Making Use of Flipped

classroom

Electricity e-aristo.hk/r/
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In this Unit, we will learn about:

simple circuits series circuits and parallel circuits Learning Science in English
(U8: Describing relationships
circuit diagrams household electricity and electrical safety between variables)
electric current power of electrical appliances E
Foundation Worksheets (U8)
voltage how to calculate the cost of electricity E
resistance efficiency of electrical appliances E Quick Revision Notes (U8)

Teaching PowerPoint (U8 by sections) e-Companion (e-Book) Answers to Textbooks (U8)

 Electrical appliances such as electric lamps, electric irons and hairdryers have surely improved our
living standard a lot since the 19th century. Yet, today’s engineers still work hard to invent new
appliances that further improve our lives in the 21st century. Let’s see two recent inventions below.

App-controlled image toaster toast with a

chosen image

An electric current will

Input the image you wish pass through specific
to ‘print’ on your toast in tiny-sized filaments in
an app. the toaster. These
filaments get hot and
Teaching notes ‘print’ the image on the
The image toaster is called toast.
Toasteroid. It was developed by
a company in U.S. toaster Teaching notes
The portable ‘car’ is
called WalkCar. It was
developed by a
Portable ‘car’ Japanese company
Cocoa Motors in 2015.
It can reach a top speed
of about 10 km/h and
can travel a distance up
to 12 km after 3 hours
of charging.

When you step on the ‘car’, an

electric current will pass through
the electric motor in the ‘car’ and
the ‘car’ starts to move.

portable ‘car’ (around 3 kg)

Imagine you are an engineer. Think about some problems you face in
daily life. Design some simple electrical devices to solve these problems
by using the knowledge learned in this Unit.

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Making Use of Electricity

Introducing electricity and
simple circuits
Section objective
Students should be aware A The importance of electricity
that the use of electricity has
improved human’s living Electricity is very important in our modern lives. We use electricity
standard a lot.
at home and school every day. We use it to light our rooms, cook
our food and wash our clothes. We also use it for transportation and
communications.

Fig. 8.1 Electricity is very important in our modern lives.

Can you imagine what our lives would be without electricity? Let us
study Fig. 8.2 to understand more. You can see that the use of
electricity has greatly changed the way we live.

a b

Fig. 8.2 How was life in the days when there was no electricity?

75
8 Making Use of Electricity Prior knowledge
In primary level, students have learned
about the concept of the flow of electricity.
Section objectives
Students should be able to
B Conditions needed for electricity to flow
• understand that an
electric cell and a closed For an electrical appliance to work, it must be connected to a source
circuit are required for
of electricity and there is a path for electricity to flow. This path is
lighting up a bulb.
• recognize that an electric called an electric circuit (or a circuit for short). An electric circuit
cell is the energy source
in a circuit.
has three basic components:

1. Source of electricity 2. Device using electrical 3. Wire connecting different

and electrical energy energy parts of the circuit together

light bulb electric wire

electric cell

fan
mains socket

Teaching notes
Other sources of electricity:
dynamo, solar cell In the experiment below, we will see how these circuit components
are connected so that electricity flows.
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Experiment 8.1 e-aristo.hk/r/

Experiment video
isexptu8i01.e ev08s1

Finding the conditions needed for electricity to flow

Material and apparatus

electric cell with a battery holder 2 connecting wire 3

light bulb 1

Procedure

1. Your teacher will give you the following circuit components.

- + - +

electric cells with a battery holder light bulb connecting wires

Cont'd

electric circuit 電路 electrical appliance 電器

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Making Use of Electricity

2. Using the circuit components given, design a circuit that makes the bulb light up.
Draw a diagram below to show your design.

electric cells

light bulb connecting wire

3. Connect the circuit after getting your teacher’s approval.

Does the bulb light up? (Answers may vary.)

4. Set up the following circuits one by one. Observe if the bulb lights up in each case and
complete the table below.
A. – + – + B. – + – +

C. D. – + – +

Your design in Step 2 A B C D

Does the bulb light up? yes no no no yes

Is there an electric cell in the circuit? yes yes yes no yes

Is there a complete path from one

yes no no no yes
end of the electric cell to the other?

Discussion

Based on your results, write the TWO conditions needed for electricity to flow.

Condition 1: There are electric cells in the circuit.

Condition 2: There is a complete path from one end of the electric cell to the other.

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8 Making Use of Electricity

Learn more In Experiment 8.1, the bulb lights up only when the circuit includes
a source of electrical energy, which is an electric cell in this case.
A dangerous circuit we
should never connect Also, there must be a complete path for electricity to flow from one
We should never connect end of the electric cell to the other. This circuit is called a
a circuit that does not closed circuit (Fig. 8.3). If there is a gap in the circuit, electricity
include any devices using
electrical energy. cannot flow. This circuit is called an open circuit (Fig. 8.4).
Otherwise, the circuit
would overheat, which
would cause danger. When electricity flows through the circuit, the chemical energy
stored in the electric cell is changed to electrical energy. The light
bulb then changes the electrical energy to light energy and thermal
energy (Fig. 8.3).

Chemical energy in the electric cell

Electrical energy

Light energy and thermal energy

Orange colour is used here to released by the light bulb
highlight the flow of electricity.
In reality, only the bulb gives
out light. Fig. 8.3 In a closed circuit with an electric cell, electricity flows. Also, energy
conversions take place at the electric cell and the light bulb.
Teaching notes
Dry cell Wet cell
Misconception
Electrolyte Students may think
paste liquid that the light bulb can
inside the cell
still light up if it is
Example lithium ion cells for mobile car battery connected to the
phones (electrolyte: lithium (electrolyte: positive pole of the
salt in organic solvent) sulphuric acid) electric cell without
connected to the
Learn more negative pole. Remind
them that it lights up
Types of electric cells only when connected
to both poles to form
Dry cell a complete circuit.
- common in gap
daily life
Fig. 8.4 An open circuit

Wet cell
- used in
A closed circuit with a source of electrical
cars
energy is required for electricity to flow.

closed circuit 閉合電路

open circuit 斷路
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1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) Electricity can flow in an open circuit with an electric cell. F

(b) An electric cell is a source of electrical energy. T

2. In each of the following circuits, does the bulb light up?

(a) (b) (c) (d)

Yes / No Yes / No Yes / No Yes / No

Teaching notes
Section objective In Q2(b), there is a flow of electricity but it does not flow through the bulb. Remind
Students should be able to students not to connect such circuit as the current will be dangerously large.
identify electrical conductors
and insulators. C Electrical conductors and insulators
Prior knowledge
Can electricity flow through all materials? Which materials allow
In primary level, students have
learned about the concepts of electricity to flow? Let us investigate in the following experiment.
electrical conductors and insulators.

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Experiment 8.2 e-aristo.hk/r/

Experiment video
isexptu8i02.e ev08s2

Which objects allow electricity to flow?

Material and apparatus

light bulb 1 objects (wooden chopstick, pen, paper clip, 1 set

electric cell with a battery holder 1 ruler, paper, key, coin)
connecting wire 3

Procedure
– + – +
1. Connect a circuit as shown on the right.

Cont'd

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8 Making Use of Electricity

2. How can you modify the circuit in Step 1 to test if the given objects allow electricity to
flow? Complete the diagram below to show the modified circuit.

– + – +

object

In the test, how do you know if the object allows electricity to flow through?
If the object allows electricity to flow through, the bulb will light up.

3. Connect the circuit after getting your teacher’s approval. Use the circuit to test each given
object and also some other objects around you. Record the results in the table below.

Does the object allow

Object Does the bulb light up?
electricity to flow through?

(Answers may vary.)

Discussion
Think of the objects that allow electricity to flow through in your test. What material are
these objects made of?
Metals

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Teaching notes Some materials allow electricity to flow through them. They are
Besides solids, some liquids
can also conduct electricity. called electrical conductors. Metals such as copper, aluminium and
For example, salt solution iron are good electrical conductors. Besides metals, some other
and mineral water are
conductors. However, materials like graphite can also conduct electricity but not as well
distilled water is an insulator. as metals can.

Materials that do not allow electricity to flow through them are

called electrical insulators. Plastic, glass and wood are examples of
electrical insulators.

graphite
copper

plastic

Fig. 8.5 Graphite conducts electricity, Fig. 8.6 Electric wires are usually made
but only weakly. of copper to conduct electricity.

Teaching notes
Aluminium is used to make
overhead power lines
because it is lighter than
copper. Also, it is less
expensive.

Learn more
Conductors in a smartphone plastic handles

As much as 15% of the Fig. 8.7 Overhead power lines are Fig. 8.8 The handles of the tools used
mass of a smartphone made of aluminium to conduct electricity. by an electrician are usually made of plastic.
comes from copper. It is
Do you know why?
used to make the parts
Plastic is an insulator and can help prevent
that conduct electricity
an electrician from getting an electric shock.
between different
components in the
smartphone.
• Materials that allow electricity to flow through them are
called conductors .
• Materials that do not allow electricity to flow through them
are called insulators .

Section objective Prior knowledge

Students should be able to D Switches In primary level, students have
understand a switch as a learned about the use of switches.
device to open or close a
circuit. We can turn an electrical appliance on or off using a switch. Let us
understand how a switch works in Experiment 8.3.

electrical conductor 導電體 graphite 石墨

electrical insulator 絕緣體
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8 Making Use of Electricity

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Experiment 8.3 e-aristo.hk/r/

Experiment video
isexptu8i03.e ev08s3

Studying how a switch works

Material and apparatus

switch (different types) 3 electric cell with a battery holder 2 circuit board 1
light bulb 1 connecting wire 3

Procedure

1. You teacher will give you a switch shown on the right.

R
Observe it carefully. Are the parts P, Q and R of the switch P
conductors or insulators?
Q
P: Insulator     Q: Conductor

R: Conductor

2. Use the switch to connect the circuit shown on the right. – +– +

3. Close the switch and observe if the bulb lights up.

4. Open the switch and observe if the bulb lights up.

5. For electricity to flow in the circuit,

(a) should the switch be open or closed? Closed

(b) complete the drawing of the switch on the right and show
the path of electricity flowing through it using a red pen.

6. Your teacher will give you other switches shown below. Observe the switches and
circle the parts which are conductors.

7. Repeat Steps 2 to 4 using these switches. Study how we can use them to start or stop the
flow of electricity in a circuit.

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Making Use of Electricity

A switch can be used to open or close a circuit. When a switch is
closed, the path for electricity to flow in the circuit is complete. In
other words, the circuit becomes a closed circuit and thus electricity
can flow through it.

When the switch is open, there is a gap in the circuit and the circuit
becomes incomplete. Thus, electricity cannot flow.

switch (closed) switch (open) gap

Fig. 8.9 (left) When the switch of the torch is closed, the circuit is complete. Electricity flows in the circuit and the torch lights
up. (right) When the switch is open, the circuit becomes incomplete and no electricity flows in the circuit.

a b c
light switch

switch of a
hairdryer
switch of a computer screen

Fig. 8.10 Some switches used in daily life

A switch is used to open or close a circuit.

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Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) Copper is a good electrical conductor. T

(b) Some non-metals can conduct electricity. T

(c) We close the switch of a circuit to make the circuit complete. T

5-minute Quiz
(Quiz 1: Introducing electricity
switch 開關 and simple circuits)

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8 Making Use of Electricity

Section objectives
Students should be able to
• recognize the circuit Circuit diagrams
symbols (electric cell,
battery, light bulb, switch,
ammeter, voltmeter,
resistor and rheostat). Where is the toilet?
• draw and interpret simple
circuit diagrams.

Fig. 8.11 Joe is reading the map

of a shopping mall to find the toilet.

On the maps of shopping malls, facilities such as toilets are usually

Classroom question
Q: Symbols are commonly represented by symbols. With the use of symbols, the map is easier
used in science. Give one to read and can help us locate the facilities more easily.
example other than
circuit symbols.
A: Chemical symbols Similarly, we can use symbols to represent circuit components.
These symbols are called circuit symbols (Table 8.1). With circuit
Teaching notes
Remind students that in the symbols and straight lines (representing connecting wires), we can
symbol of an electric cell,
draw a circuit diagram to represent a circuit. Circuit diagrams can
• the longer line represents
the positive pole; and help us show a circuit clearly and communicate with one another
• the shorter thick line
clearly.
represents the negative pole.

Circuit Circuit Circuit

Circuit component Circuit component Circuit component
symbol symbol symbol
– +

Electric cell Light bulb Resistor*

– + – + – +

– + A

Battery Ammeter* Rheostat*

closed

open – + V

Switch Voltmeter* Junction

Table 8.1 Some common circuit symbols (*We will learn more about these circuit components in later sections.)
Teaching notes
circuit symbol 電路符號 ammeter 安培計 rheostat 變阻器 Remind students to draw a dot
circuit diagram 電路圖 voltmeter 伏特計 at each T-shaped junction.
84 resistor 電阻器
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Making Use of Electricity

Actual circuit Circuit diagram

– +

– + – +

Fig. 8.12 Examples of actual circuits and their circuit diagrams

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Experiment 8.4 e-aristo.hk/r/

Experiment video
isexptu8i04.e ev08s4

Setting up circuits based on circuit diagrams

Material and apparatus

circuit board 1 light bulb 2 connecting wire 5

electric cell 2 switch 1

Procedure

Set up the following circuits one by one. Check your circuit with your teacher.

Circuit A Circuit B Circuit C

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8 Making Use of Electricity

Skills Practice Drawing circuit diagrams is an important skill. The tips for drawing
(Drawing circuit diagrams)
SPS: CM
circuit diagrams are shown below.

a b

Tip 1. Draw the connecting

wires as straight lines
using a ruler.The straight
lines should be either
vertical or horizontal.

Tip 2. The circuit is drawn

with right-angled corners.

Fig. 8.13 (a) A correctly drawn circuit diagram and (b) an incorrectly drawn circuit diagram

SPS: CM

Activity 8.1

Drawing circuit diagrams p.178

Drawing circuit
diagrams
In the space provided, draw a circuit diagram to represent each circuit below.

a
– + – +

b
– + – +

– +
V
resistor voltmeter

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Making Use of Electricity

Think about

A circuit diagram is drawn to represent an

actual circuit.

Do you know why circuit The use of circuit diagrams allows scientists and engineers to
diagrams are so useful and communicate their ideas more easily. For example, engineers often
important for industries? use circuit diagrams to represent the circuit they designed. This can
be a circuit used in a computer, an electrical appliance, or even a car.

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Match each of the following circuit components with its name and circuit symbol. The first
one has been done for you as an example.

Circuit component Name Circuit symbol

(1) (a) switch (i)

(2) (b) light bulb (ii)

(3) (c) electric cell (iii)

(4) (d) ammeter (iv)

(5) (e) resistor (v)

(1) c ii         (2) e i         (3) b iii

(4) a iv         (5) d v

5-minute Quiz
Section Quiz 8.1–8.2
(Quiz 2: Circuit diagrams) 87
8 Making Use of Electricity

Electric current

In Section 8.1, you have studied the flow of electricity in a circuit.

This flow of electricity is called electric current. Do you know what
an electric current is?

Section objective
Students should be able to
A Basic ideas of an electric current
recognize electric current as
a flow of charges. In Unit 6, we have learned that matter is made up of atoms. Each
atom has a nucleus surrounded by tiny electrons. The nucleus carries
a positive charge while electrons carry a negative charge.

Teaching notes
Size of an atom ~ 10–10 m
Size of a nucleus ~ 10–14 m electron
If an atom is magnified to the size of a stadium,
the nucleus will be the size of a pea.
(According to modern understanding, electron is nucleus
a point particle. However, its radius is calculated
to be about 10–15 m by using a model in physics.)

