revision helper u8_bilingual

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

Understanding Integrated Science for the 21st Century (3rd Edition) 最新綜合科學 — 邁進 21 世紀 (第三版)

© Aristo Educational Press Ltd 2012 © 雅集出版社有限公司 2012

1

23.1 Making electricity flow 23.1 電怎樣才會在電路中

流通？

What are the conditions needed for

electricity to flow?

通電有哪些必要條件？

Q1

A1

To make electricity flow, there must be a

source of electrical energy and the circuit

must be a closed circuit.

要令電在電路中流通，電路中必須有電

源，而電路必須是閉合電路。

A dry cell 乾電池

It is a source of electrical

energy.

是電源。

Its stored chemical energy is

converted into electrical

energy when it is connected

to a light bulb.

當用導線把它與燈泡連接

起來時，它所儲存的化學能

會轉換成電能。

A closed circuit 閉合電路

It is a complete circuit

without gap that allows

electricity to flow through it.

是完整、沒有間隙的電路，

可以讓電通過。

1

Part

1 Chapter

Revision notes 溫習筆記

Basics of electricity 23

第部分

第章

有關電的基礎概念




2

What are switches? 甚麼是開關？

Q2

A2

Switches are commonly used to control

the flow of electricity through an electrical

appliance.

日常生活中大部分電器都裝有開關，可

用來使電路斷開或閉合。

When a switch is closed, the circuit is

complete. Electricity can flow and

therefore the appliance is turned on.

當開關閉合時，電路會變得完整，電便

能夠在電路中流通，於是電器便能運作。

When a switch is open, the circuit

becomes open circuit. Electricity cannot

flow and therefore the appliance is turned

off.

當開關斷開時，會形成斷路，令電不能

夠在電路中流通，於是電器便會停止運

作。

The following are some switches used in

daily life:

以下是一些在日常生活中使用的開關：

The following are some switches used in

the school laboratory:

以下是一些在學校實驗室中使用的開

關：

push button

switch

按鈕開關

sliding switch

滑動開關




3

Materials that allow electricity to flow

through are called electrical conductors.

有些物料能讓電通過，它們稱為導電體。

Most metals such as copper, iron, steel

and aluminium are good electrical

conductors.

大部分金屬，如銅、鐵、鋼和鋁等都是

良好的導電體。

The following are some good electrical

conductors:

以下是一些良好的導電體：

Some non-metals, such as graphite and

salt water can also conduct electricity.

有些非金屬，如石墨和鹽水，它們的導

電性能亦頗佳。

Materials that do not allow electricity to

flow through are called electrical

insulators.

有些物料不能讓電通過，它們稱為絕緣

體。

Rubber, plastic, glass, wood and air are

some examples of electrical insulators.

橡膠、塑膠、玻璃、木材和空氣都是絕

緣體的例子。

The tools used by electricians have plastic

or wooden handles. This prevents them

from getting an electric shock when

working.

電工所用的工具通常具有塑膠或木的手

柄。這樣可避免他們在工作時觸電。

What materials allow electricity to

flow through?

哪些物料能讓電通過？

Q3

A3

aluminium foil

鋁箔

iron nail

鐵釘

copper wires

銅線

graphite

石墨




4

The following are some tools with plastic

or wooden handles:

以下是一些具有塑膠或木的手柄的工

具：

A3

plastic handles

塑膠手柄

wooden handle

木手柄

23.2 Electric current 23.2 電流

What is electric current? 甚麼是電流？ Q4

A4

The electric current in a metal wire is a

flow of free electrons.

金屬線中的電流是由自由電子的流動

所形成。

Free electrons are the electrons that

escape from an atom and move freely

within the conductor.

自由電子是一些能夠擺脫原子核的束

縛，在原子間自由移動的電子。

When the conductor is not connected to a

dry cell, the free electrons move in different

directions.

