Energy is stored in food, fuel and moving things, and it moves from one store to another.
Name the places energy is kept, follow it as it moves, see why some always spreads out as waste heat, tell heat from temperature, find how heat travels and how to slow it, read food energy in kilojoules, and sort fuels into renewable and not.
You are ready for this book if
You can name three things at home that get hot when they are used.
Written and drawn by attnom. Free to read, print and share.
S3.8.1 Chapter 1
Energy stores
Is there energy in a banana?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Name energy stores, such as chemical, kinetic, elastic and thermal
Give an example of each store from daily life
Amina eats a banana. A little later she runs to the well and back. She is not tired at all. Where did her running come from?
Energy is what makes things happen. It makes things move, get hot, shine and make a sound. Nothing happens without it.
Energy is kept in places we call stores. A store is not a box. It is a way a thing holds energy, ready to use. We measure energy in joules, J. The next chapter uses them.
Draw it first
Draw three energy stores
1 Draw a candle. The wax is a chemical store. Energy waits in it until the candle burns.
2 Draw a ball with an arrow. A moving thing has energy in its kinetic store.
3 Draw a cup with steam. Hot things have energy in their thermal store.
Food and fuel are chemical stores. So are the wax of a candle, a battery and a bag of rice. They hold energy until something changes them.
The energy for Amina's run was in the banana. Her body moved it from a chemical store to a kinetic store.
A ball that is up on a roof has energy too, because it can fall. This is its gravitational store. A stretched rubber band has energy because it can spring back. This is its elastic store.
Neither thing is moving. Each holds energy, because of where it is or how it is stretched.
One more store is the thermal store. A hot thing holds energy in it. The hotter the thing is, the more it holds.
Light and sound are not stores. They are ways that energy moves from place to place. You will see this in the next chapter.
See it three ways
As a picture
In words
Every store has a name from what it is. Food and fuel hold chemical energy. Moving things hold kinetic energy. Raised things hold gravitational energy. Stretched things hold elastic energy. Hot things hold thermal energy.
Try it
Hunt for stores at home
You need: paper and a pencil, a walk round your home or yard.
Be safe: Look, do not touch, anything hot. Keep away from fires, stoves and hot pots.
Draw a table with two columns: "what I found" and "store".
Look for something that holds a chemical store. Write it in the table.
Find a moving thing, a raised thing, a stretched thing and a hot thing. Add each one to the table.
Write the name of the store next to each thing.
What you will see: Most homes have all five stores. Most of them do not look like they hold energy, because they sit still. Your list will be different from ours.
If you cannot try it, this is what happens.
A wrong idea many people have
A thing that is not moving has no energy.
Why it feels right: We see energy when something moves or burns, so a still thing seems to have none.
The test that breaks it: Hold a stone high over soft sand, then let go. It lands with a thud. Before you let go, it held energy in its gravitational store.
What is true: Stored energy cannot be seen. A still thing can hold a lot, such as food, a raised stone or a stretched band.
Key words
energy
what makes things move, heat up, shine and make sounds.
stores
a way a thing holds energy, ready to use.
chemical store
energy kept in food, fuel and batteries.
kinetic store
energy of a moving thing.
gravitational store
energy of a thing held up high.
elastic store
energy of a stretched or squashed thing.
Back to your guess
The answer is A: Yes. Energy is kept in the banana, ready for you to use.
Yes. Food keeps energy in it, in a store. Your body takes that energy out when you move, grow and stay warm. Things that are not moving can still hold energy.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Which store holds the energy in a lump of coal that is not burning?
See the answer
Chemical store
Fuels such as coal, wood and gas hold chemical energy. It comes out when they burn.
If you chose A: Kinetic energy is for moving things. Coal sitting still is not moving. Go to S3.8.1.
If you chose C: Thermal energy is for hot things. Cold coal is not hot until it burns. Go to S3.8.1.
If you chose D: Elastic energy is for stretched or squashed things. Coal is neither. Go to S3.8.1.
Ravi pulls back a rubber band. Which store has the energy?
See the answer
Elastic store
A stretched band holds energy in its elastic store. When it springs back, that energy moves.
If you chose A: Gravitational energy is for things held up high. The band is stretched, not raised. Go to S3.8.1.
If you chose B: It is not moving yet. The energy waits in the stretch. It becomes kinetic only when you let go. Go to S3.8.1.
Which of these hold energy mainly in a chemical store?
See the answer
A bag of beans
Food holds energy in a chemical store. Your body takes it out when it digests the food.
If you chose B: The tea is hot, so it holds energy in its thermal store. It is not a fuel or a food in this question. Go to S3.8.1.
If you chose C: A rolling ball holds energy in its kinetic store, because it is moving. Go to S3.8.1.
A stone sits on a high shelf. Which store has the energy?
See the answer
Gravitational store
A thing held up high holds energy in its gravitational store, because it can fall.
If you chose A: It is not moving. Kinetic energy is for moving things. Go to S3.8.1.
If you chose C: A stone is not a food or a fuel. Its energy comes from being up high. Go to S3.8.1.
Tariq eats rice and then runs. In which stores is the energy, first and then later?
See the answer
Chemical store first, then kinetic store
Food is a chemical store. When the body moves, the energy goes into a kinetic store.
If you chose A: This is the wrong way round. The rice is a chemical store. Running makes a kinetic store. Go to S3.8.1.
If you chose B: The rice may be warm. But the energy for running was in the food, as a chemical store. Go to S3.8.1.
If you chose C: Rice is not held up high. The energy for running was in the rice, as a chemical store. Go to S3.8.1.
Which of these holds energy, even though it is not moving?
See the answer
The hot soup, in its thermal store
Hot things hold energy in a thermal store. The soup does not move, but it is hot.
If you chose A: Still things can hold a lot of energy. Hot soup, food and raised stones are all still. Go to S3.8.1.
If you chose B: The steam moves, but the soup itself is hot and holds more. Hot things hold thermal energy even when still. Go to S3.8.1.
Keep it
Copy this into your exercise book.
Energy is kept in stores: chemical (food, fuel), kinetic (moving), gravitational (raised), elastic (stretched) and thermal (hot). A still thing can hold energy.
Draw: A banana, an arrow, and a runner. Write chemical under the banana and kinetic under the runner.
S3.8.2 Chapter 2
Energy transfers
When a candle burns, where does its energy go?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Describe how energy moves when a candle burns or a ball falls
Draw an energy transfer diagram
Light a candle and watch it. The wax slowly gets shorter. The flame gives light and warmth. The energy that was in the wax has gone somewhere. Where?
When energy moves from one store to another, we call it an energy transfer. Things happen when energy moves. A ball falls. A candle shines.
We count energy in joules, J. Lifting a small apple up one metre moves about 1 J into its gravitational store. A candle gives out about 80 J every second.
Energy moves in different ways. A force moves it when something pushes, pulls or falls. Heating moves it from hot to cool things. Light, sound and an electric current also move it.
Draw it first
Draw a ball falling
1 Draw a ball held high. All its energy is in its gravitational store: 10 J.
2 Let it fall. Halfway down, 5 J has moved into its kinetic store. 5 J is still in the gravitational store.
3 Just before it lands, all 10 J is in the kinetic store. The total has stayed at 10 J.
The ball moved its energy by a force. The pull of gravity is a force. It pulled the ball down and moved the energy from one store to the other.
