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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


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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
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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.
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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.
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~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
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~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
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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.
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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.
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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.
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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.
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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.)
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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.
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~ 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.
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