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nota bergambar untuk pelajar tingkatan 3, matapelajaran : sains (science)Bahasa : Inggeris
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Science Form 3
Unit 8
Generation of
Electricity
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THE GENERATION OF ELECTRICAL
ENERGY
8.1
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Generator
�Electrical energy is produced by using a
generator in the power station.
�A generator can also be called as an alternator
or dynamo.
�Generators convert mechanical energy into
electrical energy.
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8.1 The Generation of Electrical Energy
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i. Thermal generator
a. Fossil fuels such as petroleum, natural gas and coal are burned
in a boiler to produce steam.
b. The steam, which is produced under high pressure, is
channeled through pipes and valves from the boiler to rotate
a steam turbine.
c. The turbine rotates at high speed and this causes the
generator to produce electrical energy.
d. Steam from the turbine is then channelled to the condenser
where the steam is cooled down with sea water to become
water again.
e. The water is returned to the boiler to go through the process
again
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8.1 The Generation of Electrical Energy
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2. Hydroelectric generators
a. Hydroelectric power plants require a constant
source of water which flows from a high position
from the dams.
b. The kinetic energy of the flowing water from the
reservoir is channeled through tunnels to rotate the
turbines.
c. The water turbines will then rotates the generator to
produce electrical energy.
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8.1 The Generation of Electrical Energy
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3. Diesel generator
a. A diesel generator uses a diesel engine that is
similar the diesel engine of a vehicle.
b. The combustion of diesel in the engines
rotates a generator to produce electrical
energy.
c. This generator produces a low power output
and is usually used to supply electrical energy
to small town and villages.
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8.1 The Generation of Electrical Energy
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4. Nuclear power plants
a. Large amount of heat energy produced by nuclear
fission in the reactor is used to convert water into
steam
b. The steam produced turns the steam turbine to drive
a generator to generate electrical energy.
c. It is expensive to build a nuclear power plant..
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8.1 The Generation of Electrical Energy
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5. Gas turbine generators
a. In a gas turbine, air is filtered and compressed by a
compressor.
b. The air is then mixed with natural gas in the
combustion chamber.
c. The mixturemixture is ignited by sparks plugs. sparks plugs.
d. The ignition causes the mixture of gas to expand.
Gas at very high pressure and temperature rotates a
turbine and generates electrical energy.
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Alternative Sources of Energy
1. Cogeneration is a technology which involves
the generation of several types of energy such
as heat and electrical energy simultaneously
from one energy source, such as biomass.
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Alternative Sources of Energy
2. The decomposition of rubbish and animal faeces
produce methane gas and alcohol.
3. Solar cells can convert light energy to electrical
energy.
4. Photovoltaic cells are the quickest and cheapest
method of supplying electrical energy to remote
households.
5. Wind energy , wave energy and geothermal
energy can also be used to rotate turbines
connected to generator
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TRANSFORMERS
8.2
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Transformers
1. Transformers are used to change the voltage of
alternating current.
2. A basic transformer consists of two sets of insulated
coil windings on opposite sides of a soft coil.
a. The core is constructed of many layers of thin iron
called laminations.
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b. The primary coil is the wire coil that
connected to the input voltage.
c. The secondary coil is the wire coil that
connected to the electrical load
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3. When alternating current flows through the
primary coil, a continually changing magnetic
• field is created and thus induce a flow of
current in the secondary coil.
4. The output voltage produced in the
secondary coil depends on
a. the input voltage
b. The number of turns of the primary coil and
secondary coil.
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Step-up transformers
a. A step -up transformer has more turns in
the secondary coil than in the primary coil.
b. The secondary voltage will be higher than the
the primary voltage.
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Step-down transformers
a. A step-down transformer has more turns in
primary coil than in the secondary coil.
b. The secondary voltage will be lower than the
primary voltage.
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B. Role of Transformers in the Transmission and
Distribution of Electrical Energy
1. Transformers are very important in the transmission
and distribution of electrical energy.
2. A step-up transformer located near the power
station to increase the
voltage of the electrical
energy generated before
it is transmitted through
the cables.
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3. A step-down transformer in the substations
decreases the suitable voltage before the
electrical energy is distributed to various
sector.
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ELECTRICAL POWER TRANSMISSION
AND DISTRIBUTION SYSTEM
8.3
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Electrical power transmission and distribution system
1. Consists of:
a. Transformer station d. Main substation
b. Branch substation e. Switch zone
c. National Grid Network
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2. The power stations usually generate electrical
energy at a voltage of 11 kV.
