CHAPTER 3 ELECTROMAGNETISM FULL NOTES

Causes of energy loss in transformers Ways to improve the efficiency of a
transformer
Leakage of magnetic field
➢ electrical energy is lost when a fraction of Wind the secondary coil on top of the primary
coil
the magnetic field produced by the primary
coil does not link with the secondary coil. Laminated soft-iron core

Transformer Input terminals
coils Output terminals

“ When the world says, ‘Give up’, Hope
”whispers, ‘Try it one more time.’

51

Exercise 8.4

1. A transformer is required to step down the 2. A step up transformer has 10 000 turns on

mains voltage of 240 V provide a 12 V its secondary coil and 100 turns on its

supply for an electric toy. If the primary coil is primary coil. An alternating current of 5.0 A

wound with 1 000 turns of wire, calculate the flows in the primary coil when it is connected

number of turns required for the secondary to a 12 V a.c supply. Calculate:

coil. a) the input power to the transformer

Ans : 50 b) the e.m.f induced across the secondary
coil.

c) the maximum current that could flow in

a circuit connected to the secondary coil

if the transformer is 100% efficient.

Ans :
a) 60 W
b) 1200 V
c) 0.05 A

52

Question 3

Diagram shows three identical bulbs of 16 V, 48 W each connected in series to the secondary coil
of a transformer. The input voltage is 240 V from the mains supply.

240 V

The primary coil has 4800 turns. The three bulbs are lighted with normal brightness.

a) What is meant by normal brightness ?
Normal brightness means the voltage supplied to bulb is the same as the voltage
labelled on the bulb

b) What is the output voltage of the transformer ?

Ans : 48 V

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c) Determine the number of turns of wire in the secondary coil.

Ans : 960 turns

d) If the transformer is 100% efficient, calculate the current in the primary coil.

Ans : 0.6 A

54

e) If the transformer is 80% efficient, calculate the current in the primary coil.

Ans : 0.75 A

f) State two ways which causes a transformer to lose energy.
Resistance of coil
Eddy current in the iron core

55

8.5 Generation And Transmission Of Electricity

Electricity is generated in power stations, transmitted (sent) through long-distance cables, and then
distributed to consumers.

Sources of energy used to generated electricity
Renewable energy sources : an energy resource that is continually replaced and will not run out.
Non-renewable sources: an energy resource that cannot be replaced once it has been used.

Renewable energy Non-renewable energy

• Biofuel • Coal
• Biomass • Crude oil
• Geothermal • Natural gas
• Hydropower • Nuclear Energy (Uranium)
• Solar energy
• Tidal power
• Wave power
• Wind power

Ways of generating electricity

• Electricity is produces using generators

• A generator has a huge magnet that is turned by a turbine.

• As the magnet turns inside a coil of wire, electricity is produced by electromagnetic induction.

• Many sources of energy are used to turn these turbines. 56

Coal-fired power • Coal is burned and the heat energy is used to produce high
station pressure steam which used to drive the generators to produce
electricity.
Gas-fired power
station • The energy changes: chemical energy → heat energy → kinetic
energy → electric energy
Hydro-electric
Power station • The burning of natural gas produces high pressure steam that is
used to drive turbines.

• The energy changes: chemical energy to heat energy to kinetic
energy to electric energy

• A huge dam is built

across rivers to contain

water.
• When the water in the

dam is allowed to flow

down to a lower level, the

kinetic energy of the

moving water is used to

drive water turbines,

which in turn drive the

generators to produce

electricity.
• The energy changes:

potential energy → kinetic
energy → electric energy

57

Nuclear power 1. Energy from nuclear fission (splitting of uranium 235 atoms) is used
station to produce high pressure steam to drive turbines
Biomass
2. The energy changes: nuclear energy → heat energy → kinetic
Solar energy → electric energy

Wind generator 1. Biomass includes rotting plants and agricultural wastes such as oil
palm kernels and household sewage that are used to produce
methane gas and alcohol.

