Websink Prep UKKS3 ScienceYear 8 chaptersScience textbooksYear 1Year 2Year 3Year 4Year 5Year 6Year 7Year 8Year 9Browse allPast papersFind a tutorBecome a tutor
KS3 Science · Year 8Chapter 11 · Energy stores, transfers and heating
Year 8 · Chapter 11

Energy stores, transfers and heating

By the end of this chapter you will be able to

  • Name the energy stores and say how energy is transferred between them
  • Explain conservation of energy, and use a Sankey diagram to show useful and wasted energy
  • Calculate the efficiency of a device from its useful output and total input
  • Describe conduction, convection and radiation, and link them to examples
  • Explain how insulation reduces the energy wasted from a building or a product

You should already know

  • Energy changes and heating (Year 7 Science, and Year 8 Chapter 10)
  • Simple division and percentages (Year 8 Chapter 1)

Where this leads

  • Waves and the electromagnetic spectrum, including infrared radiation (Year 8 Chapter 12)

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8P1 energy stores (kinetic, thermal, chemical, gravitational potential, elastic, nuclear, magnetic, electrostatic) and energy transfers; conservation of energy; Sankey diagrams (basic), 8P2 heating by conduction, convection and radiation; thermal conductivity and insulation; reducing the energy wasted from buildings and products.

Exercise P  Do you remember?

1
FWhat is 25 percent of 200? Write the number.
2
FWhat is the name of the energy that a moving object has? Write the word.
3
FIn which direction does heat travel from a hot object to a cold one? Write the word: towards the cold, or away from it.
4
FWhat is 7 times 8? Write the number.
5
FWhat is the unit of energy? Write the symbol.
11.1

Energy stores and transfers

Year 8
8P1: energy stores (kinetic, thermal, chemical, gravitational potential, elastic, nuclear, magnetic, electrostatic) and transfers between them
Video lesson: Energy stores and transfersComing soon
Story. Ali throws a ball up in the air. It slows, stops, and then falls back to his hand. “Where does the energy go?” he asks. His teacher says, “It moves from the kinetic store to the gravitational potential store as it rises, and back again as it falls.”

An energy store is a place energy can be held. Energy is transferred between stores by heating, light, sound, electrical working or mechanical working. The total energy in a closed system stays the same: this is conservation of energy. Energy is not created or destroyed; it is transferred.

The table gives eight energy stores and an example of each.

Energy storeExample
kinetica moving car
thermala hot cup of tea
chemicala battery or food
gravitational potentiala book raised above the floor
elastica stretched spring
nuclearatoms in the nucleus
magnetictwo magnets close together
electrostaticcharged objects

Key ideas

  • Energy is stored in stores; it is transferred between them.
  • Energy is conserved: the total stays the same in a closed system.
  • Kinetic store: moving objects. Gravitational potential: raised objects.
  • Energy transfers can be by heating, light, sound, or working.
Worked example 11.1.1
What store does a moving car have? Write the word.

Method. Recall the store.

  1. Recall. A moving object has energy in its kinetic store.
    kinetic store
  2. Name it. So the store is kinetic.
    kinetic
  3. Write the answer.
    kinetic

Check. A faster car has more kinetic energy. ✓

Worked example 11.1.2
A book is lifted higher. Which store does the energy go into? Write the words.

Method. Think about a raised object.

  1. Think. Raising an object gives it height, so it stores energy by its height.
    by its height
  2. Name it. So the energy goes into the gravitational potential store.
    gravitational potential
  3. Write the answer.
    gravitational potential store

Check. The higher it is, the more energy it stores. ✓

Worked example 11.1.3
In a stretched spring, which energy store holds the energy? Write the words.

Method. Recall the store.

  1. Recall. A stretched or squashed spring stores energy in its elastic store.
    elastic
  2. Name it. So the store is the elastic store.
    elastic store
  3. Write the answer.
    elastic store

Check. Elastic energy is released when the spring returns. ✓

Watch out

Watch out: energy is not used up; it is transferred. When a ball lands and warms the floor slightly, the energy has moved to the thermal store.

Exam tip

Exam tip. Name the store the energy is in, and say how it is transferred from one store to another.

Try it (with your teacher, or a grown-up at home). Roll a small ball up a slope and watch it stop and roll back. Name the store the energy is in at the top, and the store it is in at the bottom. Do not throw anything at a person.