如果導電體沒有連接至電源(例如電

池)，自由電子會向不同方向移動。

atoms in the conductor (They are fixed in position)

組成導電體的原子 (它們有固定的位置)

free electrons moving in

different directions

自由電子向不同方向移動




5

When the conductor is connected to a dry

cell and forms a closed circuit, the free

electrons move through the circuit from the

negative terminal to the positive terminal

of a dry cell.

若把導電體連接至乾電池，同時又形成

閉合電路，自由電子會由乾電池的負極

向乾電池的正極方向移動，形成電流。

flow of free electrons

自由電子的流動

free electrons are driven towards

the positive pole of the dry cell

自由電子受驅動，從負極流向正極

A4




6

How do we measure the size of

electric current?

我們如何量度電流的大小？

Q5

A5

We can use an ammeter to measure the

size of electric current.

我們可利用安培計來量度電流的大小。

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

milliamperes (mA).

較小的電流則以毫安培 (mA) 作為量

度單位。

A1000

1mA1

A

1000

1mA1

An ammeter is connected within the path

of the current to be measured.

安培計必須連接於所要量度的電流所流

通的路徑中。

dry cell

乾電池

light bulb

燈泡

ammeter

安培計

switch

開關




7

Voltage is the ‘electrical push’ that makes

free electrons flow in a circuit.

電壓是驅動自由電子在電路中流動的

「驅動力」。

The higher the voltage, the greater is the

‘electrical push’.

電壓愈高，「驅動力」便會愈大。

What is voltage? 甚麼是電壓？

Q6

A6

How do we measure voltage? 我們如何量度電壓？

Q7

A7

23.3 Voltage 23.3 電壓

‘electrical push’

電池的驅動力

dry cell

乾電池

Flow of free electrons

自由電子的流動

We can use a voltmeter to measure

voltage.

我們可利用伏特計來量度電壓。

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)。




8

A7

A voltmeter is connected across the part of

the circuit to be measured.

要量度電路中某元件兩端的電壓，必須

把伏特計以橫跨該元件兩端的形式連

接。

The higher the voltage, the greater is the

‘electrical push’.

電壓愈高，「驅動力」愈大。

Therefore a larger current flows through

the circuit and the bulb becomes brighter.

因此電路中的電流亦愈大，而電路中的

燈泡亦會愈光亮。

How does voltage affect the current

in a circuit?

電壓如何影響電路中電流的大

小？

Q8

A8

- + - +

smaller flow of free electrons

自由電子的流動較小

larger flow of free electrons

自由電子的流動較大

Measuring voltage across a light bulb

量度一個燈泡兩端的電壓。

dry cells

乾電池

light bulbs

燈泡

switch

開關

voltmeter

伏特計




9

The atoms in a conductor can oppose the

flow of free electrons.

組成導電體的粒子會阻礙自由電子

的流動。

The opposition of a material to the flow of

free electrons is called resistance.

物料對自由電子流動的阻力，稱為電

阻。

The unit of resistance is ohm (Ω). 電阻的單位是歐姆 (Ω)。

What is resistance? 甚麼是電阻？

Q9

0

A9

When resistance increases, the current in

the circuit decreases.

當電阻增加，電路中的電流會減少。

How does resistance affect the

current in a circuit?

電阻會否影響電路中電流的大

小？

Q10

A10

The resistance of a wire depends on the

material, the thickness and the length of

the wire.

導線的電阻與導線的物料、粗幼和長度

有關。

For wires made of the same material and

of the same length, a thinner wire has

higher resistance.

若比較相同物料和長度的導線，導線愈

幼，電阻愈大。

For wires made of the same material and

of the same thickness, a longer wire has

higher resistance.

若比較相同物料和粗幼的導線，導線愈

長，電阻愈大。

What factors affect the resistance

of a wire?

有哪些因素會影響導線的電

阻？

Q11

A11

23.4 Resistance 23.4 電阻




10

Resistors and rheostats are used to

control or adjust the current flowing

through a circuit.

電阻器和變阻器常用來控制或調節電

路中電流的大小。

Resistors have fixed value of resistance. 電阻器的電阻值是固定的。

The resistance of rheostats can be

changed.

