LEVEL 1 P HYSICS

MODULE 7
ENERGY
Edited by Sia Zhi Wei

吉隆坡坤成中学
Kuen Cheng High School
I
 CONTENTS
7.1 INTRODUCTION TO ENERGY 1
QUICK CHECK 1 6

7.2 ENERGY CONVERSION 7

QUICK CHECK 2 10

7.3 CONSERVATION OF ENERGY 11

QUICK CHECK 3 16

7.4 POTENTIAL ENERGY AND KINETIC ENERGY 17

QUICK CHECK 4 22

7.5 WORK 23
QUICK CHECK 5 26

7.6 POWER 27
QUICK CHECK 6 31

7.7 HEAT 32
QUICK CHECK 7 35

7.8 CONDUCTION 36
QUICK CHECK 8 39

7.9 CONVECTION 40
QUICK CHECK 9 44

I
7.10 R A D I A T I O N 45
QUICK CHECK 10 50

REFERENCE 51

II
7.1 INTRODUCTION TO ENERGY

Learning objectives
By the end of this section, you should be able to:

1. Describe where we get our energy from.

2. Know the unit of energy

7.1.1 Energy in food

We need energy from food to facilitate essential body functions and to actively engage in
various activities, the energy stored in food is usually shown on the food label.

If you drink a typical 700 ml milk tea, you have to swim for 1 hour or jog for 1 hour and 15
minutes to burn off these calories. Do you know what does calories mean?

Calorie is a unit of energy that measures how much energy a food provides to the body. We
often encounter the term "calorie" on food nutrition labels.

Do you know exactly how much energy has in 1 calorie?

Initially 1 calorie is that energy needed to heat up 1 gram of water by 1 °C. A larger unit, 1
kilocalorie (kcal), is 1000 times 1 calorie.

Figure 7.1 Coke nutrition label.

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Different foods store different amounts of energy.

Figure 7.2 Calorie content of Malaysian breakfast.1

(Photo credit - Chun Kai Nguang, Junior 2_2024)

All activities have an energy cost. Keeping your body warm, breathing, moving, and talking all
need energy. For the bones, muscles, and brains to grow, energy is also needed.

The United States government states that the average man needs 2,700 kcal per day and the
average woman needs 2,200 kcal per day.2 Not everybody needs the same number of calories
each day. People have different metabolisms that burn energy at different rates, and some
people have more active lifestyles than others.

We need more energy for all the activities that we do such as walking, running or lifting things.

The figure below shows the energy consumed for each minute of activity.

Figure 7.3 Energy consumed for each minute of activity.3

(Photo credit - Ngu Wey Ni, Lim Xuan Qi Joey, Lee Zi Yen, Fong Yanson, Junior 2_2024)

1
https://www.myfitnesspal.com/nutrition-facts-calories/
2
https://www.medicalnewstoday.com/articles/263028
3
Helen Reynolds (2013). Complete Physics for Cambridge Lower Secondary 1: Cambridge Checkpoint and beyond, Oxford University
Press.
2
An adult should take in only as much energy as they need for the activities that they do. If they
take in more energy than they need, their body stores it as fat for future use. If they eat less
than they need, then the body will use energy from its store of fat and they will lose weight.

Even though we often use the unit 'calories' in the context of food and nutrition, the SI unit of
energy is the joule (J). 1 calorie is equal to 4.18 J.

7.1.2 Energy in fuels

Fuels such as coal, oil or wood also provide us with stored energy that we can use. When we
burn wood or coal, the stored energy is released and can be used to heat up a room or to cook
food.

If we use an electric kettle to boil water, the energy used is the electrical energy. The electrical
energy also comes from the coal and oil.

Energy at different places and conditions are given different names.

Figure 7.4 Coal, oil and wood

7.1.3 Types of energy

The burning of fuel and the digestion of food involve chemical reaction, we called the stored
energy in fuel or food chemical energy.

Stored energies are not only found in food or fuels. Position of an object in the Earth’s gravity
or deformation in certain objects can also store energy. A change in the position or the shape
of an object can cause the stored energy to be released.

Figure 7.5 When does the stored energy can be release.

(Photo credit - Wong Yun Bo, Junior 2_2024)

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The energy stored in these cases are called the potential energy. The potential energy because
of the position of the object is called gravitational potential energy, whereas the potential
energy contained in a deformed object is called elastic potential energy.

For instance, when a mother lifts her baby up from the floor, the baby gains gravitational
potential energy because the baby is in the higher position. If you sit on a bed or a sofa, the
springs inside it are compressed (deformed) and the spring now store elastic potential energy.

Figure 7.6 Example of gravitational potential energy and elastic potential energy.

Figure 7.7 Does a basketball appear scary to you? How about when the
basketball is flying at high speed towards you? What make you feel the different?
(Photo credit - Ryan Chiew, Junior 2_2024)

Kinetic energy is the energy an object has because of its motion. Beside the moving basketball
described above, a swinging pendulum that moves back and forth also possesses kinetic energy
because it is in motion.

Figure 7.8 Example of kinetic energy.

Apart from the types of energy mentioned above, do you know what other types of energy?

4
Supplementary reading: Nuclear energy

The Sun produces the light and thermal energy that we need to live on Earth.
Why does the Sun radiate light and heat?

