Revision helper u8_bilingual

Part
1
Revision Helper 溫習錦囊 (Bilingual version) (雙語版本)

第部分 Revision notes 溫習筆記
Chapter
第 1
23 章 Basics of electricity
有關電的基礎概念
23.1 Making electricity flow 23.1 電怎樣才會在電路中
流通？

What are the conditions needed for 通電有哪些必要條件？
Q1
electricity to flow?

A1

 To make electricity flow, there must be a  要令電在電路中流通，電路中必須有電
source of electrical energy and the circuit 源，而電路必須是閉合電路。
must be a closed circuit.

A dry cell 乾電池
 It is a source of electrical  是電源。
energy.
 Its stored chemical energy is  當用導線把它與燈泡連接
converted into electrical 起來時，它所儲存的化學能
energy when it is connected 會轉換成電能。
to a light bulb.

A closed circuit 閉合電路
 It is a complete circuit  是完整、沒有間隙的電路，
without gap that allows 可以讓電通過。
electricity to flow through it.

Understanding Integrated Science for the 21st Century (3rd Edition) 最新綜合科學 — 邁進 21 世紀 (第三版) 1
© Aristo Educational Press Ltd 2012 © 雅集出版社有限公司 2012


Q2 What are switches? 甚麼是開關？

A2

 Switches are commonly used to control  日常生活中大部分電器都裝有開關，可
the flow of electricity through an electrical 用來使電路斷開或閉合。
appliance.
 When a switch is closed, the circuit is  當開關閉合時，電路會變得完整，電便
complete. Electricity can flow and 能夠在電路中流通，於是電器便能運作。
therefore the appliance is turned on.
 When a switch is open, the circuit  當開關斷開時，會形成斷路，令電不能
becomes open circuit. Electricity cannot 夠在電路中流通，於是電器便會停止運
flow and therefore the appliance is turned 作。
off.
 The following are some switches used in  以下是一些在日常生活中使用的開關：
daily life:
sliding switch
滑動開關

push button
switch
按鈕開關

 The following are some switches used in  以下是一些在學校實驗室中使用的開
the school laboratory: 關：



Q3 What materials allow electricity to 哪些物料能讓電通過？
flow through?
A3

 Materials that allow electricity to flow  有些物料能讓電通過它們稱為導電體。
，
through are called electrical conductors.
 Most metals such as copper, iron, steel  大部分金屬，如銅、鐵、鋼和鋁等都是
and aluminium are good electrical 良好的導電體。
conductors.
 The following are some good electrical  以下是一些良好的導電體：
conductors:

iron nail
鐵釘

copper wires
aluminium foil
銅線
鋁箔

 Some non-metals, such as graphite and  有些非金屬，如石墨和鹽水，它們的導
salt water can also conduct electricity. 電性能亦頗佳。

graphite
石墨

 Materials that do not allow electricity to  有些物料不能讓電通過，它們稱為絕緣
flow through are called electrical 體。
insulators.
 Rubber, plastic, glass, wood and air are  橡膠、塑膠、玻璃、木材和空氣都是絕
some examples of electrical insulators. 緣體的例子。
 The tools used by electricians have plastic  電工所用的工具通常具有塑膠或木的手
or wooden handles. This prevents them 柄。這樣可避免他們在工作時觸電。
from getting an electric shock when
working.



A3

 The following are some tools with plastic  以下是一些具有塑膠或木的手柄的工
or wooden handles: 具：

plastic handles wooden handle
塑膠手柄木手柄

23.2 Electric current 23.2 電流
Q4 What is electric current? 甚麼是電流？

A4

 The electric current in a metal wire is a  金屬線中的電流是由自由電子的流動
flow of free electrons. 所形成。
 Free electrons are the electrons that  自由電子是一些能夠擺脫原子核的束
escape from an atom and move freely 縛，在原子間自由移動的電子。
within the conductor.
 When the conductor is not connected to a  如果導電體沒有連接至電源(例如電
dry cell, the free electrons move in different 池)，自由電子會向不同方向移動。
directions.
atoms in the conductor (They are fixed in position) free electrons moving in
組成導電體的原子 (它們有固定的位置) different directions
自由電子向不同方向移動



A4

 When the conductor is connected to a dry  若把導電體連接至乾電池，同時又形成
cell and forms a closed circuit, the free 閉合電路，自由電子會由乾電池的負極
electrons move through the circuit from the 向乾電池的正極方向移動，形成電流。
negative terminal to the positive terminal
of a dry cell.
free electrons are driven towards
flow of free electrons
the positive pole of the dry cell
自由電子的流動
自由電子受驅動，從負極流向正極



Q5 How do we measure the size of 我們如何量度電流的大小？
electric current?

A5

 We can use an ammeter to measure the  我們可利用安培計來量度電流的大小。
size of electric current.