Teaching notes
• All the electrons, including
the ones in the atoms, are Fig. 8.14 Structure of an atom
constantly moving, but only
the free electrons can move
freely within the whole metal.
• Besides metals, some other In metals, some of the electrons can escape from the atoms
materials (e.g. graphite) also and move freely within the metal. These electrons are called
have a number of free
electrons, and can therefore free electrons.
conduct electricity.
• In HKDSE chemistry, the free electron
term ‘delocalized electron’ is
used instead of ‘free electron’.
metal

atoms

Fig. 8.15 In a metal, there are free electrons, which can move within the metal freely.

nucleus 原子核 charge 電荷

electron 電子
88 free electron 自由電子
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Making Use of Electricity

Teaching notes When a metal is connected to an electric cell to form a closed circuit,
• In an open circuit, the free
electrons are moving in the free electrons are driven to flow in a direction from the
random directions so negative pole towards the positive pole of the electric cell. This flow
there is no net flow of free
electrons and no electric of free electrons forms an electric current.
current is formed.
• In a closed circuit, there is
a net flow of free electrons
and an electric current is
formed.

Active learning direction of

Simulation the flow of
(Flow of free electrons) free electrons

Teaching notes
The orange balls
here represent the
atoms, not the
nuclei, in the metal.

Fig. 8.16 The flow of free electrons of a metal wire in a closed circuit

Before the discovery of electrons, scientists believed that an electric

current was a flow of positive charges from the positive pole of the
electric cell to the negative pole. This is called the conventional
current. Today, scientists still use this convention. In the rest of this
Unit, we will keep to this convention to study circuits.

Learn more
Electric current of lightning
flow of free electrons – + – +
An electric current forms electric current
when there is a flow of
charged particles such as
electrons. There are other
charged particles in
Nature, for example ions.
For lightning, the flow of
both free electrons and electric current
ions in air is responsible
for the electric current in flow of free electrons
the lightning.

Fig. 8.17 Directions of the electric current and the flow of free electrons in an electric circuit

Teaching notes
An electric current is formed when there is a flow of charges. In metals, the
charges are free electrons. In salt solutions, the charges are positive and
negative ions. For lightning, the charges are free electrons and ions.

electric current 電流 conventional current 傳統電流

ion 離子
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8 Making Use of Electricity

Activity 8.2

Analogies of electric currents

We can use an analogy to understand more about electric currents and electric circuits.
Two analogies are shown below.

Analogy 1: Water pipe system

connecting wire electric cell

water flow light bulb
water turbine water pipe

pump

Study the above water pipe system carefully. Complete the table below to show how the
water pipe system is similar to the electric circuit.

Item in the Item in the

How are the items similar?
water pipe system electric circuit
water free electrons move around the water pipe system or the circuit
water pipe connecting wire provide a path for the water or free electrons to flow
pump electric cell supply energy to the water or free electrons
water turbine light bulb use the energy carried by the water or free electrons

Analogy 2: Travelling lorries system

commercial kitchen

road
school

lorry

lunch box

Cont'd

analogy 比擬

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Making Use of Electricity

Study the travelling lorries system carefully. What items in the electric circuit do the items in
this system represent?

Item in the travelling lorries system Item in the electric circuit

lorries free electrons

road connecting wire

lunch box energy

commercial kitchen electric cell

school light bulb

• An electric current in metal is a flow of free electrons .

• In a closed circuit with an electric cell, the free electrons
flow towards the positive pole of the cell.

Section objectives
Skills Practice
Students should be able to
• use an ammeter to
B Measuring electric current (Measuring current and voltage)
SPS: OB, PA
measure electric current.
• state that ampere (A) is a The size of an electric current is related to the number of free
unit of electric current. electrons that flow through a point in the circuit every second. The
Small electric currents are more free electrons flowing through it, the larger the electric current.
measured in milliamperes (mA):
We can use an ammeter to measure the size of an electric current.
1 mA = 0.001 A
The unit of an electric current is ampere (A).
Teaching notes
The ammeter on the right below has two red terminals. May remind students they
are used for measuring different ranges of current.

Fig. 8.18 Ammeters used in a school laboratory Fig. 8.19 You may think of an ammeter as a device that
measures the number of free electrons passing through
the circuit each second.
ammeter 安培計
Classroom question
ampere 安培 Q: Which terminal of the ammeter on the right in Fig. 8.18 is better for measuring a small current?
A: The 1 A terminal. It is because the sensitivity is higher. 91
8 Making Use of Electricity
Fig. 8.20 shows how an ammeter should be connected in a circuit.
An ammeter has a positive terminal (red) and a negative terminal
(black). The ammeter should be connected so that the electric
current flows into the ammeter at its positive terminal and leaves
from its negative terminal.
current in
– + – +

current out A current in A

– + – +

current out
Fig. 8.20 Correct and incorrect ways to connect an ammeter in a circuit

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Experiment 8.5 e-aristo.hk/r/

Experiment video
isexptu8i05.e ev08s5

Using an ammeter to measure an electric current

Material and apparatus

circuit board 1 ammeter 1 switch 1

electric cell 2 light bulb 1 connecting wire 3

Procedure
positive
pointer
1. Look at the ammeter carefully. Identify the terminal
different parts of the ammeter as shown.
negative
terminal

2. Set up the circuit shown below. Draw the circuit diagram in the box provided.
Teaching notes
Remind students to connect the circuit components
using the wires of the same colours as those shown in
the diagram. – +

A
A

– +

p.178
Tip
Drawing circuit diagrams
Make sure the positive terminal of the ammeter is
connected to the positive pole of the electric cell.

Cont'd
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Making Use of Electricity

3. Close the switch. What is the reading of the ammeter? (Answers may vary.)

4. Set up the circuit shown below with two electric cells.

– +

(a) What is the reading of the ammeter? (Answers may vary.)

(b) Compared to Step 3, is the light bulb brighter or dimmer? Brighter

Teaching notes
May explain that when the current is larger, more free electrons flow through the light bulb every second.
Thus, more energy is transferred to the bulb and it becomes brighter.

Learn more
Classroom question Typical electric currents in common electrical appliances
Q: How is the current
flowing through an
appliance related to the
energy used (more
precisely: power) by the
appliance?
A: The more the energy
used, the larger the
current. smartphone: 0.15 A desk lamp: 0.2 A hairdryer: 5 A air conditioner: 10 A

• Electric current can be measured using an ammeter .

• Electric current is measured in amperes ( A ).

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1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) An electric current is a flow of atoms in metals. F

(b) In an electric circuit with a larger electric current, more free electrons pass
through the circuit every second. T

Cont'd

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8 Making Use of Electricity

2. Which of the following diagrams correctly shows the direction of the electric current and
the flow of free electrons in a circuit?
A. B.
direction of the
electric current

direction of the flow

of free electrons

C. D.

3. In which of the following circuits is the ammeter correctly connected?

A. B. C.
– + + –

ammeter
electric cell +
switch A A A

– + – + – – +

light bulb C

5-minute Quiz
(Quiz 3: Electric current and use of ammeters)
Section objective
Students should be able to C Heating effect and magnetic effect of
recognize the heating effect
and magnetic effect of
electric current
Why does my phone get
electric current.
1. Heating effect of electric current warmer after I have
used it for a while?

Have you noticed that your smartphones

become warmer after you have used them
for a while? You will understand more
about this in this section.

Fig. 8.21 A smartphone gets warm

when it is used.

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Making Use of Electricity

Experiment 8.6 Demonstration
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Experiment video
isexptu8i06.e ev08s6

Observing the heating effect of electric current

Material and apparatus

low voltage power supply 1 heat-proof mat 1 connecting wire 3

coiled nichrome wire (~5 cm long) 1 ammeter 1 wax

Procedure

1. Your teacher will set up the circuit shown below.

power
supply Caution
nichrome wire Do not touch the
nichrome wire.
wax
It is very hot.

ammeter

2. Switch on the power supply and slowly increase the current through the circuit.

What can you observe about the wax on the nichrome wire? The wax melts.

3. Continue to increase the current through the circuit.

What can you observe about the nichrome wire? It becomes red hot and glows.

Discussion

What is the energy conversion occurring in the nichrome wire above?

Electrical energy is converted into thermal energy (and light energy).

Teaching notes In the experiment above, when an electric current flows through the
When free electrons flow,
they collide with the atoms nichrome wire, some of the electrical energy changes to thermal
of the nichrome wire and energy. As a result, the wire heats up. This is called the
this causes the atoms to
vibrate more vigorously. heating effect of electric current. The heating effect is larger if the
Thus, the wire heats up. electric current is larger.

The heating effect of electric current is useful. Some electrical

appliances such as electric kettles make use of the heating effect
to work. We will learn more about this in Section 8.7A.

heating effect 熱效應

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8 Making Use of Electricity

When an electric current flows through a conductor, some

electrical energy is converted into thermal energy. This is called
the heating effect of electric current.

Prior knowledge 2. Magnetic effect of electric current

In primary level, students have
learned about the magnetic Besides heating effect, electric current also produces a magnetic
effect of electric current.
effect. Let us observe this in the experiment below.
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Experiment 8.7 e-aristo.hk/r/

Experiment video
isexptu8i07.e ev08s7

Observing the magnetic effect of electric current

Material and apparatus

wire loop (~5 turns) 1 compass 1 insulated copper wire (~50 cm) 1
electric cell with a battery holder 2 connecting wire 3 paper clip 10
knife switch 1 long iron nail 1 sandpaper

Procedure

Part A: Magnetic effect of a wire carrying an electric current

1. Set up the circuit as shown. The wire of the
wire loop should be placed roughly in the
same direction as the needle on the
compass.

Caution
Open the switch as long as the result is obtained. needle
wire loop compass
Otherwise, the wire loop may become very hot.

2. Close the switch briefly. What happens to the needle of the compass?
The needle of the compass changes direction.

3. Open the switch. What happens to the needle of the compass now?
The needle of the compass turns back to its original direction.

4. Repeat Step 2 by holding the compass about 3 cm above the wire.

Compared to Step 2, what can you observe about the needle of the compass?
The needle of the compass turns less.

Cont'd
96
 8

Making Use of Electricity

Part B: Magnetic effect of a coil carrying an electric current
5. Make a coil by winding 20 turns of
copper wire around a long iron nail.
insulating
Use a piece of sandpaper to remove coating
the insulating coating from the two removed

ends of the wire. iron nail

6. Set up the circuit as shown on the

right.
– + – +
coil (around
the iron nail)

Caution
Open the switch as long as the result
is obtained. Otherwise, the coil may
become very hot. compass

7. Close the switch briefly. What happens to the needle on the compass?
The needle of the compass changes direction.

8. Open the switch. What happens to the needle on the compass now?
The needle of the compass turns back to its original position.

9. Repeat Step 7 with the compass further away from the coil.
Compared to Step 7, what can you observe about the needle of the compass?
The needle of the compass turns less.

10. Place the coil near some iron paper coil (around
clips as shown on the right. the iron nail)

paper
Caution clips
Open the switch as long as the result
is obtained. Otherwise, the coil may
become very hot.

11. Close the switch briefly. What happens to the paper clips?
The paper clips are attracted by the coil.

12. Open the switch. What happens to the paper clips now?
The paper clips fall.

97
8 Making Use of Electricity

In Experiment 8.7, when an electric current flows through a wire or

a coil, the needle on a compass nearby changes direction. This
shows the magnetic effect of the electric current. The magnetic
effect is larger if the electric current is larger.

The experiment also shows that when an electric current flows

through a coil, the coil can attract iron paper clips like a magnet. This
coil is called an electromagnet.

Teaching notes
The iron in the coil strengthens
the magnetic effect. Without the
iron, the magnetic effect is
weak and not easy to be
current-carrying coil observed.

paper clips

Fig. 8.22 The coil carries a current and acts like a magnet.

e-Explore An electromagnet is useful because it acts like a magnet that can be

switched on or off. It produces a magnetic effect only when current
8.1 Maglev trains
flows through it. Electromagnets are commonly used in devices such
as electric door locks, electric bells, etc.

a b

electromagnet
When a maglev train
moves, it is levitated above
the rail tracks using
electromagnets. Also, it
speeds up or brakes by metal
using electromagnets. The
use of electromagnets is
very essential to operate
this high-speed vehicle.
Explore more on our Web.

ex08s1

e-aristo.hk/r/ Fig. 8.23 The electric door lock in (a) is operated with an electromagnet.
isexploreu8i01.e
Its structure is shown in (b).

magnetic effect 磁效應 levitate 懸浮

electromagnet 電磁鐵
98
 8

Making Use of Electricity

We will learn more about the electrical appliances making
use of the magnetic effect of electric current in Section 8.7A.

Teaching notes
An electric bell consists of an electromagnet. It attracts a metal arm in
Activity 8.3 the bell so that the arm hits the gong of the bell to produce a sound. May
use this animation to explain (open with Internet Explorer or Firefox):
e-aristo.hk/r/isteu8i01.e
Finding out how an electric bell works
An electric bell works by making use of the magnetic effect of electric
current. Search the Internet to find out how it works. Share your
findings with your classmates.

Active learning
3D model
(Electric bell) • A current-carrying wire can affect the direction of the needle
of a compass. This shows the magnetic effect

of electric current.
• A current-carrying coil acts like a magnet. It is called an
electromagnet .

e-aristo.hk/r/
e-Checkpoint
8.5 iscpu8i05.e
cp08s5

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) The larger the current, the larger the heating effect it produces. T

(b) An electromagnet operates as a magnet only when an electric

current flows through it. T

2. A powerful electromagnet can be used at a

scrapyard to separate iron and steel from
other waste. Suggest TWO reasons to
explain why an electromagnet instead of
an actual magnet is used.
• An electromagnet can be switched off to drop

the iron and steel pieces.

• The strength of the electromagnet can be

adjusted according to need.

5-minute Quiz
scrapyard 廢料場
(Quiz 4: Heating effect and magnetic
effect of electric current)
99
8 Making Use of Electricity

Voltage
Teaching notes
An electric eel has a
specialized organ
containing about 5000
cells that can produce I am an electric eel . Don’t come
electricity. Each cell can
close to me, or I will give you an
produce a voltage of about
0.1 V (an ordinary dry cell electric shock. I can do so
has a voltage of 1.5 V). because my body can act like an
These cells are like electric cell and produce a
electric cells connected in voltage up to 500 volts.
series, giving rise to a total
voltage of 500 V.

Fig. 8.24 An electric eel can produce a voltage, which makes the animals nearby get
an electric shock.

In this section, we will learn about voltage and how it affects the
electric current in a circuit.

Section objective
Students should be able to
A Basic ideas of voltage
recognize voltage as a
measure of the amount of Have you noticed the label shown below on electric cells? Do you
energy supplied to the free
know what the ‘1.5 VOLTS’ on the label means?
electrons by an electric
cell.

Fig. 8.25 The label on an electric cell

The label shows the voltage of the electric cell. Voltage is a measure
of the amount of energy supplied to the free electrons by a source
of electricity such as electric cells. For example, an electric cell
with a higher voltage supplies more energy to the free electrons
in the circuit.

Voltage is also related to the ‘electrical push’ that makes free

electrons flow in a circuit. The higher the voltage, the greater the
‘electrical push’.

voltage 電壓 electric eel 電鰻

100
 8

Making Use of Electricity

The voltage of an electric cell is a measure of the
amount of energy supplied to the free electrons by the cell.

Section objectives
Students should be able to B Measuring voltage
• use a voltmeter to
measure voltage. We can use a voltmeter to measure voltage. The unit of voltage
• state that volt (V) is a unit
of voltage. is volt (V).

