Science Form 3

Unit 8

Generation of

Electricity

yschow@smkbpja

THE GENERATION OF ELECTRICAL

ENERGY

8.1

yschow@smkbpja

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.

yschow@smkbpja

8.1 The Generation of Electrical Energy

yschow@smkbpja

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

yschow@smkbpja

8.1 The Generation of Electrical Energy

yschow@smkbpja

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.

yschow@smkbpja

8.1 The Generation of Electrical Energy

yschow@smkbpja

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.

yschow@smkbpja

8.1 The Generation of Electrical Energy

yschow@smkbpja

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..

yschow@smkbpja

8.1 The Generation of Electrical Energy

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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

yschow@smkbpja

TRANSFORMERS

8.2

yschow@smkbpja

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.

yschow@smkbpja

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

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

3. A step-down transformer in the substations

decreases the suitable voltage before the

electrical energy is distributed to various

sector.

yschow@smkbpja

ELECTRICAL POWER TRANSMISSION

AND DISTRIBUTION SYSTEM

8.3

yschow@smkbpja

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

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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

yschow@smkbpja

ELECTRICAL POWER SUPPLY AND

WIRING SYSTEM IN HOMES

8.4

yschow@smkbpja

yschow@smkbpja

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

schow@smkbpja

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

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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 .

yschow@smkbpja

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).

yschow@smkbpja

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.

yschow@smkbpja

COST OF

ELECTRICAL

ENERGY

USAGE

8.5

yschow@smkbpja

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

yschow@smkbpja

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:

yschow@smkbpja

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:

yschow@smkbpja

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:

yschow@smkbpja

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:

yschow@smkbpja

FUSE AND EARTH WIRE

8.6

yschow@smkbpja

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'.

yschow@smkbpja

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.

yschow@smkbpja

5. A suitable rating of a fuse can be determined

from the following formula:

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

SAFETY PRECAUTIONS IN THE USE

OF ELECTRICAL ENERGY

8.7

yschow@smkbpja

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.

yschow@smkbpja

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.

yschow@smkbpja

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).

yschow@smkbpja

CONSERVING ELECTRICAL ENERGY

8.8

yschow@smkbpja

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.

yschow@smkbpja

yschow@smkbpja

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.

yschow@smkbpja