See it three ways
As a picture
In words
Each bar is the same length. The energy is shared between two stores. As one store gets smaller, the other gets bigger by the same amount.
In symbols
At the top: 10 J + 0 J. Halfway: 5 J + 5 J. At the bottom: 0 J + 10 J. The total is always 10 J.
Worked example
A ball has 10 J in its gravitational store when it is held high. Halfway down, 5 J is still in that store. How much energy is in its kinetic store?
The total stays 10 J.
The gravitational store holds 5 J.
The kinetic store holds the rest: 10 - 5 = 5 J.
Answer: 5 J
Now the ball lands. The ball and the ground squash a little, and they get a tiny bit warmer. So some energy moves to the thermal store of the ball and the ground. A tiny part, far less than 1 J, goes out as sound, which you hear as a thud. A ball that bounces keeps the rest in its kinetic store.
This is a transfer diagram: stores in boxes, and the way the energy moves on each arrow.
Worked example
A ball has 20 J in its kinetic store just before it lands. It bounces up with 12 J in its kinetic store. How much went into the thermal store of the ball and ground?
The total stays 20 J.
12 J is back in the kinetic store.
The thermal store took the rest: 20 - 12 = 8 J.
Check: 12 + 8 = 20 J. That is the total again.
Answer: 8 J
The numbers for the second worked example.A burning candle moves energy out of the wax. The room gets a little warmer.
The words by heating mean energy moves from a hotter thing to a cooler one. The flame is hot, so energy moves into the cooler air around it.
Try it
Feel a candle's energy leave
You need: a candle in a dish, matches, used only by an adult, a room with no draught.
Be safe: An adult lights and blows out the candle and stays close. Tie back long hair. Keep cloth and paper away from the flame. Never hold a hand above it.
Ask an adult to light the candle on a flat table and to stay with you.
Hold your hand beside the flame, about 20 cm away. Keep your palm facing the flame.
Notice the warmth on your palm. Move your hand away and notice it fade.
Look at the wall or table near the candle. See the light on it.
Ask the adult to blow the candle out.
What you will see: You feel warmth and you see light. Both are energy leaving the wax. Hold your hand beside the flame, never above it. Hot air rises there.
If you cannot try it, this is what happens.
A wrong idea many people have
A falling ball makes new energy as it speeds up.
Why it feels right: A falling ball goes faster and faster, so it looks as though it is gaining energy.
The test that breaks it: Look at the bars. At the top the ball has 10 J. Halfway it has 5 J + 5 J. At the bottom it has 10 J. The total never grew.
What is true: The ball does not gain energy. Energy moves from the gravitational store into the kinetic store.
Key words
energy transfer
energy moving from one store to another.
joules
the units we count energy in. We write J.
by a force
a way energy moves, when things push, pull or fall.
by heating
a way energy moves, from a hotter thing to a cooler one.
transfer diagram
boxes for stores, with arrows for the way energy moves.
Back to your guess
The answer is A: It moves out into the air and the room, as warmth and light.
The energy in the wax moves out of its chemical store. It goes into the air and the room, as warmth and as light. It does not disappear.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
A ball falls from a shelf to the floor. What happens to its stores while it falls?
See the answer
Its gravitational store gets smaller and its kinetic store gets bigger.
Energy moves from the gravitational store to the kinetic store by a force. The total stays the same.
If you chose A: This is the wrong way round. The ball loses height and gains speed. Go to S3.8.2.
If you chose C: Energy is not made. As the gravitational store shrinks, the kinetic store grows by the same amount. Go to S3.8.2.
A candle burns. Where does energy move from, and to?
See the answer
From the chemical store of the wax to the thermal store of the air and room.
The wax is a chemical store. When it burns, the energy moves out by heating and by light. It warms the air and the room.
If you chose A: This is the wrong way round. The wax is the store that gets smaller. Go to S3.8.2.
If you chose B: Energy is not made from nothing. It moves out of the wax and into the surroundings. Go to S3.8.2.
If you chose C: A flame is not a store of moving energy. The wax holds the energy, and the flame lets it out. Go to S3.8.2.
A ball has 20 J in all. Partway down a fall, 8 J is in its gravitational store. How much is in its kinetic store?
See the answer
12 J
The total is 20 J. Kinetic = 20 - 8 = 12 J.
If you chose A: You added. The two stores share the total, so subtract the gravitational energy from the total. Go to S3.8.2.
If you chose B: You copied the number from the other store. The two stores are not usually equal. Subtract to find the kinetic store. Go to S3.8.2.
If you chose D: All of it is in the kinetic store only at the bottom. Here some is still in the gravitational store. Go to S3.8.2.
Mei draws a transfer diagram for a falling ball. It goes from the gravitational store to the kinetic store. What should she write on the arrow?
See the answer
By a force
Gravity pulls the ball down. A pull is a force, so the energy moves by a force.
If you chose A: The ball is not giving out light. Light moves energy only when something shines. Go to S3.8.2.
If you chose B: No hot thing is warming a cool thing here. Gravity pulls the ball, and a pull is a force. Go to S3.8.2.
If you chose C: Sound comes later, when the ball lands. While falling, gravity pulls the ball. Go to S3.8.2.
Which of these is a way that energy moves, and not a store?
See the answer
Light
Light, sound, heating and a force are ways energy moves. Chemical, kinetic and elastic are stores.
If you chose B: Chemical is a store. Food and fuel hold energy in it. Go to S3.8.1.
If you chose C: Kinetic is a store, for moving things. Light is a way energy travels. Go to S3.8.1.
If you chose D: Elastic is a store, for stretched or squashed things. Go to S3.8.1.
From an earlier chapter
Review: which of these holds energy in a chemical store?
See the answer
A tank of cooking gas
Fuels such as gas are chemical stores. The energy comes out when they burn.
If you chose A: The cart is moving, so it holds energy in a kinetic store. Go to S3.8.1.
If you chose C: A stretched band holds energy in an elastic store. Go to S3.8.1.
If you chose D: Hot tea holds energy in a thermal store. Go to S3.8.1.
Keep it
Copy this into your exercise book.
Energy moves from one store to another. A falling ball moves energy from the gravitational store to the kinetic store, by a force. The total stays the same.
Draw: Three boxes joined by arrows: gravitational store, kinetic store, thermal store. Write the way on each arrow.
S3.8.3 Chapter 3
Energy is never used up
A bicycle rolls to a stop. Where did its energy go?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain that energy is not made or destroyed, only moved
Explain why some energy always ends up as wasted heat
Kofi rides along a flat road and stops pedalling. The bicycle slows down and stops. Its kinetic store is empty. Has the energy gone?
Here is one of the biggest ideas in science. Energy is never made. Energy is never destroyed. It only moves from one store to another.
The bicycle stops, but its energy has only moved. The ground, tyres and air are a little warmer.
Every time energy moves, some of it spreads out into the surroundings. These are the air, the ground and everything else nearby. The surroundings get a little warmer.