Exercise 11A  Energy stores and transfers

1
FWhich energy store does a hot cup of tea have? Write the word.
2
FWhich store does a battery contain? Write the word.
3
FWhich store holds energy in a magnet pair that are pulled together? Write the word.
4
FWhich store holds energy in the nucleus of an atom? Write the word.
5
FWhat is the name of the principle that energy is never created or destroyed? Write the words.
6
FWhat is the name for the energy store of a raised object? Write the words.
7
FWhich store does an electrically charged object hold energy in? Write the word.
8
FWhich store is the energy in when a spring is stretched? Write the word.
9
FIs energy created when a ball is dropped and falls? Write yes or no.
10
FIs energy destroyed when a car brakes and stops? Write yes or no.
11
FName the way energy moves from a lamp into the room. Write the word: light, sound, or heating.
12
PSAli says that the total energy of a closed system goes down when a ball slows down. Is Ali right? Write yes or no.
13
PSWorking scientifically: a pupil says that a bouncing ball gets lower each bounce. Name one store the energy has moved into. Accept any store: thermal, or sound.
14
RExplain why a ball that is dropped speeds up as it falls. Write one sentence.
15
RChallenge: a closed system has 100 J in its stores at the start. After the energy is transferred, what is the total in the stores at the end? Write the number of joules.
11.2

Sankey diagrams and efficiency

Year 8
8P1: Sankey diagrams (basic) showing useful and wasted energy; efficiency as the useful output over the total input
Video lesson: Sankey diagrams and efficiencyComing soon
Story. Sam compares two lamps. “The old one feels hot and the new one feels cool,” he says. “Where does the rest of the energy go?” His teacher draws a Sankey diagram, where the width of each band shows how much energy goes where.

A Sankey diagram shows energy transfers. The width of each arrow or band is proportional to the amount of energy. The input energy splits into useful energy and wasted energy, usually as heat or sound.

The efficiency of a device is the useful energy output divided by the total energy input, multiplied by 100 to give a percentage:

efficiency = useful energy output / total energy input x 100%

input 100 Juseful 60 Jwasted 40 J (heat, sound)Sankey diagram: width shows the amount of energy
A Sankey diagram: 100 J input, 60 J useful and 40 J wasted.

The table gives the energy input and useful output of three devices.

DeviceEnergy input (J)Useful energy output (J)Efficiency (%)
electric heater50040080
LED lamp10880
old light bulb60610

Key ideas

  • A Sankey diagram shows the energy input, the useful output and the wasted energy.
  • Efficiency = useful output / total input x 100%.
  • Wasted energy is usually heat or sound.
  • A higher efficiency means less energy is wasted.
Worked example 11.2.1
In the Sankey diagram, the input is 100 J and the useful output is 60 J. How much energy is wasted? Write the number of joules.

Method. Subtract the useful output from the input.

  1. Subtract. Wasted = input minus useful = 100 minus 60.
    100 - 60
  2. Calculate. So the wasted energy is 40 J.
    100 - 60 = 40
  3. Write the answer.
    40 J

Check. 60 + 40 = 100. ✓

Worked example 11.2.2
What is the efficiency of the device in the Sankey diagram? Write the number with a percentage sign.

Method. Use the efficiency formula.

  1. Calculate. Efficiency = 60 divided by 100, then multiplied by 100.
    60 / 100 x 100
  2. Write. So the efficiency is 60 percent.
    60%
  3. Write the answer.
    60%

Check. Useful output is 60 out of 100. ✓

Worked example 11.2.3
An old light bulb has an input of 60 J and a useful light output of 6 J. What is its efficiency? Write the number with a percentage sign.

Method. Use the efficiency formula.

  1. Divide. Useful output divided by input is 6 divided by 60.
    6 / 60
  2. Multiply. Multiply by 100: 0.1 x 100 = 10.
    10
  3. Write. So the efficiency is 10 percent.
    10%
  4. Write the answer.
    10%

Check. Most of the energy becomes heat. ✓

Watch out

Watch out: efficiency is a percentage, so it is always between 0% and 100%. A device cannot use more energy than it is given.

Exam tip

Exam tip. Write the formula first, then put the useful output on the top line and the input on the bottom line.

Try it (with your teacher, or a grown-up at home). Look at the device table. For each device, calculate the wasted energy by subtracting the useful output from the input. Do not use any electrical appliance yourself.