Ammeters used in the school laboratory
學校實驗室的安培計

 The unit of electric current is ampere (A).  電流的單位是安培 (A)。
 Smaller currents are measured in  較小的電流則以毫安培 (mA) 作為量
milliamperes (mA). 度單位。
1 1
 1 mA  A  1 mA  A
1000 1000
 An ammeter is connected within the path  安培計必須連接於所要量度的電流所流
of the current to be measured. 通的路徑中。

dry cell
乾電池

switch ammeter
開關安培計

light bulb
燈泡



23.3 Voltage 23.3 電壓
Q6 What is voltage? 甚麼是電壓？

A6

 Voltage is the ‘electrical push’ that makes  電壓是驅動自由電子在電路中流動的
free electrons flow in a circuit. 「驅動力」。
 The higher the voltage, the greater is the  電壓愈高，「驅動力」便會愈大。
‘electrical push’.

Flow of free electrons
自由電子的流動

‘electrical push’ dry cell
電池的驅動力乾電池

Q7 How do we measure voltage? 我們如何量度電壓？

A7

 We can use a voltmeter to measure  我們可利用伏特計來量度電壓。
voltage.

Voltmeters used in the school laboratory
學校實驗室的伏特計
 The unit of voltage is volt (V).  電壓的單位是伏特 (V)。



A7

 A voltmeter is connected across the part of  要量度電路中某元件兩端的電壓，必須
the circuit to be measured. 把伏特計以橫跨該元件兩端的形式連
接。
- +- +

dry cells
乾電池
switch
開關 light bulbs
燈泡

voltmeter
伏特計

Measuring voltage across a light bulb
量度一個燈泡兩端的電壓。

Q8 How does voltage affect the current 電壓如何影響電路中電流的大
in a circuit? 小？
A8

 The higher the voltage, the greater is the  電壓愈高，「驅動力」愈大。
‘electrical push’.
 Therefore a larger current flows through  因此電路中的電流亦愈大，而電路中的
the circuit and the bulb becomes brighter. 燈泡亦會愈光亮。

smaller flow of free electrons larger flow of free electrons
自由電子的流動較小自由電子的流動較大



23.4 Resistance 23.4 電阻

Q9 What is resistance? 甚麼是電阻？
0
A9

 The atoms in a conductor can oppose the  組成導電體的粒子會阻礙自由電子
flow of free electrons. 的流動。
 The opposition of a material to the flow of  物料對自由電子流動的阻力，稱為電
free electrons is called resistance. 阻。
 The unit of resistance is ohm (Ω).  電阻的單位是歐姆 (Ω)。

Q10 How does resistance affect the 電阻會否影響電路中電流的大
current in a circuit? 小？
A10

 When resistance increases, the current in  當電阻增加，電路中的電流會減少。
the circuit decreases.

Q11 What factors affect the resistance 有哪些因素會影響導線的電
of a wire? 阻？

A11

 The resistance of a wire depends on the  導線的電阻與導線的物料、粗幼和長度
material, the thickness and the length of 有關。
the wire.
 For wires made of the same material and  若比較相同物料和長度的導線，導線愈
of the same length, a thinner wire has 幼，電阻愈大。
higher resistance.
 For wires made of the same material and  若比較相同物料和粗幼的導線，導線愈
of the same thickness, a longer wire has 長，電阻愈大。
higher resistance.



How do scientists make use of 科學家如何利用電阻控制電路中
Q12 resistance to control the current in a 電流的大小？
circuit?
A12

 Resistors and rheostats are used to  電阻器和變阻器常用來控制或調節電
control or adjust the current flowing 路中電流的大小。
through a circuit.
 Resistors have fixed value of resistance.  電阻器的電阻值是固定的。

Resistors with different values of resistance Resistors inside a computer
不同電阻值的電阻器電腦內的電阻器

 The resistance of rheostats can be  變阻器的電阻值可在特定範圍內調節。
changed.
 The two types of rheostats commonly used  在學校實驗室中有兩種常用的變阻器，
in the school laboratory are sliding 它們是滑動型變阻器和旋鈕型變阻器。
rheostat and rotary-type rheostat.

Sliding rheostat Rotary-type rheostats
滑動型變阻器旋鈕型變阻器



A12

How rheostats work: 變阻器的運作原理：
 A rheostat has a coil of resistance wire and  變阻器有一組電阻線圈及一個滑動接
a sliding contact: 觸：
sliding contact
滑動接觸

resistance wire
電阻線圈

 By moving the sliding contact, the length of  當把滑動接觸移到不同位置時，電流流
the resistance wire through which current 經電阻線的長度會改變。
flows can be changed.
 Therefore the resistance can be changed,  因此電阻亦會改變，令電路中的電流有
and hence the current flowing through the 所改變。
circuit can be changed.
- +- +

sliding contact 3) bulb becomes brighter
2) larger current
滑動接觸燈泡較光亮
電流較大

1) current flows through a
shorter resistance wire
電流流經的電阻線較短
- +- +

sliding contact
3) bulb becomes dimmer
2) smaller current 滑動接觸
燈泡較暗
電流較小

1) current flows through a
longer resistance wire
電流流經的電阻線較長



A12

 Examples of rheostat usage:  變阻器的應用例子：

A rheostat is used in the volume control of a hi-fi A rheostat is used in the dimmer
system switch for a lamp
音響組合的音量控制器裝有變阻器。變阻器可用來調節電燈的亮度。



