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Making Use of Electricity8

8-3EXPERIMENT 8.1

Lighting up a light bulb (Demonstration)

1. Does the bulb light up? Yes

2. Connect the apparatus as shown in Fig. (b) and (c). Observe whether the bulb light up or not in

each case.

Set-up

Fig. (b) Fig. (c)

Is the dry cell used? No Yes

Does a set-up form a complete path?

Yes No

Does the light bulb light up?

No No

3. From the results above, what conditions are required for lighting up the bulb?

A dry cell must be included and the path must be completed in the set-up.

Unit 8 Making Use of Electricity2

8-5EXPERIMENT 8.2

What kinds of objects conduct electricity?1. On the circuit board shown below, add a bulb, a dry cell and connecting wires to design a

circuit to test the whether the objects provided conduct electricity.

dry cell

bulb

connecting wire

object tobe tested

2. After obtaining approval from your teacher, use the circuit constructed to test the objects

provided. Record the results in the table below.

ObjectIs the object made of metal?

( / )Does the object conduct

electricity? (/ )

iron paper clip

nylon string

metal spoon

wooden chopstick

plastic ruler

glass rod

aluminium foil

pencil core

Discussion

1. What is the function of the bulb in your circuit?

When the bulb lights up, this means that the object tested conducts electricity .

2. What can you conclude from the results above?

Most of the objects that can conduct electricity are made of metals .


8-8EXPERIMENT 8.3

Using a switch to control the flow of electricity (Demonstration)

2. Close the switch. What happens to the circuit?

When the switch is closed, the conducting parts touch each other. The circuit

becomes closed . Electricity can flow through the circuit and therefore the bulb

lights up .

3. Open the switch again. What happens to the circuit?

The circuit becomes open again. The bulb does not light up.

8-10TEST YOURSELF 8.1A

1. (a) T (b) F

2. D 3. B

8-14Activity 8.1

Drawing circuit diagrams 1. 2.

3.


8-15EXPERIMENT 8.4

Setting up circuits using circuit diagrams1. Read carefully the circuit diagram shown in Fig. (a).

Complete the sentences below to describe the circuit.

The cell is connected to a switch , a

resistor and an ammeter , one

by one. When the switch is closed,

electricity flows through the circuit.

8-16TEST YOURSELF 8.1B

1. Link the following circuit symbols and the names of the circuit components.

Circuit symbol Circuit component

(a) • • resistor

(b) • • rheostat

(c) • • bulb

(d) • • ammeter

(e) • • battery

2. For each circuit shown below, draw the circuit diagram in the space provided.

(a) (b)


8-19Activity 8.2

Comparing a water pipe system with a circuitReferring to Fig. (a) and Fig. (b), compare the circuit with the water pipe system by completing the

table below.

Components in the water pipe system

Components in the circuit

1. Energy source water pump dry cell

2. Flow of … water free electrons

3. Path of flow water pipe connecting wire

4. Energy converter water turbine bulb

8-21EXPERIMENT 8.5

Using an ammeter to measure the current in a circuit2. The reading of the ammeter is A. (Answer varies with student)

3. (a) The reading of the ammeter is A. (Answer varies with student)

(b) The brightness of the bulb increases ( increases / decreases / remains

unchanged ).

(c) What is the relationship between the current flow through the bulb and the brightness of

the bulb?

When the current in the circuit becomes larger, the brightness of the bulb increases ( increases / decreases / remains unchanged ).

8-23EXPERIMENT 8.6

Heating effect of an electric current2. Gently increase the current flowing through the nichrome wire. What happens to the wax?

The wax melts when the current increases to a certain value.

Discussion

What can you conclude from the results above?

When a current flows through the nichrome wire, the wire is heated up . When the current

increases, the heating effect becomes stronger ( becomes stronger / becomes weaker /

remains unchanged ).


8-24EXPERIMENT 8.7

Studing the magnetic effect of current (Demonstration)

3. Place a compass and some iron paper clips near the coil. Leave the switch open.

(a) Does the compass needle turn? No

(b) Do the paper clips move? No

4. Close the switch so that a current passes through the coil.

(a) Does the compass needle turn? Yes

(b) What happens to the paper clips?