When we mention nuclear energy, it's often associated with the energy
released in a nuclear power plant or by atomic bombs. The energy radiated
from the Sun - both thermal and light energy originates from nuclear fusion
processes that is occurring inside the core of the Sun. In fact, all the energy
from other stars come from nuclear reaction in the core of the star.

Nuclear fusion is the process of fusing two light nuclei and forming a
heavier one, in the process massive amounts of energy is released.

Figure 7.9 An illustration of the process of nuclear fusion, specifically the

creation of helium from hydrogen. Four protons (hydrogen nuclei) are
combining on the left, releasing in the process two protons and two neutrons
(a helium nucleus). (Image credit: Mark Garlick/Getty Images)

Because of its tremendous hydrogen content, the Sun has maintained this
fusion rate for around four and a half billion years and will continue to do so
for a further four and a half billion years until the hydrogen in its center is
exhausted.

Edited from the source: https://www.space.com/what-is-nuclear-fusion

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QUICK CHECK 1

1 How many joules are there in 200 kJ?

________________________________________________________________________________

2 Name two things your body needs energy for when you are asleep.

________________________________________________________________________________

3 Why is it important for young children to take in more energy than they use for their
activities each day?

________________________________________________________________________________

4 A child picks up a toy from the floor. Which type of energy has the toy gained?

________________________________________________________________________________

5 A stretched elastic band is a store of which type of energy?

________________________________________________________________________________

6 Name two sources of light energy.

________________________________________________________________________________

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7.2 ENERGY CONVERSION

Learning objectives
By the end of this section, you should be able to:

1. State energy conversion in some real-life

examples.

7.2.1 Energy conversion

When your phone shows 'low battery,' you probably start thinking about finding a charger and
then locating an electric outlet to plug it in. But have you ever wondered what happens when
you plug your phone into the socket?

Figure 7.10 Low battery on the phone.

(Photo credit - Wong Wai Lam, Junior 2_2024)

When using our phones, the electrical energy comes from the chemical energy stored in the
battery. So, using a phone involves the conversion of chemical energy into electrical energy.
Now, can you explain what does the message “low battery” in the picture above mean?

When you plug your phone into a socket, you are storing more energy into your battery. During
this process electrical energy is converted into the chemical energy in the battery.

Energy conversion is the process in which one type of energy changes into another type.

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Aside from this, there are still many examples of energy conversion in our daily life. When you
do physical exercise, the chemical energy of food stored in you converts into kinetic energy and
thermal energy.

Figure 7.11 Do physical exercise.

(Photo credit - Lee Xuan Le, Junior 2_2024)

During the burning of wood, chemical energy gets converted to thermal energy and light energy.

Figure 7.12 Burning of wood.

When we switch on an electric bulb, a conversion of electrical energy to light energy and thermal
energy takes place.

Figure 7.13 Switch on an electric bulb.

Can you provide more examples of energy conversion?

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7.2.2 Conversion of gravitational potential energy into kinetic energy
David is sliding down the water slide. When he was at the top of the slide, he has gravitational
potential energy. As he moved down the slide, his gravitational potential energy changed to
kinetic energy.

How could David regain his gravitational potential energy?

Figure 7.14 David is sliding down the water slide.

Here is another example of energy conversion between gravitational potential energy and kinetic
energy.

When you throw a basketball upward, the basketball initially has kinetic energy. When the ball
rises, some of its kinetic energy is converted into gravitational potential energy. Therefore, the
ball slows down as it rises because more kinetic energy is converted into gravitational potential
energy.

Figure 7.15 A ball is thrown upward.

Now do you know how to explain why an object moves faster and faster when it fall?

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QUICK CHECK 2

1 Describe the energy conversion in:

(a) An electric kettle

(b) A torch

2 Which type of energy is represented by a rubber band.

A Thermal energy. B Elastic potential energy.
C Gravitational potential energy. D Sound energy.

3 The pictures show which energy transformation?

A Electrical energy to elastic potential energy.

B Chemical energy to thermal energy.
C Elastic potential energy to kinetic energy.
D Thermal energy to nuclear energy.

4 Which energy conversion is taking place when you build a campfire?

________________________________________________________________________

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7.3 CONSERVATION OF ENERGY

Learning objectives
By the end of this section, you should be able to:

1. State the law of conservation of energy.

2. Explain how the law applies to different
situations.

7.3.1 Conservation of energy

Have you ever played on a swing?

When the boy in the figure below start pumping, the swing moves back and forth faster and
faster, the kinetic energy of the boy increases. However, when he stops pumping, the swing
gradually comes to a stop and his kinetic energy reduces to zero. Where does his kinetic energy
go to?

Figure 7.16 A boy on a swing.

Energy cannot disappear, his kinetic energy is simply converted to other form of energy. What
form of energy do you think his initial kinetic energy has converted to?

Energy not only cannot disappear, it cannot be created too. Energy can only be
converted from one form to another.

For example, consider a roller coaster. When the coaster is at the top of its hill, it has potential
energy. As it moves down the hill, that potential energy is converted into kinetic energy and
thermal energy. As the coaster comes to a stop at the end of the ride, all of its initial potential
energy has been converted into thermal energy.

Figure 7.17 Roller coaster.

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7.3.2 Useful energy and wasted energy
When you turn on a light bulb, it lights up and it also heats up. It converts electrical energy into
light energy and thermal energy.