Exercise 11B  Sankey diagrams and efficiency

1
FWhat is the name of the diagram that shows energy transfers with bands whose widths show the amount? Write the words.
2
FWhat is energy that is not useful called? Write the word.
3
FWrite the formula for efficiency. Write the words: useful output over total input, times 100%.
4
FA device has an input of 100 J and a useful output of 60 J. What is the wasted energy, in J? Write the number.
5
FA device has an input of 500 J and a useful output of 400 J. What is its efficiency? Write the number with a percentage sign.
6
FAn LED lamp has an input of 10 J and a useful output of 8 J. What is its efficiency? Write the number with a percentage sign.
7
FAn old light bulb has an efficiency of 10%. What percentage of its energy is wasted? Write the number with a percentage sign.
8
FWhich device in the table is the least efficient? Write the name.
9
FWhich two devices in the table have the same efficiency? Write the names.
10
FIn a Sankey diagram, is the useful output band wider than the wasted band in a device that is 60% efficient? Write yes or no.
11
PSAli says a device with 100 J input and 120 J useful output is possible. Is Ali right? Write yes or no.
12
PSWorking scientifically: a pupil measures 50 J input and 30 J useful output. Calculate the efficiency. Write the number with a percentage sign.
13
RExplain why a device can never be 100% efficient in practice. Write one sentence.
14
RChallenge: a device has an efficiency of 25%. For an input of 200 J, what is the useful output? Write the number of joules.
11.3

Heating by conduction, convection and radiation

Year 8
8P2: heating by conduction, convection and radiation; thermal conductivity
Video lesson: Heating by conduction, convection and radiationComing soon
Story. Ruby puts a metal spoon into a hot drink, and the handle soon becomes hot. Her teacher says, “That is conduction. The energy passes along the spoon through the particles, and metals are good at it.” Later, the class sees a warm room heated by a radiator, and the warm air rising. That is convection.

Conduction is the transfer of energy through a solid by particles vibrating and passing energy on. Metals are good conductors because they have free electrons that carry energy quickly. Materials that conduct poorly are insulators.

Convection happens in liquids and gases. The hotter, less dense fluid rises, and cooler fluid sinks to take its place, setting up a convection current.

Radiation is the transfer of energy by infrared waves. It does not need particles, so it can cross a vacuum. Dark, matt surfaces absorb and give out more infrared than shiny surfaces.

hot endcooler endConduction: energy passes along the rod through particles.
Conduction along a rod: heat passes from the hot end to the cooler end.
heat source at the bottomhot water rises; cooler water sinks
Convection in water: hot water rises above the heat source and cooler water sinks.
hotinfrared radiationdarkshinydark, matt surfaces absorb and give out more radiation than shiny ones
Radiation: a hot object gives out infrared; a dark surface absorbs more than a shiny one.

The table gives the thermal conductivity of some materials.

MaterialThermal conductivity
coppergood conductor
steelgood conductor
glasspoor conductor
woodpoor conductor
airpoor conductor (insulator)
woolpoor conductor (insulator)

Key ideas

  • Conduction: energy passes through a solid by particles; metals conduct well.
  • Convection: fluids rise when hot and sink when cool, making currents.
  • Radiation: infrared waves, which need no particles and cross a vacuum.
  • Dark, matt surfaces absorb and give out more radiation.
Worked example 11.3.1
Which type of heat transfer needs no particles at all? Write the word.

Method. Recall the types.

  1. Recall. Radiation transfers energy by infrared waves and needs no particles.
    needs no particles
  2. Name it. So the type is radiation.
    radiation
  3. Write the answer.
    radiation

Check. The Sun heats the Earth by radiation. ✓

Worked example 11.3.2
Which type of transfer happens in a liquid as the hot liquid rises? Write the word.

Method. Recall the process.

  1. Recall. In a liquid, hot fluid rises and cooler fluid sinks, which is convection.
    convection
  2. Name it. So the transfer is convection.
    convection
  3. Write the answer.
    convection

Check. Convection happens only in fluids. ✓

Worked example 11.3.3
Is copper a good or a poor conductor? Write the words.

Method. Read the table.

  1. Read. The table lists copper as a good conductor.
    good conductor
  2. Name it. So copper is a good conductor.
    good conductor
  3. Write the answer.
    good conductor

Check. Metals conduct well. ✓

Watch out

Watch out: convection needs a fluid. Heat in a solid moves by conduction, not convection.

Exam tip

Exam tip. Name the transfer and the reason: “convection, because the hot fluid rises”.

Try it (with your teacher, or a grown-up at home). Look at the three diagrams. Say which transfer is shown in each, and give one everyday example. Do not heat any object; use the diagrams only.

