KS3 Science Study Pack: Energy

Key Knowledge

Energy is a quantity that helps scientists describe changes. A change might be an object moving faster, water becoming warmer, a lamp giving out light, a sound being made, a battery running a circuit, or an object being lifted higher. Energy is measured in joules, written as J. For larger amounts, scientists often use kilojoules, written as kJ. One kilojoule is 1000 joules.

Energy is not a substance that gets used up. It does not vanish when a device runs. Instead, energy is stored in different ways and transferred from one store to another. Scientists use the idea of energy stores and energy transfer pathways because it helps them describe what is changing and where the energy is going.

The most important rule is conservation of energy:

• energy cannot be created;
• energy cannot be destroyed;
• the total amount of energy before and after a change is the same;
• energy can be transferred to stores that are less useful for the purpose we wanted.

In everyday speech, people often say that energy is "lost" or "used up". In science, this is only acceptable if we explain what really happened: the energy has been transferred to the surroundings, often by heating and sound, and has become less useful.

Energy as a Conserved Quantity

When a change happens, energy is transferred between stores. A complete energy description should say:

1. the starting energy store;
2. the transfer pathway;
3. the final energy store or stores;
4. which output is useful and which output is wasted.

A simple structure is:

from store -> transfer pathway -> store

For example:

• A kettle heating water: chemical store in a power station fuel, or another energy resource -> electrical transfer through wires -> thermal store of the water. Some energy is also transferred to the thermal store of the kettle body and surroundings.
• A person lifting a school bag: chemical store in the person's muscles -> mechanical transfer as a force lifts the bag -> gravitational potential store of the bag increases. Some energy is transferred to thermal stores in the person's body and surroundings.
• A speaker playing music: chemical store in a battery or electrical supply -> electrical transfer to the speaker -> sound transfer to the surroundings. Some energy is transferred to thermal stores in the speaker and air.

These examples show why scientists avoid saying that energy is simply "made" or "gone". The energy has moved between stores.

Energy Stores

An energy store is a way energy can be kept in a system. A system is the object or group of objects being studied, such as a ball, a kettle, a circuit, or a car.

Energy store	Simple definition	When the store increases	Everyday example
Kinetic store	Energy stored by a moving object	The object moves faster or has more mass	A cyclist moving along a road
Thermal/internal store	Energy stored in the particles of an object	The object gets warmer or there is more material at the same temperature	Hot water in a cup
Chemical store	Energy stored in chemical substances	Fuel, food, batteries, or cells contain energy that can be transferred during reactions	Breakfast cereal, petrol, a phone battery
Gravitational potential store	Energy stored when an object is raised in a gravitational field	The object is lifted higher or has more mass	A book on a high shelf
Elastic potential store	Energy stored in a stretched or compressed object	The object is stretched or compressed more	A stretched elastic band
Magnetic store	Energy stored because of the positions of magnets or magnetic materials	Magnets are moved so that they attract or repel in a changed position	Two repelling magnets pushed closer together
Electrostatic store	Energy stored when electric charges are separated	Opposite or like charges are separated or brought into a changed position	A charged balloon near hair
Nuclear store	Energy stored inside atoms	Changes happen inside atomic nuclei	Nuclear fuel in a power station

More About Each Store

The kinetic store is linked to movement. A moving skateboard, rolling ball, bus, or running student has energy in its kinetic store. A faster moving object has more energy in its kinetic store than the same object moving slowly.

The thermal or internal store is linked to the particles inside a material. Hot water has more energy in its thermal store than the same amount of cold water. Cold objects still have thermal energy; their particles are still moving, but usually less than in hotter objects.

The chemical store is important in food, fuels, cells, and batteries. Food has energy in its chemical store, which can be transferred in the body so muscles can move and the body can stay warm. A phone battery has energy in its chemical store before the phone is used.

The gravitational potential store increases when an object is lifted. A ball at the top of a ramp has more energy in its gravitational potential store than the same ball at the bottom.

The elastic potential store increases when something is stretched or compressed. Examples include a trampoline bed, a spring, a bow, or a stretched elastic band.

The magnetic store is used when magnets attract or repel. If you push two like poles of magnets closer together, you are transferring energy mechanically into a magnetic store.

The electrostatic store is linked to separated electric charges. A balloon rubbed on a jumper can become charged and then attract small pieces of paper or hair.

The nuclear store is inside atoms. At KS3, you only need the simple idea that nuclear fuels contain energy in nuclear stores, and this can be transferred in nuclear power stations. Nuclear fuel is finite, so nuclear energy is classed as non-renewable, even though nuclear power stations release very low carbon dioxide during operation.

Energy Transfer Pathways

Energy is transferred between stores by pathways. The pathway is not a store; it is the way energy moves or changes.

Transfer pathway	How the transfer happens	Example	Useful or wasted in that example
Heating	Energy is transferred because of a temperature difference	A hot radiator warms a room	Useful if the aim is to heat the room
Radiation, including light	Energy is transferred by waves such as light or infrared radiation	Sunlight warms the Earth	Useful for solar panels or warming surfaces
Electrical transfer	Energy is transferred by an electric current	A circuit powers a lamp	Useful if the lamp gives light
Sound	Energy is transferred by vibrations travelling through a material	A speaker sends sound through air	Useful in a speaker, wasted in a noisy engine
Mechanical work	Energy is transferred when a force moves an object	A person lifts a bag	Useful if the bag needs lifting

Examples of Stores and Transfers

In a circuit with a lamp, energy is transferred electrically from a cell or battery to the lamp. The lamp transfers some energy by light to the surroundings and some by heating to the thermal store of the lamp and the air.

