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GCSE Physics · GCSEChapter 1 · Energy stores and transfers
GCSE · Chapter 1

Energy stores and transfers

By the end of this chapter you will be able to

  • Name the energy store in a given object or system
  • Describe how energy is transferred between stores, and name the way it is transferred
  • Read a Sankey diagram and say how much energy is useful and how much is wasted
  • Explain why energy is never lost, only transferred, and how insulation and lubrication reduce waste

You should already know

  • Energy as something that makes things happen (KS3 Physics)
  • Basic arithmetic with whole numbers and decimals

Where this leads

  • Efficiency, power and energy calculations (Chapter 2)
  • Specific heat capacity (Chapter 3)

GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe energy stores: kinetic, gravitational potential, thermal, elastic, chemical, nuclear, magnetic and electrostatic, Describe energy transfers by heating, mechanically, electrically and by radiation, State the law of conservation of energy, Read and draw energy transfer diagrams and Sankey diagrams, Explain energy dissipated to the surroundings and how wasted energy can be reduced.

Exercise P  Do you remember?

1
FWhat is 2 x 3.5? Write the number.
2
FHow many joules are in 1 kilojoule? Write the number.
3
FWhat is 80% of 50? Write the number.
4
FWhich is bigger, 0.2 or 1/4? Write the fraction.
5
FWhat is the name of the process that moves heat through a liquid when the warm, less dense liquid rises? Write the word.
1.1

Energy stores

GCSE
Describe energy stores: kinetic, gravitational potential, thermal, elastic, chemical, nuclear, magnetic and electrostatic
Video lesson: Energy storesComing soon
Context. Ruby is watching a ball roll down a slope in the park. “Where is the energy?” she asks. Energy is never a thing you can see. We describe it by saying which store it is in.

An energy store is a place where energy can be kept until it is needed. A moving object has energy in its kinetic store. A raised object has energy in its gravitational potential store. A hot object has energy in its thermal store. Energy is not used up when it is stored; it is simply moved from one store to another.

StoreWhat it isExample
kineticenergy of a moving objecta moving car or a thrown ball
gravitational potentialenergy of an object raised above the grounda book on a high shelf
thermalenergy stored in the particles of a hot objecta warm cup of tea
elasticenergy stored when a material is stretched or squasheda stretched catapult or a spring
chemicalenergy stored in the bonds of fuels and foodpetrol, a battery, a bar of chocolate
nuclearenergy stored in the nucleus of an atomuranium fuel in a power station
magneticenergy stored when magnets push or pull aparttwo magnets pushed together
electrostaticenergy stored when charges are separateda balloon charged by rubbing
The main energy stores
topat restheightstore: gravitational potentialmovingstore: kineticfalls
A ball at the top of a drop and at the bottom: energy moves from the gravitational potential store to the kinetic store

Key ideas

  • Energy comes in eight main stores: kinetic, gravitational potential, thermal, elastic, chemical, nuclear, magnetic and electrostatic.
  • Energy is stored, not used up.
  • A store is the place the energy is kept, not the thing that makes it move.
  • Name the store that the energy is in now.
Worked example 1.1.1
A fully charged battery is connected to nothing. What is the energy store of the energy in the battery? Write the word.

Method. Think about what the battery is made of and what it does when connected.

  1. Look at the object. A battery stores energy in its chemicals, ready to be released by a circuit.
    battery
  2. Name the store. Energy stored in chemicals is the chemical store.
    chemical store
  3. Write the answer.
    chemical

Check. A battery is not hot, so the thermal store is small. ✓

Worked example 1.1.2
A book is lifted from the floor onto a high shelf. Which store has gained energy? Write the word.

Method. Think about the height of the book.

  1. Compare heights. The book is higher above the ground than before.
    higher
  2. Name the store. Energy stored because an object is raised is the gravitational potential store.
    gravitational potential
  3. Write the answer.
    gravitational potential

Check. The book is still at the same speed, so its kinetic store did not change. ✓

Worked example 1.1.3
A stretched elastic band is let go and snaps back. The energy in the stretched band was in which store? Write the word.