變阻器的電阻值可在特定範圍內調節。

The two types of rheostats commonly used

in the school laboratory are sliding

rheostat and rotary-type rheostat.

在學校實驗室中有兩種常用的變阻器，

它們是滑動型變阻器和旋鈕型變阻器。

How do scientists make use of

resistance to control the current in a

circuit?

科學家如何利用電阻控制電路中

電流的大小？

Q12

A12

Sliding rheostat

滑動型變阻器

Resistors inside a computer

電腦內的電阻器

Rotary-type rheostats

旋鈕型變阻器

Resistors with different values of resistance

不同電阻值的電阻器




11

How rheostats work: 變阻器的運作原理：

A rheostat has a coil of resistance wire and

a sliding contact:

變阻器有一組電阻線圈及一個滑動接

觸：

By moving the sliding contact, the length of

the resistance wire through which current

flows can be changed.

當把滑動接觸移到不同位置時，電流流

經電阻線的長度會改變。

Therefore the resistance can be changed,

and hence the current flowing through the

circuit can be changed.

因此電阻亦會改變，令電路中的電流有

所改變。

A12

sliding contact

滑動接觸

resistance wire

電阻線圈

3) bulb becomes brighter

燈泡較光亮 2) larger current

電流較大

sliding contact

滑動接觸

- + - +

1) current flows through a

shorter resistance wire

電流流經的電阻線較短

2) smaller current

電流較小

1) current flows through a

longer resistance wire

電流流經的電阻線較長

3) bulb becomes dimmer

燈泡較暗

sliding contact

滑動接觸

- + - +




12

Examples of rheostat usage: 變阻器的應用例子：

A12

A rheostat is used in the volume control of a hi-fi

system

音響組合的音量控制器裝有變阻器。

A rheostat is used in the dimmer

switch for a lamp

變阻器可用來調節電燈的亮度。




13

Circuit symbols are used to represent the

components in an electric circuit.

科學家用一套電路符號來代表電路中不

同的元件。

By using circuit symbols, we can draw a

circuit diagram to represent an actual

circuit.

利用電路符號，我們便可以繪畫出一個

電路圖，以顯示一個電路中各電路元件

的接駁情況。

Circuit component

電路元件

Circuit symbol

電路符號

Circuit component

電路元件

Circuit symbol

電路符號

Dry cell 電池

Light bulb 燈泡

Battery 電池組

(a group of dry cells)

Ammeter 安培計

What are the symbols used to represent

the components in an electric circuit?

科學家用甚麼符號來代表

電路中不同的元件？

Q13

A13

24.1 Circuit symbols and

circuit diagrams

24.1 電路符號和電路圖

Chapter 24 Electric circuits

電路

+ -

+ - + -

第章




14

Circuit component

電路元件

Circuit symbol

電路符號

Circuit component

電路元件

Circuit symbol

電路符號

Connecting wire 導線

Connector 導片

Voltmeter 伏特計

Junction of connecting wires

or connectors

相接的導線或導片

Resistor 電阻器

Switch 開關

Rheostat 變阻器

A13




15

How do we draw circuit diagrams to

represent actual circuits?

如何繪畫電路圖以顯示各電路

元件的接駁情況？

Q14

A14

1. First, draw the circuit symbols of the

components in the circuit. Make sure the

terminals of the battery point are in the

correct direction.

首先，繪畫電路中各個元件的電路符號，並

確保電池的正極和負極指向正確的方向。

2. Draw vertical and horizontal straight lines

to represent wires connecting the

components.

繪畫水平線或垂直線來代表連接各元件的

導線。

3. Draw a dot at each T-shape junction.

在各個 T 形的接駁點畫上一圓點。

4. Finally, check the diagram to see if the

components are connected in the correct

order.

最後，檢查電路圖中各元件是否以正確的次

序連接。




16

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

Q15

A15

A series circuit : 串聯電路：

There is only one path for the electric

current to flow.

電路中只有一條路徑讓電流通過。

If one of the bulbs burns out or is

removed, there will be a gap in the

circuit. So no current will flow.