In cooking, energy may be transferred by heating from a hob to a pan, then to food. The useful output is the increase in the thermal store of the food. Wasted energy includes heating the pan handle, the air, and nearby surfaces.

In movement, a cyclist transfers energy from chemical stores in muscles mechanically to the bicycle. Energy is transferred to the kinetic store of the cyclist and bike. Going uphill also increases the gravitational potential store. Braking downhill transfers energy from the kinetic store to thermal stores of the brakes, tyres, road, and air.

In a falling object, the gravitational potential store decreases as the kinetic store increases. At impact, energy is transferred to thermal stores and by sound. The energy is not destroyed.

Energy Pathway Diagrams

Torch

Chemical store in battery
        |
        | electrical transfer
        v
Lamp
  |-----------------> light to surroundings (useful)
  |
  v
thermal store of lamp and surroundings (wasted)

For a torch, the useful output is light transferred to the surroundings. The wasted output is energy transferred to the thermal store of the lamp, battery, and surroundings. The battery's chemical store decreases as energy is transferred through the circuit.

Falling Object

At the top:     large gravitational potential store, small kinetic store
Falling:        gravitational potential store decreases, kinetic store increases
At impact:      energy transferred to thermal stores and sound

A falling ball speeds up because energy is transferred from its gravitational potential store to its kinetic store. When it hits the ground, some energy is transferred to thermal stores in the ball, ground, and air, and some is transferred by sound.

Wind or Hydroelectric Generation

Moving water or wind -> turbine turns -> generator -> electrical transfer -> homes

In a wind turbine, the kinetic store of moving air decreases. Energy is transferred mechanically to the turbine and then electrically to homes and the National Grid. Some energy is transferred to thermal stores and sound.

Useful and Wasted Energy

Useful energy is energy transferred in the way we want. Wasted energy is energy transferred in a way that is not useful for the intended purpose. Wasted energy does not disappear. It is still energy, but it has spread out into less useful stores, often thermal stores in the surroundings.

Device or situation	Input store or transfer	Useful output	Wasted output	Explanation
LED lamp	Electrical transfer	Light transferred to surroundings	Heating of lamp and air	Most of the useful purpose is lighting
Filament lamp	Electrical transfer	Light transferred to surroundings	Large heating of lamp and air	It gets hot, so much energy is dissipated
Kettle	Electrical transfer	Increase in thermal store of water	Heating kettle body and air, sound	The aim is to heat water
Car engine	Chemical store of fuel	Kinetic store of car	Heating engine, exhaust, tyres, sound	Engines transfer a lot of energy to surroundings
Mobile phone	Chemical store in battery	Light from screen, sound, electrical processes	Heating of phone and surroundings	A warm phone shows some energy transfer to thermal stores
Cyclist braking	Kinetic store of cyclist and bike	Slowing down safely	Heating brakes, tyres, road, air, sound	The kinetic store decreases

Conservation and Dissipation

Conservation of energy means the total amount of energy stays the same. This is true even when energy seems to have disappeared. The energy has been transferred somewhere else.

Dissipation means energy spreads out into the surroundings, usually into thermal stores. Dissipated energy is difficult to use because it is spread through many particles in the environment. For example, when a phone becomes warm, energy has been transferred to the phone's thermal store and then to the air. It would be very difficult to collect that spread-out energy and use it to fully recharge the battery.

Before-and-After Examples

When a roller coaster is at the top of a hill, it has a large gravitational potential store. As it moves down, this store decreases and its kinetic store increases. Some energy is dissipated because of friction and air resistance. Friction does not destroy energy; friction transfers energy mechanically into thermal stores of the track, wheels, brakes, and surroundings.

When an elastic band is stretched, energy is transferred mechanically into its elastic potential store. When it is released, energy is transferred mainly to the kinetic store of the band and any object it launches. Some energy is transferred by sound and heating.

When hot water cools in a cup, energy is transferred from the thermal store of the water to the thermal store of the cup and surroundings. The energy is not lost from the universe, but it becomes more spread out.

Efficiency

Efficiency describes how much of the input energy is usefully transferred. A more efficient device wastes a smaller proportion of the input energy.

efficiency = useful output energy / total input energy

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

Efficiency can be written as a decimal or a percentage. For example, an efficiency of 0.25 is the same as 25%.

Efficiency cannot be over 100%. If a device had more useful output energy than input energy, that would break the conservation of energy rule. In real devices, some energy is usually dissipated to the surroundings.

Worked Example 1: Lamp Efficiency

A lamp transfers 200 J of electrical energy. Only 50 J is usefully transferred by light.

Equation:

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

Substitution:

percentage efficiency = 50 J / 200 J x 100

Arithmetic:

50 / 200 = 0.25

0.25 x 100 = 25%

Final answer:

percentage efficiency = 25%

This means 25% of the input energy is usefully transferred by light. The remaining 150 J is wasted, mainly by heating the lamp and surroundings. It has not disappeared.

