Form 3 Science Chapter 8 Generation of Electricity

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FORM 3 SCIENCE NOTES
CHAPTER 8 GENERATION OF ELECTRICITY
8.1 Generation of Electrical Energy
Types of generators
 Generation of electricity – Process of converting some form of energy into electricity.
 Electrical energy is produced in large amounts by power stations.
 This involves the use of generators 发电机.
 Generator – Machine that produces electrical energy from mechanical energy 机械能.
 Dynamo – Generator which produces direct current.
 Alternator – Generator which produces alternating current.
 Thermal generators 热发电机
- Fossil fuels are burned in a boiler to produce steam.
- Steam produced under great pressure is channeled through pipes and valves from boiler to rotate
steam turbine.
- Turbine rotates at high speed and causes the generator to turn.
- Steam from turbine is channeled to condenser 凝结器 to be cooled with sea water.
- The steam becomes water.
 Hydroelectric generators 水力发电机
- Dams are built across rivers to store water.
- Require a constant source of water flowing from a great height in dams.
- Kinetic energy of flowing water from the reservoir is channeled through tunnels near bottom of
dam to rotate turbines.
- Water turbines will then rotate the generators.
Thermal generator
Hydroelectric generator

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 Diesel generators 柴油发电机
- Uses diesel engines that are similar to those in vehicles.
- Combustion of diesel rotates generator.
- Produces low power output.
- Usually used to supply electrical energy to small towns and villages that are far from thermal and
hydro-powered stations.
 Gas turbine generators 燃气轮机
- Filtered air is sucked into compressor 压缩机.
- Compressed air is mixed with natural gas and ignited in combustion chamber.
- Hot gas expands and rotates turbine.
- Generator connected to turbine rotates.
 Nuclear power plants 核电站
- Nuclear fission 核裂变 in reactor converts water into steam.
- Steam rotates the steam turbine.
Diesel power station
Gas turbine generators
Nuclear power plant

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Alternative sources of energy
 Wind energy
- Giant wind turbines are used to rotate generators.
 Geothermal energy
- High pressure steam in Earth is harvested by power companies to drive turbines.
 Tidal energy
- Flow of water due to high and low tides can be used to rotate generators.
 Biomass
- Biofuels and manure are the oldest form of biomass.
- In Malaysia, biodiesel from palm oil is produced to power vehicle.
- In Brazil, sugar cane is used to produce alcohol.
 Solar energy
- Provide hot water and light up buildings.
- Solar cells and solar panels convert solar energy into electrical energy.
8.2 Transformers
 Transformers 变压器 are used in transmission and distribution of electrical power.
 They change the voltage of alternating current 交流电 with little loss in energy.
 Alternating current (a.c.) – Current that flows back and forth in periodic cycles.
 Basic transformer consists of two sets of insulated coiled wires 线圈 on opposite sides of soft iron
core 铁芯.
 Core is constructed of many layers of thin iron (laminations 叠片).
 Laminated soft iron cores reduce loss of heat when an induced current 感应电流 flows through.
 Primary coil 初级线圈 – Wire coil that receives input voltage 输入电压
 Secondary coil 次级线圈 - Wire coil that produces output voltage 输出电压
 When a.c. flows through primary coil, continually changing magnetic field is created. This magnetic
field will induce flow of current in secondary coil.
 Voltage produced in secondary coil called output voltage depends on:
- Input voltage
- Number of turns of primary coil and secondary coil

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Step-up transformer 升压变压器 Aspect Step-down transformer 降压变压器
Symbol
S. coil has more turns than P. coil Number of turns in coil P. coil has more turns than S. coil
Output voltage is higher than input
voltage
Output voltage
Output voltage is lower than input
voltage
Output current is lower than input
current
Output current
Output current is higher than input
current
Difference between step-up transformer and step-down transformers
The role of transformers in the transmission and distribution of electrical energy
 Important in transmission 传输 and distribution of electrical energy.
 Step-up transformer increases voltage of electrical energy generated before it is transmitted through
cables.
 Step-down transformer decreases voltage to suit different energy requirements.
8.3 Electrical Power Transmission and Distribution System
 Transmission and distribution system consist of:
- Transformer stations
- Switch zones
- The National Grid Network
- Main substations
- Branch substations
 Electrical energy then flows to switch zone which consists of large switches.
 Switch zone controls input of electrical energy into National Grid Network.
 This is done through opening or closing of switches which will activate or deactivate transmission of
electrical energy leaving power station.
The National Grid Network
 National Grid Network 国家电网 – System of high voltage circuits that interconnects main power
stations in Peninsular Malaysia.
 Enables electrical energy to be distributed systematically, effectively and continuously to all
consumers.
 Excess electrical energy from one area is channeled to areas that require more electrical energy.
 This guarantees an uninterrupted supply of electrical energy throughout the country.