Skills Practice
(Measuring current and voltage)
SPS: OB, PA

Learn more
Voltages of different sources
of electricity
5 V 15 V

Fig. 8.26 Voltmeters used in a school laboratory

Figure 8.27 shows how a voltmeter should be connected to measure

the voltage of an electric cell. A voltmeter has a positive terminal
USB power bank: 5 V (red in colour) and a negative terminal (black in colour). The
voltmeter should be connected so that its positive terminal is
connected to the positive pole of the electric cell, and its negative
terminal to the negative pole.

mains electricity: 220 V V

− +

− +

electricity supply for

MTR trains: 1500 V
Fig. 8.27 Using a voltmeter to measure the voltage of an electric cell
Teaching notes
Same type of dry cell (e.g. zinc-carbon cell) of different sizes (e.g. A, AA) has
the same voltage. The voltage depends on the chemicals in the cell. On the
voltmeter 伏特計 other hand, the capacity is different for different sizes.
volt 伏特
101
8 Making Use of Electricity

SPS: OB, PA, CM

Experiment 8.8 e-aristo.hk/r/

Experiment video
isexptu8i08.e ev08s8

Using a voltmeter to measure voltages of electric cells

Material and apparatus
p.178
circuit board 1 electric cell 1 set connecting wires 2
Drawing circuit
voltmeter 1 diagrams

Procedure

Part A: Voltages of different electric cells positive

terminals
1. Look at the voltmeter carefully. Identify different pointer

parts of the voltmeter.

negative
terminal

2. Your teacher will give you the five different electric cells. Identify the positive and
negative poles of each cell.

A B C D E

3. Use the voltmeter to measure the voltage of each electric cell. Record the results in the
table below. Also, draw the circuit diagram in the box provided.

– + V
Tip
Make sure the
positive terminal
of the voltmeter is – +
connected to the
positive pole of the
electric cell.

Electric cell A B C D E

Voltage (V) 1.5 V 1.5 V 1.5 V 9V 1.6 V

Cont'd

102
 8

Making Use of Electricity

Part B: Voltages for different numbers of connected electric cells
4. (a) Your teacher will give you several electric
cells identical to cell A in Step 2.

(b) From the result in Step 3, write down

the voltage of one electric cell in the table
in Step 6.

5. Connect two electric cells as shown below and measure the voltage across the cells.
Record the result in the table in Step 6.

− +

− + − +

6. Repeat Step 5 to measure the voltage of three connected electric cells. Record the result
in the table below.

− +
Caution
Make sure the electric cells
− + − + − + are connected in the same
direction. Otherwise, the cells
may overheat, causing danger.

Number of electric cells Voltage (V)

1 1.5

2 3

3 4.5

103
8 Making Use of Electricity

In Experiment 8.8, when several electric cells are connected

end-to-end in the same direction, the voltage across them is higher
1.5 V than that of one single cell. The total voltage across the connected
electric
cells is the sum of the voltages of all the electric cells.
cells

torch

• Voltage can be measured using a voltmeter .

Fig. 8.28 The two connected

• Voltage is measured in volts ( V ).
1.5 V electric cells in this torch
give a total voltage of 3 V.
Section objective
Students should be able to
recognize that a battery with a C How does voltage affect current?
greater voltage will cause a
larger current to flow in a circuit.

SPS: OB, PA, IF, CM

Experiment 8.9 e-aristo.hk/r/

Experiment video
isexptu8i09.e ev08s9

Finding out how voltage affects the current in a circuit

Material and apparatus

circuit board 1 ammeter 1 switch 1

p.178
electric cell 3 voltmeter 1 connecting wire 5 Drawing circuit
light bulb 1 diagrams

Procedure

1. Set up the circuit shown below. Draw the circuit diagram in the box provided.

V
– + V

– +
A
A
– +

2. Close the switch. Record the voltmeter and ammeter readings in the table in Step 3.
Also, observe the brightness of the light bulb.

Cont'd

104
 8

Making Use of Electricity

3. Repeat Step 2 by setting up a circuit with
Caution
(a) two electric cells; and then Make sure the electric cells are connected
in the same direction. Otherwise, the cells
(b) three electric cells. may overheat, causing danger.

Number of electric cells 1 2 3

Voltmeter reading (V) 1.5 3 4.5

Ammeter reading (A) (Answers may vary.)

Brightness of the light bulb

1 2 3
(1–3; 1 = dimmest, 3 = brightest)

4. Draw a graph to show the relationship between the voltage and the current in a circuit.

Relationship between the voltage and current in a circuit

p.180
Plotting graphs

(Answers may vary, but the graph

current (A) should be a straight line passing
through the origin.)

Quick Spreadsheet

voltage (V)

Discussion

1. From your results above, how does the voltage affect the current in the circuit?
The current in the circuit increases with the voltage.

2. What happens to the brightness of the light bulb when the voltage increases?
The brightness of the light bulb increases.

105
8 Making Use of Electricity

In Experiment 8.9, when more electric cells are connected to

increase the voltage, the current in the circuit increases and the
light bulb becomes brighter.

electric cells
running down

Fig. 8.29 When the electric cells in a torch almost run down, their voltages drop.
Thus, the current decreases and the light bulb becomes dimmer.

The higher the voltage, the larger the current.

e-aristo.hk/r/
e-Checkpoint
8.6 iscpu8i06.e
cp08s6

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) The unit of voltage is ampere. F

(b) When voltage increases, the current in a circuit increases. T

2. Jason wants to measure the total voltage of two connected electric cells with a voltmeter.
Draw connecting wires in the diagram below to show how he should set up the circuit.

5-minute Quiz
Section Quiz 8.3–8.4
(Quiz 5: Voltage)

106
 8

Making Use of Electricity

Resistance

Have you ever been to a crowded

MTR station like the one on the
right? It is hard to walk in a
crowded station as the people
around you ‘resist’ your motion.
This situation is similar to the
concept that you will learn in this
section: resistance.
Fig. 8.30 A crowded station

Section objectives
Students should be able to
• recognize the differences in A Basic ideas of resistance
resistance between electrical
conductors and insulators. In Section 8.1C, you learned that electrical conductors can conduct
• state that ohm (Ω) is a unit of
resistance. electricity. Do different conductors conduct electricity equally well?

SPS: OB, PA

Experiment 8.10 e-aristo.hk/r/

Experiment video
isexptu8i10.e ev08s10

Do different electrical conductors conduct electricity equally well?

Material and apparatus

circuit board 1 ammeter 1 copper and nichrome wires (same length

electric cell 3 switch 1 and same thickness) 1 set
light bulb 1 connecting wire 4

Procedure

1. Set up the circuit shown on the right using

a copper wire. A

2. Close the switch. Observe the brightness

of the light bulb and take the reading of
the ammeter. copper wire

What is the ammeter reading?

Caution
(Answers may vary.) Make sure the electric cells are connected
in the same direction. Otherwise, the cells
may overheat, causing danger. Cont'd

107
8 Making Use of Electricity

3. Repeat Step 2 with a nichrome wire of the same length and thickness as the copper wire.

(a) Is the bulb brighter or dimmer than in Step 2? Dimmer

(b) What is the ammeter reading? (Answers may vary.)

(c) Is the current smaller or larger than in Step 2? Smaller

Conclusion

For the same length and thickness, a nichrome wire conducts electricity poorer

(better/poorer) than a copper wire.

Teaching notes
Teachers may explain resistance to students as follows: resistance
is due to the collisions between free electrons and atoms in a
material. (This is good enough for junior form students.)
In Experiment 8.10, the nichrome wire and the copper wire do not
Material Resistance (Ω)
−4
conduct electricity equally well. It is because they have different
silver 2.0 × 10 resistance. The resistance of a material measures the opposition of
−4
copper 2.1 × 10 the material to the flow of electric current. Its unit is ohm (Ω).
−3
iron 1.2 × 10 Different materials have different resistance. Conductors have low
graphite 4.0 resistance while insulators have very high resistance. Also, some
13 17 conductors, for example copper, have lower resistance than some
glass 10 –10
other conductors, for example graphite.
Table 8.2 Resistance of the
rods (of 1 m long and 1 cm thick)
of different materials
Resistance measures the opposition of a material to the
flow of electric current. It is measured in ohm ( Ω ).
Section objective
Students should be able to
understand that a higher
resistance will result in a
smaller current to flow in a
B How does resistance affect current?
circuit.
SPS: OB, PA, IF, CM

Experiment 8.11 e-aristo.hk/r/

Experiment video
isexptu8i11.e ev08s11

Studying the effect of resistance on the current in a circuit

Material and apparatus

circuit board 1 ammeter 1 resistor (different resistances) 4

electric cell 2 switch 1
light bulb 1 connecting wire 4
Cont'd

resistance 電阻
ohm 歐姆
108
 8

Making Use of Electricity

Procedure

1. Your teacher will give you four resistors. They are the circuit component with a fixed
resistance. Record the resistances of the resistors in the table in Step 4.
p.178
2. Set up the circuit below using one of the resistors. Draw the circuit
Drawing circuit diagrams
diagram in the box provided.

− + − +

A A
− +

resistor

3. Close the switch. Record the ammeter reading in the table in Step 4. Also, observe the
brightness of the light bulb.
4. Repeat Steps 2 and 3 using other resistors. Record your results below.

Resistance of the resistor (Ω)

Ammeter reading (A) (Answers may vary.)
Brightness of the light bulb
(1–4; 1 = dimmest, 4 = brightest)

5. Draw a graph to show how the current changes with the resistance.

Relationship between the resistance and current in a circuit

p.180
Plotting graphs

(Answers may vary, but the graph

current (A) should be a curved line similar to the
one here.)

Quick Spreadsheet

Conclusion resistance (Ω)

When the resistance of a circuit is higher, the current will be smaller .

resistor 電阻器

109
8 Making Use of Electricity

When the resistance of a circuit is higher, the electric current is

smaller. This is because a circuit of higher resistance has stronger
opposition to the flow of electric current.

When resistance increases, the electric current in a circuit

decreases .

e-aristo.hk/r/
e-Checkpoint
8.7 iscpu8i07.e
cp08s7

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) Insulators have high resistance. T

(b) Good conductors have zero resistance. F

(No conductors, except superconductors at very low temperatures, have zero resistance.)
2. Peter sets up the three circuits shown below. The circuit components of the circuits are
identical except that the resistors (circuit symbol: ) have different resistance.

Circuit P Circuit Q Circuit R

A 0.5 A A 0.6 A A 0.7 A

resistor P resistor Q resistor R

(a) In which circuit is the light bulb the brightest? Circuit R

(b) Arrange the resistors in descending order of resistance. P, Q, R

Section objective
Students should be able to C Factors affecting resistance
understand the effect of
length, thickness and the In Experiment 8.10, we found that different materials have different
EXTENSION

material of the wire on the

resistance of a circuit. [E] resistance. Apart from the material, what other factors affect the
resistance of a conducting wire? Does the thickness or the length
also affect the resistance? Let us design an investigation to find out.

110
 SPS: OB, IN, PA, IF, CM 8
Experiment 8.12 Experiment video

Making Use of Electricity

e-aristo.hk/r/
isexptu8i12.e ev08s12

Finding out the factors that affect the resistance of a conducting wire
Material and apparatus

circuit board 1 light bulb 1 ruler 1 p.178

electric cell 2 thin nichrome wire 1 ammeter 1 Drawing circuit
switch 1 thick nichrome wire 1 connecting wire 4 diagrams

EXTENSION
Part A: How does the thickness of a conducting wire affect its resistance?
I. Hypothesis

A thicker wire has (Answers may vary.) resistance.

II. Variable table

Independent variable Dependent variable Control variables
(the only variable that is changed) (the variable to be measured) (variables that are kept the same)

thickness of the wire

length of the wire
thickness of the wire current in the circuit
material of the wire
number of electric cells
others: light bulb

III. Procedure

1. Set up the circuit shown below using the thin nichrome wire. Draw the circuit diagram
in the box provided.
− + − +

A
A
− +

thin nichrome wire

2. Close the switch and record the ammeter reading.

Tip
3. Repeat Step 2 using the thick nichrome wire. You can use the circuit symbol
for the nichrome wire.
IV. Results

Nichrome wire used thin thick

Ammeter reading (A) (Answers may vary.)

The current is larger when a thicker wire is used. Cont'd

111
8 Making Use of Electricity

V. Conclusion

A thicker wire has lower resistance.

Part B: How does the length of a conducting wire affect its resistance?
I. Hypothesis
(Answers may vary.)
EXTENSION

II. Variable table

Independent variable Dependent variable Control variables

(the only variable that is changed) (the variable to be measured) (variables that are kept the same)

length of the wire current in the circuit thickness of the wire,

(through which the current flows) material of the wire,
number of electric cells,
light bulb

III. Procedure

Briefly write the procedure for the experiment on the lines below. Also, draw a circuit
diagram in the box to show your circuit.
1. Set up the circuit as shown using the thin nichrome

wire.

2. Adjust the distance between the clips on the

nichrome wire to 30 cm.

3. Close the switch and record the ammeter reading.

4. Repeat Steps 2 and 3 by adjusting the distance A

between the clips to 25 cm, 20 cm, 15 cm and 10 cm.

Cont'd

112
 8

Making Use of Electricity

IV. Results

Record your results in the table below.

Length of the nichrome

wire through which the 30 25 20 15 10
current flows (cm)

Ammeter reading (A) (Answers may vary.)

EXTENSION
Draw a graph to show the relationship between the length of the wire and the current
flowing through it.

Relationship between the length of the wire and the current in the circuit

p.180
Plotting graphs

(Answers may vary, but the graph

current (A) should be a curved line similar to
the one here.)

Quick Spreadsheet

length (cm)

V. Conclusion
A longer wire has higher resistance.

113
8 Making Use of Electricity

e-Explore According to the results of Experiments 8.10 and 8.12, the resistance
of a wire depends on a few factors:
8.2 Body fat scales
• Material of the wire
Different materials have different resistance. An insulator has
higher resistance than a conductor.
• Thickness of the wire
A thinner wire has higher resistance.

In our bodies, body fat and

• Length of the wire
body tissue have different A longer wire has higher resistance.
EXTENSION

resistance. Some body fat

scales work on this to
measure the amount of
body fat in our bodies.
Explore more on our Web.

ex08s2

e-aristo.hk/r/
isexploreu8i02.e

Fig. 8.31 Overhead power lines are usually made of thick wires. This is because
they need to have low resistance to carry currents over long distances.

• The thinner a wire, the higher its resistance.

• The longer a wire, the higher its resistance.

e-aristo.hk/r/
e-Checkpoint
8.8 iscpu8i08.e
cp08s8

Look at the four metal rods below. Rods A, B and C have the same thickness while
rods B, C and D have the same length.

rod A: copper

rod B: copper

rod C: nichrome

rod D: nichrome

Arrange the metal rods in descending order of resistance. D, C, B, A

5-minute Quiz
(Quiz 6: Resistance and factors affecting resistance)
114
 8
Section objective
D Resistor

Making Use of Electricity

Students should be able to
recognize the use of
resistors in a circuit. In Section 8.5B, we have learned how resistance affects current.
Engineers apply this knowledge and use resistors to control the
sizes of electric currents in circuits.

Resistors are made from A resistor is a circuit component with a fixed value of resistance.
conductors of higher
resistance such as nichrome. It is used to control or limit the sizes of currents in the circuits of
electrical appliances, for example a computer (Fig.8.33). The larger
the resistance of the resistor, the smaller the current.

Teaching notes
The colour bands on
resistors indicate their
resistances. The colour code
can be found at:

e-aristo.hk/r/
isteu8i02.e
resistor
Colour codes are also used
for some other circuit Fig. 8.32 Resistors Fig. 8.33 Circuit board of a computer
components, for example
inductors (電感器), to
indicate their ratings.