If you turn on the lamp for studying, the useful energy is the light energy and you can consider
the thermal energy as wasted energy.

Wasted energy doesn’t mean that it is ‘lost’, just that it is not the energy that is useful
to us.

Figure 7.18 Table lamp.

The diagram below gives us a picture of what is happening when the input energy convert to
other forms of energy. Beside the useful output energy, there will always be some energy wasted.

Figure 7.19 Energy conversion diagram.

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 Figure 7.20 The filament bulbs.

A filament light bulb only converts about 10% of the input energy to the useful light energy, the
rest of the 90% is "wasted" because it warms up the surrounding air without producing any light.

Figure 7.21 Conversion of electrical energy in a light bulb.

Since energy can't be created or destroyed, the total amount of output energy will always match
the total amount of input energy.

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7.3.3 Efficiency
We often come across advertisements claiming that LED lights are energy-saving. Do you know
why we say LED lights save more energy compared to traditional lighting?

Figure 7.22 Using LED lighting saves energy.

LED lamp gains more and more popularity because it converts more of its input energy into light
energy (useful energy). In fact, 80% or more of the input energy in a LED lamp is converted
into light energy.4 That is why LED lamp is normally cooler.

Since the LED converts more of its input energy into light energy, it is said to be more efficient
or its efficiency is high. Whereas, the filament lamp converts only 10% of its input energy into
useful energy, it is less efficient or that its efficiency is low.

When an electric device converts most of the electrical energy into useful energy, we say that
they are efficient or they have high efficiency.

The efficiency of a machine or device tells you how much of the energy that you put in is
converted to useful energy.

We can calculate efficiency using this equation:

Useful output energy (J)

Efficiency = × 100%
Total input energy (J)

The filament converts only 10% of its input energy into useful, what is its efficiency?

4
https://cityofmidlandmi.gov/218/Efficiency-of-LED-vs-Incandescent-Lights
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Example: For every 100 J of stored chemical energy supplied, the car engine transfers 25 J of
kinetic energy. Calculate the efficiency of the car engine.

Useful energy out (J)

Efficiency = × 100%
Total energy supplied (J)

25 J
= × 100%
100 J

= 25%

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QUICK CHECK 3

1 What is the useful energy and what is the wasted energy in these items?
(a) A hairdryer

(b) A television

(c) A kettle

2 What would you notice if you stood under a filament bulbs compared with standing under
a LED bulb?

________________________________________________________________________

________________________________________________________________________

3 Why do more efficient electrical devices save you money?

________________________________________________________________________

________________________________________________________________________

4 (a) Fill in the table below for four machines: A, B, C and D

(b) Is machine A or D more efficient? State a reason for your answer.

____________________________________________________________________

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7.4 POTENTIAL ENERGY AND KINETIC ENERGY

Learning objectives
By the end of this section, you should be able to:

1. State what is meant by potential energy and

kinetic energy.
2. Calculate potential energy and kinetic
energy.

7.4.1 Potential energy

When an object is at a certain height from the ground, the energy it has is called the
gravitational potential energy.

The two skiers have the same mass, but they slide from different heights. When both of them
reach the ground, the blue skier will move at a higher speed? Why is this so?

Figure 7.23 When they slide down, their initial gravitational

potential energy is converted into kinetic energy.

Conclusion:

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Now we have two skiers with different masses starting from the same height, the green skier
has a larger mass than the yellow skier has. Imagine that when the two skiers collide with you
at the bottom of the slope, which one has a greater impact on you?

Figure 7.24 The heaviest of two objects at the same

height has the greatest gravitational potential energy.

The gravitational potential energy of an object depends also on its mass. A mother with her
infant baby take a lift to the top of a tall building. The infant baby would gain less gravitational
potential energy than his mother.

Figure 7.25 Why using safety net is a must during construction?

(Photo credit - Lim Chao Wang, Junior 2_2024)

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7.4.2 Calculating gravitational potential energy

From the previous discussion, we found that the gravitational potential energy of an object
depends on its height from the ground and the mass it has.

The amount of gravitational potential energy of an object has can be calculated with the formula:

Gravitational potential energy = Mass x g x Height

(Where g = 10 N/kg on Earth)

Example 1: A car has a mass of 3500 kg and is suspended 2 m above the ground. How much
gravitational potential energy does it have? (Take the value of g as 10 N/kg)

Gravitational potential energy = Mass x g x Height

= 3500 kg x 10 N/kg x 2 m

= 70000 J

7.4.3 Kinetic energy

Kinetic energy is the energy possessed by a body due to its motion. A moving car, a flowing
water or air all possess kinetic energy.

Observe the two lion cubs in the figure below, which one do you think has more kinetic energy?

Figure 7.26 Two lion cubs.

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Can you imagine the extent of damages caused by a fast moving truck and by another slow
moving identical truck when they hit a stationary object?

A fast moving truck will cause more damage in an accident than a slow moving one because it
has more kinetic energy. The kinetic energy it has must be converted into other forms included
sound energy, thermal energy and also the energy to deform the object.

When two objects moving at the same speed, the object with greater mass will definitely have
greater kinetic energy. For example, a tiger is chasing a deer and they are both moving at the
same speed, the tiger will have more kinetic energy than the deer because it has more mass

Figure 7.27 Kinetic energy depends on the mass of the object.