3. The voltage is then stepped up to values as
high as 132 kV, 275 kV and 500 kV by step up
transformers.
4. The electrical energy at this high voltage
with low current in order to reduce the
power loss during the transmission over a
long distance.
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5. The electrical energy at a high voltage is
channelled to the National Grid Network
through the switch zone.
6. The electrical energy is distributed from the
National Grid Network to a series of
substations.
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7. The voltage is step down stage by stage from main
substation to branch substation before the electrical
is distributed to the consumers.
8. The voltage required for various sectors:
a. Heavy industries
b. Small industries 11 kV
c. Shops and office 415 V
d. Residential area 240 V
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ELECTRICAL POWER SUPPLY AND
WIRING SYSTEM IN HOMES
8.4
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Electrical wiring system
1. The electrical wiring system in a house supplies the
electrical energy from the mains to the electrical
appliances.
2. Consists of:
a. Electric meter
b. Mains fuse
c. Main switch
d. Circuit breakers
e. Live wire
f. Neutral wire
g. Earth wire
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Main switch
Circuit
breakers
3. The electrical wires from the mains supply
cable contain a live wire and neutral wire.
a. An alternating current at 240 V flows through
the live wire from the mains into the house.
b. The neutral wire carries the electric current
back to the mains to complete the circuit
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4. The earth wire is connected to copper plates
in the ground for safety purpose.
5. Live wire that carry the electrical
energy is connected with the mains fuse and
to the electric meter which is located outside
the house.
a. The mains fuse will break the circuit if
excessive current is supplied.
b. The electric meter measures the amount of
electrical energy used in the house. yschow@smkbpja
6. Main switch controls the electric energy from
the mains supply to the appliances.
7. The circuit breaker will cuts off the flow of
electric current under abnormal condition
cause by short circuit of leakages.
8. This prevent electric shocks and fires.
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Single-phase distribution lines and three phase
distribution linoes
1. Electrical energy can be distributed either on single-
phase or three phase or distribution lines.
2. The residential areas residential areas that that
need a low voltage electrical supply 240 V requires
only single- phase distribution lines.
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3. When using powered appliances, a thee-phase
distribution line is required
a. Electrical energy from substations is normally
distributed on three distributed on three-phase
distribution lines.
b. This system is able to supply electrical energy at a
high voltage.
c. The three-phase distribution line supplies current to
the high-powered appliances without cutting off the
circuit.
d. The single-phase distribution line will be overloaded
if high-powered appliances are connected to the
electric circuit.
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3 pin plug
1. Electrical appliances at home are connected to
power circuits through sockets.
2. Each socket has a 3-pin plug and the current that
flows from the power socket to the
• passes through the 3 passes through the 3-pin pin
plug.
3. 3-pin plug contains three pins marked
• L representing live,
• N representing neutral and
• E representing earth .
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4. The cable that connects the electrical appliance to
the 3-pin plug contains three wires.
a. The live wire is brown and is connected to the live
pin (L).
Electric current from the mains is carried to the
appliance through the live wire.
b. The neutral wire is blue and is connected to the
neutral pin (N).
Electric current flows from the appliance to the
mains through the neutral wire
c. The earth wire is yellowyellow with green green stripes and is
connected to the earth pin (E).
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5. The 3 The 3-pin plug has a pin plug has a
fuse that is connected to the live wire.
a. When excess current flows through the
fuse, the fuse melts and it breaks the circuit.
• b. In this way the fuse prevents the appliance
or the live wire from burning.
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COST OF
ELECTRICAL
ENERGY
USAGE
8.5
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A. Power, Voltage and Current Rating
of Electrical Appliances
1. Electrical appliances need electrical energy to
function.
2. The electric meter measures the amount of
electrical energy used.