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8.4 Electrical Power Supply and Wiring System in Homes
 Voltage of electrical power supplied to houses is 240V. The current produced is enough to electrocute a
person.
 Electric current supplied through mains supply cables from substation is a.c.
Parts of the electrical wiring system
 Electrical wiring in house is connected parallel.
 Three types of wires:
- Live wire (L)
- Neutral wire (N)
- Earth wire (E)
 A.c. at voltage of 240V flows through L wire from branch substation into house.
 Current flows along N wire back to branch substations
 E wire is connected to copper plates 铜板 in ground.
 A.c. is supplied to houses:
- Its voltage can be changed using transformers
 Main fuse: Controls amount of current supplied to consumers. Break the circuit if current exceeds
safe amount.
 Electricity meter: Measures the total amount of electrical energy used in house.
 Main switch: Disconnect electrical power from main supply when electrical energy is not needed.
 Circuit breakers: Prevent electrical shocks and fires as a result of short circuits and leakages.
National Grid Network

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3-pin plugs
 Electrical appliances in houses are connected to power circuit sockets through 3-pin plugs.
 3-pin plug has a fuse that is connected to live wire.
 Each wire has a certain colour which is based on international colour code.
 L wire
- Brown
- Electric current from mains is carried through live wire to appliance
 N wire
- Blue
- Electric current flows through neutral wire from appliance to mains
 E wire
- Yellow with green stripes
- When there is leakage from appliance, large current leaking from it flows through earth wire to
ground.
8.5 The Cost of Electrical Energy Usage
Electrical energy and electrical power
 Joule meter (Electricity meter) measures total amount of electrical energy used
 Electrical power – Rate of converting electrical energy into other forms of energy.
 SI unit: watt (W) / joule per second (j s-1
)
Power, voltage and current rating of electrical appliances
 Relationship between power, voltage and electric current of an appliance is:
- Power (P) = Voltage (V) × Current (I)
P (W) = V (V) × I (A)
 Electric current that flows through electrical appliance in operation can be calculated.
𝑪𝒖𝒓𝒓𝒆𝒏𝒕 (𝑰) =
𝑷𝒐𝒘𝒆𝒓 (𝑷)
𝑽𝒐𝒍𝒕𝒂𝒈𝒆 (𝑽)
Structure of 3-pin plug

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Calculating the cost of electrical energy usage
 Electrical energy used depends on;
- Power and voltage ratings of the appliances
- Length of time it is used.
 Unit: kilowatt-hour (kWh)
8.6 Fuse and Earth Wire
Fuses
 Fuse 保险丝 – Safety device which consists of a fine wire with a low melting point.
 Made of tin 锡 and lead alloy 铅合金
 Types: Cartridge fuses, wire fuses
 Wire in fuse is replaceable after it is blown.
 Wire in cartridge fuse cannot be replaced after it is blown.
Fuses in electrical wiring systems
 Fuse with a specific rating controls the electric current that flows through a circuit.
 Short circuit 短路 – Current flows through a shorter path of lower resistance.
 During short circuit, current in circuit increases and a lot of heat is generated. Fuse wire melts and
breaks circuit.
 If there is no fuse, electrical appliances may be damaged and fires may break out.
Formula triangle of power, voltage and current
Cartridge fuse

8. 8
Earth wires
 An earth wire which has a low resistance connects appliances to ground.
 Earth wire is very important safety feature used in electrical appliance.
- Should a fault or electricity leakage occur, most of the current will flow through wire to ground
instead of passing through a person.
- Large current blows fuse in the circuit. This cuts off the electricity supply and the person is
prevented from getting electric shock.
8.7 Safety Precautions in the Use of Electrical Energy
Source of electrical accidents
 Turning switch on or off with wet hands.
 Connecting too many appliances to one power source.
 Touching damaged wire when current is flowing.
What to do in the event of an electric shock
 Switch off the mains immediately
 Remove electrical source from victim
- Remove electrical source from victim by using insulator if can’t locate the mains.
 Resuscitate victim
- If victim is not breathing, carry out cardiopulmonary resuscitation
 Call an ambulance
- Cover victim with blanket, call ambulance or doctor or send victim to hospital immediately for
treatment.
Safety precautions and safety features in homes
 Do not touch switches with wet hands.
 Use earthed appliances whenever possible.
 Do not connect too many appliances to one socket.
 Switch off appliance when it is not being used.
 Use appliances that have been tested and approved by SIRIM
8.8 The Importance of Conserving Electrical Energy
Activities that lead to electricity wastage
 Opening the door of refrigerator too often
 Turning on lights and fans while not in use
 Using washing machine to was small load

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Energy efficiency
 Use energy-efficient machines in industrial process.
 Do not use electrical appliances with high power requirements for long period.
 Do not use an air conditioner to cool a room when a fan can do the same job
The need to conserve energy
 Saving money
- Save money by using fuel-efficient vehicles
 Protecting environment
- Global warming is mainly caused by burning of fossil fuels.
- Coal-fired power stations contribute to acid rain and mercury pollution
- Radioactive waste, a by-product of all nuclear power plants, is very dangerous and its disposal is a
difficult problem
 Protecting human health
- Air pollution leads to throat irritation, cough, bronchitis, and breathing problems.
- Global warming allows insects carry infectious diseases to spread to new areas.