Chapter 8 WB Solutions

Interactive Science (Second Edition) Workbook Solutions
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INTERACTIVE SCIENCE (Second Edition) 2A
Workbook Solutions
Chapter 8 MAKING USE OF ELECTRICITY
Sectional Exercise
Section 8.1
A. Concept checking p.51
1. False
2. True
3. False
4. True
5. True
B. Multiple choice p.51
1. (a) electrical conductors
(b) electrical insulators
2. (a) closed
(b) open
C. Question p.52
1. (a) electrical insulator
(b) electrical conductor
(c) electrical insulator
(d) electrical conductor
(e) electrical insulator
(f) electrical insulator
(g) electrical insulator
(h) electrical conductor
(i) electrical conductor
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2.
3. (a) (i) will not
(ii) do not form
(b) (i) will
(ii) form a closed circuit
(c) (i) will
(ii) the bulb, the dry cells and the wires form a closed circuit
(d) (i) will not
(ii) electrical energy in the circuit
4. No. The wheels of the tram are connected with the power supply through the rails, and
the circuit is complete. (or other reasonable answers)
Sections 8.2 & 8.3
A. Multiple choice p.54
1. (a) ampere
(b) volt
2. free electrons
3. 1.5
4. (a) ammeter
(b) voltmeter
5. larger
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B. Question p.55
1. (a) dry cell
(b) wire
(c) free electrons
2. (a)
(or other reasonable answers)
(b) Connect one more dry cell to the circuit, such that the positive poles of the
dry cells point to the same direction. (or other reasonable answers)
(c)
(or other reasonable answers)
(d) The ammeter in (c) has a higher reading because the total voltage of two dry cells
is higher than that of one dry cell, and hence the current is larger. (or other
reasonable answers)
3. (a)
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(b)
(c) The reading of the voltmeter is 3.0 V.
Section 8.4
A. Multiple choice p.57
1. ohm
2. smaller
3. higher
4. higher
5. rheostat
B. Multiple choice p.57
1. A
2. C
3. D
4. C
C. Question p.59
1. (a)
(Electric current of the opposite direction is also correct.)
(b) In anti-clockwise direction
(c) (i) Connect terminals B and C to the circuit.
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(ii) In clockwise direction
(d) Dimmers on a lighting system / Volume controls on hi-fi systems (or other
reasonable answers)
2. Measure the resistance of the bulb. If the measured resistance is infinitely high, the
filament is broken.
Section 8.5
A. Multiple choice p.60
1. D
2. C
3. B
4. C
B. Question p.61
1. (a) dry cell
(b) battery
(c) bulb
(d) ammeter
(e) voltmeter
(f) resistor
(g) rheostat
(h) switch
2. (a)
(b)
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(c)
(d)
(e)
Section 8.6
A. Concept checking p.63
1. True
2. True
2. False
3. False
4. False
5. True
B. Multiple choice p.63
1. D
2. B
3. B
4. B
5. A
6. B
7. C
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C. Multiple choice p.66
1. (a) parallel
(b) can
2. (a) series
(b) cannot
D. Question p.66
1.
In series In parallel
a, c, f b, d, e
2. (a)
(or other reasonable answers)
(b) Voltmeter and resistor
(c) Move the sliding contact towards Q .
(Answer depends on how the wires are connected in (a).)
(d) Connect one more dry cell to the circuit. / Replace the resistor with another one of
lower resistance. (or other reasonable answers)
Section 8.7
A. Concept checking p.68
1. True
2. True
3. False
4. False
5. True
6. False
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B. Multiple choice p.68
1. B
2. C
3. B
C. Question p.69
1. (a) (i) fuse rating
(ii) thin wire
(iii) glass wiring
(iv) metal contact cap
(b) A fuse protects an electrical appliance from being damaged by an excessive
electric current.
(c) In series
2. (a)
(b) (i) fuse rating
(ii) current
(iii) melt
(c) Yes, because a fuse of a higher melting point can allow a larger current to pass
through it.