A resistor is a circuit component with a fixed value of

resistance.

Section objectives
Students should be able to E Rheostat
• understand how a
rheostat works. [E] Sometimes, the current in a circuit needs to be adjusted. This can be
• give examples of common
done using a rheostat. A rheostat is a circuit component whose

EXTENSION
applications of rheostats
(e.g. dimmer switch, resistance can be adjusted. Fig. 8.34 shows two types of rheostats
volume control). [E]
commonly used in a school laboratory: a sliding rheostat and
a rotary-type rheostat.
a b

terminal terminal terminal

terminals
terminals

Fig. 8.34 Two types of rheostats: (a) sliding rheostat and (b) rotary-type rheostat

resistor 電阻器 rotary-type rheostat 旋鈕型變阻器

rheostat 變阻器
sliding rheostat 滑動型變阻器 115
8 Making Use of Electricity

A sliding rheostat has a coil of resistance wire, a metal bar and a

sliding contact. The figure below shows how an electric current
flows through a sliding rheostat.

sliding contact
metal bar
movable
current

current
EXTENSION

resistance wire

Fig. 8.35 Current flow in a sliding rheostat

Let us understand more about how a sliding rheostat works in the

following experiment.

SPS: OB, PA

Experiment 8.13 e-aristo.hk/r/

Experiment video
isexptu8i13.e ev08s13

Studying how a rheostat works

Material and apparatus

circuit board 1 sliding rheostat 1 ammeter 1

electric cell 2 light bulb 1 connecting wire 4

Procedure

1. Look at the sliding rheostat carefully. Identify different parts of it.

metal bar sliding contact

A B

terminal Y

terminal X terminal Z

resistance wire

Cont'd

116
 8

Making Use of Electricity

2. Set up the circuit as shown below.

sliding contact
Q
- + - +

EXTENSION
ammeter

3. Move the sliding contact of the rheostat towards Q.

(a) How does the current in the circuit change? It decreases.

(b) How does the brightness of the light bulb change? It decreases.

Discussion

1. In the diagram below, draw arrows to show the path of the electric current through the
rheostat before and after the sliding contact is moved towards Q.

P Q P Q

2. Briefly explain the changes in the current and the brightness of the bulb in the
experiment.

When the sliding contact is moved towards Q, the current flows through a
longer (shorter/longer) length of the resistance wire. Therefore,
the resistance of the rheostat increases . As a result,
the current becomes smaller and the bulb becomes dimmer .

3. Suppose the experiment is repeated with terminals X and Z (see Step 1) of the rheostat
connected to the circuit. Think about how the result of the experiment will change.
The current in the circuit and brightness of the bulb will remain unchanged. It is because the current will
flow through the entire length of the resistance wire, no matter where the sliding contact is.

117
8 Making Use of Electricity

In Experiment 8.13, we studied how a sliding rheostat works. When

we move the sliding contact, we change the length of the resistance
wire that the current flows through. This changes the resistance of
the rheostat and thus the current in the circuit.

sliding contact larger current sliding contact smaller current

move the sliding

EXTENSION

contact to the right

resistance wire resistance wire

Fig. 8.36 Moving the sliding contact to the right will increase the length of the resistance wire that the current flows through.
Therefore, the resistance of the rheostat is higher, resulting in a smaller current.

A rotary-type rheostat works in a similar way. When the sliding

contact of the rheostat is rotated, the electric current flows through
a longer or shorter length of the resistance wire. As a result, the
resistance of the rheostat changes.

move move
case
resistance wire
remove sliding
the case contact

Teaching notes
Explain to students that
• the middle terminal is connected to current current
terminals
the sliding contact.
• the terminals on either side are
connected to the resistance wire.
Fig. 8.37 Structure of a rotary-type rheostat Fig. 8.38 Current flow in a rotary-type rheostat
Teaching notes
Remind students that one wire is connected to
the middle terminal while the other can be
connected to the terminal on either side.
Rheostats are used in many devices, for example in dimmer switches
to adjust the current and thus the brightness of lamps. They are also
used as the volume controls of hi-fi systems.

118
 8
a b

Making Use of Electricity

Teaching notes
Rheostats are used hi-fi volume control
in dimmer switches
in the early days.
dimmer switch
Today, most
dimmer switches
control the
brightness of a
lamp by turning it
on and off rapidly.
This can control the
total amount of
energy transferred

EXTENSION
to the lamp and
thus its brightness. Fig. 8.39 Some applications of rheostats: (a) the dimmer switch of a lamp and (b) a hi-fi volume control

A rheostat is a circuit component whose resistance can

be changed.

e-aristo.hk/r/
e-Checkpoint
8.9 iscpu8i09.e
cp08s9

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) Resistors can be used to limit the size of the electric current in a circuit. T

E (b) Rheostats work by adjusting the voltage of the electric cells. F

E 2. Leo is connecting a circuit to control the volume of a buzzer using a rheostat. In the
diagram below, draw the connecting wires to show his circuit. The volume of the buzzer
is increased by moving the sliding contact to the right.

buzzer

5-minute Quiz
Section Quiz 8.5
(Quiz 7: Resistors and rheostats)

119
8 Making Use of Electricity

Series circuits and

parallel circuits
Section objective
Students should be able to
A Basic ideas of series circuits and
identify series circuits and parallel circuits
parallel circuits.
The circuits we have worked with so far had only one light bulb.
If there are two bulbs, there is more than one way to connect them.

One way to connect the two bulbs is one after the other to form a
single-path circuit as shown in Fig 8.40. This is called a series circuit
and the light bulbs are said to be connected in series.

Fig. 8.40 (left) Series circuit with two bulbs and (right) the circuit diagram of the
series circuit

The two light bulbs can also be connected in a circuit with branches
as shown in Fig. 8.41. This is called a parallel circuit and the light
bulbs are said to be connected in parallel.

Think about

main loop
branch

branch

Is the circuit above a series Fig. 8.41 (left) Parallel circuit with two bulbs and (right) the circuit diagram of the
circuit or a parallel circuit? parallel circuit
Teaching notes
Series circuit (because there is only one path for current to flow May use the following analogy to explain
from the positive pole of the electric cell to the negative pole.) the idea of a parallel circuit: the light bulbs
series circuit 串聯電路 in the branches of a parallel circuit are like
parallel circuit 並聯電路 the toll booths of Cross-Harbour Tunnel.
120
 8

Making Use of Electricity

SPS: OB, PA, CM

Experiment 8.14 e-aristo.hk/r/

Experiment video
isexptu8i14.e ev08s14

Studying the difference between series and parallel circuits

Material and apparatus
p.178
circuit board 1 switch 1 connector 4
Drawing circuit
electric cell 2 light bulb 2 diagrams

Procedure

1. Set up the series circuit shown below. Draw the circuit diagram in the box provided.

− + − +

2. Close the switch. Do the bulbs light up? Yes

3. Open the switch and remove one of the bulbs. Then, close the switch again.

Does the remaining bulb light up now? No

4. Set up the parallel circuit shown below. Draw the circuit diagram in the box provided.

− + − +

5. Close the switch. Do the bulbs light up? Yes

6. Open the switch and remove one of the bulbs. Then, close the switch again.

Does the remaining bulb light up now? Yes

121
8 Making Use of Electricity

From Experiment 8.14, in a series circuit of two light bulbs, when

one bulb is removed, the other goes out. This is because there is
only one path for current to flow through (Fig. 8.42). If the circuit is
open at any point, the circuit will be incomplete. No current will
flow and the bulb will not light up.

one bulb is removed the circuit is incomplete

current and no current can flow

Fig. 8.42 What happens in a series circuit when one bulb is removed

the lorries cannot

lorry
travel a bridge is cross the bridge
broken

bridge

Fig. 8.43 We can use this analogy to explain Fig. 8.42: if one of the bridges is broken, the lorries cannot cross the bridge.

In a parallel circuit of two bulbs, when one bulb is removed, the

other can still light up. This is because there is more than one branch
for current to flow through in a parallel circuit (Fig. 8.44). If one of
the branches is open, the current can still flow through the other
branch. Therefore, the bulb in the remaining closed branch will still
light up.

current

Teaching notes
When the light bulb in the
lower branch is removed, one bulb
the current can still flow
the current in the upper is removed
through this branch
branch remains
unchanged. On the other
hand, the current in the
main loop decreases; it is
not a constant.

Fig. 8.44 What happens in a parallel circuit when one bulb is removed

122
 8
lorry travel travel

Making Use of Electricity

a bridge is
broken

bridge travel

Fig. 8.45 We can use this analogy to explain Fig. 8.44: if one of the bridges is broken, the lorries will still be able to cross
using the other bridge.

Parallel circuits are useful because the device (for example a light
bulb) in each branch operates independently. The electrical
appliances in our homes are connected in parallel. We will learn
more about this in Section 8.7B.

Think about
• When light bulbs are connected in series , they are
connected one after the other on the same single path.
• When light bulbs are connected in parallel , they are
connected on different branches.
• If a gap appears in a series circuit, there is no
Should the light bulbs used (still / no) current flow in the circuit.
for Christmas lights on the
walls of buildings be
connected in series or in
• If a gap appears in a branch of a parallel circuit, there is
still (still / no) current flow in the other branches.
parallel? Why?
In parallel. In case a bulb burns
out, the others can still light up.
e-aristo.hk/r/
e-Checkpoint
8.10 iscpu8i10.e
cp08s10

1. In the diagrams below, draw connecting wires to show how to connect the light bulbs
(a) in series, and (b) in parallel.
(a) (b)

Cont'd

123
8 Making Use of Electricity

2. Study the circuit on the right. Draw a – + – +

circuit diagram below to show how you
would add a switch so that
(a) bulb P can be turned on or off without bulb P
affecting bulb Q.
(b) bulbs P and Q can be turned on or off bulb Q

at the same time.

(a) (b)

Section objectives
Students should be able to
B
• recognize that, in a series circuit, Current and voltage in series circuits
the current is the same at all points
in the circuit.
and in parallel circuits
• recognize that, in a parallel circuit,
– the current in the main loop is 1. Current and voltage in series circuits
the sum of the currents in the
branches.
– a larger current flows in the How are the currents at different points in a series circuit related?
branch with a lower resistance. How about the voltages? Let us find out in the following experiment.
– the voltage across each branch
of the circuit is the same.
SPS: OB, PA

Experiment 8.15 e-aristo.hk/r/

Experiment video
isexptu8i15.e ev08s15

Studying the currents and voltages in a series circuit

Material and apparatus

circuit board 1 switch 1 connecting wire 8

electric cell 2 ammeter 4
light bulb (identical) 2 voltmeter 3

Cont'd

124
 8

Making Use of Electricity

Procedure

1. Set up the series circuit shown below. The ammeters are used to measure the currents at
different points of the circuit.

A1 A

– +
– + – + A1
A2 A
A2
–+
A
A4 A4
–+

A3

A3
A

–+

2. (a) Close the switch. Do the two light bulbs have the same brightness? Yes

(b) Record the ammeter readings in the table below.

Ammeter A1 A2 A3 A4
Reading (A) (Answers may vary, but the readings are the same.)

(c) What can you observe about the currents at different points along the circuit?
They are the same.

3. Set up the series circuit shown below. The voltmeters are used to measure the voltages
across the electric cells and across each bulb in the circuit.
V1 V1
V

–+
– + – +

V2 V3
V V

–+ –+ V2 V3

4. (a) Close the switch. Record the voltmeter readings in the table below.

Voltmeter V1 V2 V3
Reading (V) (Answers may vary.)

(b) Are the voltmeter readings the same? No

125
8 Making Use of Electricity

From Experiment 8.15, we found that the currents are the same at
different points of a series circuit. In a series circuit, there is only
i�ea� � S���n�� one path for current to flow through and the same current flows
through all points of the circuit.
Constancy in current
current
In studying circuits with a
light bulb, some students
may think that when an
electric current passes
through the light bulb,
the size of the current
decreases. This is not true.
The size of the current
remains the same when it
passes through the light
bulb.

Teaching notes
Sum of the voltages across
the bulbs = voltage across Fig. 8.46 In a series circuit, the current at all points is the same.
the electric cells (not required
by the syllabus)

Supplementary notes However, the voltage across each light bulb and the voltage across
Voltage in series circuits the electric cells are not equal.
(See p.T23)

Teaching notes
Activity 8.4 Students may have this question about a series
circuit: Why does the current not decrease after
passing through a light bulb? The bulb ‘has used
up electricity’.
Analogy of the current in a series circuit These students mix up energy with current. This
Activity aims to correct their misconception.
We can understand more about the current in a series circuit using the travelling lorries
system analogy (on p.90) shown below. Study the analogy carefully.

lorry

school A school B
lunch box

1. After the lorries have passed school A, does the number of lorries decrease? Does the
number of lunch boxes carried by the lorries decrease?
1. The number of lorries remains the same. The number of lunch boxes decreases.
2. Hence, explain why the current remains the same when passing through each light bulb
in a series circuit. 2. The number of free electrons remains the same after passing through
each light bulb.

126
 8

Making Use of Electricity

2. Current and voltage in a parallel circuit

In the previous section, we studied the currents and voltages in

a series circuit. Will we get similar results for a parallel circuit?
Let us see in the following experiment.

SPS: OB, PA

Experiment 8.16 e-aristo.hk/r/

Experiment video
isexptu8i16.e ev08s16

Studying the currents and voltages in a parallel circuit

Material and apparatus

circuit board 1 switch 1 connecting wire 6

electric cell 2 ammeter 3 connector 5
light bulb (identical) 3 voltmeter 3

Part A: Measuring the currents in a parallel circuit

Procedure

1. Set up the parallel circuit shown below. Ammeter A1 measures the current in the main
loop, while ammeters A2 and A3 measure the currents in the two branches of the circuit.

– + – +

A
A1
–+
A1
A A2
–+ A2

A A3 A3
–+

2. Close the switch and record the ammeter readings in the table below.

Ammeter A1 A2 A3

Reading (A) (Answers may vary.)

Cont'd

127
8 Making Use of Electricity

Discussion

A1. In the above circuit, which branch should have a higher resistance? The branch with one
bulb or the branch with two bulbs?
The branch with two bulbs

A2. From your results, what can you observe about the currents in different branches?

The branch with lower resistance has a larger current.

A3. From your results, what can you observe about the sum of the currents in the two branches
compared to the current in the main loop?
The sum of the currents in the two branches is equal to the current in the main loop.

Part B: Measuring the voltages in a parallel circuit

Procedure

3. Set up the parallel circuit shown below. The voltmeter V 1 measures the voltage across the
electric cells while voltmeters V 2 and V 3 measure the voltages across different branches
of the circuit.
V1 V1
V

V2
V2
V

V3 V3
V

4. (a) Close the switch. Record the voltmeter readings in the table below.

Voltmeter V1 V2 V3

Reading (V) (Answers may vary, but the readings are the same.)

(b) What can you observe about the voltmeter readings? They are the same.

128
 8

Making Use of Electricity

In Experiment 8.16, we studied the currents and voltages in a parallel
circuit. We can now understand the results as follows.

In a parallel circuit, the current in the main loop (denoted by I m)

splits when it flows into the branches (Fig. 8.47). Part of the current
flows through one branch (denoted by I 1) and the rest flows through
the other branch (denoted by I 2). The sum of the currents in the
branches is equal to the current flowing in the main loop. We can
write this as an equation:
Can you explain this relationship
of currents by using the analogy
on p.126? I m = I 1 + I 2 (Parallel circuit)

At the junction where a main

road splits into two branches, The size of the current in each branch depends on the resistance of
some lorries take the first
the branch. The branch with a lower resistance has a larger current.
branch and the rest take the
second branch. The sum of
the numbers of lorries taking
the branches is equal to the
number on the main road. Im
I1
• Im = I1 + I2
• I1 > I2

I2
Teaching notes
Remind students that the
lower branch has a higher
Fig. 8.47 Currents in a parallel circuit resistance because it has two
bulbs (provided that all bulbs
have the same resistance).