7.4.4 Calculating kinetic energy

How much kinetic energy a moving object has depends on its mass and velocity. You can
calculate the amount of kinetic energy a moving object has with this formula:

1
Kinetic energy = x Mass × Velocity 2
2

Example 1: Eric has a mass of 40 kg and is jogging at a velocity of 1 m/s. How much kinetic
energy does he have?

1
Kinetic energy = x Mass × Velocity 2
2

1
= x 40 kg x (1 m/s) 2
2

= 20 J

20
Exercise
1 An 2 kg apple is hanging in a tree, 3 m off the ground. What is the gravitational potential
energy of the apple? (Take the value of g as 10 N/kg)

2 A car is traveling with a velocity of 40 m/s and has a mass of 1120 kg. What is the kinetic
energy of the car?

7.4.5 Mechanical energy

An airplane is flying at 250 m/s and at an altitude of 10 000 m. If the mass of the airplane is
240 000 kg, then its kinetic energy is 7 500 000 000 joules, and its gravitational potential energy
is 24 000 000 000 joules.

The sum of its kinetic energy and potential energy,

7 500 000 000 J + 24 000 000 000 J = 31 500 000 000 J

is called the mechanical energy of the airplane.

Mechanical energy of an object is the sum of its kinetic energy and its potential energy.

To start the pendulum swinging you pull it back and let it go. When you pull the pendulum back
you are also pulling it up, so its gravitational potential energy (GPE) increases. When you let it
go, it swings to a lower position and the GPE it has is converted into kinetic energy. As the ball
swings up again its kinetic energy is converted back into GPE.

Figure 7.28 A simple pendulum.

In an environment with no friction and air resistance, gravitational potential energy and kinetic
energy mutually converted to each other. However, the sum of these two energy remains
constant. We said that its mechanical energy remains constant.
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QUICK CHECK 4

1 A man and his son run to the top of a hill and stop. The mass of the man is bigger than
the mass of the boy.

(a) While they are both running at the same speed who has more kinetic energy?

____________________________________________________________________

(b) Would it be possible for them to have the same amount of kinetic energy? How?

____________________________________________________________________

____________________________________________________________________

(c) When they are at the top of the hill who has more gravitational potential energy?

____________________________________________________________________

2 A girl drops a stone down a well and listens for the splash. Draw an energy transfer
diagram for this process starting with the girl holding the ball over the well.

3 There is a bell at the top of a tower that is 45 m high. The bell weighs 100 kg. What is the
gravitational potential energy of the bell? (Take the value of g as 10 N/kg)

4 You serve a volleyball with a mass of 2 kg. The ball leaves your hand with a speed of 30
m/s. What is the kinetic energy of the ball?

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7.5 WORK

Learning objectives
By the end of this section, you should be able to:

1. Define work and identify its units.

2. Calculate work.

7.5.1 Work

Picture A

Picture B

Figure 7.29 Force applied to a rock and soccer ball.

Are there any changes to the energy of the rock after it is lifted (picture A) or the soccer ball
after it is kicked off (picture B)?

In both cases, the energy of the rock and that of the ball increases. The lifting force and the
force to kick the ball transfer energy to them. In physics, when a force transfer energy from one
object to another, we say that work is done.

In everyday life, we frequently use the term 'work' to refer to tasks, jobs, or physical activities
such as exercise. However, in the realm of science, it carries a distinct meaning that differs from
its colloquial usage.

Work is the transfer of energy by the force. For work to be done, a force must be exerted and
there must be a displacement.

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For example, the gravitational potential energy of the rock increases, work is done when you lift
up an object. The kinetic energy of the ball increases when you kick the ball. Work is done when
you kick the ball so that it moves.

If you sit in front of a desk and study very hard, do you do any work?

Figure 7.30 Study hard for exam.

(Photo credit - Loke Yi Swen, Junior 2_2024)

Figure 7.31 A car stuck in a snow drift.

If your car is stuck in a snow, you may exert a large force on it but your car still does not
move, do you do any work on your car?

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7.5.2 Calculations on work
When you push a luggage and you do work on it by the amount 100 J, say. You are transferring
100 J of energy to your luggage. How can you calculate the work done when you push your
luggage with a force over a certain displacement?

Work can be calculated using the following formula:

Work = Force x Displacement

Example 1: A person pushes on a box with 150 N of force. The box slides along the floor for 2
meters. How much work did the person do on the box? What is this work representing?

Work = Force x Displacement

= 150 N x 2 m

= 300 J

*The person transferred 300 J of his energy to the box. Therefore, the box now has
more energy.

Example 2: How much work is needed to lift a 10-kg object at constant speed to a table top 1
m above the ground? (Hint: The force you need to lift a 10-kg object at constant speed is
equal to its weight)

Force = Weight

= Mass x g

= 10 kg x 10 N/kg

= 100 N

Work = Force x Displacement

= 100 N x 1 m

= 100 J

*Note: Since work is a measure of energy transferred, the unit of work is the same as that of
energy - joule.

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QUICK CHECK 5

1 How much work is done in holding a 15 N sack of potatoes while waiting in line at the
grocery store for 3 minutes?
A 15 J B 45 J
C 0J D 5J

2 You push against the back of your friend’s car that is stuck. You push and be become very
tired. Did you do work?
A Yes, work was done.
B No, work wasn't done.