若其中一個燈泡被移走或損壞時，電

路中會有間隙，因此電流不能流通。

24.2 Series circuits and

parallel circuits

24.2 串聯電路和並聯電路

one bulb is removed

移走其中一個燈泡

Current can flow.

電流可以流通。

No current can flow.

電流不能流通。




17

A15

A parallel circuit : 並聯電路：

There are several branches through

which the electric current can flow.

電路中有數條支電路讓電流通過。

If one of the bulbs burns out or is

removed, the electric current can still

pass through other branches.

若其中一條支電路中的燈泡被移走或

損壞時，電流仍可流過其他支電路。

one bulb is removed

移走其中一個燈泡

Current can flow through all the bulbs.

電流可流過所有燈泡。

Current can still flow through the

remaining bulbs.

電流仍可流過餘下的燈泡。




18

Is there any difference between the

currents flowing through series and

parallel circuits?

流經串聯電路和並聯電路的

電流二者有沒有分別？

Q16

A16

In a series circuit, the current is the same

at different points in the circuit.

在串聯電路中，流經各點的電流都是相

等的。

In a parallel circuit, 在並聯電路中，

a) the current flowing in the main loop is

the sum of the currents flowing in the

branches.

a) 主電路的電流是支電路電流的總

和。

b) a larger current flows through the

branch with lower resistance.

b) 流經電阻較小的支電路的電流較

大。

Current in the main loop is

the sum of the currents in

the branches.

主電路的電流是支電路電

流的總和。

A smaller current flows in

the branch with higher

resistance.

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。

branches

支電路

main loop

主電路




19

25.1 How do electrical appliances

get the electricity they need?

25.1 電器如何獲得

所需的電力？

What is mains electricity? 甚麼是巿電？

Q17

A17

Mains electricity is generated in power

stations and transmitted to our homes.

市電是由發電廠所生產，然後經電纜

輸送至用戶家中。

Most electrical appliances in our homes

use mains electricity.

我們家中大部分電器都是使用市電

的。

The mains electricity in Hong Kong has a

voltage of 220 V.

香港市電的電壓為 220 V。

Different countries may have different

mains voltages.

不同國家的市電電壓或有不同。

Chapter

25 Household electricity 家居用電

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。

第章




20

How are electrical appliances

connected to the mains electricity?

家中的電器是如何接駁至

巿電的？

Q18

A18

Electrical appliances can be connected to

the mains electricity by inserting their plugs

into the mains sockets.

我們只需把電器的插頭插進市電插座，

便能把電器接上市電電源。

Most electrical appliances in Hong Kong

use three-pin plugs.

香港大部分電器的插頭都是三腳插頭。

The three pins of a three-pin plug are the

live pin, neutral pin and earth pin.

三腳插頭上的插腳分別是活線插

腳，中線插腳和地線插腳。

The pins of the plug are connected to three

wires. They are the live wire, neutral wire

and earth wire.

插頭上的插腳分別連接著三條電

線，它們分別是活線，中線和地線。

When a plug is inserted into a mains

socket, these wires are connected to the

corresponding wires in the mains socket.

把三腳插頭插進市電插座，這些插腳

便會連接至市電插座內三條相應的

電線。

earth socket

地線插孔

live socket

活線插孔

neutral socket

中線插孔

Mains socket

巿電插座

live pin

活線插腳

neutral pin

中線插腳

earth pin

地線插腳

Three-pin plug

三腳插頭

earth wire (green and yellow)

地線 (綠色和黃色)

neutral wire (blue)

中線 (藍色) live wire (brown)

活線 (棕色)

Wires inside a three-pin plug

三腳插頭內的電線




21

22

Q19

A19

How is electricity distributed in our

home?

