motor can be increased by:

1. ………………………………………………………………………………………….…..(37)

2. ……………………………………………………………………………………………..(38)

3……………………………………………………………………………………………..…(39)

3. 3 Analysing electromagnetic induction. [ ………./ 36 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.

A Electromagnetic Induction

1. When a conductor is moved to cut through a magnetic field, an (1) ……………………

…………………………………..is produced.

3. It requires a (2) …………………………………….motion between the magnet and the

coil to produce an induced current.

3. The production of electric current by a changing magnetic flux is called

(3)…………………………………….

B Induced e.m.f by a moving conductor in a magnetic field

Action Observation Inference

The wire is moved

upwards

Galvanometer pointer is

deflected to the left

……………………..flows in the wire

(4)

The wire is moved

downwards

Galvanometer pointer is

deflected to the……... (5)

…………………………………………

…………………………………….(6)

The wire is move

horizontally …………………… (7) …………………………………… (8)

Magnet is moved upwards Galvanometer pointer is

deflected to the…………..

(9)

…………………………………..

…………………………………..(10)

(11) ……………………………………can be used to determine the direction of the induced emf

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Thumb (12) (……………... )

Center finger (………………….. )

First finger (………… )

/current produced

(13)

(14)

C Induced e.m.f by coil of wire / solenoid

1. Fill in the blanks to explain what happens when the bar magnet is moved in and out of the solenoid

2. Lenz’s Law states that the (19)………………………….. of the induced emf / current

is such that its magnetic effect always (20)……………………the change producing it.

3. Faraday’s Law of electromagnetic induction states that the (21)……………………………

of the induced e.m.f. is (22)………………….……….. to the rate of change of magnetic

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Magnetic field lines are being cut by coil,(15)………………….. is induced

No deflection shown on the galvanometer.(16)…………………….. is induced

(17)………………………. induced in the opposite direction

Moving the coil towards a magnet also induces a (18)…………………...

flux linkage with the solenoid

The magnitude of the induced current in:

a straight conductor increases when: in a coil increases when:

(a) the length of conductor is

increased (a) ……………………………………..

…………………………………… (23)

(b) the strength of the magnetic

field is increased(b) ………………………......................

……………………………………(24)

(c) the relative motion between the

magnet and the coil is increased

(c)

……………………………………………

……… .……………………………… (25)

D Applications of electromagnetic induction

1. The d.c generator and a.c generator make use of electromagnetic induction to

produce (26)………………………………………….

2. Comparison between alternating and direct current (a.c / d.c)

Complete the table of comparison below.

Alternating current Direct current

Graphs

Current, I/A

(27)(28)

Direction

(29) (30)

Magnitude

(31) (32)

Examples of sources

1. (33)

2. (34)

1. (35)

2. (36)

3. 4 Transformer [ ………./ 28 x 100 = ………….%]

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Time, t/sTime, t/s

Current, I/A

A. Operating principle of a transformer

1. Label the diagram below which shows the parts of a transformer.

(3)

(1)

9( (4) (2) 9(

(5)

(6)

2. When an alternating current flows in the primary coil, an induced changing

(7) …………………. is produced in the soft iron core which is linked to secondary coil.

3. The changing magnetic flux cut by the secondary coil (8) ……..………an alternating

emf / current in the secondary coil.

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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

4. The induced alternating emf has the same (9) ………..………………as the input emf.

B. Step-up and step-down transformers

1. The output voltage depends on the (10) ……………………... of the number of turns of

primary and secondary coils.

2. The relationship between the voltages and the number of turns in primary and

secondary coils is given as follows:

3. Np is greater than Ns 4. Np is less than Ns

(11) (12)

5. In an ideal transformer, there is (13)………………………….of energy.

(14) (15)

(16)

=

12

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

………transformer…………... transformer

Input voltage Output voltage

Np Ns Np Ns

= =

C. Energy losses in a transformer

1. An ideal transformer has(17) ………………………………….. …………..efficiency.

2. But in practice, the efficiency of a tranformer is (18) ………………………….than100%.

3. The efficiency of a transformer is expressed as follows:

Efficiency = ___________________ x 100% (19)

4. Complete the table below to show the factors affecting the efficiency of a transfomer

and the ways to improve it:

Causes of energy loss Way to reduce

The (20)……………….. is magnetized and

demagnetized alternately due to a.c

current in primary coil

Energy lost as heat is known as

hysterisis loss.

Used (21) ………………..

which can be magnetized and demagnetized

easily

Changing magnetic field in the soft iron

core induces (22)……………………………

Energy is lost through (23)………………...

generated.

Used (24)………………. ……………………..

core

Current flowing through the primary and

secondary coil produces heat due to

(25…………………………….. of the wire.

Use (26)…………………………………………..

(27) …………………………of magnetic

flux in the primary coil

(28)………………………………………………..is

wound over the primary coil

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3.5 Understanding the generation and transmission of electricity

[ ………./ 33 x 100 = ……….% }

A. Sources of Energy

1. The generation of electricity comes from many sources such as :

i. ………………………(1)ii ………………………(2)iii……………………… (3)iv……………………… (4)v………………………. (5)vi……………………… (6)vii. …………………….(7)

viii……………………… (8)

B. Transmission of Electricity1. Complete the diagram below which shows the transmission of electricity to

consumers.

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450 V

(9)………. transformer

33 kV

33 kV

132 kV

Heavy industry

Light industry

11 kV

Buildings

240 V

My house

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.

(10) …………...transformers

transmission

2. The (11) ……………………………………...is a network of underground cables

and pylons connecting all the power stations and substations in the whole country to

the consumers.

3 . The advantages of the National Grid Network are:

Efficient (12)…………………..distribution according to requirements when demand

is high/ low

good energy management when there is a (13) breakdown/ interruption of supply

4. Electrical energy is transmitted from the power station to the consumer using long

transmission cables will cause power loss as (14) ………………..energy which can

be calculated as follow:

The power loss can be reduced by:

i. Reducing the (15) …………………………………….of the cables

ii. Reducing the current or (16) ……………………………the voltage in the cable

5. (17) ………………….play an important role in the transmission of electricity at a higher voltage.

C. Renewable energy

1. Energy plays a very important role in economic development but the reserves of

(18) ………………………such as oil and gas are very limited.

2. Hence, there is a modern trend of nations harnessing sources of (19) …………...

…………...which are continuously replenished naturally and they are sustainable.

3. Give the example of renewable energy:

i. …………………………………….(20)

ii. ……………………………………(21)

iii. …………………………………….(22)

iv. ……………………………………(23)

v. …………………………………….(24)

vi. …………………………………….(25)

vii. …………………………………….(26)

4. Give the example of non-renewable energy:

i. Fossil fuels

a) ……………………………..(27)

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Power plant

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

b) …………………………….(28)

c) ……………………………..(29)

5. Give the benefits of using renewable energy in our nation:

i. Avoid …………………………………(30) of fossil fuels

ii. Cleaner sources for little (31) ………………………….

iii. Avoid harming (32) …………………………..and fauna

iv. Avoid the disruption of (33) …………………..l balance

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