The voltage in each branch of a parallel circuit is the same. It is also

equal to the voltage across the electric cells.

Vc

i�ea� � S���n��

V1
Change and constancy in a
parallel circuit V1 = V2 = Vc
Suppose the circuit in
Fig. 8.48 is modified so
that it has one more V2 Supplementary notes
branch. The voltage in Voltage in parallel circuits
each branch will not (See p.T24)
change.
Fig. 8.48 Voltages in a parallel circuit

129
8 Making Use of Electricity

SPS: OB, PA, CM

Activity 8.5

Designing a simple circuit of a hairdryer

A hairdryer is a common electrical appliance. Basically, its structure includes a
heating element and a fan connected to a motor. The heating element heats
the air in the hairdryer and the fan blows the hot air out.

motor
fan

heating element

1. Design a circuit of a hairdryer that can work in two modes: warm wind mode and cool
wind mode. You can use a motor (to represent the fan), a nichrome wire (to represent the
heating element), switches and electric cells in your circuit. Draw the circuit diagram in
the space provided.

p.178
Drawing circuit diagrams

1. 3. Example of the circuit

diagram of a hairdryer with a
sliding switch.

The switch can be slid to M

M connect to terminals:
- 3 and 4 (warm wind mode)
- 2 and 3 (cool wind mode)
1 slide
- 1 and 2 (off mode) 2
(Continue below) 3
4

Tip Tip Caution

The circuit of the hairdryer you You can use the circuit symbols below: Do not touch the
design should be practical and nichrome wire.
1. M for the motor
safe to use. It is very hot.
2. for the nichrome wire

2. With your teacher’s approval, set up a circuit based on your design and test your design.

3. Search the Internet and find a circuit for a hairdryer we use in daily life.
For Question 3 (cont’d)
• The is a diode, which allows current • Mains electricity is an a.c. (current direction is changing from
to flow in one direction (to the right) but not heating element 發熱元件 time to time). However, the motor in
the other. the circuit works properly only under d.c. The four diodes
130 work together to change the current from a.c. to d.c.
 8

Making Use of Electricity

Series circuit Parallel circuit

• Same at every • Larger in the branch with

point lower resistance

Current • Current in the main loop equal

to the sum of the
currents in the branches

• Not the same across different • Same across every

circuit components branch
Voltage
• Equal to the voltage across the
electric cells

Skills Practice
(Identifying series circuits
e-aristo.hk/r/
and parallel circuits) e-Checkpoint
8.11 SPS: CS
iscpu8i11.e
cp08s11

A circuit is connected as shown below. The three light bulbs are identical.

Q R

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) The current flowing through bulb P is equal to the current flowing
through bulb Q. larger than F

(b) The current flowing through bulb Q is equal to the current flowing
through bulb R. T

(c) The current flowing through bulb P is larger than the current flowing
through bulb R. T

(d) The voltage across bulb P is equal to the voltage across the electric cells. T

2. If the current in the lower branch (with bulbs Q and R) is 0.5 A and the current drawn
from the electric cells is 1.5 A, what is the current in the upper branch (with bulb P)?
Current in the upper branch = 1.5 A – 0.5 A = 1 A

5-minute Quiz
Section Quiz 8.6
(Quiz 8: Series circuits and parallel circuits)
131
8 Making Use of Electricity

Our household electricity

Section objectives
Students should be able to
So far, we have studied some concepts of electricity. Now, let us
• be aware that many
household electrical understand how these concepts are related to the use of electricity in
appliances (e.g. hairdryer
and fan) are making use
our homes.
of the heating effect and
magnetic effect of current.
• recognize that electrical A How do electrical appliances work?
appliances are energy
converters.
1. Using the heating effect of electric current
We have learned about the
heating effect of electric current
Some electrical appliances in our homes, such as electric kettles,
in Section 8.3C.
work on the heating effect of electric current. These appliances have
heating elements, which are usually nichrome wires. When current
flows through these wires, most of the electrical energy is converted
into thermal energy for use.
Teaching notes
Nichrome is an alloy of nickel (80%) and chromium (20%),
heating element and also iron in some types of nichrome. Its melting point is
a about 1400 °C. It is highly resistant to
look high-temperature oxidation.
c
inside
b heating element
heating element

Fig. 8.49 The heating elements in (a) an electric kettle, (b) an electric stove and (c) a hairdryer

Active learning A filament light bulb also uses the filament

3D model
(Hairdryer) heating effect of electric current to
work. It contains a long thin tungsten
Teaching notes
• A filament bulb works at filament. When a current flows through
about 2500 °C. the filament, the filament becomes so
• Tungsten has the highest
melting point (3422 °C) hot that it glows and produces light.
among all metals. It can be
heated to a high Teaching notes
temperature without melting. In a filament bulb, much of the electrical energy is
This is why tungsten is used lost as themal energy and little (<10%) is converted
to make a bulb. into light energy (useful energy output). Fig. 8.50 A filament light bulb

heating element 發熱元件 tungsten 鎢

132
 8

Making Use of Electricity

SPS: OB, PA

Activity 8.6

Making a simple electric kettle

1. Your teacher will divide your class into different groups and give each
group the following materials and apparatus.

nichrome wire pencil power supply thermometer

connecting wires beaker plasticine ammeter

2. Use the apparatus above to make a simple ‘electric kettle’.

(a) Coil the nichrome wire around the pencil,

nichrome wire
leaving a 10 cm uncoiled section at each end.

10 cm 10 cm

(b) Slide off the coiled nichrome wire from the plasticine
pencil and put it into a beaker. Use plasticine to
fix the wire in position.
beaker
Caution
Make sure the coiled nichrome wire does not coiled
touch the walls or the bottom of the beaker. nichrome wire

3. Connect the ‘electric kettle’ to a power supply as

power supply
shown. Pour 200 cm 3 of water into the beaker.
Make sure the water covers the entire nichrome
thermometer
coil and the thermometer does not touch the coil.
ammeter

Caution
• Do not turn on the power supply without
your teacher’s approval.
• Do not touch the nichrome wire after the
power supply is turned on. It is very hot.

4. Your teacher will check your set-up. With your teacher’s approval, turn on the
power supply and set the voltage to 10 V. Measure and record the temperature increase
of the water after five minutes.

5. Compare your group’s result with those of other groups. Think about ways to improve the
performance of your ‘electric kettle’. Make a longer coil with more number of turns.

133
8 Making Use of Electricity

Electrical energy is converted into thermal energy for use

in the heating element in electrical appliances.

2. Using the magnetic effect of electric current

You can refer to p.98 to In Section 8.3C, we saw an application of the magnetic effect of
recall what you have studied.
electric current: electromagnets.

The magnetic effect of electric current has another use that is

important for electrical appliances. Let us find out in the following
experiment.

SPS: OB, PA

Experiment 8.17 Demonstration

e-aristo.hk/r/
Experiment video
isexptu8i17.e ev08s17

Observing the force on a current-carrying copper rod

Material and apparatus

magnets with steel yoke 1 set low voltage power supply 1 copper rod 1
parallel copper rail 1 set connecting wire 2

Procedure Caution
Do not turn on the power
supply for long. Otherwise, the
1. Your teacher will set up the following circuit. circuit will become very hot.
Teaching notes power supply copper rod
If the current direction
is reversed or the
poles of the magnets
are interchanged, the magnets
copper rod moves in
the opposite direction.

parallel copper rail

2. Switch on the power supply.

What happens to the copper rod? The copper rod moves.

3. Switch off the power supply. Remove the magnets from the set-up and repeat Step 2.

What happens to the copper rod now? The copper rod does not move.

134
 8

Making Use of Electricity

When a current-carrying conductor is placed near a magnet, a
force is produced and this force can cause the conductor to move.
This effect is used in electric motors.

a b
magnets
case
Teaching notes open the case
• Motors are also used in
mobile phones for
producing vibration alerts.
• A hi-fi loudspeaker
produces sound when its
diaphragm vibrates by
coils
making use of magnetic
effect of current. Fig. 8.51 (a) A motor and (b) its internal structure
diaphragm

Electric motors can convert electrical energy into kinetic energy.

They are used to drive the moving parts in many electrical
appliances we use at home such as electric fans, vacuum cleaners
coil and washing machines.

magnet

Learn more
Use of electric motors in
electric cars

electric motor

Fig. 8.52 The drum of a washing machine is driven by an electric motor, which works
on the magnetic effect of electric current.
Powerful electric motors
are used in electric cars.
The electric motors are
quieter than the engines
in cars that run on petrol.
Electric motors also emit
no air pollutants when
The moving parts of electrical appliances are driven by
running. This is why electric motors , which make use of
electric cars are more
environmentally friendly the magnetic effect of electric current to work.
and become more popular
in recent years.
Teaching notes
Although electric cars do not produce any emissions, the power station that generates electricity for electric cars
to use does. Also, some studies suggest that the production of electric cars cause more pollution than that of
conventional cars.
electric motor 電動機 drum 轉筒

135
8 Making Use of Electricity

connec t i o n 8.1

Making a DIY motor

Have you imagined that you can construct your
own electric motor and use it to power objects
like a toy wheel? In this project, you will construct
an electric motor, which is easily modified to
serve different purposes.
e-aristo.hk/r/ st08s1
isstemu8i01.e

Learn more 3. Electrical appliances as energy converters

Electrical appliances
invented in recent years The use of electricity is very important for our modern living.
Some electrical appliances Through the use of different electrical appliances, electrical energy
invented in recent years can be converted into different forms of energy useful for different
are shown below. Can you
identify the energy purposes. In other words, electrical appliances are energy converters
conversions involved? Can serving different purposes. For example, a rice cooker can convert
you tell how they are
useful for modern living? electrical energy into thermal energy for cooking. electrical energy
light energy (and
electrical energy thermal energy also thermal energy)

air amplifier

electric
electrical energy
wheelchair
kinetic energy

electrical energy kinetic energy (and also sound energy)

Fig. 8.53 Electrical appliances are energy converters serving many different purposes.
Can you point out the energy conversions involved in the appliances above?

Electrical appliances are energy converters

pop art toaster useful for different purposes.

Air amplifier: electrical energy kinetic energy

Electric wheelchair: electrical energy kinetic energy
136 Pop art toaster: electrical energy thermal energy
 8

Making Use of Electricity

connec t i o n 8.2

Designing and making electrical devices

Have you ever thought of making a DIY photo frame
that can rotate on its own, or a school bag alarm
that rings when the bag is opened? In this activity,
you are going to design and make some DIY
electrical devices using electronic building blocks
to solve some real life problems.
e-aristo.hk/r/ st08s2
isstemu8i02.e

e-aristo.hk/r/
e-Checkpoint
8.12 iscpu8i12.e
cp08s12

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) Most of the electrical energy is converted into thermal energy in the
heating element of electrical appliances. T

(b) An electric motor works on the heating effect of electric current. F

2. State whether the electrical appliances below work by making use of

I. the heating effect of electric current only,
II. the magnetic effect of electric current only,
III. or both effects above.

Write I, II or III in the boxes provided.

(a) electric toothbrush (b) toaster (c) hairdryer

II I III

5-minute Quiz
(Quiz 9: How electrical appliances work)

137
8 Making Use of Electricity

Section objectives
Students should be able to B Mains electricity and domestic circuits
• state the mains voltage in
Hong Kong. 1. Mains electricity
• understand the wiring of a
3-pin plug and identify the
colour coding of wires. Most electrical appliances in our homes get power from the
• explain why parallel circuits mains electricity. Mains electricity is generated in power stations
are preferred to series
circuits in domestic circuits. and transmitted to our homes through electric cables.

The mains electricity in Hong Kong has a voltage of 220 V. This

Connecting an electrical appliance
voltage is called the mains voltage. The mains voltage varies from
to an unsuitable mains voltage can countries to countries. Therefore, electrical appliances that can be
cause an explosion or fire!
used in one country may not be suitable for use in another.

Learn more
Mains voltages in different
countries
The mains voltages in some
countries are different
from that in Hong Kong.
For example, the mains
voltage in USA is 120 V.
Search the Internet to find
the mains voltages of some
other countries.
China: 220 V Fig. 8.54 In Hong Kong, electrical appliances use mains voltage of 220 V.
Japan: 100 V Teaching notes
U.K.: 230 V Some electrical appliances (especially electronic devices) can work within 100 V
U.S.: 120 V to 240 V. They can be used in different countries.

The mains electricity in Hong Kong has

a voltage of 220 V.

earth pin 2. Connecting to the mains electricity

To connect an electrical appliance to mains electricity, we need to

insert its plug into a mains socket. Fig. 8.55 shows the plug of an
electrical appliance for use in Hong Kong. The plug is called
a three-pin plug as it has three pins, namely the live pin,
the neutral pin and the earth pin. They fit into the live hole, the
live pin neutral pin neutral hole and the earth hole of the mains socket respectively
Fig. 8.55 A three-pin plug
(Fig. 8.57).

mains electricity 市電 three-pin plug 三腳插頭 earth pin 地線插腳

mains voltage 市電電壓 live pin 活線插腳
138 mains socket 市電插座 neutral pin 中線插腳
Teaching notes
The neutral and live holes of some sockets have shutters, which are closed when no plug is connected to the sockets.
8
This design aims to prevent objects, e.g. a finger, from accidentally inserting into the holes. The shutter is open when

Making Use of Electricity

an earth pin is inserted earth hole switch
into the earth hole. earth pin live pin neutral pin

switch
earth hole
live hole
neutral hole
neutral hole live hole

Fig. 8.56 A mains socket and its three holes Fig. 8.57 The pins on a plug fit into the corresponding
holes of a mains socket.
SPS: OB

Experiment 8.18 e-aristo.hk/r/

Experiment video
isexptu8i18.e ev08s18

Studying a three-pin plug and wiring the plug

Material and apparatus

three-pin plug (with cable) 1 cable 1 scissors 1 pair

three-pin plug (without cable) 1 screwdriver 1 wire stripper 1

Procedure

Part A: Studying the structure of a three-pin plug

1. Take the three-pin plug (with a cable) and remove its cover using a screwdriver.

three-pin plug

2. Identify the live pin, the neutral pin and the

earth pin
earth pin. They are connected to three wires,
namely the live wire, the neutral wire and the live pin
earth wire respectively in the plug.
neutral pin
Write down the colours of the wires.
fuse (you will
Live wire: Brown learn about it
later)
Neutral wire: Blue

Earth wire: Green-and-yellow Cont'd

139
8 Making Use of Electricity

Part B: Wiring a three-pin plug

3. Take the three-pin plug without a cable and remove its cover with a screwdriver.

4. Loosen the cable grip with a screwdriver.

three-pin plug with

no electric cable

cable grip

5. Use a pair of scissors to cut open the outer plastic cover of Caution
the cable and remove about 4 cm of it as shown below. Handle the pair of
scissors with care.
Identify the live wire, the neutral wire and the earth wire.

outer plastic cover cut along this line

about 4 cm

cut along this line wires

6. Lay the wires on the plug and cut each to a suitable length as shown below.

cut

cable grip

cable grip 電線夾

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Making Use of Electricity

7. Use a wire stripper to remove about 6 mm of copper strand
the coloured insulation from the end of each 6 mm
wire. Then, twist the bare copper strands
tightly.

Caution
Take care when twisting the copper
strands. They may hurt your fingertips.