Draw it first
Draw where a bulb's energy goes
1 A small torch bulb takes in 100 J from its cell. Draw an arrow going in.
2 Draw two arrows going out. 10 J leaves as light. 90 J leaves as heat.
3 Add up the energy coming out: 10 + 90 = 100 J. It is the same as the energy that went in.
We want light from a bulb. The light is its useful energy. The heat is wasted energy. Wasted does not mean destroyed. It means the energy went where we cannot use it.
An energy flow diagram. Each arrow is as wide as its joules. The two arrows out are together as wide as the arrow in.
The wasted heat warms the air a tiny bit. It spreads out so thin that we cannot use it again. It is still there. It is wasted, not used up.
See it three ways
As a picture
In words
For every thing, the useful energy and the wasted energy add up to the energy that went in. None of it is lost. The body moves less than it takes in, and the rest warms you.
In symbols
energy in = useful out + wasted out. For the bulb: 100 J = 10 J + 90 J.
We can say how good a thing is at its job. This is its efficiency. Efficiency = useful energy out ÷ energy in × 100. We write it as a percentage, %.
A toy motor with a small cell. The numbers for the first worked example.
Worked example
A toy motor takes in 50 J. 30 J moves the toy. The rest warms the motor and the air. How much is wasted, and what is the efficiency?
Wasted energy = energy in - useful energy.
Wasted = 50 - 30 = 20 J.
Efficiency = useful ÷ in × 100.
Efficiency = 30 ÷ 50 × 100 = 60%.
Answer: 20 J wasted, efficiency 60%
A fan. It wastes energy in two places, in the motor and at the axle. It also hums, but far less than 1 J leaves as sound.
Worked example
A fan takes in 200 J. 100 J turns the blades. 70 J warms the motor. The rest warms the axle by friction. What is its efficiency?
Useful energy out = 100 J.
Axle = 200 - 100 - 70 = 30 J.
Wasted = 70 + 30 = 100 J.
Efficiency = 100 ÷ 200 × 100 = 50%.
Check: 100 + 100 = 200 J, the same as the energy in.
Answer: 50%
Try it
Rub your hands
You need: your two hands.
Be safe: Rub gently enough that your skin does not hurt.
Hold your palms against your cheeks. Notice how warm they feel.
Rub your palms together fast for 15 seconds.
Press your palms to your cheeks again. Notice the difference.
What you will see: Your hands are warmer. Your moving hands had energy in a kinetic store. Friction moved it into a thermal store. The energy did not vanish.
If you cannot try it, this is what happens.
A wrong idea many people have
Wasted energy is destroyed.
Why it feels right: We cannot use it or find it again, so it seems to have gone.
The test that breaks it: Look at the bulb. 100 J goes in. 10 J is light and 90 J is heat. Add them up and you get 100 J. Nothing is missing.
What is true: Wasted energy is still there. It has spread into the surroundings and warmed them a tiny bit. It is too thin to use.
Key words
surroundings
the air, the ground and everything else near a thing.
useful energy
the energy that does the job we want.
wasted energy
energy that spreads into the surroundings, mostly as heat.
efficiency
useful energy out ÷ energy in × 100, as a percentage.
energy flow diagram
a picture where each arrow is as wide as its joules.
Back to your guess
The answer is C: It spreads out into the ground, the tyres and the air, as a little warmth.
The energy did not vanish. Friction moved it into the thermal store of the ground, the tyres and the air. They got a tiny bit warmer, too little to feel.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
A ball rolls along the ground and stops. What has happened to its energy?
See the answer
It has moved to the thermal store of the ball, the ground and the air.
Friction moves the energy into thermal stores. The energy is still there, spread out as a tiny bit of warmth.
If you chose B: Energy is never destroyed. It only moves to another store. Go to S3.8.3.
If you chose C: The ball did not go higher, so its gravitational store did not grow. Friction warmed the ground and the ball. Go to S3.8.3.
A small lamp takes in 60 J. It gives out 6 J as light. How much energy is wasted as heat?
See the answer
54 J
Wasted = energy in - useful = 60 - 6 = 54 J.
If you chose A: You added. Wasted energy is what is left after the light, so subtract. Go to S3.8.3.
If you chose B: That is the light, which is the useful part. The wasted part is the rest. Go to S3.8.3.
If you chose C: You divided. To find what is left, subtract the light from the energy in. Go to S3.8.3.
A motor takes in 80 J and gives 20 J of useful energy. What is its efficiency?
If you chose A: That is the wasted share. Efficiency uses the useful energy, divided by the energy in. Go to S3.8.3.
If you chose C: You divided the energy in by the useful energy, which is upside down. Divide useful by in. Go to S3.8.3.
If you chose D: You subtracted the two numbers. Efficiency is a division: useful ÷ in × 100. Go to S3.8.3.
Why can we not use the wasted heat from a bulb to light the bulb again?
See the answer
It has spread into the surroundings and is too thin to use.
Wasted heat warms the air and the walls a tiny bit. It is spread over so much that it cannot do a job.
If you chose A: Energy is never destroyed. The wasted heat is still there. Go to S3.8.3.
If you chose B: Energy does not flow back to where it came from. It spreads out into the air and things nearby. Go to S3.8.3.
A gadget takes in 150 J. Which pair of outputs is possible, if 40 J is useful?
See the answer
40 J useful and 110 J wasted
The outputs must add up to the energy in. 40 + 110 = 150 J.
If you chose B: Together these give out more energy than went in. Energy cannot be made from nothing. Go to S3.8.3.
If you chose C: Together these give out less than went in. Energy is never destroyed, so 30 J would be missing. Go to S3.8.3.
From an earlier chapter
Review: a 40 J ball is partway down a fall. 30 J is still in its gravitational store. How much is in its kinetic store?
See the answer
10 J
Kinetic = 40 - 30 = 10 J.
If you chose A: You added. The two stores share the total, so subtract. Go to S3.8.2.
If you chose C: You copied the gravitational store. The kinetic store is what is left of the total. Go to S3.8.2.
Keep it
Copy this into your exercise book.
Energy is never made or destroyed. In every transfer some spreads into the surroundings as wasted heat. Efficiency = useful out ÷ energy in × 100.
Draw: A bulb with 100 J in, 10 J light and 90 J heat. Write 10 + 90 = 100.
S3.8.4 Chapter 4
Heat and temperature
Which holds more energy, a cup of boiling water or a bath of warm water?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain the difference between heat and temperature
Read a thermometer and record temperatures in degrees Celsius
Amina touches a cup of hot tea and pulls her hand away. Then she steps into a bath that only feels warm. Which one holds more energy?
Temperature tells you how hot something is. We measure it in degrees Celsius, written °C. A thermometer is the tool that measures it.
Water freezes at 0 °C. Water boils at 100 °C at sea level. High on a mountain it boils at a lower temperature. A healthy body is about 37 °C.
Ice water, a healthy body and boiling water, on a scale from 0 to 100 °C.
To read a thermometer, look at the top of the liquid. Find the nearest number below it. Then count the small marks up from that number.
The liquid is 3 marks above 30. Mei's tea is 33 °C. Always write the unit: 33 °C.
Heat is energy moving from a hotter thing to a cooler thing. It always moves that way. It stops when both are at the same temperature.