The paper clips move towards the iron nail .

5. Open the switch again. What happens to the paper clips?

They are not attracted by the iron nail anymore .

Discussion

What can you conclude from the results of the experiment?

When a current flows through a metal coil, the metal coil behaves like a magnet

due to the magnetic effect of current. When the current is shut down, the magnetic effect

disappears .

8-25Thinking

State one advantage of using an electromagnet with a normal magnet in a door lock over a pair of

normal magnets.

If a pair of normal magnets is used, the door will be always attracted to the rim.

8-26EXPERIMENT 8.8

A moving copper rod (Demonstration)

2. Switch on the power supply. Does the copper rod move? No

3. Switch on the power supply again. What happens to the copper rod?

The copper rod moves .



1. (a) F (b) T

2.

3. Michael wants to set up a circuit with the components shown below, so that he can measure the

current in the circuit with the ammeter after the switch is closed. Complete the picture below

to show how the components should be connected.

8-30EXPERIMENT 8.9

Measuring the voltage of common dry cells

Part A Measure the voltage of different kinds of dry cells

1.

Type of dry cell

AAA dry cell AA dry cell C dry cell

Voltage (V) 1.5 1.5 1.5

+ + +

− − −


Type of dry cell

Square dry cell Button cell Mobile phone battery

Voltage (V) 9 Depends on the cell used Depends on the battery used

Part B Measure the voltage of connected dry cells

1. Voltage of each dry cell is V.

2.

Circuit Voltage (V)

(a)

(b)

(c)

Discussion

From the results above, what can you conclude?

When more dry cells are connected in the same direction, the voltage across the dry cells

increases ( increases / decreases ).

8-32EXPERIMENT 8.10

Studying the relationship between voltage and current (Demonstration)

3.

Number of dry cell

Voltmeter reading (V) Ammeter reading (A)Brightness of the bulb( low / medium / high )

1 low

2 medium

3 high

Discussion

1. The current increases ( increases / decreases / remains unchanged ).

2. The brightness of the bulb increases ( increases / decreases / remains unchanged ).

+ +− −

−

+(underneath)



1. Fill in the blanks to complete the following sentences.

The voltage of a cell measures how strong the ‘push’ can be provided by the

cell on the free electrons of a conductor. It is measured in the unit

volt (V). We can use a voltmeter to measure voltage.

2. C

3. Jenny wants to set up a circuit with the components shown below, so that she can investigate

the relationship between the voltage of the dry cells and the brightness of the bulb. Complete

the figure below to show how the components should be connected.

V

8-36Activity 8.3

Athletes on a running track

There are more ( more / less ) obstacles on track A than track B.

Therefore, it is more ( more / less ) difficult for the athletes on track A

to reach the goal line in any given time.

8-37Thinking

In fact, silver has a lower resistance than copper, but silver is seldom used to make connecting

wires. Can you suggest one reason to explain this?

Silver is more expensive.


8-38EXPERIMENT 8.11

Relationship between resistance and current2.

Resistance of the resistor (Ω)

Reading of the ammeter (A)

4. Draw a graph to show the relationship between the resistance of the resistor and the current

flowing through the circuit.

Relationship between resistance and current

(Results depends on apparatus)

24222018161412108642

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

Reading of ammeter (A)

Resistance ofthe resistor (Ω)

From the graph, describe the relationship between the resistance and the current?

When the resistance increases, the current decreases .

Extension


8-40EXPERIMENT 8.12

Investigating the factors affecting the resistance of a wire (Design)

Part A How does the length of a wire affect its resistance?

Making a hypothesis

When the length of the wire increases, the resistance ( increases / decreases ).

Designing the experiment

Independent variable Dependent variable

length of the wireresistance of the wire / brightness of the bulb / ammeter reading

Controlled variables (Put a “” in the appropriate boxes.)

number of dry cells material of the wire length of the wire

thickness of the wire components used

Others: (please specify)

Carrying out the experiment

1. Refer to Experiment 8.11, set up a circuit for measuring the resistance of a 20 cm long, thin

nichrome wire. Sketch the set-up or its circuit diagram in the space provided.