Worked Example 2: Comparing Two Devices

Device A has an input of 100 J and a useful output of 40 J.

percentage efficiency = 40 J / 100 J x 100 = 40%

Device B has an input of 100 J and a useful output of 75 J.

percentage efficiency = 75 J / 100 J x 100 = 75%

Device B is more efficient because it usefully transfers a larger percentage of the input energy and wastes a smaller percentage.

Worked Example 3: Missing Sankey Value

Input energy = 500 J

Useful output = 320 J

Wasted output:

500 J - 320 J = 180 J

Efficiency:

percentage efficiency = 320 J / 500 J x 100

320 / 500 = 0.64

0.64 x 100 = 64%

The device is 64% efficient. The remaining 180 J is transferred to less useful stores, usually thermal stores in the surroundings and sometimes by sound.

Improving Efficiency

Engineers improve efficiency by reducing wasted energy transfers. Examples include:

• insulation in homes to reduce energy transfer by heating;
• double glazing to reduce energy transfer through windows;
• draught proofing to reduce warm air escaping;
• lubrication to reduce friction in moving parts;
• streamlined shapes to reduce air resistance;
• LED lamps instead of filament lamps because LEDs transfer a larger proportion of energy by light;
• quiet motors and well-designed bearings to reduce wasted sound and heating.

Sankey Diagrams

A Sankey diagram shows energy transfers using arrows. The width of each arrow represents the amount of energy. A wider arrow means a larger amount of energy. The useful output usually continues in the main direction. Wasted outputs normally branch away.

The total input must equal the total outputs.

Input energy: 100 J
==============================> Useful output: 40 J
             \
              \===============> Wasted output: 60 J

In this diagram:

• input energy = 100 J;
• useful output = 40 J;
• wasted output = 60 J;
• total output = 40 J + 60 J = 100 J;
• efficiency = 40 J / 100 J x 100 = 40%.

Sankey Comparison: Efficient and Inefficient Lamps

LED lamp, input 100 J
========================================> Useful light: 80 J
        \
         \========> Wasted heating: 20 J

Filament lamp, input 100 J
==========> Useful light: 10 J
        \
         \======================================> Wasted heating: 90 J

The LED lamp is more efficient because a larger proportion of the input energy is usefully transferred by light. The filament lamp has a thick wasted arrow because much more energy is transferred to thermal stores. A thicker arrow does not automatically mean a better device; the useful proportion matters.

Energy Resources and Electricity Generation

Energy resources are used to generate electricity or provide heating and transport fuels. Renewable resources are replaced naturally on a human timescale. Non-renewable resources are finite and can run out.

Renewable does not mean perfect or impact-free. Renewable resources can have disadvantages, such as visual impact, habitat disruption, noise, high starting cost, location limits, or unreliable output when the weather changes.

Non-renewable resources can be reliable and energy-dense, but they can run out. Fossil fuels also release carbon dioxide when burned, which contributes to climate change. Nuclear fuel is non-renewable because the fuel is finite, but nuclear power stations produce low carbon dioxide emissions during operation.

Resource	Renewable or non-renewable	Main advantage	Main disadvantage	Reliability	Environmental impact
Solar	Renewable	Low running emissions and useful on roofs	Output changes with daylight, weather, and season	Variable	Manufacturing impacts; low emissions during use
Wind	Renewable	Good in windy UK areas, low running emissions	Output depends on wind; visual and noise concerns	Variable	Can affect views, habitats, and birds if poorly sited
Hydroelectric	Renewable	Reliable where water flow is controlled	Needs suitable valleys or dams	Often reliable	Can flood habitats and change rivers
Tidal	Renewable	Predictable tides	Expensive and limited to suitable coasts or estuaries	Predictable but not constant	Can affect estuary habitats and shipping
Wave	Renewable	Uses movement of sea waves	Technology can be expensive and exposed to storms	Variable	Possible effects on marine habitats
Geothermal	Renewable	Can provide steady heating or electricity in some areas	Location limited	Reliable where available	Low emissions but drilling has impacts
Biomass	Renewable if replanted	Can use plant material or waste	Releases carbon dioxide when burned; needs land	More controllable than wind or solar	Land use, transport, and air pollution concerns
Coal	Non-renewable	Historically reliable and easy to store	High carbon dioxide and air pollution	Reliable while fuel is available	Major climate and pollution impacts
Oil	Non-renewable	Useful transport fuel	Releases carbon dioxide and pollutants	Reliable while fuel is available	Spills, air pollution, greenhouse gases
Natural gas	Non-renewable	Reliable and quick to start	Releases carbon dioxide; finite fuel	Reliable and flexible	Lower carbon dioxide than coal, but still a fossil fuel
Nuclear fuel	Non-renewable	Very high energy output and low running carbon dioxide	Radioactive waste and high building cost	Reliable	Waste management and mining impacts

UK Location Examples

Wind power can be suitable in parts of Scotland, Wales, and coastal areas because wind speeds are often high. It may be less suitable where local communities are strongly affected by visual impact or where habitats are sensitive.

Solar panels on a school roof can be useful because the roof space is already built, and electricity can be generated during daylight. Output is lower at night and in winter, so the school still needs another electricity supply.