Exercise 11C  Heating by conduction, convection and radiation

1
FWhat is the transfer of energy through a solid by particles called? Write the word.
2
FWhat is the transfer of energy in a fluid by the movement of the fluid called? Write the word.
3
FWhat is the transfer of energy by infrared waves called? Write the word.
4
FDoes radiation need particles to transfer energy? Write yes or no.
5
FIs copper a good or a poor conductor? Write the words.
6
FIs air a good or a poor conductor? Write the words.
7
FWhich surface absorbs and gives out more infrared: dark and matt, or shiny? Write the words.
8
FIn which direction does the hot fluid move in a convection current? Write the word: up or down.
9
FIs a metal spoon in a hot drink an example of conduction? Write yes or no.
10
FDoes the Sun heat the Earth by conduction? Write yes or no.
11
PSAli says that convection can happen in a solid block of metal. Is Ali right? Write yes or no.
12
PSWorking scientifically: a pupil wants a dark, matt surface to absorb more radiation. Name one way to test this. Accept any sensible test: two cans, one dark and one shiny, at the same distance from a warm lamp.
13
RExplain why a radiator is placed low in a room. Write one sentence.
14
RChallenge: explain why a metal spoon feels hotter than a wooden spoon in the same hot drink. Write one sentence.
11.4

Insulation and reducing wasted energy

Year 8
8P2: insulation reduces heat transfer; reducing the energy wasted from buildings and products
Video lesson: Insulation and reducing wasted energyComing soon
Story. Omar wraps a beaker of warm water in different materials, and measures how much the temperature falls in 10 minutes. “Which one keeps it warmest?” he asks. His teacher says, “Compare the fall in temperature: the smaller the fall, the better the insulator.”

Insulation reduces the rate of heat transfer. Good insulators, such as wool, foam and trapped air, have a low thermal conductivity. Buildings use loft insulation, cavity wall insulation and double glazing to keep heat in and reduce the energy wasted from the building.

Products are insulated too: a kettle, a cool box and a thermos flask all reduce the transfer of energy to or from the surroundings.

The table gives the fall in temperature of warm water in a beaker after 10 minutes, for four materials. This is example class data.

InsulatorTemperature fall (degrees C)
no insulation20
aluminium foil18
wool10
thick foam6
Temperature fall in 10 minutes (example data)20no insulation18aluminium foil10wool6thick foamA smaller fall means better insulation.
Temperature fall after 10 minutes for four insulators; the smaller the fall, the better the insulation.

Key ideas

  • Insulation reduces the rate of heat transfer.
  • A smaller temperature fall means better insulation.
  • Trapped air is a good insulator.
  • Insulation reduces the energy wasted from buildings and products.
Worked example 11.4.1
Using the table, what is the fall in temperature for no insulation? Write the number of degrees C.

Method. Read the table.

  1. Read. The row for no insulation shows a fall of 20 degrees C.
    20
  2. Name it. So the fall is 20 degrees C.
    20
  3. Write the answer.
    20 degrees C

Check. No insulation loses the most heat. ✓

Worked example 11.4.2
Using the table, how much less does the water cool with wool than with no insulation? Write the number of degrees C.

Method. Subtract the wool fall from the no-insulation fall.

  1. Read. No insulation falls 20 degrees C, and wool falls 10 degrees C.
    20 and 10
  2. Calculate. The difference is 20 minus 10.
    20 - 10 = 10
  3. Write the answer.
    10 degrees C

Check. 10 + 10 = 20. ✓

Worked example 11.4.3
Which material is the best insulator in the table? Write the name.

Method. Find the smallest fall.

  1. Look. The smallest fall in the table is 6 degrees C.
    6
  2. Name it. So the best insulator is thick foam.
    thick foam
  3. Write the answer.
    thick foam

Check. Thick foam keeps the water warmest. ✓

Watch out

Watch out: insulation does not stop heat transfer completely. It slows it down, so the temperature still falls, just more slowly.

Exam tip

Exam tip. Say “insulation reduces heat transfer by conduction and convection”, and name the material with a low thermal conductivity.

Try it (with your teacher, or a grown-up at home). Look at the insulation table. Rank the four materials from the best insulator to the worst. Do not heat any water yourself; your teacher does the class test.