供應至我們家中的電力是如何分

配至家中各個插座的？

A summary of the wires in the three-pin

plugs and mains sockets:

以下是三腳插頭和巿電插座中電線的

一覽表：

Wire

電線

Colour

顏色

Function

功能

Live wire

活線

Brown

棕色

Carries a voltage which drives an electric current

through the electrical appliance

帶有電壓，能驅動電流在電器內流通

Neutral wire

中線

Blue

藍色

Provides a return path for the current

為電流提供回路

Earth wire

地線

Green and yellow

綠色和黃色

For safety

是安全裝置

A18

Electricity is transmitted to our home

through an electric cable. The electricity is

first transmitted to a consumer unit, and

then distributed among the sockets in a

ring circuit.

電力是透過電纜輸送至家中。電力會

先輸送至供電箱，然後供電箱會把電

力分配至家中環形電路中各個插座。

neutral wire

中線

earth wire

地線

live wire

活線

mains sockets

巿電插座

consumer unit

供電箱

The ring circuit in a room

房間內的環形電路




22

The sockets in a ring circuit are connected

in parallel.

環形電路中的插座是以並聯方式連

接。

There are two paths for current to flow to

each socket in a ring circuit. This reduces

the load on the wires in each path. Also,

when one of the paths is damaged, current

can still flow through the other path.

環形電路提供了兩條路徑讓電流從

供電箱流至每個插座，減少了每條電

線的負荷。同時，即使其中一條路徑

損壞了，電流仍能循另一條路徑流至

電器。

A19

Connected to consumer unit

連接至供電箱

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)

neutral wire

中線

live wire

活線

關掉關掉

接通電源




23

25.2 Electrical safety 25.2 電力安全

Q20

A20

What are the potential hazards of

using electrical appliances?

使用電器有甚麼潛在危險？

When a current flows through a conductor,

some electrical energy is changed to heat

energy. The conductor will become hot.

This is called the heating effect of electric

current.

當電流通過導電體時，有部分電能

會轉換成熱能，導電體因而變熱，

這現象稱為電流的熱效應。

A larger current produces more heat. 電流愈大，所產生的熱能亦愈多。

When the circuit inside an electrical

appliance is faulty, or when an electric

appliance is not used properly, the electric

current may become too large.

當電器中的電路出現毛病，或我們

不正確地使用電器時，電路中的電

流可能會變得很大。

As a result, a large amount of heat is

produced and may cause a fire.

大量熱能因而產生，並可能釀成火

警。

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。

wire

發熱線




24

Q21

A21

7

The two common situations that may cause

the current to become too large in a circuit

are – overloading and short circuit.

超負荷和短路是兩個會導致電路中

的電流變得過大的常見原因。

About overloading: 有關超負荷：

When too many appliances are connected

to the same mains socket, and are

switched on at the same time, the current

drawn from the mains socket may become

too large. This is called overloading.

當把多件電器接駁至同一個市電插

座，而且各電器在同時運作時，通過

插座的電流可能會很大，造成超負

荷。

To avoid overloading, 要避免超負荷，

we should not connect too many

electrical appliances to the same socket.

我們應避免把多件電器連接至同

一個市電插座。

we should connect electrical appliances

that draw large current to different mains

socket.

我們應把一些使用時需要較大電

流的電器連接至不同的市電插座。

About short circuit: 有關短路：

A short circuit can cause a large current

because it provides an easier path for

current to flow through.

由於短路提供了一條電阻非常小的

路徑讓電流通過，因此電路中的電流

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

becomes loose or worn out.

當電器的電線鬆脫或變得破舊時。

dust builds up on circuits. 當電器的電路上積聚過多塵埃。

What may cause the current to

become too large?

在甚麼情況下，電路中的電流會

變得過大？

Without a short circuit, the current in the

circuit is normal.

若沒有短路發生，電路中的電流大小是正

常的。

A large current flows when there is a

short circuit caused by a loose wire.

當電器的電線鬆脫或變得破舊時，電路中

的電流變得過大。




25

Examples of commonly used electrical

safety devices are fuse, circuit breaker

and earth wire.

常見的用電安全裝置有保險絲、斷路

器和地線。

1. Fuse 1. 保險絲

A fuse and its internal structure

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

current from flowing through a circuit.