8. Loosen the screw at the end of the earth pin. Insert the twisted end of the earth wire into
the hole in the earth pin. Then, tighten the screw to hold the end of the wire in place.
Repeat for the other two wires. Make sure that there are no loose strands.

check that there are no loose strands

screw

pin

copper hole
strand

9. Tighten the screws of the cable grip to fix the cable in position. Ask your teacher to check
your wiring.

10. Replace the cover of the plug and tighten the screw that holds it in place.

In the experiment above, we see the three wires in a plug. They are
the live wire, the neutral wire and the earth wire, and they have
brown, blue, and green-and-yellow colours respectively.

Teaching notes
• Before 1977, the colours for
live, neutral and earth wires
were red, black and green
respectively. The colour
code was amended in later
years. One reason for this is
that some electricians have
red-green colour-blindness.
The colour code in H.K.
earth wire
today started to be used
from 2007. live wire
• Electricians in Hong Kong neutral wire
commonly use the terms
‘火線’ and ‘水線’ for the live
Fig. 8.58 The wiring of a mains socket and a three-pin plug
and earth wires.
wire stripper 剝線鉗

141
8 Making Use of Electricity

Teaching notes In fact, only the live and neutral wires are responsible for delivering
The current direction in a
domestic circuit changes electricity to electrical appliances. The live wire carries the voltage
periodically—it sometimes that drives the current flowing through the appliance. The neutral
flows from the live wire to the
neutral wire and sometimes wire provides a return path for the current to complete the circuit.
flows in the opposite The earth wire is a safety device only. You will learn more about its
direction. It is an a.c.
(alternating current). function in Section 8.8B.
heating element

N L

electric kettle

Fig. 8.59 The live (L) and neutral (N) wires deliver the electricity to the electric kettle.

The pins of a three-pin plug are connected to the live ,

neutral and earth wires.

e-aristo.hk/r/
e-Checkpoint
8.13 iscpu8i13.e
cp08s13

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) The mains voltage in Hong Kong is 220 V. T

(b) The earth wire provides a return path for the electric current. F
neutral
2. The diagram below shows the inner part of an electric plug. In the diagram, draw the
live wire, the neutral wire and the earth wire. Label the wires and write down their colours.

earth wire
(colour: green-and-yellow)

electric plug

neutral wire (colour: blue)

live wire (colour: brown)

142
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Making Use of Electricity

3. Domestic circuits

An electrical appliance gets mains electricity when it is connected to

a mains socket, but how is the mains electricity supplied to the
mains sockets?

Mains electricity is transmitted from power plants to our home

through an electric cable. The cable is connected to the consumer
unit of our home, from which electricity is distributed to mains
Fig. 8.60 A consumer unit
sockets. Let us look at the domestic circuit in the figure below to
understand more.

consumer unit
connected to the power plant

electric cable mains

socket
live wire
neutral wire
earth wire

Fig. 8.61 How electricity is distributed to mains sockets in a domestic circuit

The domestic circuit above can be re-drawn as shown in Fig. 8.62.

From the figure, you can see that the mains sockets (with the
electrical appliances) are connected in parallel. You will understand
more about the reason behind after completing Experiment 8.19.

Teaching notes
The circuit on the right is
called a ring circuit. The consumer unit
concept of a ring circuit is
out of the syllabus of 2018. connected to
the power plant

live wire

neutral wire

Fig. 8.62 In a domestic circuit, the electrical appliances are connected in parallel.
(The mains sockets and the earth wire are not shown in this diagram.)

143
8 Making Use of Electricity

SPS: OB, PA

Experiment 8.19 e-aristo.hk/r/

Experiment video
isexptu8i19.e ev08s19

Should electrical appliances be connected in parallel or in series?

Material and apparatus

circuit board 1 knife switch 2 electric cell 2

light bulb 2 rheostat 1 connecting wire 6

Procedure

Part A: Connected in parallel

1. (a) Set up the circuit shown on the right. Set the resistance
of the rheostat to zero. B1 S1

(b) Ask your teacher to check your connection.

B2 S2

2. Close switches S 1 and S 2. Does bulb B 1 light up? Does bulb B 2 light up? Record your results
in the table in Step 3.

3. Repeat Step 2 by closing or opening switches S 1 and S 2 as shown in the table below. Record
your results in the table.

Switch S1 Switch S2 Does bulb B 1 light up? Does bulb B2 light up?

closed closed yes yes

closed open yes no

open closed no yes

open open no no

4. (a) Can bulb B 1 be turned on alone without turning on bulb B 2? Yes

(b) Can bulb B 1 be turned off alone without turning off bulb B 2? Yes

5. Close switches S 1 and S 2. Increase the resistance of the rheostat to change the brightness
of bulb B 1.

Does changing the brightness of bulb B 1 affect the brightness of bulb B 2? No

Cont'd

144
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Making Use of Electricity

Part B: Connected in series
6. (a) Set up the circuit shown below. Set the resistance of the rheostat to zero.

(b) Ask your teacher to check your connection.

B1 S1 B2 S2

7. Repeat Steps 2 and 3 using the circuit above. Record your results in the table below.

Switch S1 Switch S2 Does bulb B1 light up? Does bulb B2 light up?

closed closed yes yes

closed open no no

open closed no no

open open no no

8. (a) Can bulb B 1 be turned on alone without turning on bulb B 2? No

(b) Can bulb B 1 be turned off alone without turning off bulb B 2? No

9. Close switches S 1 and S 2. Increase the resistance of the rheostat to change the brightness
of bulb B 1.

Does changing the brightness of bulb B 1 affect the brightness of bulb B 2? Yes

In Experiment 8.19, the light bulbs can operate independently when

they are connected in parallel but not when connected in series.

Similarly, the mains sockets in a domestic circuit are connected in

parallel so that the electrical appliances can operate independently.
For example, if you turn off a fan at home, other appliances can
still work.

Domestic circuits are parallel circuits so that the electrical

appliances can operate independently .

5-minute Quiz
(Quiz 10: Mains electricity and domestic circuits) 145
8 Making Use of Electricity

Electrical safety with

household electricity

Fig. 8.63 When too many air conditioners in our homes are switched on at the same time, the circuit breaker may be open
and the electricity would then be cut off.

Section objectives Have you had the experience above? The circuit breaker is a safety
Students should be able to
• understand the danger of device that protects us from accidents related to the use of
overloading in the use of electricity. You will learn about electrical safety in this section.
universal adaptors.
• understand the condition
leading to short circuits A Potential hazards in using electricity
and its danger.
Each year, over 700 fires are caused by electrical accidents in
Hong Kong. Electrical accidents are not rare.

Activity 8.7

Searching for news about electrical accidents

Search for a news article about an electrical
accident in Hong Kong. Paste the article in the
box provided and discuss the following
questions with your classmates.

1. What was the cause of the accident?

2. What damages resulted from the accident?

3. How can we prevent this kind of accident?

From Activity 8.7, you can see that there are two common causes of
electrical accidents: overloading and short circuits.

circuit breaker 斷路器

146
 8
1. Overloading

Making Use of Electricity

We often connect more than one electrical appliance to a mains
socket using a universal adaptor. However, this creates potential
hazards. Let us see how they arise in the experiment below.
SPS: OB, PA

Experiment 8.20 e-aristo.hk/r/

Experiment video
isexptu8i20.e ev08s20

Studying the effect of added devices when connected in parallel

Material and apparatus

light bulb 3 ammeter 1 connecting wire 2

knife switch 3 electric cell 2 connector 5

Procedure

1. Set up the circuit shown on the right.

2. Close switch S 1 to turn on light bulb B 1. A

B1 S1
What is the ammeter reading? (Answers may vary.)

3. Close S 1 and S 2 to turn on light bulbs B 1 and B 2. B2 S2

What is the ammeter reading? (Answers may vary.)

B3
4. Close S 1, S 2 and S 3 to turn on the three light bulbs. S3

What is the ammeter reading? (Answers may vary.)

Conclusion
The current in the main loop increases with the number of bulbs connected in parallel.

In the experiment above, the current in the main loop increases with
the number of light bulbs turned on. Similarly, if we connect too
many electrical appliances to a universal adaptor, the current drawn
from the mains socket may become too large. This is called
overloading. The circuit will become very hot and accidents such as
fires or even an explosion may result.
Classroom questions
Q: Some electrical appliances usually connect to a mains socket alone.
Fig. 8.64 Avoid overloading a Name one of such appliance.
mains socket. A: Air conditioner/Electric heater
Q: Why should we avoid connecting this appliance to the same socket with
other appliances?
overloading 超負荷 A: Avoid overloading as this appliance draws a large current
147
8 Making Use of Electricity

Connecting too many electrical appliances to a mains socket

causes overloading and the current will be too large.

2. Short circuits

A short circuit is another common cause of electrical accidents. Let

us understand more in the experiment below.

SPS: OB, PA

Experiment 8.21 Demonstration

e-aristo.hk/r/
Experiment video
isexptu8i21.e ev08s21

What is a short circuit?

Material and apparatus

circuit board 1 ammeter 1 connecting wire 5

switch 1 electric cell 3
light bulb 1 nichrome wire 1

Procedure

1. Your teacher will set up the circuit shown on

the right. A

2. Close the switch.

(a) Does the bulb light up? Yes nichrome wire

(b) What is the ammeter reading? (Answers may vary.)

3. Your teacher will open the switch and connect

a wire in parallel with the light bulb. A

4. Close the switch. nichrome wire

(a) Does the bulb light up? No wire

(b) What is the ammeter reading? (Answers may vary.) Caution

• Do not touch the nichrome wire.
(c) What can you observe about the nichrome It is very hot.
• Do not close the switch for long.
wire? This may cause the circuit to
become very hot, resulting in
The nichrome wire becomes hot and glows. danger.

148
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Making Use of Electricity

In Experiment 8.21, when a wire is connected in parallel across
the light bulb, the bulb goes out and the current increases sharply.
This is because the wire has very low resistance and thus provides a
‘much easier’ path for the current to flow. As a result, nearly all
current flows through the wire instead of the bulb. This is called a
short circuit (Fig. 8.65). As the resistance of a short circuit is very
low, the current becomes very large.

Fig. 8.65 In a short circuit, the electric current

can be very large.

Short circuits can occur inside an electrical appliance if its wires are
loose or worn out (Fig. 8.66). For example, in the electric kettle in
Fig. 8.67b, nearly all the current flows through the loose wire instead
of the heating element. This leads to a very large current so the
circuit becomes very hot and may catch fire.

copper strands plastic insulation electric wire

Fig. 8.66 If electric wires are worn out or damaged, copper

strands inside may touch each other, causing a short circuit.

a heating element b a short circuit occurs

N L N L

The size of the The current is very large

current is normal. and the circuit is very hot.

Fig. 8.67 An electric kettle (a) in normal operation and (b) with a short circuit

short circuit 短路

149
8 Making Use of Electricity
Short circuits can also occur in a circuit with dust built up on it, or
in faulty electronic devices such as defective mobile phones.

dust

Fig. 8.68 In a circuit board, a short circuit may Fig. 8.69 This defective mobile
occur due to the dust built up on the board. phone burned due to a short circuit.

If a wire is connected in parallel across a heating element, a

short circuit occurs. The current may
become very large and the circuit overheats.

Section objectives
Students should be able to
B Electrical safety devices
• recognize fuses and
circuit breakers as devices To ensure safety in using electricity, household circuits and electrical
for protecting circuits.
• recognize the importance appliances are installed with different kinds of safety devices. Look
of the use of earth wire. at some examples below.

1. Fuses

Fuses can be found in electric plugs, universal adaptors,

extension units and electrical appliances.

Fig. 8.70 A fuse Fig. 8.71 The circuit symbol of a fuse

a b

fuse
fuse

Fig. 8.72 The fuses in (a) a plug and (b) a universal adaptor

fuse 保險絲 extension unit 拖板

150
 8

Making Use of Electricity

We can see how a fuse works in the experiment below.

SPS: OB, PA

Experiment 8.22 Demonstration

e-aristo.hk/r/
Experiment video
isexptu8i22.e ev08s22

Using a fuse to protect a circuit

Material and apparatus

0.25 A fuse (transparent) 1 light bulb 1 connecting wire 4

low voltage power supply 1 ammeter 1

Procedure

1. Your teacher will give you a fuse. Examine the inside of it carefully.

What can you observe? There is a thin metal wire inside the fuse.

2. Your teacher will set up the circuit shown below using the fuse you examined in Step 1.

power supply

power supply

A
A

0.25 A fuse

3. Switch on the power supply. Slowly increase the current from zero to 0.5 A. Observe the
brightness of the light bulb.

How does the brightness of the light bulb change?

The bulb becomes brighter and then suddenly goes out.

4. Switch off the power supply. Remove the fuse and examine the inside of it carefully.

What can you observe? The thin wire inside the fuse has broken/melted.

5. Your teacher will connect the bulb to the power supply (set at a low voltage) to test if the
bulb can still light up.

Can the bulb still light up? Yes

151
8 Making Use of Electricity

Teaching notes A fuse protects a circuit by breaking the circuit when the current is
• Thomas Edison first
patented a fuse in 1880. too large. A fuse contains a thin metal wire (Fig. 8.73). If the current
• Fuse wires are usually flowing through it is too large, the wire becomes hot and melts.
made of alloys containing
nickel, chromium and iron. We say ’the fuse blows‘. As a result, the circuit becomes open and
• A fuse is installed in the the flow of current stops. This prevents the circuit from overheating.
live wire so that the
electrical appliance can
be disconnected from the Each fuse has a fuse rating (Fig. 8.74). It shows the maximum current
live wire, which is the that can flow through the fuse without blowing it. For example, a
‘dangerous’ wire carrying
voltage, in case the fuse 5 A fuse allows a current of 5 A or less to flow. It will blow if the
blows. current is larger than 5 A.

thin metal wire

Fig. 8.73 The thin metal wire in a fuse Fig. 8.74 Fuses with different fuse ratings
Classroom question
Q: Can a fuse protect an
appliance (e.g. the rice To protect a circuit, we should use a fuse with a fuse rating
cooker below) if the fuse is slightly higher than the current in the circuit under normal
connected in parallel with
the heating element? Why? operation. For example, the rice cooker in Fig. 8.75 draws a current
A: No, because a large current of 4 A under normal operation. Therefore, we can use a 5 A fuse to
may pass through the
heating element without protect it.
blowing the fuse.
a b
Fuse rating suitable Fuse rating too high

rice cooker rice cooker

blown not blown

5 A Fuse 13 A Fuse

Fig. 8.75 Under normal operation, the rice cookers above have a current of 4 A. In the case of a large current
of 10 A, the circuit of the rice cooker in (a) is broken as a 5 A fuse is used, but in (b) the circuit is not broken as a
13 A fuse is used. In (b), the large current does not stop flowing and the fuse fails to protect the circuit.

fuse rating 保險線額定值

152
 Teaching notes
8
Remind students that the

Making Use of Electricity

electrician checks and fixes
Once a fuse has blown, it cannot be used again, but needs to be
the fault that causes the fuse replaced with a new one. However, we should ask an electrician to
to blow. Replacing the fuse
without fixing the fault poses
fix the fault in the circuit or appliance, and replace the fuse.
potential hazards.

Activity 8.8

Choosing a suitable fuse for electrical appliances

The table below shows the currents drawn by three electrical appliances in normal operation.
Choose a suitable fuse (0.5 A, 1 A, 3 A or 5 A) for each appliance.

Electrical appliance Current drawn under normal operation Fuse rating

microwave oven 4.5 A 5A

rice cooker 2A 3A

television 0.8 A 1A

A fuse will blow and break a circuit if the

Teaching notes
May use this animation to allow current is above its fuse rating.
students to see how a certain
type of circuit breaker works:
e-aristo.hk/r/isteu8i03.e
2. Circuit breakers

Circuit breakers are used in the consumer units of our homes.