3 If a group of workers can apply a force of 1 000 newtons to move a crate 20 meters, what
amount of work will they have accomplished?

4 The door to the classroom weighs 3 kg. How much work is done when Michael pushes on
the door to the classroom with 15 newtons of force, pushing the door 1.3 meters?

5 How far does a car that weighs 5 000 N go if 18 000 J of work is done on it?

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7.6 POWER

Learning objectives
By the end of this section, you should be able to:

1. Define power and identify its units.

2. Calculate power.

7.6.1 Power
If your classroom is on the second floor, you may need to climb stairs. If you are climbing at a
constant speed, for every step you take your leg will exert a force equal to your weight.
Therefore, the work you will do for every step equals your weight multiply by the height of each
step.

Work for every step = Weight of a person × height of each step

Why do you feel more tired when you run up the stairs than when you walk slowly up the stairs?

When you walk slowly up the stairs, you are doing work slower than when you run up the stairs.
You do more work in a unit time when you run up the stairs. We say that you are working at a
higher power when you run up the stairs.

Figure 7.32 Climbing stairs.

(Photo credit - Yeow Jey Yee, Junior 2_2024)

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The work done on a body can be quick or slow, how fast a work is done can be expressed by
power. Power is a measure of the amount of work that can be done in 1 second.

Running up the stairs requires a short duration and high power, resulting in a higher energy
consumption per second. In contrast, walking up the stairs involves a longer duration and lower
power, resulting in a lower energy consumption per second.

7.6.2 Calculations on power

If the work done by a person while climbing a wall is 4000 J in 40 seconds. How much work did
this person do in 1 second? And what does this represent?

The power can be calculated using this formula:

𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒
Power =
𝑇𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛

The unit for power is watt. Its symbol is W, 1 watt equals 1 joule of work per second.

Example 1: When Ivan pushes a box with a force of 20 N as far as 10 m, work has been done
by Ivan.

(a) Determine the work done by Ivan.

(b) If Ivan uses 5 seconds to push the box, calculate the power generated by him.

(a) Work = Force x Displacement

= 20 N x 10 m

= 200 J

𝑊𝑜𝑟𝑘
(b) Power =
𝑇𝑖𝑚𝑒

200 𝐽
=
5𝑠

= 40 W

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Hands on activity: Calculating stair climbing power

1. Measure the vertical height of the set of stairs.

2. Determine your body mass.

3. Time the number of seconds it takes you to walk up and run up the stairs.

4. Determine the amount of work done and power.

Height of one step (m)

Number of steps climbed

Total height of staircase

(m)

Mass of climber (kg)

Weight of climber (N)

Climbing Walk up Run up

stairs

Trials 1 2 3 Average 1 2 3 Average

Time (s)

29
Work done (Walk up) Work done (Run up)

Power (Walk up) Power (Run up)

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QUICK CHECK 6

1 You move a 12-newtons box up a 6-meter ramp. In order to find out how much power you
created, what would you need to know?

________________________________________________________________________

2 If you run up the stairs slower, you will ___________________.

A do more work B do less work
C generate more power D generate less power

3 A 40-kg person is climbing 3 m up a flight of stairs in 6 seconds. How much power did
they exert climbing the stairs?

4 The Power of a runner who consumed 160 J is 40 seconds is 4 watts. What will be the
power once twice the time is applied?

5 A 400 N object is in a 32,000 N elevator that rises 30 m in 2 min. How much power is
needed for the elevator's trip?

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7.7 HEAT

Learning objectives
By the end of this section, you should be able to:

1. Explain that heat is the flow of energy from

hot materials to cold materials.
2. Describe that molecules in a material begin
to vibrate (or move) more quickly when the
material is heated.

7.7.1 Heat
What would happen if you leave either a hot drink or a cold drink on the table and wait for a
while?

Figure 7.33 Transfer of thermal energy.

Heat is the transfer of thermal energy from warmer object to a cooler object. Energy transfer
is everywhere and is closely related to our daily lives.

When you touch a piece of metal that is hotter than your hand, your hand gains thermal energy
and feel hot. When you touch a piece of metal that is colder than your hand, your hand loses
thermal energy and feel cold.

Energy flow will continue until the two substances are at the same temperature.

Figure 7.34 Thermal energy always moves from a hot object to a cold object.
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You are holding two ice cream cones together. They are both at exactly 0 °C. Does any
thermal energy flow between the two ice cream scoops?

Figure 7.35 Two scoops ice cream.

7.7.2 Thermal energy and temperature

An ice cube has a temperature of -5°C. Does it have any thermal energy?

Air consists of air particles. From observations, these air particles are always in continuous
motion, and therefore they have kinetic energy. The total kinetic energy of the particles in the
air is the thermal energy discussed above.

At higher temperatures the air particles move faster and therefore the total kinetic energy of
the air particles is higher. The temperature of the air is related to the total kinetic energy of the
air particles.

We know that all other substances are made out of small particles. Just like the air, the particles
in these substances are also vibrating or moving around continuously. The total kinetic energy
of the particles in these substances depends on temperature too.

Figure 7.36 Thermal energy is the total kinetic energy of moving particles of matter.

The thermal energy of an object is the total kinetic energy of the individual molecules that
make it up. When the temperature of the object increases, its total kinetic energy is higher. We
say that the object now has more thermal energy.