保險絲是用來避免電路中的電流變

得過大。

Each fuse contains a very thin metal wire.

The metal wire becomes very hot and

melts when the current is too large.

保險絲內有一條熔點低的幼金屬

線，當流經它的電流過大時，金屬線

便會因過熱而熔斷。

As a result, the current stops flowing. This

prevents the circuit from becoming

overheated and thus prevents damage on

the circuit, or even a fire.

電流於是不再流通。這樣能防止電路

過熱，避免發生火災。

Fuses are commonly used to protect a

circuit from damage caused by

overloading or short circuit.

電路一旦出現超負荷或短路，保險絲

便會熔斷，以避免電器受損。

After a fuse burns out, it cannot be used

again and need to be replaced.

熔斷了的保險絲不能再使用，需要更

換新的保險絲。

Q22 What are the electrical safety devices

commonly used and how do they

work?

有哪些常見的用電安全裝

置？它們是如何運作的？

metal cap

金屬外殼

thin metal wire

幼金屬線

A22




26

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

shows the maximum current that can flow

through the fuse without burning it.

保險絲上標示了的數值稱為保險絲

額定值，它代表保險絲所能承受的最

大電流。

The fuse used in an appliance should have

a fuse rating slightly higher than the current

drawn by the appliance during normal

operation.

我們所選用的保險絲，其額定值應略

高於電器正常運作時所需的電流。

If the fuse rating is too high, the appliance

will be damaged before the fuse burns out.

若保險絲的額定值太高，當電流過大

時，電器會在保險絲熔斷前受到損

壞。

a large current flows

流過保險絲的電流很大

A short circuit in the electric kettle causes a

large current to flow through the fuse.

電水煲發生短路，流過保險絲的電流會很大。

short circuit 短路

the fuse melts

保險絲熔斷

no current flows

沒有電流流通

The large current causes the fuse to heat up

and melt. The circuit becomes open and the

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。

A22




27

A22

Example: 例子：

Under normal condition, the following

rice cooker draws a current of 3 A.

下圖的電飯煲在正常運作時所需

的電流為 3 A。

A 5 A fuse is used to limit the maximum

normal flowing current to 5 A.

如果所安裝的是 5 A 保險絲，可以

把電流的最大值限制在 5 A。

When a large current of 10 A flows

through the circuit, the 5 A fuse burns

out and the current stops flowing. The

circuit is protected from damage.

當有 10 A 的電流流過電路，5 A 保

險絲會熔斷，電流會不再流通。電

路因而不會受損壞。

But when a 13 A fuse is used, it does

not burn out under the 10 A current.

The large current will flow through and

damage the circuit. The fuse rating is

too high.

如果所安裝的是 13 A 保險絲，當

有 10 A 的電流流過電路，保險絲

亦不會熔斷。過大的電流令電路受

到損壞。保險絲的額定值太高。

5 A fuse

5 A 保險絲

The circuit is protected

from damage.

電路不會受到損壞。

The 5 A fuse burns out under

a large current. The current

stops flowing.

當 10 A 電流流過，5 A 保險絲

會熔斷，電流會不再流通。

Fuse rating is suitable.

使用合適額定值的保險絲

13 A fuse

13 A 保險絲

The circuit will be damaged.

過大的電流流過電路，令電路

受到損壞。

The 13 A fuse does not burn

out even if the current is too

large.

當 10 A 電流流過，13 A 保險

絲不會熔斷。

Fuse rating is too high.

使用額定值過高的保險絲




28

A22

2. Circuit breaker 2. 斷路器

The function of a circuit breaker is similar

to that of a fuse.

斷路器的作用和保險絲相似。

When the current in a circuit is larger than

the rating of the circuit breaker, the switch

of the circuit breaker turns to the ‘off’

position. This breaks the circuit and

protects it from damage by the large

current.

斷路器同樣有一個額定值，當電路中

的電流超過額定值時，斷路器的開關

會自動斷開，切斷電流，因此可防止

電路因電流過大而受損。

The circuit can be reconnected by turning

the switch back to the ‘on’ position.