Tidal power may be suitable in an estuary with a large tidal range. It is predictable, but building a tidal scheme can be expensive and may affect mudflats, birds, fish, and shipping routes.

Natural gas power stations can respond quickly when electricity demand rises or wind output drops. However, they release carbon dioxide and use a finite fuel.

Real-World Examples

A mobile phone charging has energy transferred electrically to the phone, increasing the chemical store of its battery. When the phone is used, energy is transferred from the battery's chemical store to the screen by light, to the speaker by sound, and to thermal stores in the phone and surroundings.

A kettle transfers energy electrically to a heating element. Energy is transferred by heating to the water. The useful output is the increase in the water's thermal store. Wasted outputs include heating the kettle body, the air, and the surface it stands on.

A cyclist pedalling uphill transfers energy from chemical stores in muscles to the kinetic store of the cyclist and bicycle. Going uphill also increases the gravitational potential store. When cycling downhill and braking, energy is transferred from the kinetic store to thermal stores of the brakes, tyres, road, and surrounding air.

A roller coaster at the top of a track has a large gravitational potential store. As it moves down, energy is transferred to the kinetic store. Friction and air resistance transfer some energy to thermal stores and by sound.

A packed lunch has energy in chemical stores. The body transfers this energy so muscles can work and body temperature can be maintained.

A bus journey uses fuel with a chemical energy store. As the bus moves, energy is transferred to the kinetic store of the bus and passengers. Much energy is also transferred to thermal stores of the engine, exhaust gases, tyres, road, and air, and by sound.

Practical Investigation: Insulation and Cooling

Scientists can investigate which material is the best insulator by measuring how quickly hot water cools. An insulator reduces energy transfer by heating.

Thermometer
    |
    v
  _______
 |       |   insulating material
 | water |  [wrapped around cup]
 |_______|

Example Method

1. Put 100 cm3 of hot water into a cup.
2. Wrap the cup in one insulating material.
3. Place a thermometer in the water.
4. Record the starting temperature.
5. Record the temperature every 2 minutes for 10 minutes.
6. Repeat the test at least three times for each material.
7. Compare the temperature decrease for each material.

Working Scientifically

Feature	Example for insulation investigation
Independent variable	Type of insulating material around the cup
Dependent variable	Temperature decrease of the water, or final temperature after a set time
Control variables	Volume of water, starting temperature, cup type, thickness of material, room temperature, time intervals, thermometer position
Repeatability	Repeat each material several times and calculate a mean
Safety	Use water that is hot but not boiling; wear eye protection if required; keep cups stable; handle hot water carefully
Possible improvement	Use a lid with a hole for the thermometer, use a digital temperature probe, or control room draughts

Accuracy means how close a measurement is to the true value. A calibrated digital thermometer may improve accuracy. Precision means how close repeated readings are to each other. Repeatability means the same person using the same method gets similar results. Reliability is improved when repeated results are consistent and the method is well controlled.

Data Task 1: Cooling Investigation

Two cups contained 100 cm3 of hot water. Cup A was wrapped in insulating material. Cup B was not insulated.

Time in minutes	Cup A temperature in °C	Cup B temperature in °C
0	80	80
2	76	72
4	73	65
6	70	59
8	68	54
10	66	70

Questions

1. Which cup transferred energy to the surroundings more slowly?
2. Use values from the table as evidence.
3. Which reading is probably an anomaly?
4. How should the anomaly be handled?
5. What would the line graph look like for each cup?
6. Identify the independent variable, dependent variable, and three control variables.

Model Answers

1. Cup A transferred energy to the surroundings more slowly.
2. Cup A cooled from 80 °C to 66 °C in 10 minutes, a decrease of 14 °C. Cup B cooled from 80 °C to 54 °C by 8 minutes, a decrease of 26 °C, so it cooled faster.
3. The Cup B reading at 10 minutes, 70 °C, is probably an anomaly because it increases after the water had been cooling.
4. The reading should be checked, repeated if possible, and not used in a mean if there is a clear reason to think it is a mistake.
5. Cup A would have a gentler downward line. Cup B would have a steeper downward line, except for the anomalous point at 10 minutes.
6. Independent variable: insulation or no insulation. Dependent variable: water temperature over time or temperature decrease. Control variables: volume of water, starting temperature, cup type, room temperature, time intervals, and thermometer position.

Data Task 2: Efficiency Calculation Table

Complete the table by calculating wasted energy and percentage efficiency. Round sensibly where needed.

Device	Total input energy in J	Useful output energy in J	Wasted energy in J	Efficiency as a percentage
LED lamp	120	96	24	80%
Filament lamp	120	12	108	10%
Electric motor	250	175	75	70%
Speaker	80	18	62	22.5%
Kettle	900	765	135	85%
Games console	360	288	72	80%

Worked Row: Speaker

Wasted energy:

80 J - 18 J = 62 J

Percentage efficiency:

18 J / 80 J x 100 = 22.5%

This means 22.5% of the input energy is usefully transferred as sound. The rest is transferred to less useful stores, mostly thermal stores.

Data Task 3: Appliance Power

Power is the rate of energy transfer. It is measured in watts, W. One watt means one joule is transferred each second.

