CHAPTER 3: ELECTROMAGNETISM

3.1 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.

3.1.1 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

__________________ and the other four curled fingers points to direction of its

__________________

2. Using the Right hand grip rule, draw the direction of current flow and pattern of

magnetic fields lines formed.

3.1.2 Magnetic Field due to a Current in a Circular Coil

1

- +

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 ____________

3.1.3 Magnetic Field due to a Current in a Solenoid

1. A _______________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 direction of current flow is _______________,

the polarity at end P is _____________

ii. Look from end Q of the solenoid, the current flow is ___________________, the

polarity at end Q is ______________

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

3.1.4 Factors that affects the strength of an electromagnet

2

P

Q

+-

- +

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

to the solenoid, the ___________the strength of that electromagnet.

Factors Condition Number of paper clips

attracted

The strength of

electromagnetic field

Number of turns increase

Electric current increase

Use of normal iron-core

-nil-Use of soft-iron core

3.1.5 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__________________.

ii. The soft iron armature is________ towards the electromagnet and the hammer hits

the gong

iii. At the same time, the contact is ______ and the soft iron core ______ its magnetism.

iv. The __________ brings the armature back to its original position

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

3 .2 Understanding the force on a current-carrying conductor in a magnetic field.

3

[ ……/ 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.

3.2.1 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 ________.

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

direction of force, F acting on the conductor.

4

+

N

+ =

N

SS

3. _______________________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.____________________________________

ii.____________________________________

iii.____________________________________

3.2.2 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 _______

force acts on the arm AB whereas a __________ force acts on the arms CD according

to Fleming’s Left Hand rule.

2. Draw the catapult field formed and show with arrows the direction of the forces acting

on arms AB and CD.

5

Thumb ( ) First Finger ( )

Centre Finger ( )

Carbon brush

commutator

3. Complete and draw the four stages of the motion of a direct current motor in

operation

Degree : 00

Current flow through carbon brushes: Yes /

No

Arm AB : upwards / downwards

Arm CD : upwards / downwards

Rotation : clockwise direction

Degree : 0

Current flow through carbon brushes : Yes /

No

Arm AB : right / left

Arm CD : right / left

Rotation : ____________due to inertia

Degree : 0

CoilN

S

B

A 23

4

1

current

C

D

6

N S

AB CD

Current flow through carbon brushes : Yes /

No

Arm AB : upwards / downwards

Arm CD : upwards / downwards

Rotation : _____________

Degree : 0

Current flow through carbon brushes : Yes /

No

Arm AB : right / left

Arm CD : right / left

Rotation : _____________due to inertia

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

1. ___________________________________

2. ___________________________________

3. ___________________________________

4. ___________________________________

3. 3 Analysing electromagnetic induction. [ ……./ 39 x 100 = …………..% ]

7

Thumb ( )

Center finger ( )

First finger ( )

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.

3.3.1 Electromagnetic Induction

1. When a conductor is moved to cut through the magnetic flux, an _______________ is

produced.

2. It requires a _____________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

____________________________________.

3.3.2 Induced e.m.f by a moving conductor

Action Observation Inference

The wire is moved

upwards

Galvanometer deflect to

left

The wire is moved

downwards

The wire is move

horizontally

Magnet is moved upwards

___________________rule can be used to determine the direction of the ___________induced current produced

8

3.3.3 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

____________law states that the direction of the induced current is such that it always

_______ ___ the change producing it.

9

Magnetic field lines are being cut . _________ induced

No deflection on the galvanometer.____________ is induced

___________ induced in opposite direction

Moving the coil towards a magnet also induces _____________

3.3.4 Faraday’s Law of electromagnetic induction

1. Faraday’s Law states that the magnitude of the induced e.m.f. is _________________

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. ____________________________________________________

II. ____________________________________________________

III. ____________________________________________________

3. The magnitude of the induced current in a coil increases when:

I. _________________________________________________

II. _________________________________________________

III. _________________________________________________

3.3.5 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 _____________ ,

___________ or wind turbine.

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

produce _______________________

10

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

Complete the table of comparison below.

AC Current DC Current

Graphs

Current, I/A

Direction

Examples of sources

1.

2.

1.

2.

3. 4 Transformer [ ……/ 33 x 100 = …………..% ]

11

Time, t/sTime, t/s

Current, I/A

3.4.1 Operating principle of a transformer

1. Complete the diagram below

1. When an alternating current flows in the primary coil, it produces _______________

which link the primary coil and the secondary coil

2. The magnetic flux linkage to the ___________________ coil is cut.

3. The changing magnetic flux cut by the secondary coil ___________ a current in the

secondary coil.

4. When the current in the primary coil decreases, the magnetic field will __________

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 ___________________ of the same

frequency is induced in the secondary coil.

III.4.2 Step-up and step-down transformers

12

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

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

and secondary coils.

2. For an ideal transformer, the relationship between the _______________ and the

_________ of the number of turns in primary and secondary coils is given as follows:

3 If Ns is greater than Np, then Vs is ____________ than Vp This type of transformer is

a ________________ transformer

4. If Ns is less than Np, then Vs is ________ than Vp. This type of transformer is a

________________transformer

5. For example, if the turns ratio is 1:50, the output voltage is stepped up _____ times

.

If we consider an ideal transformer, there is ____loss of energy.

=

3.4.3 Energy losses in a transformer

13

Power supplied to the primary coil = Power used in the

secondary coil

Comparing with the transformer equation

Input voltage,Vp = 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

= =

1. An ideal transformer has _________ efficiency.

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

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

Changing magnetic flux induces current in

the soft iron core.

Heat is produced.

Heat loss

As the number of turn increases, the

resistance of conductor also increases.

Heat is produced.

The core is magnetized and demagnetized

alternately due to a.c current in primary

coil

Energy lost as heat.

Flux leakage

Leakage of magnetic flux in the primary

coil

3.5 Understanding the generation and transmission of electricity

14

---------

[ …../ 40 x 100 = …………]

3.5.1: Generation and Transmission of Electricity

1. Sources of Energy

The generation of electricity comes from many sources such as :

a. ……………………

b. ……………………

c. ……………………

d. ……………………

e. ……………………

f. ……………………

g. ……………………

h. ……………………

3.5.2 Transmission of Electricity

1. The diagram below shows a model of the transmission of electricity from an ac

source.

15

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.

2. Complete the diagram below which shows the transmission of electricity to

consumers.

3. The _________________ 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 ______ is reduced,and increases the efficiency of transmission

-- efficient ________ distribution according to requirements when demand is high/ low

-- good energy management when there is a _____________/ 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 ______ energy. Power loss can

be calculated as follow

6. The power loss can be reduced by:

i. Reducing the ____________ of the cables

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

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

16

__ V

_________ transformer

_________ transformer

__ kV

Power plant

__ kV

____ kV

Heavy industry

Light industry

__ kV

Buildings

___V

My house

Pheat =I2RI = current flows in the cableR = resistance of the cable

__________ transformers

transmission

3.5.3 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 ______________

energy.

Renewable energy sources are continually replenished naturally and they are

sustainable.

3. Give the example of renewable energy:

i ………………………

ii. ……………………..

iii. …………………….

iv. ……………………..

v. …………………….

vi. …………………….

vii. ……………………...

4. Give the example of non-renewable energy:

i. Fossil fuels

a) _____

b) _____

c) _____

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

i. Avoid ____________ of fossil fuels

ii. Cleaner sources for little _____________

iii. Avoid harming _________ and fauna

iv. Avoid the disruption of _________________ balance

17