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Mastering Science Answers to Student’s Book 2A Unit 8 Unit 8 Making use of electricity Laboratory Activity 8.1 (p.81) 1 a Yes. b Yes. c Yes. 2 a No. b Yes. c No. 3 a b c Yes. No. No. A complete path and a source of electrical energy (e.g. a cell) are necessary for current to flow. Laboratory Activity 8.2 (p.84) 2 3 Material Does it conduct current? Cotton thread Rubber band © Oxford University Press 2011 Aluminium Wooden strip chopstick ~1~ Iron nail Copper wire Mastering Science Answers to Student’s Book 2A Unit 8 Section quiz 1 (p.87) 1 T 2 T 3 T 4 F Laboratory Activity 8.3 (p.92) 3 Number of cells Reading of the ammeter (A) 1 2 3 (No absolute answers, but the current should increase when more cells are used.) 4 The more cells in the circuit, the brighter the bulb. The more cells in a circuit, the larger (larger/smaller) the current in the circuit and the brighter (brighter/dimmer) the bulb. Laboratory Activity 8.4 (p.95) Cell Voltage (V) Zinc carbon D-cell 1.5 (1.3–1.5) Alkaline D-cell 1.5 (1.3–1.5) Button cell 3.0 (2.7–3.0) Rechargeable cell 1.3 (1.2–1.4) Connected cells Voltage (V) Two D-cells connected in the same direction (the positive terminal of one cell is connected to the negative terminal of the other cell) 3.0 Two D-cells connected in opposite directions (the positive terminal of one cell is connected to the positive terminal of the other cell) 0.0 Three D-cells connected in the same direction 4.5 © Oxford University Press 2011 ~2~ Mastering Science Answers to Student’s Book 2A Unit 8 When more cells are connected in the same direction, the voltage across the cells will be higher (higher/lower). Laboratory Activity 8.5 (p.96) 3 Reading of the voltmeter (V) Reading of the ammeter (A) (Depend on the experimental results) The higher the voltage, the larger (larger/smaller) the current flowing through the circuit. Laboratory Activity 8.6 (p.99) 3 Resistance of the resistor (Ω) Size of the current (mA) (Depend on the experimental results) For a resistor connected to a battery (a power supply with a fixed voltage), the higher the resistance of the resistor, the smaller (larger/smaller) the current flowing through the resistor. Laboratory Activity 8.7 (p.100) 2 (Depends on the sample given) 3 (Depends on the sample given) Discussion 1 The circuit with the copper wire. 2 Nichrome wire. 3 To make the experiment a fair test. © Oxford University Press 2011 ~3~ Mastering Science Answers to Student’s Book 2A Unit 8 Wires made of different materials have different resistance values. A copper wire has a lower (higher/lower) resistance than a nichrome wire of the same length and thickness. Laboratory Activity 8.8 (p.101) I Length of a wire Identifying variables b length of wire c Number of cells Thickness of wire Material of wire Length of wire Carrying out the experiment and recording the results 2 Length of the nichrome wire (cm) Reading of the ammeter (A) 20 15 10 (Depend on the experimental results) Drawing a conclusion The longer the wire, the smaller the current. In other words, a longer wire has a higher resistance. II Thickness of a wire Identifying variables a the size of current flowing through the circuit b thickness of wire c number of cells, material of wire, length of wire, etc. © Oxford University Press 2011 ~4~ Mastering Science Answers to Student’s Book 2A Unit 8 Carrying out the experiment and recording the results 3 4 1 Connect the circuit as shown. Keep the two clips clipped onto a thin nichrome wire 10 cm apart. 2 3 Close the switch and record the reading of the ammeter. Replace the wire with a thicker nichrome wire. Repeat steps 1 and 2. Results 5 Thickness of the nichrome wire Reading of the ammeter (A) Thin Thick (Depend on the experimental results) Drawing a conclusion The thinner the wire, the smaller the current. In other words, a thinner wire has a higher resistance. Laboratory Activity 8.9 (p.105) 1 (Depends on the experimental result) 2 a The brightness increases. b Increase. c decreases, decreases, increases, increases © Oxford University Press 2011 ~5~ Mastering Science Answers to Student’s Book 2A Unit 8 Section quiz 2 (p.107) 1 a volt b V c ohm d Ω e ampere f A 2 a C b B c C Classroom Activity 8.1 (p.111) Circuit diagram A Circuit diagram B Section quiz 3 (p.113) 1 a bulb b ammeter c voltmeter d resistor e switch f rheostat © Oxford University Press 2011 ~6~ Mastering Science Answers to Student’s Book 2A 2 Unit 8 Circuit diagram a b Laboratory Activity 8.11 (p.115) 1 4 5 a (Depend on the experimental results) b A1 = A2 = A3 It does not light up. A1 = 0; A2 = 0; A3 = 0 (p.116) In a series circuit, the current is the same (the same/different) at different points in the circuit. If a series circuit is broken at any point, current will not (not/still) flow in the circuit. © Oxford University Press 2011 ~7~ Mastering Science Answers to Student’s Book 2A Unit 8 Laboratory Activity 8.12 (p.116) 1 A1 represents the current that passes through branch PQ (the battery/branch PQ/branch RS). A2 represents the current that passes through branch RS. A3 represents the current that passes through the battery. 