Household Electricity

1. E5: Household Electricity
Textbook Page 76

2. 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

3. Look at the picture…
What is this?
Where is it located?
What it does?

4. How does Singapore get its electricity?
Power stations
Generate electricity from oil and natural gas
Transmit to homes and industries all over the
country

5. What is POWER?
120 V
7 W
W = watts

6. 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

7. POWER
Power is measured in watts (W)
Larger unit = kilowatt (kW)
1 kW = 1000 W

8. Power of electrical appliances
Different electrical appliances have different
power ratings
Do Inquiry time - pg79

9. Power from power stations
Power stations need to generate a lot of
power
Measured in megawatts (MW)

10. 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

11. 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.

12. 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

13. 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.

14. 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

15. Do inquiry time and
checkpoint
Textbook Page 81

16. 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

17. Electricity can be dangerous
Main dangers:
Electric shock
Electrical fire

18. Electricity can be dangerous
Frayed and damaged wires
If the person touches a bare wire, current
flows through the body
Electric shock

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. Objectives
At the end of this lesson, you will be able to:
State how we can save electricity

28. 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

29. Do checkpoint
Page 87