Chapter
第 24 章 Electric circuits
電路
24.1 Circuit symbols and 24.1 電路符號和電路圖
circuit diagrams

Q13 What are the symbols used to represent 科學家用甚麼符號來代表
the components in an electric circuit? 電路中不同的元件？

A13

 Circuit symbols are used to represent the  科學家用一套電路符號來代表電路中不
components in an electric circuit. 同的元件。
 By using circuit symbols, we can draw a  利用電路符號，我們便可以繪畫出一個
circuit diagram to represent an actual 電路圖，以顯示一個電路中各電路元件
circuit. 的接駁情況。

Circuit component Circuit symbol Circuit component Circuit symbol
電路元件電路符號電路元件電路符號

+ -

Dry cell 電池 Light bulb 燈泡

+ -+ -

Battery 電池組
Ammeter 安培計
(a group of dry cells)



A13

Circuit component Circuit symbol Circuit component Circuit symbol
電路元件電路符號電路元件電路符號

Connecting wire 導線

Voltmeter 伏特計
Connector 導片

Resistor 電阻器

Junction of connecting wires
or connectors
相接的導線或導片

Switch 開關
Rheostat 變阻器



How do we draw circuit diagrams to 如何繪畫電路圖以顯示各電路
Q14
represent actual circuits? 元件的接駁情況？

A14

1. First, draw the circuit symbols of the 2. Draw vertical and horizontal straight lines
components in the circuit. Make sure the to represent wires connecting the
terminals of the battery point are in the components.
correct direction.
繪畫水平線或垂直線來代表連接各元件的
首先，繪畫電路中各個元件的電路符號，並導線。
確保電池的正極和負極指向正確的方向。

3. Draw a dot at each T-shape junction. 4. Finally, check the diagram to see if the
components are connected in the correct
在各個 T 形的接駁點畫上一圓點。 order.

最後，檢查電路圖中各元件是否以正確的次
序連接。



24.2 Series circuits and 24.2 串聯電路和並聯電路
parallel circuits

Q15 What are series and parallel circuits? 甚麼是串聯電路和並聯電路？

A15

A series circuit : 串聯電路：

 There is only one path for the electric  電路中只有一條路徑讓電流通過。
current to flow.
 If one of the bulbs burns out or is  若其中一個燈泡被移走或損壞時，電
removed, there will be a gap in the 路中會有間隙，因此電流不能流通。
circuit. So no current will flow.

one bulb is removed
移走其中一個燈泡

Current can flow. No current can flow.
電流可以流通。電流不能流通。



A15

A parallel circuit : 並聯電路：

 There are several branches through  電路中有數條支電路讓電流通過。
which the electric current can flow.
 If one of the bulbs burns out or is  若其中一條支電路中的燈泡被移走或
removed, the electric current can still 損壞時，電流仍可流過其他支電路。
pass through other branches.

one bulb is removed
移走其中一個燈泡

Current can flow through all the bulbs. Current can still flow through the
電流可流過所有燈泡。 remaining bulbs.
電流仍可流過餘下的燈泡。



Is there any difference between the 流經串聯電路和並聯電路的
Q16
currents flowing through series and 電流二者有沒有分別？
parallel circuits?
A16

 In a series circuit, the current is the same  在串聯電路中，流經各點的電流都是相
at different points in the circuit. 等的。

 In a parallel circuit,  在並聯電路中，
a) the current flowing in the main loop is a) 主電路的電流是支電路電流的總
the sum of the currents flowing in the 和。
branches.
b) a larger current flows through the b) 流經電阻較小的支電路的電流較
branch with lower resistance. 大。

Current in the main loop is
the sum of the currents in
main loop the branches.
主電路主電路的電流是支電路電
流的總和。

A smaller current flows in
the branch with higher
branches resistance.
支電路流經電阻較大的支電路的
電流會較小。

A larger current flows in the branch
with lower resistance.
流經電阻較小的支電路的電流會較大。



Chapter
第 25 章 Household electricity
家居用電
25.1 How do electrical appliances 25.1 電器如何獲得
get the electricity they need? 所需的電力？

Q17 What is mains electricity? 甚麼是巿電？

A17

 Mains electricity is generated in power  市電是由發電廠所生產，然後經電纜
stations and transmitted to our homes. 輸送至用戶家中。
 Most electrical appliances in our homes  我們家中大部分電器都是使用市電
use mains electricity. 的。
 The mains electricity in Hong Kong has a  香港市電的電壓為 220 V。
voltage of 220 V.
 Different countries may have different  不同國家的市電電壓或有不同。
mains voltages.