(suggested answer)

20 cm long, thenichrome wire

– +


Recording the results

Length of the wire (cm) Ammeter reading (A)Brightness of bulb(brighter / dimmer)

20 brighter

50 dimmer

Drawing a conclusion

When the thickness of the wire increases, the resistance increases .

Part B How does the thickness of a wire affect its resistance?

Making a hypothesis

(Answers vary with students)

Designing the experiment

Independent variable Dependent variable

thickness of the wire resistance of the wire / brightness of the bulb / ammeter reading

Controlled variables (Put a “” in the appropriate box(es).)

number of dry cells material of the wire length of the wire

thickness of the wire components used

Others: (please specify)

Recording the results

Ammeter reading (A)Brightness of bulb(brighter / dimmer)

thicker wire brighter

thinner wire dimmer

Drawing a conclusion

When the thickness of the wire increases, the resistance decreases.


Discussion

1. What is the relationship between the brightness of the bulb and the resistance of the wire?

The brighter the bulb, the lower the resistance of the wire.

2. Why a copper wire cannot be used for testing in this experiment?

The resistance of copper is very low. The effects of the length and thickness on the resistance are not significant.

8-44EXPERIMENT 8.13

The working principle of rheostats (Demonstration)

Part A Sliding rheostat

2.

3. (a) What happens to the brightness of the bulb?

The brightness of the bulb increases gradually.

(b) How can you explain your observation?

When the sliding contact is moved away from terminal C, the resistance of the rheostat

decreases gradually .

Part B Rotary-type rheostat

2. (a) What happens to the brightness of the bulb?

The brightness of the bulb increases gradually.

(b) How can you explain your observation?

When the rotating contact is turned from terminal B to terminal A, the resistance of the

rheostat decreases gradually .


8-47TEST YOURSELF 8.2C

1. (a) F (b) F (c) T

2. A 3. D

4. The circuit is connected to terminals A and C of the rheostat. When the rotating contact

is turned towards terminal B, the length of the resistance wire that the current flows

through increases . As the resistance of the rheostat increases , the current

decreases .

8-49EXPERIMENT 8.14

Investigating a series circuit

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

3. Open the switch. Remove one of the bulbs and then close the switch again.

Does the remaining bulb light up? No

8-50EXPERIMENT 8.15

Measuring the current in a series circuit2. Close the switch. Compare the brightness of bulbs X and Y.

They have the same brightness.

3. Record the readings of the ammeters in the table below.

Ammeter A1 A2 A3

Reading (A) (Depends on the components used)

From the results obtained, what can you tell about the current in a series circuit?

The currents at different points of a series circuit are the same (the same / different).


8-51EXPERIMENT 8.16

Measuring the voltage in a series circuit2. Close the switch. Record the readings of the voltmeters in the table below.

Voltmeter V1 V2 V3

Reading (V) (Depends on the components used)

(a) What is the sum of the readings of V2 and V3? V

(b) What is the relationship between the readings of V1, V2 and V3?

V1 = V2 + V3

3. (a) How does brightness of bulbs X and Y change when bulb Z is added?

The brightness of bulbs X and Y decreases .

(b) Record the readings of the voltmeters in the table below.

Voltmeter V1 V2 V3 V4


What is the sum of the readings of V2, V3 and V4? V

Discussion

What can be concluded from the results above?

In a series circuit, the total voltage across the bulbs is equal to the voltage of the dry cell

.

Also, when more bulbs are connected in series, the brightness of each bulb decreases .

8-53Thinking

If we connect many bulbs in series to make a Christmas light, what would happen to the brightness

of the bulbs?

The voltage across each bulb will be very low, the bulbs become very dim.


8-54EXPERIMENT 8.17

Investigating a parallel circuit

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

3. Open the switch. Remove one of the bulb and then close the switch again.

Do all remaining bulbs light up? Yes

8-56EXPERIMENT 8.18

Measuring the currents in parallel circuits

Part A Relationship between the currents flowing through the main loop and the branches

2. Close the switch. Compare the brightness of bulbs X and Y.

They have the same brightness.

3. (a) Record the readings of the ammeters in the table below.

Ammeter A1 A2 A3


(b) Calculate the sum of the readings of A2 and A3. Is the result equal to the reading of A1?