Exercise 11D  Insulation and wasted energy

1
FWhat is the name for a material that reduces heat transfer? Write the word.
2
FWhich kind of insulation is put in the roof space of a house? Write the words.
3
FWhat is the name for the two layers of glass that reduce heat loss from windows? Write the words.
4
FUsing the table, what is the fall in temperature for wool? Write the number of degrees C.
5
FUsing the table, what is the fall in temperature for aluminium foil? Write the number of degrees C.
6
FUsing the table, which material gives the smallest fall in temperature? Write the name.
7
FIs trapped air a good insulator? Write yes or no.
8
FDoes insulation remove heat transfer completely? Write yes or no.
9
FName one product that uses insulation to keep energy in. Accept any one: a thermos flask, a cool box, or a kettle.
10
PSWorking scientifically: name one control variable in the insulation test. Accept any one: the volume of water, the starting temperature, or the time.
11
PSRuby says the foil gave the best insulation because it is shiny. Is Ruby right? Write yes or no.
12
RExplain why a building with thick walls and loft insulation wastes less energy. Write one sentence.
13
RChallenge: explain why a thermos flask is made with a vacuum between two walls. Write one sentence.

Chapter summary

SkillMethod
Energy storesKinetic, thermal, chemical, gravitational potential, elastic, nuclear, magnetic, electrostatic.
ConservationEnergy is transferred between stores; the total in a closed system stays the same.
EfficiencyUseful output / total input x 100%. Wasted energy is the rest.
Heat transferConduction (solids), convection (fluids), radiation (infrared, no particles needed).
InsulationLow thermal conductivity, trapped air; a smaller temperature fall is better.

Can you…?

  • I can name the energy stores and say how energy is transferred between them
  • I can explain conservation of energy and use a Sankey diagram
  • I can calculate the efficiency of a device from its useful output and input
  • I can describe conduction, convection and radiation, with examples
  • I can explain how insulation reduces wasted energy

Key terms

energy store
a place energy can be held, such as kinetic or thermal
conservation of energy
energy is transferred but not created or destroyed
Sankey diagram
a diagram in which the width of each band shows the amount of energy
efficiency
the useful energy output divided by the total input, as a percentage
conduction
transfer of energy through a solid by particles passing it on
convection
transfer of energy in a fluid by the movement of the fluid
radiation
transfer of energy by infrared or other electromagnetic waves
thermal conductivity
how well a material conducts heat
insulation
material that reduces the transfer of heat

Exercise 11Z  Check-up

1
What is the energy store of a moving object? Write the word.
2
Which energy store does a raised object have? Write the words.
3
What is the name of the principle that energy is transferred but not created or destroyed? Write the words.
4
A device has an input of 200 J and a useful output of 50 J. What is the wasted energy, in J? Write the number.
5
A device has an input of 200 J and a useful output of 50 J. What is its efficiency? Write the number with a percentage sign.
6
What is the transfer of energy by infrared waves called? Write the word.
7
Is convection possible in a solid? Write yes or no.
8
Which material is listed as a good conductor in the conductivity table: copper or wood? Write the name.
9
Using the insulation table, what is the fall in temperature for thick foam? Write the number of degrees C.
10
Explain why a metal spoon in a hot drink gets hot at the handle. Write one sentence.

Total 15 marks. Allow about 15 minutes.

Exercise 11X  Practice test questions

1
FName the energy store that a stretched elastic band has. Write the words.[1 mark]
2
PSA device has an input of 400 J and a useful output of 300 J. Calculate the efficiency. Write the number with a percentage sign.[2 marks]
3
FGive the name of the transfer of energy in a solid by particles passing energy on. Write the word.[1 mark]
4
RDescribe the transfer of energy by convection in a beaker of water heated at the bottom. Write one sentence.[2 marks]
5
FWhich store holds energy in an atom’s nucleus? Write the word.[1 mark]
6
PSUsing the insulation table, what is the difference in temperature fall between no insulation and thick foam? Write the number of degrees C.[2 marks]
7
RName one way energy is wasted from a house, and one insulation that reduces it. Accept any sensible pair: heat through the walls, with cavity wall insulation.[2 marks]
8
FA device is 25% efficient. What percentage of its energy is wasted? Write the number with a percentage sign.[1 mark]
9
RExplain why radiation can heat the Earth from the Sun across space. Write one sentence.[2 marks]
10
FName the energy store that a moving car has. Write the word.[1 mark]
← Chapter 10: Rates of reaction and energy changesChapter 12: Waves: sound, light and the electromagnetic spectrum →

Websink Prep UK is an independent site. It is not affiliated with, endorsed by or sponsored by AQA, Pearson Edexcel, OCR, WJEC, Eduqas, CCEA, SQA, GL Assessment, CEM or the Department for Education. All exam board names and trademarks belong to their owners. Past papers are linked from the exam boards' own websites and are not hosted here.

About Websink Prep UK · Find a tutor · Privacy · Terms · Cookie settings