Electrical appliances for use in Hong Kong can work at a voltage of 220V.
在香港使用的電器都能在 220 V 電壓下運作。



Q18 How are electrical appliances 家中的電器是如何接駁至
connected to the mains electricity? 巿電的？

A18

 Electrical appliances can be connected to  我們只需把電器的插頭插進市電插座，
the mains electricity by inserting their plugs 便能把電器接上市電電源。
into the mains sockets.
 Most electrical appliances in Hong Kong  香港大部分電器的插頭都是三腳插頭。
use three-pin plugs.

earth pin
earth socket 地線插腳
地線插孔

neutral socket live pin neutral pin
中線插孔活線插腳中線插腳
live socket
活線插孔
Mains socket Three-pin plug
巿電插座三腳插頭

earth wire (green and yellow)
地線 (綠色和黃色)

neutral wire (blue)
live wire (brown)
中線 (藍色)
活線 (棕色)

Wires inside a three-pin plug
三腳插頭內的電線

 The three pins of a three-pin plug are the  三腳插頭上的插腳分別是活線插
live pin, neutral pin and earth pin. 腳，中線插腳和地線插腳。
 The pins of the plug are connected to three  插頭上的插腳分別連接著三條電
wires. They are the live wire, neutral wire 線，它們分別是活線，中線和地線。
and earth wire.
 When a plug is inserted into a mains  把三腳插頭插進市電插座，這些插腳
socket, these wires are connected to the 便會連接至市電插座內三條相應的
corresponding wires in the mains socket. 電線。



A18

 A summary of the wires in the three-pin  以下是三腳插頭和巿電插座中電線的
plugs and mains sockets: 一覽表：
Wire Colour Function
電線顏色功能
Carries a voltage which drives an electric current
Live wire Brown
through the electrical appliance
活線棕色
帶有電壓，能驅動電流在電器內流通
Neutral wire Blue Provides a return path for the current
中線藍色為電流提供回路
Earth wire Green and yellow For safety
地線綠色和黃色是安全裝置

Q19 How is electricity distributed in our 供應至我們家中的電力是如何分
home? 配至家中各個插座的？
A19

 Electricity is transmitted to our home  電力是透過電纜輸送至家中。電力會
through an electric cable. The electricity is 先輸送至供電箱，然後供電箱會把電
first transmitted to a consumer unit, and 力分配至家中環形電路中各個插座。
then distributed among the sockets in a
ring circuit.

mains sockets
巿電插座 live wire
活線

consumer unit
供電箱

neutral wire
中線

earth wire
The ring circuit in a room 地線
房間內的環形電路

22


A19

 The sockets in a ring circuit are connected  環形電路中的插座是以並聯方式連
in parallel. 接。
 There are two paths for current to flow to  環形電路提供了兩條路徑讓電流從
each socket in a ring circuit. This reduces 供電箱流至每個插座，減少了每條電
the load on the wires in each path. Also, 線的負荷。同時，即使其中一條路徑
when one of the paths is damaged, current 損壞了，電流仍能循另一條路徑流至
can still flow through the other path. 電器。

Connected to consumer unit
連接至供電箱

live wire
活線

關掉關掉

neutral wire
中線

接通電源

Simplified drawing of a ring circuit (the earth wire is not shown)
環形電路的簡化繪圖 (沒有顯示地線)



25.2 Electrical safety 25.2 電力安全

Q20 What are the potential hazards of 使用電器有甚麼潛在危險？
using electrical appliances?

A20

 When a current flows through a conductor,  當電流通過導電體時，有部分電能
some electrical energy is changed to heat 會轉換成熱能，導電體因而變熱，
energy. The conductor will become hot. 這現象稱為電流的熱效應。
This is called the heating effect of electric
current.
 A larger current produces more heat.  電流愈大，所產生的熱能亦愈多。

wire
發熱線

Electric current is used to heat the wire of this cutter,
so it can cut through a foam board.
當發泡膠切割器的發熱線通電後會變熱，因此可用來切
割發泡膠片。

 When the circuit inside an electrical  當電器中的電路出現毛病，或我們
appliance is faulty, or when an electric 不正確地使用電器時，電路中的電
appliance is not used properly, the electric 流可能會變得很大。
current may become too large.
 As a result, a large amount of heat is  大量熱能因而產生，並可能釀成火
produced and may cause a fire. 警。



Q21 What may cause the current to 在甚麼情況下，電路中的電流會
become too large? 變得過大？

A21
7

 The two common situations that may cause  超負荷和短路是兩個會導致電路中
the current to become too large in a circuit 的電流變得過大的常見原因。
are – overloading and short circuit.

