household electricity
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E5: Household Electricity
Textbook Page 76
Objectives
At the end of this lesson, you will be able to: Understand what is power stations and energy
sources State its electrical power and its units Differentiate between power of electrical
appliances and power from power stations
Look at the picture…
What is this? Where is it located? What it does?
How does Singapore get its electricity? Power stations Generate electricity from oil and natural gas Transmit to homes and industries all over the
country
What is POWER?
120 V 7 W W = watts
What is POWER?
The larger the watts the brighter the bulb 100 W is brighter than 60 W bulb. 100 W has more power The power of an electrical appliance is the
amount of electrical energy it converts to other forms of energy in one second
POWER
Power is measured in watts (W) Larger unit = kilowatt (kW) 1 kW = 1000 W
Power of electrical appliances Different electrical appliances have different
power ratings Do Inquiry time - pg79
Power from power stations
Power stations need to generate a lot of power
Measured in megawatts (MW)
How do power companies know how much electricity we consume? Electricity meters Unit for electricity meters = kilowatt-hour
(kWh) Energy consumed (in kWh) = Power (in kW)
X Time (in hours) Cost of electrical energy = units of electrical
energy used (kWh) x Cost per unit
Calculation Exercises
An electric kettle has a power rating of 2400 W. It has been used to keep water warm for 2 hrs. Calculate the cost of the electricity used if each unit costs is 17 cents.
Calculation Exercises - Answer Power of appliance = 2400 W = 2.4 kW Time in use = 2 h Energy consumed = 2.4 x 2 = 4.8 kWh Cost of electricity = 4.8 x 0.17 = $0.82
Calculation Exercises
A fan has a power rating of 3600 W. It has been used to keep room cool for 720 mins. Calculate the cost of the electricity used if each unit costs is 20 cents.
Calculation Exercises - Answers Power of appliance = 3600 W = 3.6 kW Time in use = 720 mins = 12 h Energy consumed = 3.6 x 12 = 43.2 kWh Cost of electricity = 43.2 x 0.20 = $8.64
Do inquiry time and checkpoint
Textbook Page 81
Objectives
At the end of this lesson, you will be able to: State examples when electricity can be
dangerous State the safety features in circuits and appliances State the safety precautions when using electricity
Electricity can be dangerous
Main dangers: Electric shock Electrical fire
Electricity can be dangerous
Frayed and damaged wires If the person touches a bare wire, current
flows through the body Electric shock
Electricity can be dangerous
Overloading Adapters are used to connect more than one
appliance to a main socket Large current causes the wire to get hot When the current is too large, the circuit is
said to be overloaded
Electricity can be dangerous
Short circuits Broken or bare wire in a circuit may touch
another wire in the circuit causes a short circuit
Cause fire in the wires or in the appliance
Electricity can be dangerous
Wet Conditions If a person touches a damaged electrical wire
or appliance with wet hands, an electric current may flow through his body – electric shock
Safety features in circuits and appliances Fuses
It is placed in the circuit It is a short length wire that melts and breaks
when the current in the circuit is too large Thus causes circuit to open And no current flow Found in electrical appliances,
electric plugs and fuse boxes
Safety features in circuits and appliances Circuit Breakers
Many buildings use circuit breakers Special switch If current in a circuit becomes too large, the switch
will turn off by itself
Safety features in circuits and appliances Main Switch
Switches on and off the main electric supply to the whole building
If there is a fire, switch off the main switch
Safety features in circuits and appliances Earth Wire
Electricity flows through the live wire but not the earth wire
Earth wire is for safety Earth wire connected to metal casing, most
current flows to the ground through the earth wire instead
So you will not get electric shock
Safety precautions when using electricity Never touch bare or broken wires Never touch appliances/switches with wet hands Never overload a circuit Never use electric appliances in wet places Do not push anything into sockets Never put nails into walls near switches, sockets
and wires Do not use electrical appliances with old or frayed
wires
Objectives
At the end of this lesson, you will be able to: State how we can save electricity
What can we do to save electricity? Switch off lights, electrical appliances when
not in used Use energy efficient lamps Set thermostat of air conditioner at a higher
temperature
Do checkpoint
Page 87
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