Method. Decide what happens when a material is stretched.

  1. Look. Stretching an elastic band stores energy when the band changes shape.
    changes shape
  2. Name the store. Energy stored when a material is stretched or squashed is the elastic store.
    elastic store
  3. Write the answer.
    elastic

Check. When the band snaps back, the elastic store transfers to the kinetic store. ✓

Watch out

Watch out: “heat” is a transfer, not a store. A hot cup of tea has energy in its thermal store. Say the store, not the transfer, when you are asked where the energy is.

Exam tip

Exam tip. Start your answer with “The energy is in the… store.” Then name the store exactly, using the words in the key ideas box.

Exercise 1A  Naming energy stores

1
FName the energy store of a moving skateboard. Write the word.
2
FName the energy store of a hot radiator. Write the word.
3
FName the energy store of a compressed spring. Write the word.
4
FName the energy store of the food in a sandwich. Write the word.
5
FName the energy store of a magnet pushed towards another magnet with the same poles facing. Write the word.
6
FA stone is held at the top of a cliff. Name the store with the energy in it. Write the word.
7
FMatch each object to the store. Write the word for each.
(a)a stretched bow
(b)a stone at the top of a hill
(c)nuclear fuel in a power station
8
RA pupil says, “A moving car has heat in it.” Explain why this is not the best way to describe the car’s energy. Accept any sentence that says the moving car has kinetic energy and heat is a transfer, not a store.
9
PSA ball is thrown upwards. At its highest point it stops moving for an instant. Which store holds the energy at this point? Write the word.
10
PSA toy car is pulled back and released. It drives forward up a ramp and stops. Name the store at the top of the ramp. Write the word.
11
PSName the two stores that a moving, hot bicycle wheel has. Write two words.
12
PSA battery is connected to a lamp and the lamp lights. Give the store the energy starts in and the store it ends in. Write two words.
13
RExplain in one sentence why energy is said to be stored, rather than used up. Accept any sentence that says energy moves from one store to another and is not destroyed.
14
PSA cup of hot tea is left on a table and cools down. Name the store that the energy leaves, and the store that gets it. Write two words.
15
FWhich store is used when a spring is pulled out and then let go? Write the word.
16
FWhich store holds energy in a battery-powered torch before it is switched on? Write the word.
1.2

Energy transfers

GCSE
Describe energy transfers by heating, mechanically, electrically and by radiation
Video lesson: Energy transfersComing soon
Context. Arjun turns on the kettle and then holds his hand over it. “The water is getting energy,” he says. “But how does the energy get from the kettle to the water, and to my hand?” The answer is a transfer.

An energy transfer moves energy from one store to another. There are four main ways it can happen: heating, mechanically (a force does work), electrically (a current carries energy) and by radiation (through waves, such as infrared or light). Energy can be transferred by more than one way at the same time.

Way of transferShort nameExample
by heating (conduction, convection, radiation)heatinga hob warms a pan of water
by forces doing work (pushing, lifting, stretching)mechanicala person lifts a box
by electric currentelectricala lamp takes energy from a cell
by waves (light, infrared and others)radiationthe Sun warms your face
The four ways energy is transferred

Key ideas

  • The four ways are: heating, mechanically, electrically and by radiation.
  • Heat moves from hot to cold. The energy moves, the store does not need to.
  • Radiation can travel through empty space; heating needs particles or contact.
  • A transfer always starts from one store and ends in another.
Worked example 1.2.1
A cold pan is placed on a hot hob. Energy moves from the hob into the pan. Name the transfer. Write the word.

Method. Think about how the hob touches the pan.

  1. Look. The hob touches the pan, so energy moves through the metal by heating.
    contact
  2. Name it. Energy moving from a hot object to a cooler one by contact is heating.
    heating
  3. Write the answer.
    heating

Check. The pan gets warm, so its thermal store increases. ✓

Worked example 1.2.2
A person pushes a trolley along the floor. Name the transfer that moves energy from the person to the trolley. Write the word.