The particles that made up all objects are always in constant motion no matter how low its
temperature is. Therefore, all objects must have thermal energy.

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7.7.3 Quantity of thermal energy
If we add the same amount of thermal energy in the following cases, which will have a higher
rise in temperature?

Two cups of water with different masses. Cups of water and oil with the same mass.

How much energy does it take to raise the temperature of an object?

1 The mass of the object: In a greater mass there are more particles. You need to transfer
more energy to get them all moving or vibrating faster. For example, it takes more energy to
raise the temperature of a large pot of water by 1 degree Celsius compared to a small cup of
water.

2 What it is made of: The particles in different materials have different masses. If the
particles are more massive you need more energy to get them moving or vibrating faster. For
instance, if we have the same mass of oil and water, it would take more energy to raise the
temperature of the water by 1 degree Celsius compared to the oil.

7.7.4 Transfer of thermal energy

We have already seen how heating is a way of transferring energy from one object to another.
This can occur in three different ways - by conduction, convection or radiation. We will explore
these three ways of transferring thermal energy in the following section.

Figure 7.37 Methods of thermal energy transfer.

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QUICK CHECK 7

1 Why does your hand feel cold as an ice cube melts on it?
A Cold energy is transferred from the ice to your hand

B Thermal energy is transferred from your hand to the ice

C Thermal energy can't transfer between your hand and ice

D Thermal energy is transferred from the ice to your hand

2 Why does it take more energy to heat up 1 kg of cold water than 0.5 kg of cold water to
the same temperature?

________________________________________________________________________

________________________________________________________________________

3 Complete the table by ticking the one or both columns for each statement.

Thermal energy Temperature

Measured in joules.
Measured with a thermometer.
Measured in degrees Celsius.
Increases if you heat something for longer.

4 It takes 4200 J to raise the temperature of 1 kg of water by 1 °C.

(a) How much energy in kJ would it take to raise the temperature of 1 kg of water by
2 °C?

_____________________________________________________________

(b) How much energy in kJ would it take to raise the temperature of 3 kg of water by
1 °C?

_____________________________________________________________

35
7.8 CONDUCTION

Learning objectives
By the end of this section, you should be able to:

1. State the names of some conductors and

insulators.
2. Explain why some materials feel warmer
than others.

7.8.1 Conduction
Conduction is a method that energy is transferred from a hotter region to a colder region. As
you heat up an iron bar, as shown in the figure below, the iron atoms at the left end absorb
energy and vibrate more vigorously.

The iron atoms at the left end then collide with neighboring atoms and the energy can be passed
down to the right end. We say that thermal energy is conducted from the left end (hotter region)
to the right end (colder region).

Figure 7.38 Process of conduction transferring the kinetic

energy of more vigorous particles to less vigorous particles.

7.8.2 Good and poor thermal conductors

Thermal energy is transferred through solids via conduction. Some solids conduct thermal
energy more efficiently than others. For example, steel is a good thermal conductor, which
is why saucepans are often made of steel, it efficiently transfers thermal energy to the food
inside the pan.

On the other hand, many non-metals do not conduct thermal energy poor very well, they are
poor thermal conductors (This type of material also called thermal insulator). This does not
mean that they do not conduct at all, but that thermal energy is transferred very slowly through
them. Materials like paper, cloth, wood, and plastic are examples of thermal insulators. This is
why saucepan handles are commonly made of wood or plastic.

Is liquid and gas a good or poor thermal conductor?

36
Liquids like water are poor conductors of thermal energy compared with solids. You can see this
in the demonstration as shown in the figure below. The ice cube at the bottom of the test tube
will be melting slowly even though the water at the top is boiling.

Figure 7.39 Water is not good at conducting thermal

energy, hence the ice will be melting slowly.

Air is also a very bad thermal conductor. It is used in certain design to keep people warm.
Clothing and bedding that can keep body warm normally has lots of trap air.

Fur, feathers and wool trap air between individual strands and fibers. This helps animals such
as sheep, polar bears and birds, insulate themselves against cold winter conditions.

Figure 7.40 A red-bellied woodpecker fluffs her feathers

and visits a suet feeder to stay warm on a cold day.
(Photo credit - Courtney Celley/USFWS)

Clothing made of several thin layers of fabric with trapped air in between, often called ski
clothing, is commonly used in cold climates because it is light, fashionable, and a very effective
thermal insulator.

Figure 7.41 Ski jacket.

37
Why liquids and gases are bad thermal conductor?

Thermal energy is conducted by the collisions of neighboring particles. Liquids and gases do not
conduct thermal energy well because their particles are much further apart than the particles in
a solid.

7.8.3 Feeling warm or cold

In early morning, which part of the table feels colder to you, the metal part or the wooden part?
Do you think that they have different temperatures?

Figure 7.42 Classroom table.

Maybe you will be tempted to say that different parts of the table are at different temperatures
because they felt differently. The fact is that our skin detects the thermal energy transfer to us
than the temperature.

Metal conducts thermal energy better than wood. When you touch the metal surface, thermal
energy is conducted away from your hand to the metal faster.

However, thermal energy is conducted slower from your hand when you touch the wooden part.
That is a reason the metal part feels colder even though they are at the same temperature.

Think about it!

At room temperature, we perceive the aluminium as colder than the wood, is this true?