3. Electrical power is a measure of the rate at
which electrical energy is
consumed or converted
into other forms of energy
per unit time. yschow@smkbpja
a. The S.I. unit for electrical power is watt (W) or joule
per second (J per second) (J/s).
b. One watt is the electrical energy that is supplied at
the rate of one joule per second
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4. Different electrical appliances convert electrical
energy at different rates.
a. For example, a 2 kW electrical iron will convert 2 000
joules of electrical energy per second into heat
energy.
b. The longer the iron is used, the greater the electrical
energy that is converted into heat energy.
c. The amount of elect electrical energy used can be
calculated using the following formula:
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Electrical energy (J) = Electrical power (W) x Time (s)
5. The relationship between power, voltage and
electric current can be represented by the
following equation:
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Power (P) = voltage (V) x current (I)
B. Calculating the cost of electrical energy usage
1. The usage of electrical energy depends on the
power rating of an appliance and the time the
appliance is being used.
2. The amount of electrical energy that an
electrical appliance uses can be determined
by using the following formula:
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Energy (kWh) = Power ( kW) x Time (hour)
a. A joule is a very small unit and is not used to
measure the electrical energy used by
consumers.
b. Tenaga Nasional Berhad uses the kilowatt
hour (kWh) to calculate the amount of energy
used.
c. 1 unit of electrical energy = 1 kW h.
Examples:
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FUSE AND EARTH WIRE
8.6
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Fuses
1. A fuse is a safety device safety device which consists of a
thin piece of wire that gets heated up and melts when the
current flowing through it is higher than its rating.
2. When the fuse wire melts and breaks the circuit, the fuse is
'blown'.
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3. Fuses usually have the following ratings: uses
usually have the following ratings: 1 A,2 A, 5 A,
10 A and 13 A.
4. The most suitable fuse has a rating that is
slightly higher than
the maximum current
that normally flows
through an electrical
appliance.
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5. A suitable rating of a fuse can be determined
from the following formula:
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Current = Power / Voltage
Fuse in electrical wiring systems
1. In a home electrical wiring system, a fuse
functions as a safety device.
2. A fuse prevents current exceeding its rating
from flowing through the cable and wires in
the household circuits.
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Earth Wires
1. The earth wire is another safety device in the
electrical wiring system.
2. The earth wire has a low resistance and
connects the metal casings of electrical
appliances such as refrigerators, kettles,
electric irons and
televisions directly
to the ground.
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3. When a 3 -pin plug of an electrical appliance
is plugged into a socket, the metal casing of
the electrical appliance is connected by the
earth wire to the ground.
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4. If an electrical fault occurs, and someone
touches the metal casing which is earthed, a
large current flows into the earth wire
instead of passing through the person.
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SAFETY PRECAUTIONS IN THE USE
OF ELECTRICAL ENERGY
8.7
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Sources of electrical accidents
1. Electrical accidents may be caused by a short circuit
overloading or negligence.
a. The use of electrical appliances without earth wires
or with faulty earth wires can cause electric shocks
and even in death.
b. b. Short circuits occur when the live wire touches the
neutral wire. This provides a shorter path of low
resistance for the current to flow through.
c. When a large amount of current flows through the
wires the wires may overheat and produce electrical
sparks. yschow@smkbpja
d. Cables with torn insulator parts and exposed
live wires may also result in electric shocks.
e. The use of broken 3 -pin plugs can also cause
electric shocks.
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f. Touching switches, electrical sources and
electrical appliances which are being used
with wet hands can cause electric shocks.
g. Do not connect too many appliances to one
socket.
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Safety devices
A miniature circuit breaker (MCB)
is an electromagnetic switch that
operates like a fuse but does not blow.
Another safety device that is installed in the
household wiring is the
Earth Leakage Circuit
Breaker (ELCB).
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CONSERVING ELECTRICAL ENERGY
8.8
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Ways to conserve electrical energy
1. Electrical energy should be used carefully and efficiently to
save fuel and to lower the cost of generation of electrical
energy.
2. The examples of saving energy
a. Use machines that are efficient in industrial processes.
b. Use machines and appliances with low power requirements
instead of those that require more power.
c. Switch off appliances such as lights, fans and televisions when
not in use.
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Recycling things like
paper, plastic, and
soda cans helps
save energy.
Factories use less
energy to make
products from
recycled materials.If the lights and
TV are on, but
there's no one in
the room, that's
wasting energy.
Planting a tree
will help shade
your house from
the hot sun, and
it will make
oxygen to help
us breathe.Leaving the
refrigerator
door open
wastes
energy.
The need to conserve energy
1. Things that we do during our leisure time
involve the use of electrical energy.
2. The economy of our country would be badly
affected should there be an energy crisis.
3. The manufacturing sector which depends
heavily on electrical energy cannot operate on
a full scale.
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