Appliance	Energy transferred in 10 seconds	Power
Kettle	20 000 J	2000 W
Toaster	9000 J	900 W
LED lamp	100 J	10 W
Games console	1500 J	150 W

The kettle transfers energy fastest because it has the greatest power. This does not mean it is always wasteful; it means it transfers a large amount of energy each second.

Questions:

1. Which appliance has the lowest power?
2. How many joules does the toaster transfer each second?
3. Why is power not the same as energy?

Model answers:

1. The LED lamp has the lowest power.
2. The toaster transfers 900 J each second.
3. Energy is the quantity being transferred, measured in joules. Power is how quickly energy is transferred, measured in watts.

Data Task 4: Energy Resource Comparison

A coastal town is choosing how to generate more electricity locally. It is comparing wind power, natural gas, solar power, and tidal power.

Resource	Approximate running cost	Carbon dioxide emissions during operation	Reliability	Start-up time	Local environmental impact
Wind	Low	Very low	Variable; depends on wind	Fast when windy	Visual impact, noise concerns, possible wildlife effects
Natural gas	Medium to high because fuel is bought	Medium to high	High while fuel is supplied	Fast	Air pollution and greenhouse gas emissions
Solar	Low	Very low	Variable; daylight and weather dependent	Fast in daylight	Uses roof or land area; manufacturing impacts
Tidal	Low once built	Very low	Predictable but not constant	Linked to tide times	Can affect estuary habitats and shipping

Balanced Evaluation

Wind power could be suitable for a coastal town because coastal areas are often windy, running emissions are very low, and running costs are low after construction. However, wind output is variable and local people may object to visual impact or noise.

Natural gas is reliable and can start quickly when demand rises, but it releases carbon dioxide and uses a finite non-renewable fuel. It may be useful as backup, but it is not the best choice if the main aim is to reduce greenhouse gas emissions.

Solar power can work well on school, home, and warehouse roofs, but output is lower in winter, at night, and in cloudy weather. Tidal power is predictable and suitable only if the coastal town has the right tidal range and estuary conditions. It may have strong local environmental impacts.

A balanced conclusion might be: the town should use wind and solar where suitable, investigate tidal power carefully because of habitat impacts, and use natural gas only as backup while storage and grid connections are improved.

Graph Interpretation Task

The bar chart below is shown as text. It compares the percentage efficiency of four devices.

Efficiency (%)
100 |
 90 |                         █
 80 |        █                █
 70 |        █        █       █
 60 |        █        █       █
 50 |        █        █       █
 40 |        █        █       █
 30 |        █        █       █
 20 |        █        █       █
 10 | █      █        █       █
  0 +--------------------------------
       A      B        C       D

A = filament lamp, B = LED lamp, C = motor, D = kettle

Questions:

1. Which device is most efficient?
2. Which device is least efficient?
3. Describe the trend between filament and LED lamps.
4. Give one possible reason why the filament lamp is inefficient.
5. If device C has an input of 200 J and is 70% efficient, what is its useful output?

Model answers:

1. The kettle, device D, is most efficient at about 90%.
2. The filament lamp, device A, is least efficient at about 10%.
3. The LED lamp is much more efficient than the filament lamp because a larger proportion of its input energy is transferred by light.
4. A filament lamp transfers a large proportion of energy to thermal stores because the filament gets very hot.
5. useful output = 70 / 100 x 200 J = 140 J. The useful output is 140 J.

Practical Design Task: Best Insulator

Plan a fair test to find which material is the best insulator for a cup of hot water.

Your plan should include:

• the independent variable;
• the dependent variable;
• at least three control variables;
• a clear method;
• a safety note;
• repeat readings;
• how you would identify or handle anomalies;
• one improvement to make the results more accurate or reliable.

Model Plan

The independent variable is the insulating material wrapped around the cup. The dependent variable is the temperature decrease after 10 minutes. Control variables include the volume of water, starting temperature, type of cup, thickness of insulation, room temperature, whether a lid is used, and time intervals.

Method:

1. Measure 100 cm3 of hot water into the same type of cup.
2. Record the starting temperature with a thermometer.
3. Wrap the cup in one layer of the chosen insulating material.
4. Record the temperature every 2 minutes for 10 minutes.
5. Repeat the test three times for that material.
6. Repeat the method for each material.
7. Calculate the mean temperature decrease for each material.

Safety: use hot water carefully, keep cups away from the edge of the bench, and do not use boiling water unless the teacher instructs you.

An anomaly is a result that does not fit the pattern, such as a temperature increasing during cooling. I would repeat that measurement and only exclude it from the mean if there was a clear reason, such as misreading the thermometer.

Improvement: use a digital temperature probe connected to a data logger. This improves precision because readings can be taken at regular intervals, and it reduces reaction-time errors.