Section 8.8
A. Multiple choice p.70
1. B
2. C
3. B
4. D
5. A
6. A
7. A
B. Fill in the Blanks p.73
1. 220 V
2. (a) earth
(b) neutral / live
(c) live / neutral
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3. (a) earth
(b) earth / ground
(c) leakage
4. overloaded
C. Question p.73
1. (a) live
(b) brown
(c) neutral
(d) blue
(e) earth
(f) yellow and green
(g) fuse
2. (a) A short circuit happens.
(b) After the switch is closed, a path with a very low resistance is formed. The electric
current flowing through the fuse becomes very large. The fuses melts and breaks
the circuit.
Section 8.9
A. Concept checking p.74
1. True
2. False
3. True
4. False
5. True
6. False
7. False
8. True
9. True
B. Question p.74
1. (a) (i) power
(ii) voltage
(b) Electric current flowing through bulb A = 40 W / 220 V = 0.18 A
Electric current flowing through bulb B = 60 W / 220 V = 0.27 A
2. Power of the rice cooker = 5 A × 220 V = 1100 W
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3. (a) Electric current = 500 W / 220 V = 2.27 A
(b) Electric current = 1000 W / 220 V = 4.55 A
(c) Resistor B has a lower resistance because it allows a larger current to flow through
it.
4. (a) Electric current = 800 W / 220 V = 3.64 A
(b) 0.8 × 6 = 4.8 kW h
(c) 4.8 kW h × 0.9 × 30 = $129.6
5. (a) 800 × 0.06 × 16 = 768 kW h
(b) 800 × 0.005 × 16 = 64 kW h
(c) (768 – 64) × $0.9 = $633.6
(d) For a filament bulb, more energy is lost as heat to the surroundings. (or other
reasonable answers)
6. (a) 10923 – 10121 = 802
(b) Cost per unit = $705.76 / 802 = $0.88
(c) Amount saved = 3 × 1.2 kW × 120 h × $ 0.88 = $380.16
Section 8.10
A. Concept checking p.77
1. False
2. False
3. False
4. False
5. True
B. Question p.78
1.
Electrical appliances working on the
magnetic effect of an electric current
Electrical appliances working on the
heating effect of an electric current
a, b, c, d, f, g a, e, h, i
2. (a) When an electric current flows through the coil, a magnetic effect is produced. The
metal coil behaves like a magnet to attract the iron parts of the waste.
(b) No, because plastic cannot be attracted by the current-carrying metal coil /
electromagnet.
(c) The crane can be used to attract iron and steel objects and separate them from
household rubbish. (or other reasonable answers)
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Integrated Exercise
A. Concept checking p.79
1. False
2. True
3. False
4. False
5. True
6. True
7. False
8. False
9. True
10. True
11. False
12. True
B. Multiple choice p.80
1. B
2. D
3. B
4. C
5. B
6. D
7. D
8. C
9. B
10. C
11. B
12. C
13. C
14. C
15. B
C. Question p.84
1. (a) Aluminium is a conductor.
(b) It is an indicator of the presence of an electric current in the circuit. /
If the bulb lights up, the sample tested is a conductor. (any one)
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(c) (i) The bulb lights up: Iron paper clip, gold coin, pencil lead
The bulb does not light up: Glass, plastic bag, cotton ball
(ii) Iron paper clip, gold coin and pencil lead are conductors, while the others are
insulators.
(d) Connect an ammeter in series with the light bulb. The best conductor will give the
largest reading of the ammeter.
2. (a) (i)
(ii) Rheostat
(b)
(c) The reading of the ammeter decreases.
3. (a) Wire P : earth wire
Wire Q : neutral wire
Wire R : live wire
(b)
Wire of the
cooker
Wire of the
plug
P B
Q C
R A
(c) The yellow and green wire is the earth wire. It can protect people from getting an
electric shock. If there is a leakage of electric current to the metal case, and the earth
wire is not connected, a person who touches the metal case will get an electric shock.