斷路器可以重複使用，只要把斷路器

的開關閉合，便可重新接通電路。

A circuit breaker

斷路器

A consumer unit with many circuit breakers inside,

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。

switches of

circuit breakers

斷路器的開關




29

3. Earth wire 3. 地線

An earth wire can protect us from an

electric shock.

地線可以保護我們避免觸電意外。

It connects the metal case of an appliance

to the earth. This is called earthing.

地線把電器的金屬外殼連接至地

下，這稱為接地。

When we touch a faulty appliance without

an earth wire:

假設某電器活線鬆脫，而且沒有安裝

地線，當我們觸摸它時：

When we touch a faulty appliance with an

earth wire:

假設某電器活線鬆脫，但已經安裝了

地線，當我們觸摸它時：

3) An electric current

flows from the live wire

to the ground through

our body.

電流由活線經身體流至

地下。

2) The metal case

carries a voltage.

金屬外殼帶有電壓。

1) The live wire touches

the metal case.

活線鬆脫，接觸到電器的

金屬外殼。

4) We will not get an electric

shock even if we touch the

metal case.

即使我們觸摸金屬外殼，亦不會

觸電。

3) The fuse burns out

and breaks the circuit.

由於電流很大，保險絲會

熔斷，令電路斷開。

2) A large current flows from the live

wire to the ground through the earth

wire.

電流由活線經地線流至地下。

1) The live wire touches the metal case.

活線鬆脫，接觸到電器的金屬外殼。

A22




30

Q23

A23

What safety precautions should we

take when using electricity?

在使用市電時，我們應採取甚麼

安全措施？

Do’s 應做的事

1. Use three–pin plugs with earth wire

connected.

1. 使用已連接地線的三腳插頭。

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

damaged wires, plugs or mains sockets.

3. 找註冊電業技師更換損壞的電線、插

頭和插座。

4. Ask a registered electrician to repair

electrical fault.

4. 如出現電力故障，應找註冊電業技師

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

extension units.

2. 不要使用損壞的插頭、萬能插蘇或拖

板。

3. Do not pull the cable to remove a plug

from a socket.

3. 不要拉動電器的電線來拔出插頭。

4. Do not touch electrical appliances, mains

sockets or switches with wet hands.

4. 切勿用濕的手接觸電器、插座或開關。

5. Do not use electrical appliances in a

bathroom.

5. 不要在浴室內使用電器。

6. Do not place electric wires near hot

objects.

6. 切勿把電線放近發熱物體，如暖爐和煮

食爐等。




31

25.3 Cost of electricity 25.3 計算電費

Q24

A24

How can we compare the energy use

of different electrical appliances?

如何比較不同電器所耗用的

電能？

The power of an electrical appliance is

the amount of electrical energy

transferred to the appliance per second.

電器的功率代表每秒有多少電能供

給該電器。

The unit of power is watt (W). A larger unit

of power is kilowatt (kW).

1 kW = 1000 W

功率的單位是瓦特 (W)，而較大的功

率單位是千瓦特 (kW)。

1 kW = 1000 W

The relationship between the power of an

electrical appliance and the current

flowing through it:

current (A) = power (W)

voltage (V)

電器的功率與流經該電器的電流之

間的關係：

電流 (A) = 功率 (W)

電壓 (V)

At a given voltage, an electrical appliance

with a greater power draws a larger

current.

在電壓不變的情況下，功率較大的電

器運作時所需的電流較大。

For an electrical appliance that draws a

larger current, a thicker cable with lower

resistance is used to prevent the cable

from overheating.

在運作時需要較大電流的電器，會採

用較粗的電線，以減低電阻，避免電

線過熱。

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。




32

Net exchange of gases

Q25

A25

How much do we pay for the electricity

we use?

電力公司如何計算電費？

The cost of electricity is calculated based on

the amount of electrical energy used.

