1. Basic Definitions Power 1 Circuit 2
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Basic Definitions
Power : the rate at which energy is transferred from one form to another
By considering the Ohm’s law, electrical power is

2. Domestic circuit Power 2 Circuit 2
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Domestic circuit
Label on domestic electrical equipment :
(e.g. “12V 24W)
Example
Calculation
Electrical power rating for common electrical appliance.
Table

3. Domestic circuit Domestic circuit 1 Circuit 2
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Domestic circuit
Direct current (d.c.) and Alternating current (a.c.)
Frequency (50 Hz) and e.m.f. (220 V) of a.c. used in Hong Kong
Electrical cable
Live wire, Neutral wire and Earth wire
Photo
Wiring inside an iron
Diagram

4. Domestic circuit Domestic circuit 2 Circuit 2
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Domestic circuit
Household electric wiring plan
Photo
Ring circuit
Diagram
Lamp on a staircase
Diagram
Safety Rules

5. END of Circuit 2

6. Power 2 Circuit 2 Next Slide A lamp has a label “200 V 100 W”
(a) What is the resistance of the lamp?
(b) What is the current passing through the lamp under normal operating p.d.?
(c) What is the electrical power of the lamp if a p.d. 100 V is applied across the lamp?

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Power 2
Circuit 2
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A table of electrical power rating for common electric appliance :
Appliance
Power rating (W)
Fan
Iron
Kettle
Television
Washing machine 500
Water heater

9. Domestic circuit 1 Circuit 2 Next Slide
Three-hole socket and three-pin plug are shown in the following photos.

10. Domestic circuit 1
Circuit 2
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Live wire (brown) : its potential is varying between +ve and -ve as a.c. Hence, the current is made to flow backwards and forwards through the circuit.
Neutral wire (blue) : completes the circuit by providing path for the current to flow. It is earthed and its potential is 0.
Earth wire (green & yellow) : connects the metal body of the electrical appliance to the earth to act as a safety device.

12. Domestic circuit 1 Circuit 2 Next Slide
In the following figure, an iron is used as an example to show the wiring inside an electrical appliance.
fuse
switch
200 V a.c. (50 Hz)
neutral
earth
live

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Switch should be fitted in the live wire, not the neutral wire.
Fuse : a thin wire which melts and opens the circuit if the current exceeds a specified value.
The function of earth wire is shown in the following diagram
earth
with earth wire
without earth wire
current

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A household electric wiring plan is shown below.

15. Domestic circuit 2 Circuit 2 Next Slide
Ring circuit : Each of the live, neutral and earth wire of the cable forms a ‘ring’ and the mains sockets are connected to them.
An advantage of the ring main is that the current to each socket flows through 2 paths and the danger of over heating in the wires could be avoided even thinner wires are used.
All the sockets are connected to this ring main circuit in parallel pattern. If one of the sockets is damaged, other sockets would not be affected.

16. Back to Domestic circuit 2 Circuit 2 Click Back to live wire
neutral wire

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Lamp on a staircase : Two-way switches are connected as shown in the following figure. To complete the circuit the switches must either be both up or both down. If they are at opposite positions, the lamp is switched off. Therefore, the lamp could be controlled by both switches.
2-way switch