Method. Think about what the person does to the trolley.

  1. Look. The person applies a force and the trolley moves a distance.
    force and distance
  2. Name it. Work done by a force is a mechanical transfer.
    mechanical
  3. Write the answer.
    mechanical

Check. Work done = force x distance, so the energy transfers mechanically. ✓

Worked example 1.2.3
Energy from a cell moves through a wire to a lamp. Which transfer is this? Write the word.

Method. Think about what carries the energy along the wire.

  1. Look. A current carries the energy from the cell to the lamp.
    current
  2. Name it. Energy carried by a current is an electrical transfer.
    electrical
  3. Write the answer.
    electrical

Check. A lamp then transfers the energy by light and heating. ✓

Watch out

Watch out: a transfer names how the energy moves. Do not confuse it with a store, which names where the energy is kept.

Exam tip

Exam tip. If the question asks for a transfer, use one of the four words: heating, mechanical, electrical or radiation.

Exercise 1B  Naming energy transfers

1
FName the transfer when a hot drink warms your hands. Write the word.
2
FName the transfer when a person lifts a box onto a shelf. Write the word.
3
FName the transfer when energy moves through the wires of a kettle. Write the word.
4
FName the transfer when the Sun warms your face through the air. Write the word.
5
FWhich way of transfer needs no particles between the two objects? Write the word.
6
PSA hair dryer blows hot air onto hair. Name the transfer that warms the hair. Write the word.
7
PSA cyclist uses chemical energy from food to pedal a bike. Name the first transfer in the chain. Write the word.
8
PSName the transfer that takes energy from a battery to a torch bulb. Write the word.
9
RA pupil says, “Energy moves through a vacuum by heating.” Explain why this is wrong. Accept any sentence that says heating needs particles and a vacuum has none, so radiation is the transfer.
10
PSTwo transfers happen when a lamp is switched on: electrical and then another. Name the second transfer. Write the word.
11
FWrite the transfer for each event. Write the word for each.
(a)a skateboarder pushes off the ground
(b)a warm radiator heats a room by the air moving
(c)a phone charger sends energy to a phone
12
PSA hob heats a pan of water. Name two transfers that happen on the way from the hob to the water. Write two words.
13
RExplain why radiation from the Sun is needed for life on Earth, in one sentence. Accept any sentence that says the Sun transfers energy to Earth by radiation through space.
1.3

Energy transfer diagrams and Sankey diagrams

GCSE
Read and draw energy transfer diagrams and Sankey diagrams
Video lesson: Energy transfer diagrams and Sankey diagramsComing soon
Context. Mia is comparing two lamps for her bedroom. The shop says one is more useful. “How can I see where the energy goes?” she asks. A Sankey diagram shows the energy in as one wide band, and each part that comes out as a band of its own.

In a Sankey diagram the width of each band shows the amount of energy. The band on the left is the total energy in. The bands on the right show where the energy goes. The bands on the right always add up to the band on the left, because energy is conserved. A diagram without numbers can still show the idea, but numbers make the diagram easy to read.

Energy in a filament lamp (example numbers)input 100 Jlight: 10 Jthermal, wasted: 90 J
A Sankey diagram for a filament lamp. The useful output is the light; the rest is wasted
Energy in an electric kettle (example numbers)input 100 Jheats the water: 80 Jwasted to the room: 20 J
A Sankey diagram for an electric kettle

Key ideas

  • The total energy in equals the total energy out, including the wasted energy.
  • The wasted energy is the energy transferred to the surroundings in a way we do not want.
  • The useful energy is the energy transferred to where we want it.
  • Wasted energy = total energy in - useful energy out.
Worked example 1.3.1
A lamp has 100 J of energy in. 10 J is light and 90 J is wasted as thermal energy. How much energy is wasted? Write the number of joules.