Draw it first
Draw hot and cool water mixing
1 Draw a cup of hot water at 80 °C and a cup of cool water at 20 °C. Use the same amount in each.
2 Pour them together. Energy moves from the hot water to the cool water.
3 They end at one temperature, about halfway between: about 50 °C. Heat stopped moving.
The energy in a thermal store depends on two things. One is the temperature. The other is how much of the stuff there is. Thermal energy is the energy in a thermal store.
Tea cooling in a room at 20 °C. Heat moves out of the tea until it is as cool as the room.
Worked example
Use the graph. How many degrees did the tea cool in the first 10 minutes? How many in the next 10?
At 0 minutes the tea is 80 °C. At 10 minutes it is 63 °C.
First 10 minutes: 80 - 63 = 17 °C.
At 20 minutes it is 51 °C. Next 10 minutes: 63 - 51 = 12 °C.
The tea cools by less each time, because it is closer to the room temperature.
Answer: 17 °C, then 12 °C
See it three ways
As a picture
In words
The cup is hotter. The bath is cooler, but it holds a hundred times as much water. A bigger amount of warm water can hold more thermal energy than a small amount of hot water.
In symbols
Warming 1 cup of water (about 250 mL) by 1 °C takes about 1 kJ. Above 20 °C: the cup holds 1 × 80 kJ. The bath holds 100 × 20 kJ.
Worked example
The room is at 20 °C. Which holds more thermal energy: 1 cup of water at 100 °C, or 100 cups at 40 °C? Warming 1 cup of water (about 250 mL) by 1 °C takes about 1 kJ.
The cup is 80 °C above the room. The bath is 20 °C above the room.
Energy = number of cups × degrees above the room × 1 kJ.
Cup: 1 × 80 = 80 kJ.
Bath: 100 × 20 = 2000 kJ.
The bath holds far more thermal energy, even though it is cooler.
Answer: The bath: 2000 kJ, against 80 kJ for the cup
Try it
Fool your hands
You need: three bowls, cool water, warm water: an adult mixes a little hot water from a pot into cool water.
Be safe: An adult makes the warm water and checks that it feels comfortable on their wrist. Never use hot water.
Put cool water in the first bowl and warm water in the third bowl. Mix some of each in the middle bowl.
Put your left hand in the cool bowl and your right hand in the warm bowl. Wait 30 seconds.
Put both hands in the middle bowl. Notice how each hand feels.
What you will see: The middle bowl is at one temperature. Yet it feels warm to the hand that was cool, and cool to the hand that was warm. Your hands feel heat moving, not temperature. A thermometer is fairer.
If you cannot try it, this is what happens.
A wrong idea many people have
Heat and temperature are the same thing.
Why it feels right: People say "hot" for both a high temperature and a lot of heat.
The test that breaks it: The cup is at 100 °C and the bath at 40 °C. The cup has the higher temperature. The bath holds 2000 kJ above room temperature, against 80 kJ for the cup.
What is true: Temperature tells how hot a thing is. Heat is energy moving from hot to cool. A thing's thermal energy depends on its temperature and on how much of it there is.
Key words
temperature
how hot something is, in degrees Celsius.
thermometer
a tool that measures temperature.
heat
energy moving from a hotter thing to a cooler one.
thermal energy
the energy in a thermal store. It depends on temperature and amount.
Back to your guess
The answer is C: The bath, because there is so much more water.
The bath holds far more thermal energy. Hot is only part of the story. How much water there is matters too.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Room temperature is 20 °C. Which holds more thermal energy? 1 cup of water at 90 °C, or a bucket of 20 cups at 50 °C?
See the answer
The bucket
Cup: 1 × 70 = 70 kJ. Bucket: 20 × 30 = 600 kJ. The bucket holds more.
If you chose B: Temperature is only half the story. The bucket has twenty times as much water. Go to S3.8.4.
If you chose C: They do not. Work out cups × degrees above the room for each one. Go to S3.8.4.
A hot spoon is put into a bowl of cool water. Which way does energy move?
See the answer
From the spoon into the cool water.
Heat always moves from the hotter thing to the cooler thing. The spoon cools and the water warms until both are at one temperature.
If you chose A: There is no flow of cold. Energy only moves from the hotter thing to the cooler thing. Go to S3.8.4.
If you chose C: Energy does move. It keeps moving until the spoon and the water are at the same temperature. Go to S3.8.4.
What temperature does this thermometer show?
See the answer
27 °C
The liquid is 2 marks above 25. Each mark is 1 °C. So the reading is 25 + 2 = 27 °C.
If you chose A: You counted two marks up from 20. The liquid is higher: count up from 25. Go to S3.8.4.
If you chose B: You counted down from 25 by two. The liquid is above 25, so count up. Go to S3.8.4.
If you chose C: You read the number above the liquid. Count the small marks up from 25. Go to S3.8.4.
The air is 24 °C in the morning and 31 °C in the afternoon. How many degrees warmer is it in the afternoon?
See the answer
7 °C
Change = 31 - 24 = 7 °C.
If you chose A: You added the two temperatures. To find how much warmer, subtract. Go to S3.8.4.
If you chose B: That is the afternoon temperature. The question asks for the change. Go to S3.8.4.
What does the thermal energy in a pot of water depend on?
See the answer
Its temperature and how much water there is.
Thermal energy depends on both. A hotter thing holds more, and a bigger amount holds more.
If you chose B: Temperature matters, but so does the amount. A big pot of warm water holds more than a small cup of hot water. Go to S3.8.4.
If you chose C: The amount matters, but so does the temperature. The same amount holds more energy when hotter. Go to S3.8.4.
From an earlier chapter
Review: a gadget takes in 70 J and gives 20 J of useful energy. How much is wasted?
See the answer
50 J
Wasted = 70 - 20 = 50 J.
If you chose A: You added. Wasted energy is what is left over, so subtract the useful energy. Go to S3.8.3.
If you chose C: That is the useful energy. The wasted energy is the rest. Go to S3.8.3.
Keep it
Copy this into your exercise book.
Temperature tells how hot a thing is, in °C. Heat is energy moving from hot to cool. Thermal energy depends on temperature and on how much there is.
Draw: A thermometer with the liquid at 27 °C. Mark 20, 25 and 30 on it.
S3.8.5 Chapter 5
How heat travels
Why does the metal spoon in hot tea get hot, while the wooden spoon stays cool?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain conduction, convection and radiation with examples from home
Compare how fast metal and wooden spoons warm in hot water
Ravi stirs his tea and leaves a metal spoon in the cup. When he lifts it out, the handle is hot. His wooden spoon stood in the same cup, and its handle is cool.
Heat moves from hot things to cool things in three ways. They are called conduction, convection and radiation. We will take them one at a time.
Conduction is heat moving through a solid, from one particle to the next. In the hot end of a spoon the particles shake hard. They bump the next ones and pass the energy along.
Draw it first
Draw heat moving along a spoon
1 Draw a cup of hot water. Stand a metal spoon and a wooden spoon in it.
2 Draw an arrow up the metal handle. Heat moves along the metal from the water.
3 Write: heat travels up the metal. The wooden handle stays cool.
A conductor lets heat pass through it easily. Metals are good conductors. An insulator lets heat pass through slowly. Wood, plastic, cloth and air are insulators.