About overloading: 有關超負荷：
 When too many appliances are connected  當把多件電器接駁至同一個市電插

to the same mains socket, and are 座，而且各電器在同時運作時，通過
switched on at the same time, the current 插座的電流可能會很大，造成超負
drawn from the mains socket may become 荷。
too large. This is called overloading.
 To avoid overloading,  要避免超負荷，
 we should not connect too many  我們應避免把多件電器連接至同
electrical appliances to the same socket. 一個市電插座。
 we should connect electrical appliances  我們應把一些使用時需要較大電
that draw large current to different mains 流的電器連接至不同的市電插座。
socket.

About short circuit: 有關短路：
 A short circuit can cause a large current  由於短路提供了一條電阻非常小的
because it provides an easier path for 路徑讓電流通過，因此電路中的電流
current to flow through. 會變得很大。
 In daily life, short circuits may occur when  在日常生活中，短路會發生於
 the wiring of an electric appliance  當電器的電線鬆脫或變得破舊時。
becomes loose or worn out.
 dust builds up on circuits.  當電器的電路上積聚過多塵埃。

Without a short circuit, the current in the A large current flows when there is a
circuit is normal. short circuit caused by a loose wire.
若沒有短路發生，電路中的電流大小是正當電器的電線鬆脫或變得破舊時，電路中
常的。的電流變得過大。



What are the electrical safety devices 有哪些常見的用電安全裝
Q22
commonly used and how do they 置？它們是如何運作的？
work?
A22

 Examples of commonly used electrical  常見的用電安全裝置有保險絲、斷路
safety devices are fuse, circuit breaker 器和地線。
and earth wire.

1. Fuse 1. 保險絲

metal cap
金屬外殼

thin metal wire
A fuse and its internal structure 幼金屬線

保險絲及其內部構造

How fuses work: 保險絲如何運作：
 A fuse is used to prevent too large a  保險絲是用來避免電路中的電流變

current from flowing through a circuit. 得過大。
 Each fuse contains a very thin metal wire.  保險絲內有一條熔點低的幼金屬

The metal wire becomes very hot and 線，當流經它的電流過大時，金屬線
melts when the current is too large. 便會因過熱而熔斷。
 As a result, the current stops flowing. This  電流於是不再流通。這樣能防止電路

prevents the circuit from becoming 過熱，避免發生火災。
overheated and thus prevents damage on
the circuit, or even a fire.
 Fuses are commonly used to protect a  電路一旦出現超負荷或短路，保險絲
circuit from damage caused by 便會熔斷，以避免電器受損。
overloading or short circuit.
 After a fuse burns out, it cannot be used  熔斷了的保險絲不能再使用，需要更
again and need to be replaced. 換新的保險絲。



A22

the fuse melts
保險絲熔斷

short circuit 短路

a large current flows no current flows
流過保險絲的電流很大沒有電流流通

A short circuit in the electric kettle causes a The large current causes the fuse to heat up
large current to flow through the fuse. and melt. The circuit becomes open and the
電水煲發生短路，流過保險絲的電流會很大。 current stops flowing.
電流過大會令保險絲因過熱而熔斷，電路因而
斷開，電流不再流通。

Choosing the right fuse: 選擇合適的保險絲：

Fuses with different fuse ratings
不同額定值的保險絲

 The value on a fuse is called fuse rating. It  保險絲上標示了的數值稱為保險絲
shows the maximum current that can flow 額定值，它代表保險絲所能承受的最
through the fuse without burning it. 大電流。
 The fuse used in an appliance should have  我們所選用的保險絲，其額定值應略
a fuse rating slightly higher than the current 高於電器正常運作時所需的電流。
drawn by the appliance during normal
operation.
 If the fuse rating is too high, the appliance  若保險絲的額定值太高，當電流過大
will be damaged before the fuse burns out. 時，電器會在保險絲熔斷前受到損
壞。



A22

 Example:  例子：
 Under normal condition, the following  下圖的電飯煲在正常運作時所需
rice cooker draws a current of 3 A. 的電流為 3 A。
 A 5 A fuse is used to limit the maximum  如果所安裝的是 5 A 保險絲，可以
normal flowing current to 5 A. 把電流的最大值限制在 5 A。
 When a large current of 10 A flows  當有 10 A 的電流流過電路，5 A 保
through the circuit, the 5 A fuse burns 險絲會熔斷，電流會不再流通。電
out and the current stops flowing. The 路因而不會受損壞。
circuit is protected from damage.