Method. Use the fact that the total in is the useful plus the wasted.

  1. Take the numbers. Total in = 100 J. Useful light = 10 J.
    100 J in, 10 J useful
  2. Subtract. Wasted = 100 - 10 = 90 J.
    100 - 10 = 90
  3. Write the answer.
    90 J

Check. 10 J useful + 90 J wasted = 100 J in. ✓

Worked example 1.3.2
A kettle takes 100 J from the mains. The water gets 80 J of heating. How much is wasted? Write the number of joules.

Method. Wasted energy is what is left over after the useful energy.

  1. Take the numbers. Total in = 100 J. Useful = 80 J.
    100 J in, 80 J useful
  2. Subtract. Wasted = 100 - 80 = 20 J.
    100 - 80 = 20
  3. Write the answer.
    20 J

Check. 80 J + 20 J = 100 J. ✓

Worked example 1.3.3
A torch has 60 J of energy in and gives out 12 J of light. The rest is wasted as thermal energy. How much is that? Write the number of joules.

Method. Find the energy that is not the useful light.

  1. Take the numbers. Total in = 60 J. Useful light = 12 J.
    60 J in, 12 J light
  2. Subtract. Wasted = 60 - 12 = 48 J.
    60 - 12 = 48
  3. Write the answer.
    48 J

Check. 12 J + 48 J = 60 J. ✓

Watch out

Watch out: the wasted energy is not “lost”. It is transferred to the surroundings, usually as thermal energy, and it is still there. The total energy in a diagram always stays the same.

Exam tip

Exam tip. When you read a Sankey diagram, add up the outputs. If they do not match the input, you have misread a band.

Exercise 1C  Reading Sankey diagrams

1
FIn the lamp Sankey diagram, how much energy goes in? Write the number of joules.
2
FIn the lamp Sankey diagram, how much energy is useful? Write the number of joules.
3
FIn the kettle Sankey diagram, how much energy heats the water? Write the number of joules.
4
FIn the kettle Sankey diagram, how much energy is wasted to the room? Write the number of joules.
5
FA machine has 500 J of energy in and 350 J is useful. How much is wasted? Write the number of joules.
6
FA radio has 40 J of energy in and 25 J is wasted. How much useful energy is there? Write the number of joules.
7
PSCopy and complete: in a Sankey diagram the input band is 60 J. One output is 35 J and the other output is missing. Find the missing output. Write the number of joules.
8
PSTwo Sankey diagrams are drawn for two lamps with the same input of 100 J. Lamp A gives 15 J of light. Lamp B gives 5 J of light. Which lamp wastes more energy? Write A or B.
9
PSA box has 200 J of energy in. 50 J is useful. Draw a Sankey diagram in words: list the two output bands with their values. Write the two values in joules, separated by a comma.
10
RExplain in one sentence how a Sankey diagram shows that energy is conserved. Accept any sentence that says the output bands add up to the input band.
11
RA pupil says a Sankey diagram with a 90 J band for wasted energy and a 10 J band for useful energy is wrong because the wasted band is too big. Is the pupil right? Write yes or no.
12
RIn the torch Sankey diagram, the input is 60 J and the useful light is 12 J. Find the fraction of the input that is wasted as a fraction. Write it in lowest terms.
13
PSA Sankey diagram shows 300 J in and 240 J wasted. How much energy is useful? Write the number of joules.
1.4

Conservation of energy and reducing wasted energy

GCSE
State the law of conservation of energy, Explain energy dissipated to the surroundings and how wasted energy can be reduced
Video lesson: Conservation of energy and reducing wasted energyComing soon
Context. Tom’s house is cold in winter. The heating costs a lot. His dad says the warm air escapes through the walls and the roof. “So is the energy lost?” Tom asks. The energy is not lost: it is dissipated into the surroundings.

The law of conservation of energy says that energy cannot be created or destroyed. It can only be transferred from one store to another. When energy is transferred to the surroundings in a form that is spread out and hard to use, we say it is dissipated. Dissipated energy is usually thermal energy spread into the air, the floor and the machine itself.