(d) When the cooker is switched on, the electric current flowing through the cooker is
larger than 3 A . The fuse melts and breaks the circuit.
4. (a) Money saved = (0.04 – 0.003) kW × 500 h × $0.9 = $16.65
(b) Amount of electrical energy consumed = 1.5 kW × 0.5 h = 0.75 kW h
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5. (a) In parallel
(b)
(c) Total current = 100 W / 220 V + 1500 W / 220 V = 0.45 A + 6.82 A = 7.27 A
(d) (i) Total current = 7.27 A + 1300 W / 220 V = 13.18 A
(ii) The total current (13.18 A) is larger than the fuse rating (13 A), so the fuse
melts and breaks the circuit.
(iii) No. If he connects five electrical appliances of low power to the extension
unit, and if the total current that flows through the mains socket is smaller
than 13 A, they can work properly at the same time.
6. (a) (i),(ii)
Electrical
appliance
Power (W) Working
current (A)
Suitable fuse
rating (A)
Lamp 100 0.43 1
Washing machine 1400 6.09 10
Air-conditioner 1500 6.52 10
Refrigerator 200 0.87 1
LCD TV 130 0.57 1
Computer 150 0.65 1
Electric water
heater
2700 11.74 13
Iron 1000 4.35 5
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(b) (i)
Electrical
appliance
Quantity Power (W) Monthly usage
(hours)
Units of
electricity used
Lamp 7 100 200 140
Washing
machine
1 1400 50 70
Air-conditioner 3 1500 100 450
Refrigerator 1 200 720 144
LCD TV 2 130 200 52
Computer 1 150 150 22.5
Electric water
heater
1 2700 30 81
Iron 1 1000 10 10
Total units of electricity used 969.5
(ii) Total cost = (400 × $0.86) + (400 × $0.90) + (169.5 × $1.00) = $873.5
(c) (i) Air-conditioner
(ii) Replace filament bulbs with compact fluorescent lamps. (or other reasonable
answers)
Quick Review
1. Electric current, voltage and resistance p.90
(a) open
(b) closed
(c) voltage
(d) larger
(e) resistance
(f) smaller
(g) current
(h) brighter
2. Ammeter, rheostat and fuse p.90
(a) rheostat
(b) resistance
(c) fuse
(d) fuse rating
(e) broken / open
(f) fuse
(g) (electric) current
(h) black
(i) positive
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3. Series and parallel circuits p.91
(a) parallel
(b) will not
(c) will not
(d) will still
(e) will not
(f) will still
4. Socket, plug and earthing p.91
(a) earth
(b) neutral
(c) live
(d) earth
(e) neutral
(f) live
(g) live / neutral
(h) neutral / live
(i) earth
(j) earth / ground
STSE Article Reading p.92
(a) In parallel
(b) Power = 5.4 V × 0.25 A = 1.35 W
(c) It uses renewable energy. / It is environmentally friendly.
It is portable. (or any two reasonable answers)
(d) Its battery is made of materials of low resistance.
(e) The voltage and the power supplied by the charger are too small.
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Little Einstein’s Corner p.94
1. (a)
Switch 1 Switch 2 Lamp (on/off)
open open off
open closed on
closed open on
closed closed on
(b)
Switch 1 Switch 2 Lamp (on/off)
A A on
A B off
B A off
B B on
(c) Dora’s design
(d) In Paul’s design, when one switch is closed, he cannot use the other switch to turn
off the lamp.
(e)
2. (a) Resistance = 200 V / 4 A = 500
(b) Current = 220 V / 500 = 0.44 A
(c) In (a), power = 200 V × 0.4 A = 80.0 W
In (b), power = 220 V × 0.44 A = 96.8 W
(d) The bulb applied with 220 V is brighter because its power is higher.
(e) Increasing the mains voltage to 220 V can increase the power of electrical
appliances.