How fast heat moves through each material.One set of readings of the top of each handle, taken with a thermometer, once a minute.
Worked example
Use the table. By how many degrees did the metal handle warm in 4 minutes? How about the wooden handle?
After 4 minutes the metal handle is 38 °C. It started at 20 °C.
Metal: 38 - 20 = 18 °C.
Wood: 21 - 20 = 1 °C.
The metal handle warmed 18 times as much.
Answer: metal 18 °C, wood 1 °C
Convection is heat moving in a liquid or a gas. Water at the bottom of a pot is heated. It spreads out a little. A cupful of it then weighs less than a cupful of the cool water above, so it rises.
Warm water rises and cool water sinks to take its place. The loop spreads the heat through the pot.Air does the same. This is why the air near the ceiling is warmer than the air near the floor.
Radiation is heat that travels as invisible waves called infrared. It needs no material at all. It crosses empty space. The Sun warms the Earth with light and with infrared.
The space between the Sun and Earth is almost empty. Radiation crosses it, so no conduction or convection is needed.
A dark, dull surface takes in radiation best. It also gives out radiation best. A shiny, light surface sends most of it back. This is why a black can warms faster in the sun than a shiny one.
Example readings. Both cans started at 25 °C. The black can ends hotter.
See it three ways
As a picture
In words
Conduction moves heat through solids, such as a hot spoon. Convection moves heat in liquids and gases, such as warm air over a stove. Radiation moves heat as infrared, even through empty space, such as the Sun on your skin.
Try it
Race a metal and a wooden spoon
You need: a metal spoon and a wooden spoon of the same length, a cup, hot water, poured by an adult, a clock, or a friend who counts seconds.
Be safe: An adult pours the hot water and stays close. Never grab a spoon that has stood in hot water. Test it gently with one finger first, and take your finger away at once if it feels hot.
Ask an adult to pour hot water into the cup. Let it stand for five minutes first if it came from a kettle.
Stand both spoons in the cup with their handles leaning on the rim. Start counting.
After one minute, touch the top of each handle very briefly with one finger. Do not hold on.
Test again after two and three minutes. Write down which handle feels warmer each time.
What you will see: The metal handle warms up within a minute or two. The wooden handle stays cool for much longer. Heat travels fast along the metal and slowly along the wood.
If you cannot try it, this is what happens.
A wrong idea many people have
A metal chair is colder than a wooden chair in the same room.
Why it feels right: Metal feels much colder when you touch it, so it seems colder.
The test that breaks it: Both chairs have stood in the room all day. A thermometer reads the same on both. Metal feels colder because it carries heat away from your hand fast.
What is true: Both are at the same temperature. Metal is a good conductor, so it takes heat from your hand quickly. Wood is an insulator and takes it slowly.
Key words
conduction
heat moving through a solid.
convection
heat moving in a liquid or gas, as warm fluid rises.
radiation
heat moving as infrared, with no material needed.
conductor
a material that lets heat pass through it easily.
insulator
a material that lets heat pass through slowly.
infrared
invisible waves that carry heat.
Back to your guess
The answer is C: Heat travels along metal much better than along wood.
Heat moves along metal quickly. It moves along wood very slowly. Wood does warm up, but only after a long time.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
The handle of a metal spoon in hot tea gets hot. Which way is the heat travelling?
See the answer
By conduction
Heat moves through a solid by conduction, from particle to particle. Metal conducts heat well.
If you chose B: Convection happens in liquids and gases that can flow. A spoon is a solid, so its heat conducts through it. Go to S3.8.5.
If you chose C: Radiation travels as infrared through space. Here the heat passes through the metal itself. Go to S3.8.5.
The bottom of a pot of water is heated. What does the warm water do?
See the answer
It rises, and cooler water sinks to take its place.
Warm water has spread out, so a cupful of it weighs less, and it rises. Cooler water sinks in to replace it. This loop is convection.
If you chose A: Warm water has spread out, so a cupful of it weighs less than a cupful of cool water. So it rises. Go to S3.8.5.
If you chose C: The warm water is free to move. It has spread out, and it rises. Go to S3.8.5.
How does the Sun's warmth reach the Earth across empty space?
See the answer
By radiation
Radiation travels as infrared and needs no material. It crosses empty space.
If you chose A: Conduction needs a material to pass through. Space between the Sun and Earth is almost empty. Go to S3.8.5.
If you chose B: Convection needs a liquid or gas that can flow. Space has almost none. Go to S3.8.5.
Which is the best handle for a cooking pot?
See the answer
Wood, because it is an insulator.
Wood lets heat pass slowly, so the handle stays cool enough to hold.
If you chose B: Metal is strong, but it conducts heat to your hand fast. It could burn you. Go to S3.8.5.
If you chose C: They do not. Metal warms much faster than wood, as the spoons showed. Go to S3.8.5.
A learner tests three spoons in hot water. After 2 minutes the handles read as shown. How many degrees warmer is the copper handle than the steel handle?
See the answer
8 °C
Copper 41 °C and steel 33 °C. 41 - 33 = 8 °C.
If you chose A: You added the two readings. To find how much warmer, subtract. Go to S3.8.5.
If you chose B: You took the wooden spoon instead of the steel one. Read the steel row. Go to S3.8.5.
If you chose C: That is how much the copper handle warmed from 20 °C. The question compares it with steel. Go to S3.8.5.
From an earlier chapter
Review: a cup of tea is 88 °C when it is poured. Half an hour later it is 46 °C. How many degrees did it cool?
See the answer
42 °C
Cooling = 88 - 46 = 42 °C.
If you chose A: You added the two temperatures. The cooling is the difference, so subtract. Go to S3.8.4.
If you chose C: That is the temperature at the end. The question asks how much it fell. Go to S3.8.4.
Keep it
Copy this into your exercise book.
Conduction: heat through solids, fast in metals. Convection: warm liquid or gas rises, cool sinks. Radiation: infrared crosses empty space. Insulators slow heat down.
Draw: A metal spoon and a wooden spoon in a cup, with an arrow up the metal one.
S3.8.6 Chapter 6
Keeping warm and cool
How can I keep my tea warm for longer?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Test which material keeps hot water warm longest
Explain how a wet clay pot keeps water cool
Mei pours hot tea for her grandmother. It must still be warm when she wakes up. Mei wraps the cup in a cloth. Does that help?
Wool, cloth, feathers and foam are insulation. They hold many tiny pockets of still air. Still air is a poor conductor, and it cannot carry heat by convection. So heat leaves slowly.
Draw it first
Draw a cup wrapped in cloth
1 Draw a cup of hot water. Draw long arrows for the heat escaping from it.
2 Wrap it in a cloth. Dots show the tiny pockets of trapped air.
3 Now draw short arrows. The cloth slows the heat down, so the tea stays warm longer.
How can we find the best wrap? We do a fair test. We change only one thing, which is the wrap. We keep everything else the same.
A fair test. Use the same cup, the same amount of water and the same time for every wrap.Example results. Every cup started at 80 °C in a room at 20 °C. Cloth keeps the water warmest.
