Science Form 3

Science Form 3 Unit 8 Generation of Electricity yschow@smkbpja

8.1

THE GENERATION OF ELECTRICAL ENERGY 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 mixture is ignited by 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

8.2

TRANSFORMERS

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

8.3

ELECTRICAL POWER TRANSMISSION AND DISTRIBUTION SYSTEM yschow@smkbpja

Electrical power transmission and distribution system 1. Consists of: a. Transformer station b. Branch substation c. National Grid Network

d. Main substation e. Switch zone

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

8.4

ELECTRICAL POWER SUPPLY AND WIRING SYSTEM IN HOMES 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 Main switch d. Circuit breakers e. Live wire f. Neutral wire g. Earth wire Circuit schow@smkbpja 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 singlephase 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 yellow with 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

8.5

COST OF ELECTRICAL ENERGY USAGE

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: Electrical energy (J) =

Electrical power (W)

yschow@smkbpja

x Time (s)

5. The relationship between power, voltage and electric current can be represented by the following equation: Power (P) = voltage (V) x current (I)

yschow@smkbpja

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: Energy (kWh) = Power ( kW) yschow@smkbpja

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

8.6

FUSE AND EARTH WIRE

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: Current = Power / Voltage

yschow@smkbpja

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

8.7

SAFETY PRECAUTIONS IN THE USE OF ELECTRICAL ENERGY 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

8.8

CONSERVING ELECTRICAL ENERGY

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

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.

yschow@smkbpja

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