4 5 a b c a b (Depend on the experimental results) A2 A1 + A2 = A3 (Depend on the experimental results) The other bulb in branch PQ does not light up while bulb Z is unaffected. Section quiz 4 (p.118) 1 Bulbs D and E. 2 3 They have the same brightness. Bulbs D and E have the same brightness. Bulb C is brighter than bulbs D and E. Laboratory Activity 8.13 (p.119) 2 (Depends on the experimental result) 3 4 a The reading increases. b It melts. The wire glows/becomes red hot. Discussion Electrical energy → heat energy (p.120) When a current passes through a conductor, the temperature of the conductor increases (increases/decreases). Some electrical energy is changed to heat energy. © Oxford University Press 2011 ~8~ Mastering Science Answers to Student’s Book 2A Unit 8 Laboratory Activity 8.14 (p.122) 5 a The metal wire in the fuse melts. b When the metal wire in the fuse melts, the circuit becomes open and the current stops flowing. No current flows through the bulb and the bulb goes out. The bulb is not blown. 6 Number printed on the fuse (A) Fuse A Fuse B Fuse C 0.05 0.1 0.2 Largest current recorded (A) (Depend on the experimental results) (p.123) If the current passing through the fuse is greater (greater/smaller) than the fuse rating, the fuse will blow and the current will be cut off to prevent the circuit from overheating. Section quiz 5 (p.124) 1 heat 2 heating Laboratory Activity 8.15 (p.127) 2 a earth wire b neutral wire c fuse d live wire 3 Earth wire: green and yellow Live wire: brown Neutral wire: blue 5 7 8 Live wire. Safety shutters. a They open. b The shutters prevent people from putting fingers into the socket holes and getting an electric shock. © Oxford University Press 2011 ~9~ Mastering Science Answers to Student’s Book 2A Unit 8 (p.129) The pins of a plug are connected to wires of different colours: Wire Colour Earth Green and yellow Live Brown Neutral Blue (p.130) The earth wire connects the metal case of an appliance to the earth. It protects us from an electric shock by earthing. Laboratory Activity 8.16 (p.132) 3 One bulb (one ‘branch’) Two bulbs (two branches) Three bulbs (three branches) Reading of the ammeter (A) (Depend on the experimental results) When the number of branches increases, the current flowing through the cell increases (increases/decreases). Laboratory Activity 8.17 (p.134) 2 a Yes. b (Depends on the experimental result) 4 a No. b c (Depends on the experimental result) The nichrome wire heats up and glows. The wire connected in parallel with the bulb provides a path with a very low (nearly zero) resistance. This results in a large current flowing through the wire. The large current causes the nichrome wire to heat up and glow. © Oxford University Press 2011 ~ 10 ~ Mastering Science Answers to Student’s Book 2A Unit 8 Classroom Activity 8.2 (p.136) 1 A large current flowing through the circuit causes the circuit to overheat and may cause a fire. 2 A fuse should be installed in the plug. If a large current flows through the circuit due to short circuit, the fuse blows and cuts the circuit. 3 No. The three-pin plugs contain the earth wire, which can protect users from getting an electric shock. Classroom Activity 8.3 (p.138) (2) Mandy may damage the plug by pulling the cable to remove the plug from a socket. (3) Tommy’s wet hand may conduct current and he may get an electric shock. (4) The socket is overloaded. The circuit may overheat and this may cause a fire. Section quiz 6 (p.140) 1 a live b green and yellow c Y 2 d a b c earth, electric shock Correct. The connections between the wires and the plug are loose. Wrong connection: the earth wire and the neutral wire are interchanged. Classroom Activity 8.4 (p.142) Electrical appliance Rice cooker Hair dryer Electric fan Electric iron Microwave oven Power rating (Answers vary with Ss.) Which of the above electrical appliances consumes the most energy in the same period of time? (Answers vary with Ss.) © Oxford University Press 2011 ~ 11 ~ Mastering Science Answers to Student’s Book 2A Unit 8 Classroom Activity 8.5 (p.144) 1 2 The Hongkong Electric Co., Ltd.: $412 = $0.967 426 CLP Power Hong Kong Limited: $1082 $1082 = = $0.910 1189 (400 600 189) Average amount of electrical energy used every day = 426 = 13.3125 units 32 Section quiz 7 (p.145) a Energy consumed = 2200 W × 8 h = 2.2 kW × 8 h = 17.6 kW h b Energy consumed = 120 W × 30 min = 0.12 kW × 0.5 h = 0.06 kW h c Total energy consumed by the air conditioner in July = 17.6 × 31 = 545.6 kW h Total cost for the air conditioner in July = $0.9 × 545.6 = $491.04 (2200 W) (power) d Current = = = 10 A 220 V voltage Laboratory Activity 8.18 (p.146) 2 They are attracted by the magnet. 4 5 They are attracted by the coil. They are not attracted by the coil. When a current passes through a coil of wire wound around an iron nail, the coil and the nail work like a magnet. Laboratory Activity 8.19 (p.148) 2 It moves. © Oxford University Press 2011 ~ 12 ~ Mastering Science Answers to Student’s Book 2A Unit 8 Revision exercise A Multiple-choice questions (p.158) 1 C 2 B 3 A 4 D 5 C 6 C 7 C B 1 2 3 True or false questions (p.159) T F F 4 5 6 F F T C a Question (p.159) Bulbs X and Y. b c d e Series circuit. Bulbs X and Y. Parallel circuit. Switches A, B and C. © Oxford University Press 2011 ~ 13 ~