Common Misconceptions

Misconception	Scientific correction
Energy is destroyed when it is used.	Energy cannot be created or destroyed. It is transferred between stores.
Energy is lost from the universe.	Energy is transferred to the surroundings and becomes less useful, but it still exists.
Wasted energy disappears.	Wasted energy is energy not usefully transferred for the intended purpose.
Only moving objects have energy.	Energy can be stored in kinetic, thermal, chemical, gravitational potential, elastic potential, magnetic, electrostatic, and nuclear stores.
Energy and force are the same thing.	A force is a push or pull. Energy is a quantity measured in joules that describes stores and transfers.
Cold objects have no thermal energy.	All objects have thermal/internal stores, but hotter objects usually have more energy in that store.
Renewable energy has no disadvantages.	Renewable resources can have location limits, variable output, cost, material use, and environmental impacts.
Nuclear energy is renewable.	Nuclear fuel is finite, so nuclear energy is non-renewable, although it has low carbon dioxide emissions during operation.
Efficiency can be over 100%.	Useful output energy cannot be greater than total input energy because energy is conserved.
A thicker Sankey arrow always means the device is better.	Arrow width shows amount of energy. Efficiency depends on the useful proportion of the input.
Power means the same as energy.	Power is the rate of energy transfer. Energy is measured in joules; power is measured in watts.
Friction destroys energy.	Friction transfers energy mechanically into thermal stores of objects and surroundings.

Key Vocabulary

Term	Student-friendly definition	Unit or example
Energy	A quantity used to describe changes and stored or transferred in different ways	joule, J
Joule	The unit of energy	J
Kilojoule	1000 joules	kJ
Store	A way energy is kept in a system	kinetic store
Transfer pathway	A way energy moves between stores	heating, electrical transfer
Kinetic store	Energy store of a moving object	moving car
Thermal/internal store	Energy store linked to particle motion and temperature	hot water
Chemical store	Energy store in food, fuels, cells, and batteries	petrol, lunch
Gravitational potential store	Energy store of an object raised in a gravitational field	book on a shelf
Elastic potential store	Energy store in stretched or compressed objects	spring
Magnetic store	Energy store linked to magnets and magnetic materials	repelling magnets
Electrostatic store	Energy store when charges are separated	charged balloon
Nuclear store	Energy store inside atoms	nuclear fuel
Conservation of energy	Energy cannot be created or destroyed	total input equals total output
Dissipation	Energy spreading to less useful stores in the surroundings	heating air
Useful energy	Energy transferred in the intended way	light from a lamp
Wasted energy	Energy not usefully transferred for the intended purpose	heating from a speaker
Efficiency	The fraction or percentage of input energy usefully transferred	0.8 or 80%
Power	Rate of energy transfer	watt, W
Watt	One joule transferred each second	1 W = 1 J/s
Renewable	Replaced naturally on a human timescale	wind
Non-renewable	Finite and can run out	coal
Independent variable	Variable deliberately changed	insulation material
Dependent variable	Variable measured	temperature decrease
Control variable	Variable kept the same for a fair test	volume of water
Accuracy	How close a measurement is to the true value	calibrated thermometer
Precision	How close repeated readings are to each other	similar repeated temperatures
Repeatability	Same method and person get similar results	repeated trials
Reliability	Results are trustworthy because repeats and controls are good	consistent evidence
Anomaly	A result that does not fit the pattern	cooling water suddenly gets hotter

Exam-Style Questions

Multiple Choice

1. Which unit is used for energy? A. watt
   B. joule
   C. newton
   D. degree Celsius

2. Which statement is correct? A. Energy is destroyed when a battery goes flat.
   B. Energy is a force that pushes objects.
   C. Energy can be stored and transferred.
   D. Energy only exists in moving objects.

3. A ball is held above the ground. Which store has increased compared with the ball on the ground? A. gravitational potential store
   B. electrostatic store
   C. nuclear store
   D. sound store

4. A lamp transfers 100 J of energy. It transfers 20 J by light and 80 J by heating. What is its efficiency? A. 10%
   B. 20%
   C. 80%
   D. 120%

5. What does a wider arrow mean in a Sankey diagram? A. The device is always more efficient.
   B. More energy is being transferred.
   C. Energy is being created.
   D. The output is always wasted.

6. Which resource is non-renewable? A. wind
   B. tidal
   C. natural gas
   D. solar

7. Which sentence correctly compares energy and power? A. Energy and power mean the same thing.
   B. Power is the rate of energy transfer.
   C. Energy is measured in watts.
   D. Power is measured in joules.

8. Why does a cyclist's brakes become warm when braking? A. Energy is destroyed by friction.
   B. Energy is transferred from the kinetic store to thermal stores.
   C. Energy is created inside the brakes.
   D. Cold objects have no energy.

Fill in the Blanks

Use these words: conserved, joules, thermal, chemical, useful, wasted, renewable, power, control, anomaly.

1. Energy is measured in ________.
2. Energy cannot be created or destroyed, so it is ________.
3. Food and fuels contain energy in their ________ stores.
4. Energy transferred to the surroundings by heating often increases ________ stores.
5. Energy transferred in the intended way is called ________ energy.
6. Energy not transferred in the intended way is called ________ energy.
7. A resource replaced naturally on a human timescale is ________.
8. The rate of energy transfer is called ________.
9. A variable kept the same in a fair test is a ________ variable.
10. A result that does not fit the pattern is an ________.

Short-Answer Questions

1. Describe the energy pathway for a mobile phone being charged.
2. Describe the energy changes when a skateboarder rolls down a ramp.
3. Explain why "the energy has been lost" is not a complete scientific explanation.
4. Give two ways to reduce wasted energy in a home.
5. Why is an LED lamp usually more efficient than a filament lamp?
6. Explain why renewable resources can still have environmental impacts.
7. Why is nuclear fuel non-renewable?
8. What is the difference between accuracy and precision?