The 5 A fuse burns out under
Fuse rating is suitable. a large current. The current
使用合適額定值的保險絲 stops flowing.
當 10 A 電流流過，5 A 保險絲
會熔斷，電流會不再流通。

The circuit is protected
5 A fuse from damage.
5 A 保險絲電路不會受到損壞。

 But when a 13 A fuse is used, it does  如果所安裝的是 13 A 保險絲，當
not burn out under the 10 A current. 有 10 A 的電流流過電路，保險絲
The large current will flow through and 亦不會熔斷。過大的電流令電路受
damage the circuit. The fuse rating is 到損壞。保險絲的額定值太高。
too high.

The 13 A fuse does not burn
Fuse rating is too high.
out even if the current is too
使用額定值過高的保險絲
large.
當 10 A 電流流過，13 A 保險
絲不會熔斷。

The circuit will be damaged.
過大的電流流過電路，令電路
13 A fuse
受到損壞。
13 A 保險絲



A22

2. Circuit breaker 2. 斷路器

switches of
circuit breakers
A circuit breaker A consumer unit with many circuit breakers inside, 斷路器的開關
斷路器 which help to protect circuits in home from damage
by large currents.
家中的供電箱裝有多個斷路器，以防止家中各個電
路因電流過大而受損。

 The function of a circuit breaker is similar  斷路器的作用和保險絲相似。
to that of a fuse.
 When the current in a circuit is larger than  斷路器同樣有一個額定值，當電路中
the rating of the circuit breaker, the switch 的電流超過額定值時，斷路器的開關
of the circuit breaker turns to the ‘off’ 會自動斷開，切斷電流，因此可防止
position. This breaks the circuit and 電路因電流過大而受損。
protects it from damage by the large
current.
 The circuit can be reconnected by turning  斷路器可以重複使用，只要把斷路器
the switch back to the ‘on’ position. 的開關閉合，便可重新接通電路。



A22

3. Earth wire 3. 地線
 An earth wire can protect us from an  地線可以保護我們避免觸電意外。

electric shock.
 It connects the metal case of an appliance  地線把電器的金屬外殼連接至地
to the earth. This is called earthing. 下，這稱為接地。
 When we touch a faulty appliance without  假設某電器活線鬆脫，而且沒有安裝
an earth wire: 地線，當我們觸摸它時：
1) The live wire touches 2) The metal case 3) An electric current
the metal case. carries a voltage. flows from the live wire
活線鬆脫，接觸到電器的金屬外殼帶有電壓。 to the ground through
金屬外殼。 our body.
電流由活線經身體流至
地下。

 When we touch a faulty appliance with an  假設某電器活線鬆脫，但已經安裝了
earth wire: 地線，當我們觸摸它時：
1) The live wire touches the metal case.
活線鬆脫，接觸到電器的金屬外殼。

2) A large current flows from the live 3) The fuse burns out
wire to the ground through the earth and breaks the circuit. 4) We will not get an electric
wire. 由於電流很大，保險絲會 shock even if we touch the
電流由活線經地線流至地下。熔斷，令電路斷開。 metal case.
即使我們觸摸金屬外殼，亦不會
觸電。



Q23 What safety precautions should we 在使用市電時，我們應採取甚麼
take when using electricity? 安全措施？
A23

Do’s 應做的事
1. Use three–pin plugs with earth wire 1. 使用已連接地線的三腳插頭。
connected.
2. Use fuses with suitable fuse ratings. 2. 使用合適額定值的保險絲。
3. Ask a registered electrician to replace 3. 找註冊電業技師更換損壞的電線、插
damaged wires, plugs or mains sockets. 頭和插座。
4. Ask a registered electrician to repair 4. 如出現電力故障，應找註冊電業技師
electrical fault. 來檢查及維修。

Don’ts 不應做的事
1. Do not overload sockets. 1. 不要讓插座「超負荷」。
2. Do not use damaged plugs, adapters or 2. 不要使用損壞的插頭、萬能插蘇或拖
extension units. 板。
3. Do not pull the cable to remove a plug 3. 不要拉動電器的電線來拔出插頭。
from a socket.

4. Do not touch electrical appliances, mains 4. 切勿用濕的手接觸電器、插座或開關。
sockets or switches with wet hands.
5. Do not use electrical appliances in a 5. 不要在浴室內使用電器。
bathroom.
6. Do not place electric wires near hot 6. 切勿把電線放近發熱物體，如暖爐和煮
objects. 食爐等。



25.3 Cost of electricity 25.3 計算電費
Q24 How can we compare the energy use 如何比較不同電器所耗用的
of different electrical appliances? 電能？
A24