A circuit breaker is an automatic switch that has a function similar
to a fuse. It has a rating showing the maximum current that can
flow through it. If the current exceeds the rating, the circuit breaker
will switch off by itself to break the circuit.

Unlike a fuse, a circuit breaker need not be replaced after breaking

the circuit. It can be reset (switched on again) to restore the circuit.
circuit
breakers

Fig. 8.76 Circuit breakers in

a consumer unit
A circuit breaker will turn off by itself and break a
circuit if the current is above its rating.

circuit breaker 斷路器

153
8 Making Use of Electricity

3. Earth wire

In Section 8.7B, we have worked with the earth wire in a plug. It is

a safety device that connects the metal casing of an electrical
appliance to the ground. This is called earthing.

What might happen if an electrical appliance does not have an earth

wire? Suppose this appliance has a fault which causes a live wire to
touch the metal casing. If we touch the metal casing, the current
will flow through our body to the ground. We will then get an
electric shock.

hand

an exposed live wire current

touches the metal casing

Fig. 8.77 We will get an electric shock if we touch this faulty appliance that has no
earth wire.

Teaching notes
• The resistance of the earth If the faulty appliance has an earth wire, the current will flow
wire is much smaller than
that of our bodies. Thus, through the earth wire to the ground instead of through our body.
the current will flow This protects us from getting an electric shock. In most cases, the
through the wire instead of
our bodies to the ground. current also causes the fuse to blow and thus the circuit is broken
• Electrical appliances with before we touch the appliance.
double insulation do not
need to have an earth wire
as the circuit and the wires
inside them are separated
from the casing with
plastic.

hand
Symbol of double insulation an exposed live wire current
touches the metal casing

Fig. 8.78 The earth wire protects us from getting an electric shock.

The earth wire is connected to the metal casing

of an appliance. It protects us from getting an electric shock.

earthing 接地

154
 Section objective
Students should be able to state
Prior knowledge
In primary level, students have learned about
8
safety precautions in using electricity. the safety precautions when using electricity.

Making Use of Electricity

C
Active learning
Useful website Safety precautions in using electricity
(Advice on using
electricity safely)
Although the use of fuses, circuit breakers and earth wires helps us
use electricity safely, it is important to take the following precautions
when using electricity.

spa
ce
space

space

Use electrical appliances Leave enough space around Ask a registered electrician
with a three-pin plug (that electrical appliances to to replace damaged wires
includes the earth wire). prevent overheating. and plugs, etc.

wet hand

Do not overload mains Never touch switches or Do not use electrical

sockets. electrical appliances with appliances in wet places.
wet hands.

electric heater

Do not use damaged plugs Never pull on a cable to Do not place electric cables
and universal adaptors, etc. remove a plug from a mains near a heater or stove.
socket.
Fig. 8.79 Safety precautions of using electricity

155
8 Making Use of Electricity

Some safety precautions:

• To charge the device, use the chargers provided by the manufacturer of
Activity 8.9 the device. Also, use the battery provided by the manufacturer.
• Pay attention to whether the device is unusually hot.
• Do not use the device when it is connected to a mains socket for charging.
What safety precautions should we take when using electronic devices?
We often use electronic devices such as smartphones and
portable power banks. What safety precautions should we take to
avoid electrical accidents related to these devices? Design a
poster or make a video clip offering advice to increase the
awareness on electrical safety.

e-aristo.hk/r/
e-Checkpoint
8.14 iscpu8i14.e
cp08s14

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) Overloading can cause a dangerously large current in a circuit. T

(b) When a short circuit occurs, the resistance of the circuit is very high. F

(c) The current flowing through an appliance under normal operation is

higher than the rating of the fuse. (The fuse rating should be slightly higher than F
the current.)
smaller
(d) An earth wire can prevent overheating of a circuit due to large current. F
people from getting an electric shock
2. Kate and her family have little awareness of electrical safety. Study the picture of their
home below and circle the safety hazards.

5-minute Quiz
Section Quiz 8.7–8.8
156 (Quiz 11: Electrical safety with household electricity)
 8

Making Use of Electricity

Power, energy and the cost
of electricity
How come the cost of
electricity is so high?
Do you know how the electricity
company calculates the cost of electricity
we use? How can we choose electrical
appliances to reduce the cost of
electricity? Let us find out in this section.

EXTENSION
Section objectives
Students should be able to
• understand power as the
electrical energy transferred Fig. 8.80 A housewife reading
to an electrical appliance an electricity bill
per second:
energy
power = [E]
time
• state that watt (W) is a unit A Power of electrical appliances
of power. [E]
Active learning Have you noticed the following labels on electrical appliances?
Simulation
(Power, energy and the
cost of electricity)

Teaching notes Fig. 8.81 Labels on two electrical appliances

• The unit of power (watt)
was named after Scottish
engineer James Watt The labels show the powers of the two appliances. Power is the
(1736–1819), who
invented a steam engine. amount of energy transferred to the electrical appliance per second.
• May introduce the prefix of The unit of power is watt (W). A larger power can be measured in
units k which means 1000.
Other examples of units kilowatts (kW).
with the prefix k:
– 1 km = 1000 m
– 1 kg = 1000 g 1 kW = 1000 W

The relationship between power, energy and time can be shown

with the formula below.

energy (J)
In Unit 5 of Book 1B, we learned power (W) =
that the unit of energy is joule (J). time (s)

power 功率
watt 瓦特
kilowatt 千瓦特 157
8 Making Use of Electricity

Teaching notes Different electrical appliances have different powers. The greater
May use the booking fee of
a badminton court to help
the power, the more electrical energy that is transferred to the
students distinguish energy appliance in a given period of time.
and power:
cost to book the power We can calculate the energy transferred to an electrical appliance
court per hour in a given period of time if we know the power of the appliance. By
total cost to book energy
the court for a the formula of power,
period of time
energy (J) = power (W) × time (s)
EXTENSION

Example 8.1
A 12 W lamp has been switched on. Find the electrical energy it uses in 30 minutes.

Solution
Power = 12 W
Time = 30 min = 30 × 60 s = 180 s
Energy (J) = power (W) × time (s)
= 12 W × 180 s
= 2160 J
The electrical energy used by the lamp is 2160 J.

Activity 8.10

Calculating the electrical energy used by electrical appliances

1. The table below shows the powers of four electrical appliances in Joe’s home and how
long each is used per day. Calculate the electrical energy used by each appliance per day.

Electrical appliance Power (W) Time of use (h) Electrical energy used (J)

air conditioner 1500 8 43 200 000 (or 4.32 × 107)

table lamp 10 5 180 000 (or 1.8 × 105)

electric kettle 2000 1 7 200 000 (or 7.2 × 106)

television 180 2 1 296 000 (or 1.296 × 106)

2. Which of these electrical appliances uses the most electrical energy

(a) per day? Air conditioner

(b) per second? Electric kettle

158
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Making Use of Electricity

Power is the amount of energy transferred
to an electrical appliance per second. It is measured in
watts ( W ) or kilowatts ( kW ).

Section objectives
Students should be able to B Cost of electricity
• recognize that a kilowatt-hour
meter and a joulemeter are 1. Electrical energy calculated in kilowatt-hours

EXTENSION
used to measure the electrical
energy used. [E]
• state that kilowatt-hour (kWh) We have learned how to apply a formula to calculate the amount of
is a unit of electrical energy electrical energy used by an electrical appliance. In fact, the
for calculating the cost of
electricity. [E] electrical energy can be measured using a joulemeter or
• understand the inter- a kilowatt-hour meter.
conversion between joule and
kilowatt-hour. [E] a b
• calculate the cost of electricity
from the amount of electrical
energy used. [E]

Fig. 8.82 (a) A joulemeter and (b) a kilowatt-hour meter

Joulemeters are usually used in a laboratory. Kilowatt-hour meters

are usually used by electricity companies to measure the amount of
electrical energy we use at home. The amount is measured in
kilowatt-hours (kW h). 1 kW h is the amount of electrical energy used
by a 1000 W (or 1 kW) electrical appliance in one hour (h). In fact,
1 kW h is equal to one unit of electricity shown on the electricity bill.

Misconception
Students may think that kilowatt-hour (kW h) is Unit
a unit of power because of the ‘kilowatt’ in the
name. Remind them that it is a unit of electrical
400
energy. May compare the following:
kilowatt-hour (kW h) – a unit of electrical energy
kilowatt (kW) – a unit of power

Fig. 8.83 Each unit of electricity used is equal to 1 kW h of electrical energy.

joulemeter 焦耳計
kilowatt-hour meter 千瓦時計
kilowatt-hour 千瓦小時 159
8 Making Use of Electricity

2. Conversion between units of joules and

kilowatt-hours

Both the joule and kilowatt-hour are units of electrical energy.

These two units can be inter-converted. From what you have learned
before,

1 kW h = energy used by a 1000 W appliance in 1 hour

= power (W) × time (s)
= 1000 W × 60 × 60 s
EXTENSION

= 3 600 000 J

The conversion between the kilowatt-hour and joule is given by:

1 kW h = 3 600 000 J

3. Calculating electrical energy in kilowatt-hours

e-Explore using the formula for power
8.3 Proof of the formula for
finding the electrical energy On p.158, we learned how to use a formula to calculate the
in kilowatt-hours
electrical energy, in joules, used by an electrical appliance. We can
The formula for finding the
electrical energy in also use the same formula to calculate the electrical energy in
kilowatt-hours can be kilowatt-hours. First, we need to change the units of power and time
proved using what we have
studied. to kilowatts (kW) and hours (h) respectively. Then, we can use the
Explore more on our Web. formula below to calculate the electrical energy:
ex08s3
electrical energy (kW h) = power (kW) × time (h)
e-aristo.hk/r/
isexploreu8i03.e Teaching notes
May compare the following equations:
energy (J) = power (W) × time (s)
Example 8.2 electrical energy (kW h) = power (kW) × time (h)

Kate is cleaning the floor at home with a 1500 W vacuum cleaner. Find the electrical
energy it uses in 15 minutes.

Solution
1500
Power = 1500 W = kW = 1.5 kW
1000
15
Time = 15 min = h = 0.25 h
60
Electrical energy (kW h) = power (kW) × time (h)
= 1.5 kW × 0.25 h
= 0.375 kW h
The electrical energy used by the vacuum cleaner is 0.375 kW h.
160
Teaching notes
In Hong Kong, the cost of
H.K. Electric
e-aristo.hk/r/isteu8i04.e
CLP
e-aristo.hk/r/isteu8i05.e 8
electricity (residential) is
calculated under the progressive 4. Calculating the cost of electricity

Making Use of Electricity

method. Details can be found on
the right.
The kilowatt-hour meter in our home measures our electricity usage.
The electricity company can then calculate the cost of electricity
using the formula below.

1 unit of electricity stands for 1 kW h

of electrical energy used.
cost of electricity = units of electricity used × cost per unit
Skills Practice
(Calculating the cost of electricity)
SPS: IF

EXTENSION
Example 8.3
Leo used 500 units of electricity last month. If the cost per unit is $1.3, what is the cost
of electricity?

Solution
Cost of electricity = units of electricity used × cost per unit
= 500 × $1.3
= $650

SPS: IF

Activity 8.11

Calculating the cost of electricity

The following shows an electricity bill.

DOMESTIC TARIFF Bill Type & Merchant Code No. : 02

Bill issued on (DD-MM-YY) From 06-06-18 Thru 06-08-18 Deposit Held

06-08-18 For 62 days of electricity consumption $990.00
Due date
Electricity Charge (Bimonthly) @ 81.7 c 400 units 326.8
@ 94.5 c 600 units 567 21-08-18
@ 109.8 c 155 units 170.19
Fuel Clause @ 22.4 c 258.72
Rebate @ 0c 0
Odd Cents Brought Forward 0.79 Total Amount Due
Odd Cents Carried Forward -0.50
$1,323.00
Total Amount $1,323.00

1. Fill in the orange boxes above by calculating the corresponding costs.

2. As shown above, the cost of electricity per unit increases with the electricity usage.
Suggest ONE reason to explain why the cost is charged in this way.
This encourages people to save electricity.

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8 Making Use of Electricity

SPS: OB, PA, IF

Experiment 8.23 Demonstration

e-aristo.hk/r/
Experiment video
isexptu8i23.e ev08s23

Finding the cost of electricity used by an electric kettle to boil water

Material and apparatus

electric kettle 1 kilowatt-hour meter 1 stopwatch 1

EXTENSION

Procedure

1. Fill an electric kettle with water. Then, connect the kettle to a mains socket
via a kilowatt-hour meter.

kilowatt-hour meter

to a mains socket
electric kettle

2. Record the reading of the kilowatt-hour meter in the table in Step 4.

3. Switch on the electric kettle and start the stopwatch at the same time.

4. When the water boils and the electric kettle shuts off automatically, stop the stopwatch.

(a) How long is the electric kettle switched on? (Answers may vary.)

(b) Record the reading of the kilowatt-hour meter in the table below.

Reading of the kilowatt-hour meter

Before the experiment (Answers may vary.)

After the experiment (Answers may vary.)

What is the electrical energy supplied to the electric kettle, in kW h?

(Answers may vary.)

Cont'd

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Making Use of Electricity

Discussion

1. Suppose that the cost of electricity is $1.3 per unit. Calculate the cost of electricity used
to boil the water in the experiment.
(Answers may vary.)

2. (a) Convert the electrical energy used by the electric kettle from kW h to J.
(Answers may vary, but this formula is used: 1 kW h = 3 600 000 J)

EXTENSION
(b) Hence, calculate the power in W of the electric kettle using the formula of power.
energy (J)
(Answers may vary, but this formula is used: power (W) = )
time (s)

• The electrical energy used by an appliance can be measured

using a joulemeter or a kilowatt-hour meter .
• For the household electricity, 1 unit of electricity means
1 kilowatt-hour ( kW h ) of electrical energy used.

e-aristo.hk/r/
8.15 iscpu8i15.e e-Checkpoint
cp08s15

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) The power of an appliance is the total amount of electrical energy it uses. F

(b) Kilowatt-hour (kW h) is a unit of power. F

energy
2. Kate forgets to turn off a 60 W desk lamp before going to
bed. The desk lamp remains switched on for 8 hours.

(a) Calculate the total electrical energy wasted in kW h.

60
Electrical energy wasted = power × time = kW × 8 h
1000
= 0.48 kW h

(b) Calculate the cost of electricity wasted. (Given: cost of electricity is $1.3 per unit)
Cost of electricity wasted = 0.48 × $1.3 = $0.624

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8 Making Use of Electricity

Section objective C Efficiency of electrical appliances

Students should be able to
recognize the efficiency of an
electrical appliance as the ratio The two light bulbs in Figure 8.84 have the same brightness.
of useful power output to power However, the LED bulb requires much less power than the filament
input: useful power
efficiency = output power input bulb. Do you know why?

×100% [E] 7 W LED bulb 60 W filament bulb

EXTENSION

Fig. 8.84 The LED bulb requires much less power to give the same brightness as the
filament bulb.