Energy in a torch (example numbers)input 60 Jlight: 12 Jthermal, wasted: 48 J
Energy in a torch. The useful output is the light; the thermal energy is dissipated to the surroundings

Key ideas

  • Energy cannot be created or destroyed. It is conserved.
  • Energy is dissipated to the surroundings, usually as thermal energy.
  • Insulation reduces the energy transferred by heating, so less energy is wasted.
  • Lubrication reduces friction, so less energy is wasted as thermal energy in moving parts.
Worked example 1.4.1
A pupil says that 30 J of energy has disappeared from a machine with 100 J in and 70 J out. Is energy destroyed? Write yes or no.

Method. Use the law of conservation of energy.

  1. Check. Energy is conserved, so 100 J in must equal the energy out plus any dissipated.
    100 = 70 + 30
  2. Decide. The 30 J has been dissipated to the surroundings, not destroyed.
    dissipated
  3. Write the answer.
    no

Check. 70 J + 30 J = 100 J. ✓

Worked example 1.4.2
A cold wall has no insulation. Thermal energy moves out of a room through the wall. Name the material that is added to the wall to reduce this loss. Write the word.

Method. Think about a material that slows heat flow.

  1. Look. Heat flows out of the room because the wall lets energy through.
    heat flows out
  2. Choose. Insulation slows the transfer of thermal energy.
    insulation
  3. Write the answer.
    insulation

Check. Insulation reduces the wasted energy from heating. ✓

Worked example 1.4.3
A bicycle chain has no oil and gets hot. Which process is reduced by oiling the chain? Write the word.

Method. Think about why the chain gets hot.

  1. Look. The chain rubs on the teeth, and the rubbing makes thermal energy.
    rubbing
  2. Name. Rubbing that wastes energy is friction; oil reduces it.
    friction
  3. Write the answer.
    friction

Check. Lubrication reduces friction and so reduces thermal energy wasted. ✓

Watch out

Watch out: “energy is used up” is wrong. Energy is conserved. Say that energy is transferred or dissipated, and name where it goes.

Exam tip

Exam tip. To reduce wasted energy, name the cause (heating, friction) and the method (insulation, lubrication). Give both.

Exercise 1D  Conservation, dissipation and reducing waste

1
FWhich law says energy cannot be created or destroyed? Write the name.
2
FEnergy that spreads out into the surroundings in a way that is hard to use is called what? Write the word.
3
FWhat reduces the energy lost from a house by heating? Write the word for the material.
4
FWhat reduces the thermal energy made by rubbing two moving parts? Write the word.
5
FA machine has 500 J in. 420 J is useful. How much energy is dissipated? Write the number of joules.
6
FA pupil says “the wasted energy from a car goes into the petrol.” Is the pupil right? Write yes or no.
7
PSName the two ways in which energy is typically dissipated in a moving bicycle. Write two words.
8
PSA house has 6000 J of thermal energy lost per second through the walls and 2000 J per second through the roof. How much is lost in total per second? Write the number of joules.
9
PSInsulation is added so that the loss through the walls falls from 6000 J to 4500 J per second. How much less energy is lost? Write the number of joules.
10
PSTwo machines do the same job. Machine X has no lubrication. Machine Y is lubricated. Which machine wastes less energy as thermal energy? Write X or Y.
11
RA pupil says “we cannot reduce wasted energy because energy is always lost.” Explain why this is wrong. Accept any sentence that says energy is dissipated, not lost, and wasted energy can be reduced by insulation or lubrication.
12
RA pupil measures 100 J into a machine and finds the useful output is 65 J. The pupil says 35 J disappeared. Explain why the pupil is not correct. Accept any sentence that says the 35 J was dissipated to the surroundings.
13
RA roof with 100 mm of insulation loses less heat than one without. Describe in one sentence how the insulation reduces the wasted energy. Accept any sentence that says the insulation slows the transfer of thermal energy out of the house.