The sum of readings of A2 and A3 is equal to the reading of A1

(c) What can you conclude from the results above?

In a parallel circuit, the current in the main loop is equal to the sum of the

currents in all the branches.

Part B Relationship between resistance and current

1. Which branch, 1 or 2, has a higher resistance? 2

2. Close the switch. Compare the brightness of bulbs X and Y again.

Bulb X is brighter than bulb Y

3. Record the readings of the ammeters in the table below.

Ammeter A1 A2 A3



4. (a) In Fig. (b), which branch, 1 or 2, has a larger current passing through it? 1

(b) What can you conclude from the result above?

In a parallel circuit, a larger current flows through the branch with lower resistance.

8-58EXPERIMENT 8.19

Measuring the voltages in parallel circuits2. (a) Record the readings of the voltmeters in the table below.

Voltmeter V1 V2 V3


(b) Are the readings of the voltmeters the same? Yes

3. (a) Record the readings of the voltmeters in the table below.

Voltmeter V1 V2 V3


(b) Do the readings of the voltmeters remain the same when compared with Step 2?

Yes

Discussion

What can you conclude from the results above?

The voltages across different branches of a parallel circuit are always the same .

8-59TEST YOURSELF 8.3

1. (a) series circuit (b) parallel circuit (c) series circuit

2. Do you think Cherry is correct? Why?

Incorrect ( Correct / Incorrect ). It is because in a series circuit, the current at any

point is the same .

3. C 4. B


8-65Activity 8.4

Mains voltages in different countries

U.S.A.: 120 V United Kingdom: 230 V

China: 220 V Japan: 100 V

Argentina: 220 V Australia: 230 V

8-66EXPERIMENT 8.20

Studying the structure of a three-pin plug2. Write down the colors of the three wires in the table below.

Wire live wire neutral wire earth wire

Colour brown blue yellow and green

8-70Thinking

What would happen to the appliances if all the sockets are connected in series instead of in

parallel?

When one appliance is switched off, all the other appliances will be switched off too. Also, all appliances work at a voltage

less than 220 V.

8-72EXPERIMENT 8.21

Studying the cause of short circuit (Demonstration)

1. (b) Close the switch.

(i) Does the bulb light up? Yes

(ii) The reading of the ammeter is A.

2. (b) Now close the switch again.

(i) Does the bulb light up? No

(ii) Observe the reading of the ammeter. How does the current in the circuit change

when compared with that in Step 1?

The current increases ( increases / decreases / remains unchanged).

(Depends on the components used)


(iii) Observe what happens to the steel wire. How can you explain this phenomenon?

The steel wire turns red . It is because the current flowing through

the steel wire is large so that the wire becomes hot .

8-75EXPERIMENT 8.22

Studying the working principle of a fuse (Demonstration)

3. Increase the voltage slowly. How does the brightness of the bulb change?

The brightness of the bulb increases gradually. Finally, it goes out .

4. Switch off the power supply and observe the fuse. What happens to the wire in it?

The wire in the fuse melts .

5. Connect the bulb to the power supply directly at a low voltage.

(a) When the power supply is switched on, can the bulb light up? Is the bulb burnt out?

The bulb can ( can / cannot ) light up. It is not ( is / is not ) burnt

out.

(b) If no fuse is used in Step 3, do you think the bulb will eventually burn out? Yes

Discussion

When the current in the circuit becomes too large, the wire in the fuse melts so that the

circuit becomes open . The bulb is not damaged by the large current.

8-76Thinking

What are the disadvantages of using a fuse with rating much larger than the normal working current

of the appliance?

If the fuse rating is much larger than the normal working current, when an electrical appliance becomes faulty so that a large

current passes through it, the current may be not large enough to burn out the fuse. The fuse is unable to protect the

circuit.

8-77Thinking

Can you state one advantage of using circuit breakers over fuses to protect household circuits?

Circuit breakers can be reset easily without replacement, while fuses must be replaced once they are blown.

Extension



1. (a) T (b) T (c) T

2. (a) Earth wire (b) Neutral wire (c) Live wire

3. Wire Colour code Description

live brown It is connected to the mains voltage of 220 V.

neutral blue It provides a return path for current to flow.

earth yellow and green It is a safety device.