電力公司會根據用戶所耗用的電

能來計算電費。

The amount of electrical energy used

depends on the power of the electrical

appliances and the period of time they are in

use. It is measured in kilowatt-hours (kW h).

電器所耗用的電能與其功率及運

作時間有關。耗用的電能以千瓦小

時 (kW h)作為量度單位。

Electrical energy used (kW h)

= power (kW) × time (h)

耗用的電能 (kW h)

= 功率 (kW) × 時間 (h)

The electricity company uses a kilowatt-hour

meter to measure the amount of electrical

energy we use.

電力公司會利用千瓦時計來量度

用戶所耗用的電能。

A kilowatt-hour meter

電力公司所使用的千瓦時計

Each unit of electricity used is equal to 1 kW h of electrical

energy.

電費單上會顯示用電度數，一度電相當於 1 kW h 的電能。




33

25.4 How do electrical

appliances work?

25.4 家中的電器是如何

運作的？

Q26

A26

How is the heating effect of electric

current applied on electric appliances?

如何將電流的熱效應應用在

電器上？

Electrical appliances that make use of the

heating effect of electric current contain

resistance wires of high resistance.

家中有些電器會應用了電流的熱效

應，它們內部都裝有高電阻的電阻

線。

When a current passes through the wires,

most of the electrical energy is changed to

heat energy.

當電流流經電阻線時，大部分電能

會轉換成熱能，使電阻線發熱。

Examples: hair dryers, electric irons,

electric kettles, etc.

例子：風筒、電熨斗、電水煲。

The filament of a bulb has high resistance.

When a current pass through the filament, it

becomes so hot that it glows and give out

light.

燈泡內的鎢絲也具有高電阻。當電

流流經燈絲時，燈絲的溫度會升高

達 2500 °C，由於溫度很高，燈絲會

發亮。

resistance wires

電阻線




34

Q27

A27

How is the magnetic effect of electric

current applied on electric appliances?

如何將電流的磁效應應用在

電器上？

When an electric current flows through a

coil, the coil behaves like a magnet. It is

called an electromagnet.

當電流通過金屬線圈時，線圈便具

有磁鐵般的性質。我們稱通了電的

線圈為電磁鐵。

Examples of electric appliances that use

electromagnets: doorbells, electric door

locks, etc.

電磁鐵的應用十分廣泛，常用於電

門鐘和電子門鎖等裝置。

electromagnet

電磁鐵

door

門

metal

金屬塊

An electromagnet is used in

an electric door lock.

電磁鐵常用於電子門鎖中。

A coil with current flowing through behaves like a

magnet.

當電流通過金屬線圈時，線圈便具有磁鐵般的性質。

paper clips

萬字夾

coil

金屬線圈




35

Q28

A28

How is the force on a wire carrying

electric current applied on electric

appliances?

如何將磁鐵和載電流導線所產

生的力應用在電器上？

When a conductor carrying a current is

placed near a magnet, a force is produced

and causes the conductor to move.

當電流通過置於磁鐵附近的導電體

時，便會有一股力作用於載電流的

導電體上，使其移動。

Scientists make use of this in the design of

motors and loudspeakers.

科學家把上述的原理，應用於電動

機和揚聲器等裝置中。

An electric fan which includes a motor.

在電風扇內裝有電動機。

The structure of a loudspeaker

揚聲器的構造

diaphragm

膜片

coil

線圈

magnet

磁鐵




36

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 □

Revision exercise 單元練習

2

Part

第部分




37

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) □




38

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.

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 □

1

3

2




39

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. 3 A

B. 4 A

C. 5 A

D. 9 A □

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 □




40

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.

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________




41

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.




42

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

1. (a) Current (A) = Power (W) Voltage (V)

= 1100 W 220 V

= 5 A

(b) Electrical energy used (kW h) = Power (kW) × Time (h)

= 1.1 kW × 6

60 h

= 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

Part




43

2.

revision helper u8_bilingual

Education

flow of electricity

makesfree electrons

closed circuit

position free electrons

aristo educational press

open circuit

acomplete circuit

unit of voltage