Calculation Questions

1. A motor has an input energy of 300 J. It usefully transfers 210 J. Calculate its percentage efficiency.
2. A speaker transfers 500 J of energy. It usefully transfers 125 J as sound. Calculate the wasted energy and efficiency.
3. A kettle transfers 2400 J of electrical energy. 2040 J is usefully transferred to the thermal store of the water. Calculate the percentage efficiency.
4. A device is 60% efficient and has an input energy of 800 J. Calculate the useful output energy.
5. A toy car transfers 150 J to its kinetic store and 50 J to thermal stores and sound. Calculate the total input energy and the efficiency.

Diagram Interpretation

Use the Sankey diagram.

Input energy: 200 J
==============================> Useful output: 130 J
             \
              \===========> Wasted output: ? J

1. Calculate the wasted output.
2. Calculate the percentage efficiency.
3. Explain what the wasted output means.
4. State whether this device is more or less efficient than a 50% efficient device.

Use the energy pathway.

Chemical store in fuel -> mechanical transfer in engine -> kinetic store of car
                                      |
                                      v
                         thermal stores and sound in surroundings

5. Identify the useful output.
6. Identify two wasted outputs.
7. Explain why the total energy after the transfer equals the total energy before the transfer.

Data Interpretation

A class compares two bouncing balls. They drop each ball from 1.00 m and measure the rebound height.

Ball	Drop height in m	Rebound height in m	Notes
A	1.00	0.82	quiet bounce
B	1.00	0.48	louder bounce
C	1.00	0.76	normal bounce
D	1.00	1.20	measurement may be wrong

1. Which ball appears to transfer the most energy back into gravitational potential store after bouncing?
2. Which ball dissipates more energy, A or B? Use evidence.
3. Which result is probably an anomaly?
4. Explain why repeat readings would improve reliability.
5. Suggest two control variables.

Longer Questions

1. A battery-powered torch is switched on for five minutes. Explain the energy transfers in the torch. Use the ideas of energy stores, transfer pathways, useful output, wasted output, and conservation of energy. Your answer should be about 6 marks.

2. A school wants to replace old filament lamps with LED lamps. The filament lamps are 10% efficient and each transfers 600 J of energy in a short test. The LED lamps are 80% efficient and transfer the same total input energy in the test. Use calculations and scientific explanation to justify which lamp is better for lighting classrooms. Include one limitation of using only this data. Your answer should be about 6-8 marks.

3. Plan an investigation to find which material is the best insulator for a cup of hot water. Include variables, control variables, method, safety, repeat readings, anomalies, and one improvement. Your answer should be about 6-8 marks.

4. A coastal town is choosing between wind power and a natural gas power station. Wind has very low emissions but variable output. Natural gas is reliable and quick to start but releases carbon dioxide. Write a balanced evaluation and recommend a choice or combination. Your answer should use reliability, emissions, running cost, and local impact.

Model Answers

Multiple Choice Answers

1. B. Joule is the unit of energy.
2. C. Energy can be stored and transferred.
3. A. Lifting the ball increases its gravitational potential store.
4. B. 20 J / 100 J x 100 = 20%.
5. B. A wider Sankey arrow shows a larger energy transfer.
6. C. Natural gas is a finite fossil fuel.
7. B. Power is the rate of energy transfer.
8. B. Friction transfers energy mechanically to thermal stores.

Fill in the Blanks Answers

1. joules
2. conserved
3. chemical
4. thermal
5. useful
6. wasted
7. renewable
8. power
9. control
10. anomaly

Short-Answer Model Answers

1. When a mobile phone is charged, energy is transferred electrically from the mains supply to the phone. The chemical store of the battery increases. Some energy is wasted by heating the charger, phone, and surroundings.
2. As the skateboarder rolls down the ramp, the gravitational potential store decreases and the kinetic store increases. Some energy is transferred to thermal stores and by sound because of friction and air resistance.
3. "Lost" is incomplete because energy cannot be destroyed. A better explanation is that energy has been transferred to the surroundings, often to thermal stores, and has become less useful.
4. Loft insulation and double glazing reduce energy transfer by heating. Draught proofing also reduces warm air escaping.
5. An LED lamp transfers a larger proportion of its input energy by light and a smaller proportion to thermal stores than a filament lamp.
6. Renewable resources can still need land, materials, roads, dams, or turbines. They may affect habitats, views, noise levels, or local communities.
7. Nuclear fuel is mined from finite materials. These fuels can run out, so nuclear fuel is non-renewable.
8. Accuracy is closeness to the true value. Precision is closeness of repeated readings to each other.

Calculation Model Answers

1. Equation: percentage efficiency = useful output energy / total input energy x 100
   Substitution: 210 J / 300 J x 100
   Arithmetic: 210 / 300 = 0.70, so 0.70 x 100 = 70%
   Final answer: the motor is 70% efficient. This means 70% of the input energy is usefully transferred.