4. C

5. The earth wire is connected to the metal case of an electrical appliance. When the

live wire touches the metal case accidentally, the earth wire provides a path

for current to flow to the ground. This protects us from an electric shock .

8-83Activity 8.8

Calculating the energy used by different electrical appliances

Electrical appliance Power rating (W)Energy used by the appliance

in one minute (J)

1. air-conditioner 1 500 90 000

2. desk lamp 20 1 200

3. water heater in bathroom 3 000 180 000

4. refrigerator 150 9 000

5. washing machine 700 42 000

6. rice-cooker 1 000 60 000

7. television set 400 2 400

8. vacuum cleaner 200 1 200

Among these electrical appliances, which of them uses the most energy in the same period of time?

Which of them uses the least?

uses the most energy, while uses the least.


8-85Activity 8.9

Comparing the energy labels of electrical appliances of different models1. Which of the air-conditioners has a higher efficiency? A

2. Which of the air-conditioners consumes less electrical energy? A

3. If the air-conditioners are of the same price, which one would you recommend to Doris?

Explain your answer.

A. It is because it has a higher efficiency and it consumes less electrical energy. It is much more energy-efficient

than B.

8-88EXPERIMENT 8.23

Calculating the cost of electricity (Demonstration)

2. Record the initial reading of the kilowatt-hour meter in the table below.

Initial reading of the kilowatt-hour meter

(kW h)

Final reading of the kilowatt-hour meter

(kW h)

Energy used by the electric kettle ( = Final reading – Initial reading)

(kW h)

5. Calculate the power of the electric kettle.

Power = ( ) kW h

( ) min

= ( ) kW h

( ) h

= kw

6. Cost of electricity = kW h × $0.9 / kW h = $


1. (a) F (b) T (c) F

2. Efficiency = ( 1110 ) W

( 1850 ) W × 100%

= 60 %


3. The picture below shows the electricity bill of Tim’s family last month.

RESIDENTIAL TARIFF

Bill Issued on: (DD-MM-YY)

06-05-18

Energy charge:

Rate Unit Charge ($)

$0.817 per unit

$0.945 per unit

400

520

Total amount:

Meter No:

XXXXYYYY

Previous Reading

96231

Present Reading

97151

Units Used

920

920

Let’s save energy for

a green world!

Due Date

21-05-18

Thank you for your payment

$520 on

14-03-18

From 08-03-18 to 06-05-18

For 60 days of usage

Bill Type & Merchant No.: 02

326.80

491.40

818.20

(a) The cost of electricity is calculated by ‘rate × units of electricity consumed’. Use the

relationship to complete the missing values in the bill.

(b) Calculate the average cost of each unit of electricity.

Average cost = $( 818.20 )

( 920 ) units

= $ 0.889 / unit

(c) When the amount of electricity consumed is larger than a certain value, electricity is

charged at a higher rate. Suggest one reason to explain it.

It is to encourage people to save electricity.

8-99REVISION EXERCISE

A. Multiple-choice Questions1. C

5. A

2. D

6. C

3. B

7. A

4. B


B. Short-answer Questions 8.

A

9. (a) Two of the pins are wired incorrectly. Which are they?

X and Y

(b) Simon corrected the mistake. When the electrical appliance is under normal operation,

which pin has no current flowing through it?

X

10. (a) Compare the brightness of bulbs X and Y. Explain your answer.

They have the same brightness, because the voltage across them are the same.

(b) When the switched is closed, both bulbs goes out. Name this phenomenon.

short circuit

(c) Suggest one safety device that can protect the circuit when the phenomenon in Part (b)

occurs. Explain briefly how it can protect the circuit.

A fuse. When short circuit occurs, an excessively large current flows through the circuit.

The wire in the fuse melts to break the circuit.

11. (a) What was the total energy consumed by the television and the washing machine

throughout the week?

Total energy = (0.2 + 0.6) kW × 35 h = 28 kW h

(b) The electricity is charged at $0.9 per unit, what is the cost of using these electrical

appliances in the week?

Cost = 28 kW h × $0.9

= $25.2

(c) If Mr. Chan’s television has a useful power output of 160 W, find the efficiency of the

television.

Efficiency = 160200

× 100%

= 80%

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