Worked example
Use the bars. How many degrees did the bare cup lose in 30 minutes? How many did the cloth cup lose? How much warmer is the cloth cup?
Bare cup: 80 - 42 = 38 °C lost.
Cloth cup: 80 - 58 = 22 °C lost.
The cloth cup lost 38 - 22 = 16 °C less.
Answer: bare 38 °C, cloth 22 °C, 16 °C warmer
The cloth cup cools slowly. Compare this with the bare cup in chapter 4, which had cooled to 42 °C by 30 minutes.
A vacuum flask uses every trick at once. Its double wall has a gap with no air, so heat cannot conduct or convect across it. Shiny walls send radiation back. The stopper stops warm air escaping from the top.
No air in the gap means no conduction or convection. Shiny walls cut radiation.
Now keeping things cool. In many hot places people store drinking water in a clay pot. The clay has tiny holes. A little water seeps through to the outside.
Water seeps through the clay and evaporates from the outside.
To evaporate, water needs energy. It takes that energy from the pot and the water inside. So the water in the pot gets cooler.
See it three ways
As a picture
In words
Energy leaves the pot with the water that evaporates. The pot loses energy, so it cools. It cools the water inside.
It works best when the air is dry and moving, because water evaporates fast. In humid air the water evaporates slowly, so the pot cools less.
Example readings. The pot water is cooler than the air, and cooler still on a dry day.
Worked example
The air is 34 °C. How many degrees cooler is the pot water on a dry day? On a humid day?
Dry day: 34 - 27 = 7 °C cooler.
Humid day: 34 - 32 = 2 °C cooler.
The pot does its job better on the dry day, by 5 °C.
Answer: dry day 7 °C, humid day 2 °C
Try it
Which wrap keeps water warm longest?
You need: three identical cups, a piece of cloth and a piece of paper, hot water, poured by an adult, a clock, a thermometer, if you have one.
Be safe: An adult pours the hot water and stays close. Never put a finger in the water. Carry nothing hot.
Wrap one cup in cloth and one in paper. Leave the third bare.
Ask an adult to pour the same amount of hot water into each cup. Mark the level with a pencil first.
Leave them for 20 minutes. Do not touch them.
Ask the adult to take off the wraps and touch the side of each cup for a moment. Or use a thermometer if you have one. Rank the cups from warmest to coolest.
What you will see: The cloth cup stays warmest and the bare cup is coolest. The paper cup is in between. Your cups may differ a little, but the order should be the same.
If you cannot try it, this is what happens.
A wrong idea many people have
A woollen jumper makes heat.
Why it feels right: You feel warm when you put one on, so the jumper seems to give out warmth.
The test that breaks it: Wrap one cup of cold water in a woollen sock and put a bare cup beside it. After an hour the wrapped water is no warmer than the bare one. The wool made no heat.
What is true: Wool is an insulator. It traps the heat your own body gives out, and it slows that heat from escaping.
Key words
insulation
a material that holds still air and slows heat down.
fair test
a test where only one thing is changed.
vacuum flask
a bottle with a double wall and no air in the gap.
Back to your guess
The answer is C: Wrap it in cloth or wool, which traps still air.
Wrap it in cloth or wool. These trap still air, and still air passes heat on very slowly. A metal tin would let the heat out fast.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Why does a woollen cloth keep a cup of tea warm?
See the answer
It traps still air, and still air passes heat on very slowly.
Wool holds many pockets of still air. Air is a poor conductor and cannot circulate, so heat leaves slowly.
If you chose B: Wool makes no heat. It only slows down the heat that is already in the tea. Go to S3.8.6.
If you chose C: A good conductor would carry heat away fast. Wool is an insulator. Go to S3.8.5.
Amina wraps a cloth round one cup, and paper round a bigger cup with more water. She says cloth is best. Why is her test not fair?
See the answer
She changed two things: the wrap and the amount of water.
A fair test changes only one thing. Here both the wrap and the amount of water changed, so you cannot tell which made the difference.
If you chose A: That is not the problem. The problem is that she changed more than the wrap. Go to S3.8.6.
If you chose C: Paper is a fine thing to test. The test just needs the same cup and the same water. Go to S3.8.6.
Two cups start at 90 °C. After 30 minutes the cloth cup is 62 °C and the foil cup is 53 °C. How many degrees warmer is the cloth cup?
See the answer
9 °C
62 - 53 = 9 °C.
If you chose A: You added the two readings. To find how much warmer, subtract. Go to S3.8.6.
If you chose B: That is how much the cloth cup lost. The question asks how much warmer it is than the foil cup. Go to S3.8.6.
If you chose C: That is how much the foil cup lost. Compare the two readings instead. Go to S3.8.6.
How does a wet clay pot keep its water cool?
See the answer
Water seeps out and evaporates, and takes energy from the pot and the water.
Evaporating water needs energy. It takes it from the pot and the water inside, which cool as a result.
If you chose A: There is no such thing as made cold. Cooling is energy leaving, not cold arriving. Go to S3.8.6.
If you chose B: An insulator would only slow the warming. It could not make the water cooler than the air. Go to S3.8.6.
When does a wet clay pot cool its water best?
See the answer
In dry air that is moving.
Dry, moving air takes the water vapour away. The water evaporates fast, so more energy leaves the pot.
If you chose B: In humid air the water evaporates slowly. Slow evaporation takes away little energy. Go to S3.8.6.
If you chose C: A closed, damp room is humid. The water cannot evaporate quickly, so the pot cools little. Go to S3.8.6.
From an earlier chapter
Review: warm air rises above a heater and cool air sinks to take its place. What is this way of moving heat called?
See the answer
Convection
Convection is heat moving in a liquid or a gas as warm fluid rises and cool fluid sinks.
If you chose A: Conduction is heat moving through a solid. Air can flow, so this is something else. Go to S3.8.5.
If you chose C: Radiation is infrared that needs no material. Here the moving air carries the heat. Go to S3.8.5.
Keep it
Copy this into your exercise book.
Insulators trap still air and slow heat down. A fair test changes one thing. A wet clay pot cools water as water evaporates and takes energy away, best in dry air.
Draw: A cup in a cloth with short arrows. Next to it a wet pot with arrows going out.
S3.8.7 Chapter 7
Energy from food
Why am I so hungry after playing all day?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Compare the energy in foods using a table in kilojoules
Explain why active people need more food energy
Kofi plays football from morning until the sun is low. By the evening he is very hungry. His little sister sat in the shade all day and is only a little hungry.
Food is a chemical store. The energy in food is counted in kilojoules, written kJ. One kilojoule is 1000 joules. Your body takes the energy out of food and uses it.
Foods do not all hold the same amount. We compare them by the food energy in the same weight, usually 100 g. The numbers here are about right, not exact.
About how many kilojoules are in 100 g of each food.Oil holds the most energy in the same weight. It is a fat. Fruit holds less, even when it tastes sweet.
Now the other side. Moving, thinking, growing and keeping warm all take energy out of your body. You are always using some, even when you sleep.