Teaching notes
• Efficiency of
An LED bulb converts much of the electrical energy into light energy.
– LED bulb: as high as 80% However, a filament bulb converts little of the electrical energy into
– Filament bulb: <10%
• LED (light-emitting diode)
light energy and much is converted into thermal energy. Thus, an
（發光二極管）is composed LED bulb requires less power input to produce the same brightness
of two different
semi-conductors joined as a filament bulb. In other words, the LED bulb has a higher
together, forming a junction. efficiency.
When charges flow across
the junction, energy is
released as light. As LED bulb filament bulb
heating is not involved
in the light emission, light energy light energy
much less energy is
lost as heat. electrical energy
thermal electrical energy
energy

junction thermal
energy

LED Fig. 8.85 Sankey diagrams of an LED bulb and a filament bulb giving out the same amount of
light energy

You have already learned that: In Unit 5 of Book 1B, you learned how to find the efficiency of an
useful energy output electrical appliance using energy. The efficiency can also be found
efficiency =
total energy input
using power. In terms of power, the efficiency of an electrical
×100%
appliance is the ratio of the useful power output to the power input:

useful power output

efficiency = × 100%
power input
efficiency 效率

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Making Use of Electricity

Example 8.4
A filament bulb has a useful power output of 7 W. If it requires a power input of 85 W,
find the efficiency of the bulb.

Solution
useful power output
Efficiency = × 100%
power input
7W

EXTENSION
= × 100%
85 W
= 8.2%

When designing electrical appliances, engineers need to think of

ways to improve the efficiency of the appliances so that they require
less electrical energy to operate. This helps conserve energy.

Activity 8.12

Energy labels of refrigerators

Study the following energy labels of two different refrigerators.

Refrigerator A Refrigerator B

1. Which refrigerator uses energy more efficiently? Refrigerator B

2. How much more electrical energy is consumed in a year by using the less efficient
refrigerator above?
330 kW h – 243 kW h = 87 kW h

Cont'd
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8 Making Use of Electricity

3. Your friend Amy is considering buying refrigerator A because it is less expensive than
refrigerator B. What advice would you give her?
Refrigerator A is less efficient and thus requires a higher cost of electricity to run.

Refrigerator B can help save money in the long run.

4. Air conditioners are widely used electrical appliances. Suggest ONE way to reduce
energy used by air conditioners.
EXTENSION

Do not set the temperature too low.

Active learning
Useful website
(Energy Label Net) The efficiency of an electrical appliance is the ratio of the
useful power output to the power input of the appliance.

e-aristo.hk/r/
e-Checkpoint
8.16 iscpu8i16.e
cp08s16

1. Write ‘T’ for a true statement and ‘F’ for a false statement.
(a) The efficiency of electrical appliances cannot exceed 100%. T

(b) For an electrical appliance of 60% efficiency, 60% of its power input is
wasted when the appliance is running. (40% of its power input is wasted.) F
useful output
2. Look at the Sankey diagrams of three light bulbs below.

light bulb P light bulb Q light bulb R

light energy 0.8 J light energy 0.8 J light energy 0.6 J

electrical energy electrical energy electrical energy

40 J 4J 9J

thermal energy thermal energy thermal energy

39.2 J 3.2 J 8.4 J

(a) Which of the above bulbs has the highest efficiency? Light bulb Q
(b) Calculate the efficiency of the bulb in (a).
useful power output 0.8 W
Efficiency = ×100% = × 100% = 20%
power input 4W

5-minute Quiz
Section Quiz 8.9
166 (Quiz 12: Power, energy and the cost of electricity)
Have you heard of the scientist Thomas Edison? He was a scientist
in the 19th and early 20th centuries who invented many electrical
devices such as light bulbs. He is often considered to be the greatest Thomas Edison (1847–1931)
inventor of all time.

1 I am now learning
about eggs hatching. 2 When Edison was only 24

I invented a typewriter.

Edison was very curious since he was a child. He asked

many questions and did experiments to find answers.
Edison was very creative and kept inventing new things.

3 In 1876
I set up this laboratory with a
team of over twenty scientists.
4 In 1879
This is the light bulb
I invented.

In eight years, Edison’s team Edison invented the first light bulb
invented more than 400 devices. after fourteen months of hard work.

5 In 1882

Edison opened the first power plant,

which supplied electricity for street lights
in London. These street lights used light
bulbs invented by him.

T hink about
1. What characteristics made Edison a successful inventor? List some
of them. 1. Edison was very curious. Also, he was very creative.
2. Search the Internet to find more examples of Edison’s inventions.
Which one do you think is Edison’s greatest invention?
2. Examples of Edison’s inventions:
Phonograph (留聲機), early alkaline battery, first motion pictures
167
8 Making Use of Electricity

Concept map Padlet e-Concept Map (U8)

Electricity

can flow involves transmitted from energy

through concepts of power stations to involved
our homes is called

closed circuit mains electricity E electrical energy

can be used
can be safely by
connected in
avoiding

using

series parallel overloading short circuit

fuse earth wire

affects affects circuit

electric
voltage resistance breaker
current

has

unit
E kilowatt-hour
heating magnetic depends on
effect effect a wire’s energy
transferred
per unit time
E power
used in

electrical
appliance

E • material
• thickness
• length

168
 8

Making Use of Electricity

Unit summa
summary
ry
8.1 Introducing electricity and simple circuits
1. A closed circuit with a source of electrical energy is required for electricity to flow.

2. Materials that allow electricity to flow through them are called electrical conductors.

3. Materials that do not allow electricity to flow through them are called electrical insulators.

4. A switch is used to open or close a circuit.

8.2 Circuit diagrams

5. Circuit symbols are used to represent circuit components.

electric cell battery light bulb switch (closed) switch (open)

A V

connecting wire / connector ammeter voltmeter resistor rheostat

6. A circuit diagram is drawn to represent an actual circuit.

8.3 Electric current

7. An electric current in metal is a flow of free electrons.

8. In a closed circuit with an electric cell, the free electrons flow towards the positive pole of the cell.

9. Electric current can be measured using an ammeter.

10. Electric current is measured in amperes (A).

11. When an electric current flows through a conductor, some electrical energy is converted into
thermal energy. This is called the heating effect of electric current.

12. A current-carrying wire can affect the direction of the needle of a compass. This shows the magnetic
effect of electric current.

13. A current-carrying coil acts like a magnet. It is called an electromagnet.

169
8 Making Use of Electricity

8.4 Voltage
14. The voltage of an electric cell is a measure of the amount of energy supplied to the free electrons
by the cell.

15. Voltage can be measured using a voltmeter.

16. Voltage is measured in volts (V).

17. The higher the voltage, the larger the current.

8.5 Resistance
18. Resistance measures the opposition of a material to the flow of electric current. It is measured in
ohms (Ω).

19. When resistance increases, the electric current in a circuit decreases.

E 20. The thinner a wire, the higher its resistance. The longer a wire, the higher its resistance.

21. A resistor is a circuit component with a fixed resistance.

E 22. A rheostat is a circuit component whose resistance can be changed.

8.6 Series circuits and parallel circuits

23. Series circuit Parallel circuit
Two light bulbs connected in series Two light bulbs connected in parallel

main loop
branch

branch

• If a gap appears in the circuit, there is • If a gap appears in a branch of the circuit,
no current flow in the circuit. there is still current flow in the other branch.

• The current is the same at every point • A larger current flows in the branch with lower
in the circuit. resistance.

• The current in the main loop is equal to the

sum of the currents in the branches.

• The voltage is not the same across • The voltage is the same across every branch.
different circuit components.
• The voltage across each branch is equal to the
voltage across the electric cells.

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Making Use of Electricity

8.7 Our household electricity
24. Electrical energy is converted into thermal energy for use in the heating elements in electrical
appliances. The moving parts of electrical appliances are driven by electric motors, which make use
of the magnetic effect of electric current to work.

25. The mains voltage in Hong Kong is 220 V.

26. The pins of a three-pin plug are connected to the live

earth wire
wire, neutral wire and earth wire.
neutral wire live wire
27. Domestic circuits are parallel circuits so that the
electrical appliances can operate independently.

8.8 Electrical safety with household electricity

28. Connecting too many electrical appliances to a mains socket causes overloading.

29. In a short circuit, the current is very large and the circuit is very hot.

30. A fuse will blow and break a circuit if the current is above its fuse rating. A circuit breaker will turn
off by itself and break a circuit if the current is above its rating.

31. The earth wire is used to protect us from getting an electric shock.

E 8.9 Power, energy and the cost of electricity

32. Power is the amount of energy transferred to an electrical appliance per second. It is measured in
watts (W) or kilowatts (kW).

33. The relationship between power, energy and time can be shown using the formula below.

energy (J)
power (W) = or energy (J) = power (W) × time (s)
time (s)

34. Electrical energy can be measured using a joulemeter or a kilowatt-hour meter. Electrical energy
can be measured in kilowatt-hours (kW h). It can be calculated using the formula below.

electrical energy (kW h) = power (kW) × time (h)

35. For the household electricity, 1 unit of electricity means 1 kilowatt-hour of electrical energy used.

36. The efficiency of an electrical appliance is the ratio of the useful power output to the power input
of the appliance.

useful power output

efficiency = × 100%
power input

171
8 Making Use of Electricity

Key terms e-aristo.hk/r/

isedict.e
e-Dictionary
di08

closed circuit (閉合電路) p.78 parallel circuit (並聯電路) p.120

electric circuit (電路) p.76 series circuit (串聯電路) p.120
electrical conductor (導電體) p.81
electrical insulator (絕緣體) p.81
earth pin (地線插腳) p.138
open circuit (斷路) p.78
electric motor (電動機) p.135
switch (開關) p.83
heating element (發熱元件) p.132
live pin (活線插腳) p.138
circuit diagram (電路圖) p.84 mains electricity (市電) p.138
circuit symbol (電路符號) p.84 mains socket (市電插座) p.138
mains voltage (市電電壓) p.138
neutral pin (中線插腳) p.138
ammeter (安培計) p.91
three-pin plug (三腳插頭) p.138
ampere (安培) p.91
electric current (電流) p.89
electromagnet (電磁鐵) p.98 circuit breaker (斷路器) p.153
electron (電子) p.88 earthing (接地) p.154
free electron (自由電子) p.88 fuse (保險絲) p.150
heating effect (熱效應) p.95 fuse rating (保險絲額定值) p.152
magnetic effect (磁效應) p.98 overloading (超負荷) p.147
nucleus (原子核) p.88 short circuit (短路) p.149

E
volt (伏特) p.101 efficiency (效率) p.164
voltage (電壓) p.100 joulemeter (焦耳計) p.159
voltmeter (伏特計) p.101 kilowatt (千瓦特) p.157
kilowatt-hour (千瓦小時) p.159
kilowatt-hour meter (千瓦時計) p.159
ohm (歐姆) p.108
power (功率) p.157
resistance (電阻) p.108
watt (瓦特) p.157
resistor (電阻器) p.115
E rheostat (變阻器) p.115
E rotary-type rheostat
(旋鈕型變阻器) p.115
E sliding rheostat (滑動型變阻器) p.115

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Making Use of Electricity

Unit exercise Unit Test (U8) Online Test (U8)

A. True or false (1 mark each)

Write ‘T’ for a true statement and ‘F’ for a false statement.

1. All non-metals are electrical insulators. F

2. In a series circuit, the current is the same at every point. T
3. A fuse is used to prevent a dangerously large current flowing through a circuit. T
E 4. To produce light of certain brightness, a light bulb of a higher efficiency requires a smaller
power input to work. T

B. Multiple-choice (1 mark each)

Choose the correct answer for each question. 3. The photo below shows an ammeter
connected in series with a light bulb.
1. Rods made of different materials are
connected between points P and Q in the
circuit diagram below.
Battery

Which rod would cause the bulb to light?

A. Copper rod
B. Wood rod
C. Glass rod
D. Plastic rod A
(TIMSS 2011) From the ammeter reading,
2. Which of the following statements about the A. the current through the bulb is 0.3 A.
current in a closed circuit is correct? B. the current through the bulb is 1.5 A.
A. Free electrons flow from the positive C. the voltage across the bulb is 0.3 A.
pole of the electric cell towards the D. the voltage across the bulb is 1.5 A.
negative pole.  A
B. Free electrons flow from the negative
pole of the electric cell towards the
positive pole.
C. Nuclei flow from the positive pole of the
electric cell towards the negative pole.
D. Nuclei flow from the negative pole of
the electric cell towards the positive
pole. B

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8 Making Use of Electricity

4. Which of the following statements about an E 7. The diagram below shows a sliding rheostat.
electromagnet are correct?
sliding contact
(1) It works by making use of the heating
P Q
effect of current.
(2) It can be ‘turned off’ by cutting off the
terminal Y
current.
terminal X terminal Z
(3) Its strength increases with the current
flowing through it.
A. (1) and (2) only
B. (1) and (3) only The rheostat is connected in a circuit. Under
C. (2) and (3) only which of the following settings will the
D. (1), (2) and (3) C
rheostat have the lowest resistance?
Terminals of the Position of the
5. The diagram shows an iron nail with an rheostat connected sliding contact
insulated wire coiled around it. The wire is A. X and Y P
connected to a dry cell. B. X and Z P
C. X and Y Q
D. X and Z Q
 A

8. Three identical light bulbs are connected to a

battery as shown in the diagram. The arrow
indicates the direction of the current flow.
Battery
What will happen to the nail when an
electric current flows through the wire?
A. The nail will melt. Direction
B. Electric current will flow through the of current
nail. flow

C. The nail will become a magnet.

D. Nothing will happen to the nail. C Bulb 1 Bulb 2
(TIMSS 2007)
E 6. Which of the following nichrome rods has
the highest resistance?
Bulb 3
A. A nichrome rod of length 10 cm and
thickness 5 mm Which statement is true?
B. A nichrome rod of length 20 cm and A. The current in Bulb 1 is greater than the
thickness 5 mm current in Bulb 2.
C. A nichrome rod of length 10 cm and B. The current in Bulb 1 is greater than the
thickness 3 mm current in Bulb 3.
D. A nichrome rod of length 20 cm and C. The current in Bulb 2 is the same as the
thickness 3 mm D current in Bulb 3.
D. The current in Bulb 2 is the same as the
current in Bulb 1. D
(TIMSS 2011)

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Making Use of Electricity

9. Which of the following statements about an 10. In the circuit below, the current through
earth wire is correct? each light bulb in normal operation is 0.5 A.
A. The colour of an earth wire is brown.
B. An earth wire is used to protect us from
getting an electric shock.
C. An electrical appliance without an
earth wire cannot function normally.
D. An earth wire carries the mains
voltage. B Which of the following is the most suitable
rating of the fuse in the circuit?
A. 0.5 A
B. 1A
C. 2A
D. 3 A C

C. Questions (12 marks)

1. A circuit is shown below. Draw its circuit diagram in the space provided. (2 marks)

V
V

A
A

fuse

Correct circuit symbols (1m)

2. Study the circuit below. Light bulbs X, Y and Z are identical. Correct circuit (1m)

Y Z

(a) The current drawn from the electric cell is 1.2 A and the current through bulb Y is 0.4 A. What
are the currents through bulbs X and Z? (3 marks)
Current through bulb X = 1.2 A – 0.4 A (1m) = 0.8 A (1m)

Current through bulb Z = current through bulb Y = 0.4 A (1m)

(b) If a copper wire is connected in parallel with bulb X, what would happen to the current drawn
from the electric cell? Explain your answer briefly. (2 marks)
The current would become very large (1m) because the circuit becomes a short circuit. (1m)

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8 Making Use of Electricity

3. A 1500 W microwave oven works on the mains voltage of Hong Kong.

(a) What is the mains voltage of Hong Kong? (1 mark)

220 V (1m)

E (b) The microwave oven is turned on for 15 minutes. Calculate the electrical energy it uses.
 (2 marks)
Electrical energy used = power × time = 1500 W × 15 × 60 s (1m) = 1 350 000 J (1m)

E (c) The useful power output of the microwave oven is 960 W. Calculate its efficiency. (2 marks)
useful power output 960 W
Efficiency = ×100% = × 100% (1m) = 64% (1m)
power input 1500 W

Score: /26

176