 The power of an electrical appliance is  電器的功率代表每秒有多少電能供
the amount of electrical energy 給該電器。
transferred to the appliance per second.
 The unit of power is watt (W). A larger unit  功率的單位是瓦特 (W)，而較大的功
of power is kilowatt (kW). 率單位是千瓦特 (kW)。
1 kW = 1000 W 1 kW = 1000 W
 The relationship between the power of an  電器的功率與流經該電器的電流之
electrical appliance and the current 間的關係：
flowing through it: 功率 (W)
電流 (A) =
power (W) 電壓 (V)
current (A) =
voltage (V)
 At a given voltage, an electrical appliance  在電壓不變的情況下，功率較大的電
with a greater power draws a larger 器運作時所需的電流較大。
current.
 For an electrical appliance that draws a  在運作時需要較大電流的電器，會採
larger current, a thicker cable with lower 用較粗的電線，以減低電阻，避免電
resistance is used to prevent the cable 線過熱。
from overheating.

electric fan electric heater
電風扇電暖爐

The electric heater, which has a higher power than the electric fan, has a thicker cable.
電暖爐的功率比電風扇大，所採用的電線也較粗。



Q25 How much do we pay for the electricity 電力公司如何計算電費？
we use?
A25

 The cost of electricity is calculated based on  電力公司會根據用戶所耗用的電
the amount of electrical energy used. 能來計算電費。
 The amount of electrical energy used  電器所耗用的電能與其功率及運
depends on the power of the electrical 作時間有關。耗用的電能以千瓦小
appliances and the period of time they are in 時 (kW h)作為量度單位。
use. It is measured in kilowatt-hours (kW h).
 Electrical energy used (kW h)  耗用的電能 (kW h)
= power (kW) × time (h) = 功率 (kW) × 時間 (h)
 The electricity company uses a kilowatt-hour  電力公司會利用千瓦時計來量度
meter to measure the amount of electrical 用戶所耗用的電能。
energy we use. Net exchange of gases

A kilowatt-hour meter Each unit of electricity used is equal to 1 kW h of electrical
電力公司所使用的千瓦時計 energy.
電費單上會顯示用電度數，一度電相當於 1 kW h 的電能。



25.4 How do electrical 25.4 家中的電器是如何
appliances work? 運作的？
Q26 How is the heating effect of electric 如何將電流的熱效應應用在
current applied on electric appliances? 電器上？
A26

 Electrical appliances that make use of the  家中有些電器會應用了電流的熱效
heating effect of electric current contain 應，它們內部都裝有高電阻的電阻
resistance wires of high resistance. 線。
 When a current passes through the wires,  當電流流經電阻線時，大部分電能
most of the electrical energy is changed to 會轉換成熱能，使電阻線發熱。
heat energy.
 Examples: hair dryers, electric irons,  例子：風筒、電熨斗、電水煲。
electric kettles, etc.

resistance wires
電阻線

 The filament of a bulb has high resistance.  燈泡內的鎢絲也具有高電阻。當電
When a current pass through the filament, it 流流經燈絲時，燈絲的溫度會升高
becomes so hot that it glows and give out 達 2500 °C，由於溫度很高，燈絲會
light. 發亮。



Q27 How is the magnetic effect of electric 如何將電流的磁效應應用在
current applied on electric appliances? 電器上？

A27

 When an electric current flows through a  當電流通過金屬線圈時，線圈便具
coil, the coil behaves like a magnet. It is 有磁鐵般的性質。我們稱通了電的
called an electromagnet. 線圈為電磁鐵。
 Examples of electric appliances that use  電磁鐵的應用十分廣泛，常用於電
electromagnets: doorbells, electric door 門鐘和電子門鎖等裝置。
locks, etc.
electromagnet
電磁鐵

coil
金屬線圈

metal
paper clips
金屬塊
萬字夾

door
門

A coil with current flowing through behaves like a An electromagnet is used in
magnet. an electric door lock.
當電流通過金屬線圈時線圈便具有磁鐵般的性質。，電磁鐵常用於電子門鎖中。



Q28 How is the force on a wire carrying 如何將磁鐵和載電流導線所產
electric current applied on electric 生的力應用在電器上？
appliances?
A28

 When a conductor carrying a current is  當電流通過置於磁鐵附近的導電體
placed near a magnet, a force is produced 時，便會有一股力作用於載電流的
and causes the conductor to move. 導電體上，使其移動。
 Scientists make use of this in the design of  科學家把上述的原理，應用於電動
motors and loudspeakers. 機和揚聲器等裝置中。

diaphragm
膜片

coil
線圈

magnet
磁鐵

An electric fan which includes a motor. The structure of a loudspeaker
在電風扇內裝有電動機。揚聲器的構造


Part

第 2 部分 Revision exercise 單元練習
A. True or false questions 是非題
Write ‘T’ for a true statement or ‘F’ for a false statement in each box provided.
細閱以下句子，正確的在方格內填上「T」，不正確的則填上「F」。