Draw it first
Draw how much energy an hour of each activity uses
1 Draw a short bar for resting. In an hour it moves about 300 kJ out of the body.
2 Draw a longer bar for walking. It moves about 1000 kJ in an hour.
3 Draw the longest bar for running. It moves about 2500 kJ in an hour. The numbers are rough, for a grown person.
The more you move, the more energy leaves your body each hour. So active people need more food energy to put it back. Growing bodies also need food energy, to build new parts.
Muscles turn only about a quarter of the food energy into movement. The rest is heat, and it keeps you warm.
Worked example
Tariq eats 200 g of cooked rice and 100 g of cooked beans. How much food energy is that?
Rice: 200 g is 2 lots of 100 g. Each lot is about 550 kJ.
Rice energy = 2 × 550 = 1100 kJ.
Beans: 1 × 500 = 500 kJ.
Total = 1100 + 500 = 1600 kJ.
Answer: about 1600 kJ
See it three ways
As a picture
In words
To find the energy in a meal, work out how many lots of 100 g there are. Multiply by the kJ in 100 g. Do this for each food and add up.
In symbols
energy = (grams ÷ 100) × kJ per 100 g. For the rice: 2 × 550 = 1100 kJ.
Worked example
Joseph walks for 3 hours. Walking moves about 1000 kJ an hour. How much bread, in grams, holds the same energy? Bread has about 1000 kJ in 100 g.
Energy moved = 3 × 1000 = 3000 kJ.
Lots of 100 g of bread = 3000 ÷ 1000 = 3.
Bread = 3 × 100 = 300 g.
Answer: 300 g
Try it
Feel your body move energy
You need: a clock or a friend who counts seconds, flat ground.
Be safe: Stop at once if you feel dizzy or any pain. Jog on flat ground with space around you.
Sit still for two minutes. Press two fingers on your wrist, below the thumb, until you feel the pulse.
Count the beats for 15 seconds. Multiply by 4. Write the number.
Jog on the spot for one minute. Then count the pulse for 15 seconds again and multiply by 4.
What you will see: Your heart beats faster after jogging. Your muscles are moving more energy, so they need more oxygen and food energy. Your numbers will differ from ours.
If you cannot try it, this is what happens.
A wrong idea many people have
Only sweet foods have energy.
Why it feels right: Sugar gives a quick feeling of energy, so sweet things seem to be the energy foods.
The test that breaks it: Look at the table. Cooking oil is not sweet, yet it has about 3700 kJ in 100 g. Bread and beans are not sweet, and a banana is sweet but has only about 370 kJ.
What is true: Starch, sugar, fat and protein all hold energy. Fat holds the most in the same weight.
Key words
kilojoules
the units we count food energy in. We write kJ. 1 kJ is 1000 J.
food energy
the energy kept in a food, in its chemical store.
Back to your guess
The answer is C: Playing moves energy out of my body, so I need food to put it back.
Playing moves energy out of your body, into movement and heat. Food is a chemical store that puts energy back in.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
A table gives the kJ in 100 g of four foods. Which food has the most energy in the same weight?
See the answer
groundnuts
Groundnuts have about 2400 kJ in 100 g. That is the biggest number in the table.
If you chose A: You may have picked the first row. Compare every number, not only the top one. Go to S3.8.7.
If you chose B: Milk is a drink, so it can seem light. But 100 g of milk holds far less than 100 g of groundnuts. Compare the numbers. Go to S3.8.7.
If you chose C: A potato is bulky, but bulk is not energy. Read the kJ numbers. Go to S3.8.7.
A boiled egg has about 600 kJ in every 100 g. How much food energy is in 300 g of egg?
See the answer
1800 kJ
300 g is 3 lots of 100 g. 3 × 600 = 1800 kJ.
If you chose B: You multiplied the grams by the kJ. First find how many lots of 100 g there are, then multiply. Go to S3.8.7.
If you chose C: You added. The kJ are for each 100 g, so multiply by the number of lots of 100 g. Go to S3.8.7.
If you chose D: That is only for the first 100 g. There are more lots of 100 g in this amount. Go to S3.8.7.
Why do people who run or dig all day need more food energy?
See the answer
Their bodies move more energy out, so they need more food to put it back.
Movement moves energy out of the chemical stores in the body. Food puts energy back in.
If you chose A: Food does not change. The body simply moves more energy out when it is active. Go to S3.8.7.
If you chose C: People differ. More movement takes more energy out of the body, so active people need more. Go to S3.8.7.
Digging moves about 1200 kJ out of the body each hour. How much energy does 4 hours of digging move?
See the answer
4800 kJ
4 hours × 1200 kJ = 4800 kJ.
If you chose A: You divided. Each hour moves the same amount, so multiply the rate by the hours. Go to S3.8.7.
If you chose B: You added kJ and hours, which cannot be added. Multiply the kJ in one hour by the number of hours. Go to S3.8.7.
In what unit is the energy in food counted?
See the answer
Kilojoules, kJ
Food energy is counted in kilojoules. 1 kJ is 1000 J.
If you chose A: Kilograms measure mass, which is how much stuff there is. Energy is counted in joules or kilojoules. Go to S3.8.7.
If you chose B: Degrees Celsius measure temperature, not energy. A cold food can hold a lot of energy. Go to S3.8.7.
If you chose C: Newtons measure a force. Energy is measured in joules. Go to S3.8.7.
From an earlier chapter
Review: a body takes in 200 J from food and moves with 40 J. What is the efficiency?
See the answer
20%
Efficiency = 40 ÷ 200 × 100 = 20%.
If you chose B: That is the wasted share. Efficiency uses the useful energy divided by the energy in. Go to S3.8.3.
If you chose C: You divided the energy in by the useful energy, upside down. Divide useful by in, then multiply by 100. Go to S3.8.3.
Keep it
Copy this into your exercise book.
Food is a chemical store, counted in kilojoules. Foods hold different amounts. Active people move more energy out of their bodies, so they need more food energy.
Draw: Three bars: resting, walking, running. Make each one longer. Write kJ beside each bar.
S3.8.8 Chapter 8
Fuels and energy resources
Will we ever run out of fuel?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Sort energy resources into renewable and non-renewable
Explain how burning fuels adds carbon dioxide to the air
Compare cooking with wood, gas and sunlight
Amina's family cooks on a wood fire. Her cousin cooks on gas. At Ravi's school the cooks use a box that catches the Sun. All of them are using energy to heat food.
A fuel is a chemical store that we burn to get heat. Wood, charcoal, coal, oil and gas are fuels. An energy resource is anything we get useful energy from. That includes fuels, sunlight, wind and flowing water.
Some resources are renewable. They keep coming, or are replaced as fast as we use them. Sunlight, wind and flowing water are renewable.
Others are non-renewable. Coal, oil and gas took millions of years to form. Once we burn them, they are gone for any time we can imagine.
Draw it first
Draw a sort of energy resources
1 Draw a line down a page. Write renewable on the left and non-renewable on the right.
2 List the resources that keep coming: sunlight, wind and flowing water.
3 List the fuels that took millions of years to form: coal, oil and gas.
Wood sits in the middle. It is renewable only if trees are planted as fast as they are cut down. If not, forests shrink and the wood runs out.
Wood has a row of its own. It is renewable only when trees are replanted as fast as they are cut.