Now, if you place two identical ice cubes on an aluminium and a wooden tray, which
ice cube will melt faster?

Figure 7.43 Ice melt experiment: Aluminium vs. Wood.

38
QUICK CHECK 8

1 List the correct order of conduction.

I They collide more vigorously with neighboring particles
II Particles at the heated end gain kinetic energy and vibrate vigorously.
III Particles at cooler end vibrate vigorously.
A I, II, III B III, II, I
C II, I, III D I, III, II

2 What would happen if you heated water in a saucepan made of a material that is not a
good conductor of thermal energy?

________________________________________________________________________

________________________________________________________________________

3 A student is investigating what makes a good insulator. She wraps different materials
around a can of hot water and measures the temperatures drop of the water over 10
minutes. Here are her results.

(a) Which material is the best insulator?

____________________________________________________________________

(b) Which material is the worst insulator?

____________________________________________________________________

(c) The student cuts a piece of the best insulating material. She notices that there are a
lots of pockets or air in it. Explain why that will help the material to insulate the cup.

_____________________________________________________________

_____________________________________________________________

39
7.9 CONVECTION

Learning objectives
By the end of this section, you should be able to:

1. Describe what happens in convection.

7.9.1 Convection
In the previous section, we mentioned that water is not a good thermal conductor. However,
have you noticed that when we boil water, all of it quickly warms up, not just the bottom part?
Why is that?

Figure 7.44 Convection- Boiling water

Water is a liquid which can flow freely. When water is being boiled, the bottom part of the water
gets heated up and they will flow to the upper part. In this way, thermal energy is transferred
to the upper part and cause the water to boil quickly.

In module 6, we learned that density is how compact the particles in a material are
arranged. When objects are heated up, the particles move/vibrate more vigorously and
therefore the spacing between the particles are larger.

When you heat a liquid or gas, what happens to the density of the liquid or gas?

Figure 7.45 How thermal energy affects the particles of liquid.

40
 Figure 7.46 Convection current.

The water in contact with the bottom of the pan gets warmer. The particles in the warmer water
are moving faster than the particles in the cooler water above.

The particles in the warmer water move further apart and becomes less dense. Then the warmer
water rises, cooler water (denser) moves down to take its place.

Eventually all the water in the pan is circulating up and down. The water as a whole gets hotter
and hotter and the water will boil. The circulation of water that is set up in this way is called a
convection current.

Convection is the transfer of energy through the movement of fluid from the hotter region to
the colder region.

Figure 7.47 Air Conditioner.

Why are air conditioning units always placed at the upper part of rooms?

41
Air conditioners are used constantly on hot days. The process of cooling down a room employs
the concept of convection.

The air which is colder is released by the air conditioners. Now, this colder air is denser than the
warmer air in the room, and, hence, it sinks. The warmer air, being less dense, rises and is
drawn in by the air-conditioner. As a result, a convection current is set up and the room is cooled
down.

If air conditioners were to be placed near the floor, the cooler air would get stuck at the bottom
and the room would not be cooled effectively.

Figure 7.48 Radiator heater.

Radiator heaters work by using convection to heat a room. They are typically used in cold-
weather countries. Should they be installed at the upper part or lower part of the room?

42
7.9.2 Sea breeze and land breeze
In a hot day, if you are facing the sea on the beach, you will feel that breeze is blowing towards
you. The breeze flows from the sea towards the land, this is called sea breeze.

Breeze is formed when air flow. During the daytime, the land gets hotter than the water surface
on the sea. The air on the land then rises to leave space for the cooler air from the sea to take
its place. This formed the flow of air we called sea breeze.

Figure 7.49 Sea breeze.

At night, the temperature of the land drops faster and get colder than the sea. The warmer air
above the sea rises and the cooler air from the land blows in to take its place. This is how land
breeze is formed.

Figure 7.50 Land breeze.

Thus, the sea breeze and the land breeze are formed as a result of convection due to uneven
temperatures.

43
QUICK CHECK 9

1 When air is heated, what happens to its mass?

A Increases
B Decreases
C Remains the same

2 The figure below shows a refrigerator. The cooling unit is placed at the top. The cooling
unit cools the air near it.

(a) What happens to the density of the air as it cools?

____________________________________________________________________

(b) How does the cold air move?

____________________________________________________________________

3 Explain why there are no convection currents in solids.

________________________________________________________________________

________________________________________________________________________

4 The copper sheet is very hot and placed in a vertical position as shown in the diagram below.
A student places her hands at equal distances from the sheet, as shown in the figure below.
Explain why her hands are not heated by convection.

________________________________________________________________________________

________________________________________________________________________________

44
7.10 RADIATION

Learning objectives
By the end of this section, you should be able to:

1. Know some sources of infrared radiation and

the similarities between light and infrared.
2. Describe how infrared is transmitted,
absorbed and reflected.

7.10.1 Radiation
If you touch the burning wood, your palm feels hot because thermal energy is transferred to
your palm by conduction. If you place your palm above the fire, the heat you feel is caused by
the transfer of thermal energy to you by convection. Now, if you place your palm beside the fire
but not touching anything hot, your palm still feels hot. There must be a third method where
thermal energy can be transferred to you.

Figure 7.51 Warm hand by fire.

