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