What happens when a fuel burns? Wood, coal, oil and gas all contain carbon. The carbon joins with oxygen from the air. This makes carbon dioxide, a gas, which goes into the air.
A burning fuel takes oxygen from the air. It gives out heat and carbon dioxide.
Carbon dioxide in the air traps some of the heat that rises from the ground. More carbon dioxide traps more heat. That warms the whole Earth a little.
The carbon dioxide works like a blanket. Some heat still escapes, but a thicker blanket sends more back down.
Trees take carbon dioxide from the air as they grow. So wood burned from trees that are replanted can balance out. Wood from forests that are not replanted cannot.
Now cooking. Every fire wastes energy. Only some of the energy in the fuel goes into the pot. An open wood fire puts about 15 J in the pot for every 100 J of wood. A gas stove puts about 60 J.
An open wood fire. Most of the energy warms the air.A gas stove. A lot more of the energy goes into the pot.
Worked example
A pot of beans needs 4800 kJ. An open wood fire is about 15% efficient. How many kJ must the wood hold, and how many kg of wood is that? Dry wood holds about 16000 kJ in each kg.
The same pot needs 4800 kJ. A gas stove is about 60% efficient. How many kJ must the gas hold? How many times as much fuel energy did the wood fire need?
4800 ÷ 60 = 80.
80 × 100 = 8000 kJ.
The wood fire needed 32000 kJ. 32000 ÷ 8000 = 4 times as much.
Answer: 8000 kJ, and the wood fire needs 4 times as much
A solar cooker uses no fuel at all. A shiny surface sends sunlight onto a dark pot. The dark pot takes in the radiation and heats up. It makes no smoke and no carbon dioxide. But it needs strong sunshine, and it cooks slowly.
See it three ways
As a picture
In words
Wood and gas both make carbon dioxide when they burn. Gas is non-renewable. Wood is renewable only when trees are replanted. Sunlight makes no carbon dioxide while you cook.
Try it
Catch the Sun with a dark bottle
You need: two clear plastic bottles of the same size, with caps, water from the same jug, black paper or cloth, and shiny foil or a white cloth, a sunny spot on the ground.
Be safe: Never look at the Sun. If you use foil, never look at sunlight bouncing off it. Keep it turned away from anyone's eyes.
Fill both bottles with the same cool water and put on the caps.
Wrap one bottle in black paper. Wrap the other in shiny foil, or in a white cloth.
Stand them side by side in the sunshine for one hour. Turn the foil away from faces.
Unwrap both. Hold the back of your hand against the outside of each bottle. Feel which one is warmer.
What you will see: The black bottle is warmer. Dark surfaces take in more of the Sun's radiation. Shiny foil or white cloth sends most of it away. Your bottles may differ a little.
If you cannot try it, this is what happens.
A wrong idea many people have
Wood is natural, so burning it adds no carbon dioxide.
Why it feels right: Trees are alive and part of nature, so their smoke seems harmless.
The test that breaks it: Think of a wood fire. It gives off heat, light and smoke. Wood contains carbon, so burning it makes carbon dioxide, like any other fuel.
What is true: Burning wood adds carbon dioxide to the air. Growing trees take it back in, but only if trees are replanted as fast as they are cut.
Key words
fuel
a chemical store we burn to get heat.
energy resource
anything we get useful energy from, such as fuels, sunlight and wind.
renewable
an energy resource that keeps coming or is replaced as fast as we use it.
non-renewable
an energy resource that took millions of years to form and will run out.
carbon dioxide
a gas made when fuels with carbon burn. It traps heat in the air.
Back to your guess
The answer is B: Sunlight and wind will not run out, but coal, oil and gas will.
Coal, oil and gas took millions of years to form. We use them far faster than they form, so they will run out. Sunlight and wind keep coming.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Which of these energy resources is renewable?
See the answer
Sunlight
Sunlight keeps coming every day, so it is renewable. Coal, oil and gas are non-renewable.
If you chose A: Coal took millions of years to form. It cannot be replaced as fast as we burn it. Go to S3.8.8.
If you chose B: Gas took millions of years to form. Once we burn it, it is gone. Go to S3.8.8.
If you chose C: Oil took millions of years to form, so it is non-renewable. Go to S3.8.8.
A family burns gas to cook. What gas does burning add to the air, and what does more of it do?
See the answer
Carbon dioxide, which traps more heat and warms the Earth.
Gas contains carbon. It burns to make carbon dioxide. More carbon dioxide traps more heat.
If you chose A: Burning uses up oxygen from the air. It does not add it. Go to S3.8.8.
If you chose B: Carbon dioxide traps heat, so it warms the Earth. It does not cool it. Go to S3.8.8.
On a sunny day a family wants to cook with no smoke and no carbon dioxide. Which is best?
See the answer
A solar cooker
A solar cooker uses sunlight and burns no fuel. It makes no smoke and no carbon dioxide. It needs strong sun.
If you chose B: A wood fire makes smoke and carbon dioxide, and it is renewable only if the trees are replanted. Go to S3.8.8.
If you chose C: A gas stove burns gas, which makes carbon dioxide and is non-renewable. Go to S3.8.8.
A pot needs 1800 kJ. An open fire is about 15% efficient. How many kJ must the fuel hold?
See the answer
12000 kJ
Fuel energy = 1800 ÷ 15 × 100 = 12000 kJ.
If you chose A: You multiplied by the efficiency. A fire wastes energy, so the fuel must hold more than the pot gets. Divide. Go to S3.8.3.
If you chose B: You added a percentage to kJ, which cannot be added. Divide the useful energy by the efficiency, then multiply by 100. Go to S3.8.3.
If you chose C: That would be a fire that wastes nothing. This fire wastes most of its energy, so it needs much more. Go to S3.8.3.
Is wood a renewable energy resource?
See the answer
Yes, but only if trees are replanted as fast as they are cut.
Trees grow back, so wood can be renewable. If we cut more than we replant, it runs out.
If you chose A: Trees grow back. Wood can be renewable if new trees are planted as fast as old ones are cut. Go to S3.8.8.
If you chose C: Natural does not mean renewable. If trees are cut faster than they grow, forests shrink and the wood runs out. Go to S3.8.8.
From an earlier chapter
Review: a food has about 800 kJ in every 100 g. How much food energy is in 300 g of it?
See the answer
2400 kJ
300 g is 3 lots of 100 g. 3 × 800 = 2400 kJ.
If you chose A: You multiplied the grams by the kJ. First find how many lots of 100 g there are. Go to S3.8.7.
If you chose B: You added. The kJ are for each 100 g, so multiply by the number of lots of 100 g. Go to S3.8.7.
Keep it
Copy this into your exercise book.
Renewable: sunlight, wind, water, and wood if replanted. Non-renewable: coal, oil, gas. Burning fuels makes carbon dioxide, which traps heat. A solar cooker burns nothing.
Draw: A page split in two: renewable and non-renewable. List three resources under each.
Key words, A to Z
Every key word in this book, what it means, and the chapter that teaches it.
by a force
a way energy moves, when things push, pull or fall. S3.8.2
by heating
a way energy moves, from a hotter thing to a cooler one. S3.8.2
carbon dioxide
a gas made when fuels with carbon burn. It traps heat in the air. S3.8.8