1. An open circuit allows electricity to flow through it. □
2. When a conductor is connected to a dry cell and forms a closed circuit, the free □
electrons move from the positive terminal to the negative terminal.
3. With the same circuit components, the higher the voltage, the larger is the current □
flowing through a circuit.
4. In a series circuit, there is more than one path for the electric current to flow. □
5. In a parallel circuit, the current flowing in the main loop is the sum of the currents flowing □
in the branches.
6. The neutral wire is a safety device. □
7. The sockets in a ring circuit are connected in parallel. □
8. An earth wire connects the metal case of an appliance to the earth. □
9. Fuses can help to prevent short circuit. □
10. 1 kilowatt = 10000 watt □

B. Multiple-choice questions 多項選擇題
Choose the correct answer for each of the following questions.
選出正確的答案，然後在空格內填上代表該答案的英文字母。

1. Which of the following are necessary to make electricity flow?
(I) A closed circuit
(II) A source of electricity
(III) An open circuit
(IV) A switch
A. (I) and (II) only
B. (II) and (III) only
C. (I), (II) and (IV) only
D. (II), (III) and (IV) only □



2. Peter wants to measure the voltage across a light bulb. Which of the following connections is
correct?
A. B.

C. D.

□

3. John has set up a simple circuit.

Which of the following methods can make the bulb brighter?
(I) Using a bigger battery of the same voltage.
(II) Connecting more dry cells to the circuit.
(III) Using a dry cell with a higher voltage.
B. (I) and (III) only
C. (II) and (III) only
D. (I), (II) and (III) □



4. Which of the following statements about resistance are correct?
(I) A shorter wire has a smaller resistance.
(II) A thicker wire has a larger resistance.
(III) The resistance of a wire remains unchanged when the battery voltage of the circuit
increases.
(IV) The resistance of a wire increases when the current flowing through it increases.
B. (I) and (III) only
C. (II) and (III) only
D. (I) and (IV) only □

5. Refer to the following circuit diagram. The arrows represent the current in each part of the
circuit.

1

2

3

Arrange the size of the current in descending order (from large to small). Which of the following
is correct?
A. 1, 2, 3
B. 1, 3, 2
C. 2, 3, 1
D. 3, 2, 1 □

6. The respective colours of live wire, neutral wire and earth wire of a three-pin plug are:
A. blue, brown, green & yellow
B. green & yellow, blue, brown
C. brown, blue, green & yellow
D. blue, green & yellow, brown □



7. Study the following circuit:

What will happen to the ammeter reading when the switch is closed?
A. The ammeter reading will increase.
B. The ammeter reading will decrease, but will not become zero.
C. The ammeter reading will remain unchanged.
D. The ammeter reading will become zero. □

8. Which of the following statements describe(s) the advantage(s) of a ring circuit correctly?
(I) If one of the sockets cannot function, others can still function well.
(II) Currents flowing through each socket will not affect each other.
(III) It provides two paths for the current to flow to each socket and reduces the load on the
wires in each path.
A. (I) only
B. (I) and (II) only
C. (I) and (III) only
D. (I), (II) and (III) □

9. A microwave oven draws a current of 4 A under normal condition. What fuse rating is the most
suitable for the fuse that protects the microwave oven’s circuit?
A. 3A
B. 4A
C. 5A
D. 9A □

10. What is/are the function(s) of an earth wire?
(I) To prevent short circuit
(II) To prevent overloading
(III) To prevent electric shock when we touch a faulty appliance
A. (I) only
B. (III) only
C. (I) and (II) only
D. (II) and (III) only □



C. Short questions 短問題
1. Look at hair dryer below.

(a) Calculate the current drawn by the hair dryer when it is connected to a 220 V mains power
supply.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________

(b) The hair dryer is turned on for 6 minutes. How much electrical energy is used?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________

(c) Suppose the unit cost of electricity is $0.8, calculate the cost of electricity for using the hair
dryer for 6 minutes.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________



2. Your teacher provides you with the following materials and asks you to design a lighting circuit.
Materials Quantity
1.5 V dry cell 2
switch 1
small light bulbs 3
rheostat 1
connecting wires Unlimited
The requirements of the design are as follows:
(1) If the switch is pressed, all the bulbs will light up at the same time.
(2) If one bulb burns out, the other bulbs can still light up.
(3) The brightness of one bulb can be adjusted.

Draw your circuit diagram in the space below.



Part Answers 答案
A. True or false questions 是非題
1. F
2. F
3. T
4. F
5. T
6. F
7. T
8. T
9. F
10. F

B. Multiple-choice questions 多項選擇題
1. A
2. C
3. C
4. B
5. B
6. C
7. A
8. D
9. C
10. B

C. Short questions 短問題
Power (W)
1. (a) Current (A) =
Voltage (V)

1100 W
=
220 V
= 5A
(b) Electrical energy used (kW h) = Power (kW) × Time (h)
6
= 1.1 kW × h
60
= 0.11 kW h
(c) Cost of electricity ($) = Electrical energy used (kW h) × unit cost ($/kW h)
= 0.11 kW h × $0.8/kW h
= $0.088



2.


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