1. ASSIGNMENT BOOK Unit 8 Part 3 Part 3 Take Up the Challenge 1. Corridor lighting can be controlled by 2 two-way switches ( 雙切開關 ), i.e., you can switch on the lighting at one end of the corridor and switch it off at the other end. Circuit symbol of a two-way switch

2. ASSIGNMENT BOOK Unit 8 Part 3 You are provided with the following materials: 1 small bulb2 two-way switches 1 dry cellsome connecting wires Use the materials provided to design a simple corridor lighting system as described above. Draw your circuit diagram below. (2 marks)

3. ASSIGNMENT BOOK Unit 8 Part 3 2. The picture below shows the light bulbs for candle lamps. These bulbs are available at the supermarkets. filament support filament cap

4. ASSIGNMENT BOOK Unit 8 Part 3 Study the picture and answer the following questions. 220-230 V, 40 W Operating current = power / voltage = 40 / 220 = 0.18 A (a) What is the power rating of the light bulbs? (2 marks) __________________________________ E (b) What is the operating current of the light bulbs? (2 marks) __________________________________ __________________________________ E

5. ASSIGNMENT BOOK Unit 8 Part 3 (c) The filament is made of tungsten. Why is the filament not made of iron, gold or silver? (Hint: The melting points of iron, gold, silver and tungsten are 1500 o C, 1070 o C, 960 o C and 3700 o C respectively.) (2 marks) ____________________________________ ____________________________________ ____________________________________ The filament is made of tungsten so that a higher filament temperature can be reached and the bulb will be brighter.

6. ASSIGNMENT BOOK Unit 8 Part 3 (d) There is a fuse inside the cap of the bulb. When there is an excessively large current flowing through the bulb, both the fuse and the filament will melt. What is the purpose of installing a fuse inside the bulb? (2 marks) _____________________________________ _____________________________________ _____________________________________ To make sure that the filament supports do not carry a voltage after the bulb filament melts.

7. ASSIGNMENT BOOK Unit 8 Part 3 3. Study the circuit below: electric iron 4 A hair dryer 3 A A B cassette recorder 0.2 A 220 V live wire neutral wire X

8. ASSIGNMENT BOOK Unit 8 Part 3 (a) The above diagram shows three appliances connected to a ring mains circuit. Their operating currents are as shown. What are the currents in the main cable and the ring mains circuit? (2 marks) ____________________________________ ____________________________________ Main cable: 4 + 3 + 0.2 = 7.2 A (b) Suppose the live wire is accidentally broken at position X, will the appliances still work? (1 mark) ____________________________________ Yes Ring mains circuit: 7.2 A / 2 = 3.6 A

9. ASSIGNMENT BOOK Unit 8 Part 3 (c) What would happen if there was a short circuit between points A and B? (1 mark) Only the hair dryer will not work. No large current results in the main cable. The hair dryer will not work. A large current results in the main cable. All the appliances will not work. A dangerously large current results in the main cable. 

10. ASSIGNMENT BOOK Unit 8 Part 3 (i) The power rating of the electric iron = 4 X 220 = _____________ (ii) The power rating of the hair dryer = _____________ = _____________ (iii) The power rating of the cassette recorder = _____________ = _____________ 880 W 3 X 220660 W 0.2 X 22044 W (d) Calculate the following. E

11. ASSIGNMENT BOOK Unit 8 Part 3 (880 + 660 + 44) x 3 / 1,000 = 4.752 kW h (e) If all the appliances are switched on for 3 hours, how many kilowatt-hours of electricity will be consumed? ( 2 marks) ____________________________________ E

12. ASSIGNMENT BOOK Unit 8 Part 3 Science Focus 4. Read the following article and answer the questions. Efficiency of light bulbs Light bulbs can change electrical energy into light and heat energy. The more electrical energy is changed into light energy, the higher the efficiency of the light bulb. The efficiency of light bulbs is measured in ‘lumens ( 流明 ) per watt’ Lumen (lm) is the measure of the brightness produced. Therefore, the efficiency of the light bulb is higher if its ‘lumens per watt’ is higher.

13. ASSIGNMENT BOOK Unit 8 Part 3 Shown below is a table which compares the efficiencies of some of the bulbs commonly found in our daily life. Single filament bulb ( 白熾燈泡 ) Tungsten- halogen bulb ( 鹵鎢燈 ) Fluorescent bulb ( 螢光燈泡 ) Sodium bulb ( 鈉燈 ) Type of bulb Average efficiency (lumens / watt) Average lifetime (hours) 9 1,000 25 2,0007,50020,000 80120

14. ASSIGNMENT BOOK Unit 8 Part 3 As you can see from the table, fluorescent bulbs are far more efficient and long lasting than filament bulbs. Fluorescent bulbs can also be made to give out different colours of light by the addition of different chemicals to the coating of the bulb. However, the filament bulb is cheap and easy to replace. Although its efficiency is not high, it can provide enough light for a small room. These explain why the filament bulbs are very popular in Hong Kong. On the other hand, although the efficiency of tungsten-halogen bulbs is not very high, they are also quite popular because of their small sizes.

15. ASSIGNMENT BOOK Unit 8 Part 3 (a) Why is the efficiency of a filament bulb not high? (1 mark) _____________________________________ _____________________________________ It is because it also changes part of the electrical energy supplied to it into heat. (b) What are the advantages of using fluorescent bulbs in lighting? (2 marks) _____________________________________ _____________________________________ _____________________________________ They are more efficient and long lasting. Also it can be made to give out different colours of light.

16. ASSIGNMENT BOOK Unit 8 Part 3 (c) What is the major advantages of using filament bulbs in lighting? (2 marks) ___________________________________ It is cheap and easy to replace. (d) What is the advantage of tungsten-halogen bulbs over the others? (1 mark) ___________________________________ Their sizes are small.

17. ASSIGNMENT BOOK Unit 8 Part 3 (e) In Hong Kong, some streets are illuminated by yellow sodium lamps. Streets in Hong Kong are illuminated by yellow sodium lamps.

18. ASSIGNMENT BOOK Unit 8 Part 3 Suggest TWO reasons to explain why the government uses sodium bulbs? (2 marks) ______________________________________ ______________________________________ ______________________________________ ______________________________________ Its efficiency is very high. Therefore it can save electricity. Also its lifetime is extremely long compared with the others.