Chapter summary

SkillMethod
Name an energy storeAsk where the energy is kept now: kinetic, gravitational potential, thermal, elastic, chemical, nuclear, magnetic or electrostatic
Name an energy transferHeating, mechanical, electrical or radiation
Read a Sankey diagramInput = total of outputs; useful = input minus wasted
Use conservation of energyEnergy is never created or destroyed; it is dissipated when it spreads out
Reduce wasteInsulation reduces heating; lubrication reduces friction

Can you…?

  • I can name the energy store of an object
  • I can name a transfer as heating, mechanical, electrical or radiation
  • I can read a Sankey diagram and work out the wasted energy
  • I can explain conservation of energy in my own words
  • I can say how insulation and lubrication reduce wasted energy

Key terms

energy store
a place where energy is kept until it is needed
energy transfer
movement of energy from one store to another
dissipated
spread out into the surroundings, usually as thermal energy, and hard to use
Sankey diagram
a diagram where the width of each band shows the amount of energy
conservation of energy
energy cannot be created or destroyed, only transferred
insulation
a material that slows the transfer of thermal energy
lubrication
a substance such as oil that reduces friction between moving surfaces

Exercise 1Z  Check-up

1
Name the energy store of a moving tennis ball. Write the word.
2
Name the energy store of a raised weight. Write the words.
3
Name the transfer when a hot radiator warms a room by contact with the air. Write the word.
4
Name the transfer when energy from the Sun reaches the Earth. Write the word.
5
A lamp has 100 J in and gives out 20 J of light. How much energy is wasted? Write the number of joules.
6
Which energy store does a stretched elastic band use? Write the word.
7
State the law that says energy is never created or destroyed. Write the name.
8
Which material is used to reduce the energy lost through a loft? Write the word.
9
A machine has 300 J in and 210 J useful. How much energy is dissipated? Write the number of joules.
10
A pupil says “the energy in a kettle is used up when the water boils.” Explain why this is wrong. Accept any sentence that says the energy is transferred and conserved, not used up.

Total 26 marks. Allow about 26 minutes.

Exercise 1X  Exam-style questions

1
FName the energy store of a moving car. Write the word.[1 mark]
2
FA lamp has 100 J of energy in. 8 J is light and the rest is wasted. Calculate the energy wasted. Write the number of joules.[2 marks]
3
PSA hairdryer transfers energy from the mains. Name the transfer that warms the hair and the transfer that moves the air. Write two words.[2 marks]
4
PSA Sankey diagram for a motor has 400 J in and 280 J useful. Calculate the energy dissipated. Write the number of joules.[2 marks]
5
PSDescribe the energy transfers in a battery-powered torch from the battery to the light. Write two transfers, in order.[2 marks]
6
RExplain how a Sankey diagram shows the law of conservation of energy. Use the words input, output and dissipated in your answer. Accept any answer that uses these three words and says the outputs add up to the input.[3 marks]
7
PSA house loses 9000 J per second through its walls, and 3000 J per second through its roof. Insulation in the roof cuts the roof loss to 1000 J per second. Calculate the total energy lost per second after the insulation. Write the number of joules.[3 marks]
8
RA pupil says “wasted energy is destroyed by friction.” Explain why this is wrong, and say what happens to the energy. Accept any answer that says the energy is dissipated to the surroundings as thermal energy.[3 marks]
9
RA wind-up toy has elastic energy when it is wound up. Describe the energy stores and the transfers as the toy moves across the floor. Accept any answer that names the elastic store, the kinetic store and mechanical transfer, and thermal energy dissipated by friction.[4 marks]
10
REvaluate the claim: “a fully insulated house has no wasted energy.” Give two reasons why this claim is not right. Accept any answer that says energy can still be lost through windows and doors and that some energy is needed for ventilation, or any other valid reason.[4 marks]
Chapter 2: Efficiency, power and energy calculations →

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