2. Heat energy from the burning of biomass is used to generate
electricity

1. Solar energy originates from the sun
2. Solar cells use sun energy to generate small electrical energy to

power calculators.
3. Solar panels use sun energy to generate heat energy to heat water

at home.

1. High speed wind can be used to turn the blades of large windmills
which will in turn drive generators to produce electricity

“Two things are infinite: the universe and human
stupidity; and I'm not sure about the universe.”

― Albert Einstein

58

The Transmission of Electricity

1. Electrical energy is transmitted from power station to the consumer using long transmission
cables or power lines.

2. Electrical energy is transmitted at a high voltage and use alternating current.
3. A step-up transformer increases the voltage to as high as 110,000 or 132, 000 V.
4. Step-down transformers are used to decrease the voltage before being delivered to the

consumers.
5. The long tick cables used as transmission lines are made of copper or aluminium.

Energy loss in electricity transmission cables deduce the advantage of high voltage
transmission

1. The heating effect due to the resistance of the cables causes loss of electrical energy as heat
energy.

2. When a current flows in a cable, the power loss, P through heating is P = I2R where R is the
resistance of the cable.

3. The power loss can be reduced by
(a) reducing the resistance of the cables
(b) reducing the current in the cables

4. In order to reduce resistance of the cables, a thick copper or aluminium is used. But thick
cables are very expensive and will be too heavy to be supported securely.

5. The loss of power in the transmissions of electricity is reduced by reducing the current in the
cables.

PLoss = I2R

59

The power to be transmitted by the cables is

PTrans = VI

where I = current in the cables, V = voltage across the cables.

6. This means that the current in the cables is inversely proportional to the voltage of the cables
for a certain value of power transmission.

7. When power is transmitted at lower voltage, the current in the cables is large.
8. The greater the current in the cables, the greater the power loss.

“All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.

From the ashes a fire shall be woken,
A light from the shadows shall spring;
Renewed shall be blade that was broken,

The crownless again shall be king.”

― J.R.R. Tolkien, The Fellowship of the Ring

60

Exercise 8.5 2. Electric power is transmitted from a power
station to a town by a transmission cables
1. Find the power loss in a transmission with a total resistance of 50 Ω. If the power
cable when 20 kW electric power is station generates 8 MW of power, calculate
transmitted through a cable with the power loss in the cables if power is
resistance of 1.5 Ω at a voltage of transmitted at a voltage of
a) 200 V a) 80 kV
b) 10 kV. b) 400 kV
Ans :
a) 15 kW Ans :
b) 6 W a) 0.5 MW

b) 20 kW

61

National Grid Network

1. National Grid Network is a 132 kV 25 kV
network system of high
voltages cables which Heavy Industries Domestic
connects all the power 33 kV 240 V
stations in the country to the
consumers. Light Industries
11 kV
2. In Malaysia, electricity is
generated at 11 kV – 33 kV at Offices
the power station. It is then 415 V
stepped up in a transformer to
132 kV before it is sent to the
grid network.

3. The purpose of increasing the
voltage to 132 kV is to lower
the current flowing through
the grid, and this reducing
power loss during
transmission.

4. The high voltage in the grid is
subsequently reduced by sub
station transformers for
distribution to local users.

5. Heavy industries will be supplied with power at 33 kV. Light industries will be supplied with
power at 11 kV. Offices will be supplied with power at 415 V while domestic users will be
supplied with power at 240 V

62

If a particular station breaks The grid network also enables less efficient
down or is shut down for
2 stations to be shut down at off-peak period,
1 maintenance work, the other
thereby reducing the overall cost of
stations can supply electrical generation and transmission.
energy to the affected area.

Importance of the 4
National Grid
Network Power stations in areas
where the demand is low
can supply electricity to
areas where the demand
is high.

3 Power stations can be located

Cost of generation of electricity is 5 outside city limits so that air and
reduced because high voltage
transmission reduces the current environmental pollution can be
flowing through the cables and reduced.
hence reduces power loss in the
cables.

