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

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

4. Circuit 2Domestic circuit 1 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 Next Slide Electrical cable Live wire, Neutral wire and Earth wire Wiring inside an iron Diagram Photo

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

6. END of Circuit 2

7. Circuit 2 Next Slide Power 2 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?

8. Circuit 2 Click Back to Power 2 Back to

9. Circuit 2 Click Back to Power 2 A table of electrical power rating for common electric appliance : Back to AppliancePower rating (W) Fan 150 Iron 1000 Kettle 2000 Television 100 Washing machine 500 Water heater 3000

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

11. Circuit 2 Next Slide Domestic circuit 1

12. Circuit 2 Click Back to Domestic circuit 1 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. Back to 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.

13. Circuit 2 Next Slide Domestic circuit 1 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

14. Circuit 2 Click Back to Domestic circuit 1 Switch should be fitted in the live wire, not the neutral wire. Back to 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

15. Circuit 2 Click Back to Domestic circuit 2 A household electric wiring plan is shown below. Back to

16. Circuit 2 Next Slide Domestic circuit 2 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.

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

18. Circuit 2 Click Back to Domestic circuit 2 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. Back to 2-way switch