The thermal energy felt by your palm is not transmitted by conduction because air is a bad
thermal conductor. It is also not transmitted by convection because the air warmed up by the
fire rises. In this case, thermal energy is transferred by radiation.

The energy from the Sun reached the Earth by radiation. When you go outside on sunny days
you feel the warmth of the Sun, this is because of the infrared radiation from the Sun.

Infrared radiation travels from the Sun to the Earth, just like light. Radiation is different from
conduction and convection because it does not need a material for the thermal energy to be
transferred. It can be transferred through vacuum.

45
How would you prove that radiation does not require a medium to transfer?

Figure 7.52 View of Earth from space with moon and sunlight in background.

7.10.2 Infrared thermometer

To screen passengers for Covid-19 and other contagious diseases, some airports use infrared
thermometer to identify travelers that have fevers, without having to measure individual body
temperatures by contact. So how do the infrared thermometer work?

Figure 7.53 Infrared thermometer allows for picturing the invisible.

Normally we cannot tell the temperature of the travelers by looking at them. The image of the
travelers is taken with infrared thermometer, it absorbs infrared to produce an image that shows
the differences in temperature.

All objects, including the human body emit infrared radiation, but the radiation that they emit
depends on the temperature. As an object's temperature increases, it emits higher amounts of
radiation.

While the infrared thermometer can detect higher temperatures, they can't screen for COVID-
19 itself. Someone running to catch a flight can have a higher body temperature. A fever also
does not necessarily mean someone is sick with COVID-19, so airports need to do further
screening once they spot passengers with high temperatures.
46
7.10.3 Reflecting and absorbing radiation
During hot weather, most people think about what to wear to keep cool. Have you ever
considered what colors to wear?

Whenever radiation falls on a surface, some of the radiation will be absorbed by the surface and
the other are reflected by the surface.

Smooth and shiny surfaces reflect more energy than absorbing them. Rough and dull surfaces
absorb more energy than reflecting them.

Figure 7.54 Good and bad reflectors and absorbers of radiation.

In hot and sunny countries, houses are often painted white to reflect infrared radiation and
maintain lower indoor temperatures. This choice of color helps to minimize the absorption of
infrared radiation.

Figure 7.55 Painting buildings white is a centuries-

old method of staving off heat in countries like
Greece.

47
Solar panels utilize the Sun's radiation to generate electricity. They are typically darker in colour
because they absorb the radiation falling on them, and very little are reflected.

Figure 7.56 Solar panel.

By strategically employing different colors for houses and solar panels, residents can optimize
energy usage and ensure comfortable living conditions in warm climates.

48
Supplementary reading: Thermos flask

A thermos flask has become an indispensable part of our lives. These thermos
flasks are designed to keep our drinks at a consistent temperature for a long time
period.

But have you ever wondered how these flasks work? How exactly do they keep
your liquids hot or cold for an extended time period?

Figure 7.57 Different layers in the thermos flask act as

resistance to all the modes of energy transfer.

A thermos flask is nothing but one bottle inside another bottle, separated by a
vacuum layer in between. The vacuum layer prevents heat transfer through
conduction and convection; since conduction and convection require the presence of
any medium to transfer thermal energy.

The outer part of the inner glass layer and the inner part of the outer glass layer are
coated with reflecting material. The reflective coating minimizes the emissivity or
absorptivity of the material. And it acts as a poor radiator and a good reflector of
heat radiation.

The additional insulating material between the outer glass layer and the casing
further adds resistance to conduction heat transfer.

Edited from the source: https://www.eigenplus.com/physics-behind-thermos-flask/

49
QUICK CHECK 10

1 Which best describes thermal energy transfer by radiation?

A A slice of bacon frying in a pan. B A pot on a stove boiling water.
C A camp fire warming hands. D A hot air balloon.

2 Tom puts a metallic cover over his car’s windshield after parking. How does this control
the sun’s rays during a sunny day?
A It prevents heat energy from leaving the car.
B It controls the temperature by providing shade.
C It reflects the sun’s rays away from the windshield.
D It absorbs heat in the windshield protector and cools the car.

3 The figure below shows a thermos flask.

(a) What type of surface should the inner and outer walls of the flask be made? State a
reason for your answer.

____________________________________________________________________

____________________________________________________________________

(b) The lids of thermos flasks are usually screw lids. Suggest a material that can be used
to make the screw lid of thermos flask and explain why it is a good material.
_____________________________________________________________

_____________________________________________________________

50
References

Helen Reynolds (2013). Complete Physics for Cambridge Lower Secondary 1: Cambridge
Checkpoint and beyond, Oxford University Press.

Tom Duncan and Heather Kennett (2002). Cambridge IGCSE Physics. Hodder Education (GB)

David Sang (2014). Cambridge IGCSE® Physics Coursebook. Cambridge University Press.

Dong Zong (2013). Science Textbook Junior Middle. United Chinese School Committee’s
Association of Malaysia.

Ethiopia Learning. Grade 7 Physics. https://ethiopialearning.com/

CK-12 Flexbooks. https://flexbooks.ck12.org/

https://www.r3fitness.com.my/

https://health.family.my/

https://selftution.com/

https://byjus.com/

https://www.siyavula.com/

https://www.solarschools.net/

https://studiousguy.com/

https://www.nbcnews.com/

https://www.13wmaz.com/

https://www.bbc.co.uk/

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