63

Alternating Overhead or underground?
current, a.c
1. High voltage cables are the cheapest way
1. An alternating current is used of sending power over long distances.
in the transmission of
electrical energy because its 2. However, to prevent sparking, the only
voltage can be easily effective way of insulating the cables is to
increased or decreased with keep huge air spaces around them
transformers.
3. The cable have to be suspended from
2. A transformer cannot function pylons.
with a direct current
Transmission
issues

High voltage or Charge leakage
low voltage? between the cables and the earth

Electrical power is transmitted at a high 1. To prevent charge leakage, the aluminium
voltage so that cables are supported by high metal pylons.
• The current in the cables is smaller
• The loss of power due to heating of the 2. The metal support of the pylon is earthed so
that it is safe to workers and the public who
cables is minimized. come into contact with them.

64

Theft Danger of being struck by
light aircraft
Stricter laws should be enforced
to deter thieves from stealing Lights and special markers must be
the aluminium cables. attached to the pylons.

Transmission
issues

Cost of cables Danger of being struck by lightning

1. Copper cables have low resistance 1. The pylons carry lightning conductors
but are of high cost that are properly fixed into the ground

2. Aluminium cables are usually used as 2. The cables are properly fixed with
they are light, have low resistance porcelain support so that the cables do
and cost less. not touch the pylons.

65

Non-renewable sources of Renewable energy sources From the sun: solar panels, solar cells,
energy such as crude oil, are those which originate biomass, wind and wave energy,
coal and natural gases are from the sun or the earth hydroelectric power
fast depleting. Alternatives and will last as long as the
must be found to replace Solar System itself. 4 From the earth: geothermal
such sources to ensure
continuous supply of 23 energy, energy from the tides.
power in the future.

1

Importance of Renewable sources
renewable energy
5 of energy do not

pollute the
environment.

“ Any new fact or insight that I may have found
has not seemed to me as a 'discovery' of

P. V. Subrahmanyan mine, bu. t rather something that has always 66
Chandrasekhar
”been there and that I had chanced to pick up
(1910 - 1995)

Effects on the environment caused by the use of various sources to generate electricity

Energy Advantages Disadvantages
resource
1. Renewable energy 1. Building of dams can upset the
Hydro

2. Water is freely available in large balance of the ecosystem

quantities 2. Not suitable in areas where

3. Clean and does not pollute the natural disasters such as

environment earthquakes may occur

4. Water stored in dams can be used 3. Not suitable in flat and dry

to irrigate farms regions.

5. Dams can be used to control

floods Dams can be promoted as

recreational centre

Oil and natural 1. At present it is still available in 1. Non-renewable energy
gas large quantities 2. Discharges harmful gases that

2. Can be transported to the location can pollute the environment
where it is to be used 3. High cost

Coal 1. Plenty of resources available for 1. Non-renewable energy
up to 200 years 2. Discharges harmful gases that

2. Can be transported to the location can pollute the environment
where it is to be used

67

Energy Advantages Disadvantages
resource

Nuclear 1. A small amount uranium can 1. High cost of building
release a large quantity of energy 2. Waste in the form of used fuel rods

2. Minimum discharge of carbon which are very hot and highly
dioxide radioactive
3. Hot water discharged causes thermal
3. Reactor in nuclear power stations pollution to the environment
can be used to produce useful 4. Risk of accidents which may lead to the
radioisotopes to be used in leakage of large amounts of radioactive
industry, medicine and agriculture substances to the environment.

Biomass 1. Renewable energy 1. Requires large storage space far from
2. Reduces problem of disposal of human population if the biomass is
animal dung or sewage.
organic waste

Wind 1. Renewable energy 1. Requires many windmills which cover a
2. Clean and does not pollute larger area

environment 2. Generates high level of noise

“I fear not the man who has practiced 10,000 kicks 68

once, but I fear the man who has practiced one kick
10,000 times.”

– Bruce Lee