2. Wasted energy equation: wasted energy = input energy - useful output energy
   Substitution: 500 J - 125 J = 375 J
   Efficiency equation: percentage efficiency = useful output energy / total input energy x 100
   Substitution: 125 J / 500 J x 100 = 25%
   Final answer: 375 J is wasted and the speaker is 25% efficient.

3. Equation: percentage efficiency = useful output energy / total input energy x 100
   Substitution: 2040 J / 2400 J x 100
   Arithmetic: 2040 / 2400 = 0.85, so 0.85 x 100 = 85%
   Final answer: the kettle is 85% efficient. This means most of the energy is usefully transferred to the water's thermal store.

4. Equation: useful output energy = efficiency x total input energy
   Use efficiency as a decimal: 60% = 0.60
   Substitution: 0.60 x 800 J = 480 J
   Final answer: useful output energy is 480 J.

5. Total input energy: 150 J + 50 J = 200 J
   Efficiency equation: percentage efficiency = useful output energy / total input energy x 100
   Substitution: 150 J / 200 J x 100 = 75%
   Final answer: total input energy is 200 J and the toy car is 75% efficient.

Diagram Model Answers

1. Wasted output: 200 J - 130 J = 70 J.
2. Efficiency: 130 J / 200 J x 100 = 65%.
3. The wasted output is energy transferred to less useful stores, such as thermal stores in the surroundings or sound.
4. This device is more efficient than a 50% efficient device because 65% is a larger useful proportion.
5. The useful output is the increase in the kinetic store of the car.
6. Wasted outputs include energy transferred to thermal stores of the engine and surroundings, and energy transferred by sound.
7. The total energy is the same before and after because energy is conserved. It has been transferred between stores, not destroyed.

Data Interpretation Model Answers

1. Ball A appears to transfer the most energy back into gravitational potential store because it rebounds to 0.82 m, the highest sensible rebound height.
2. Ball B dissipates more energy than A because it only rebounds to 0.48 m from a 1.00 m drop, while A rebounds to 0.82 m. Ball B also has a louder bounce, suggesting more energy transferred by sound.
3. Ball D is probably an anomaly because its rebound height is 1.20 m, higher than the drop height, which would suggest more useful energy than input.
4. Repeat readings improve reliability because they show whether the pattern is consistent and allow a mean to be calculated.
5. Control variables include drop height, surface, ball size, release method, measuring method, and room conditions.

Longer Model Answers

1. In a battery-powered torch, energy starts in the chemical store of the battery. When the torch is switched on, energy is transferred electrically through the circuit to the lamp. The useful output is energy transferred by light to the surroundings because the purpose of the torch is to provide light. Some energy is wasted because it is transferred to the thermal store of the lamp, battery, and surrounding air. This wasted energy does not disappear; it becomes less useful because it spreads out into the surroundings. The total energy is conserved, so the decrease in the battery's chemical store equals the energy transferred by light, heating, and any other small outputs.

2. For the filament lamp, useful output is 10 / 100 x 600 J = 60 J. Wasted energy is 600 J - 60 J = 540 J. For the LED lamp, useful output is 80 / 100 x 600 J = 480 J. Wasted energy is 600 J - 480 J = 120 J. The LED lamp is better for lighting classrooms because it transfers much more energy usefully by light and much less energy to thermal stores. It is more efficient, so less energy is wasted for the same input. A limitation is that the data only compares energy in a short test; it does not include cost, lifetime, brightness quality, or manufacturing impacts.

3. The independent variable is the insulating material. The dependent variable is the temperature decrease after a fixed time, such as 10 minutes. Control variables include the volume of water, starting temperature, cup type, thickness of insulation, room temperature, use of a lid, and time intervals. I would put 100 cm3 of hot water into the same type of cup, wrap the cup in the chosen material, record the starting temperature, and then measure the temperature every 2 minutes for 10 minutes. I would repeat each material three times and calculate a mean. For safety, I would use hot but not boiling water, keep cups stable, and handle hot water carefully. An anomalous reading, such as a temperature increase during cooling, should be checked and repeated. An improvement would be to use a digital temperature probe and data logger for more precise readings.

4. Wind power has very low carbon dioxide emissions during operation and can be suitable for a coastal town because coastal areas are often windy. Running costs are low once turbines are built. However, wind output is variable, so the town may need storage, grid support, or backup generation. Wind turbines can also have visual, noise, and wildlife impacts. Natural gas is reliable and quick to start, so it can meet demand when wind is low. However, it releases carbon dioxide and uses a finite fuel. A balanced recommendation is to use wind power as a main local renewable resource if the site is suitable, supported by storage, wider grid connections, and possibly natural gas as a short-term backup while lower-carbon reliable options are developed.

Revision Checklist

Use this checklist to assess your understanding.

I can...	Confident	Need more practice
define energy as a quantity measured in joules
explain that energy is stored and transferred, not used up
state the conservation of energy rule
name kinetic, thermal, chemical, gravitational potential, elastic potential, magnetic, electrostatic, and nuclear stores
describe when each energy store increases
identify heating, radiation, electrical transfer, sound, and mechanical work as transfer pathways
write energy pathways using store -> transfer -> store
identify useful and wasted energy outputs
explain dissipation to thermal stores in the surroundings
calculate wasted energy from input and useful output
calculate efficiency as a decimal
calculate percentage efficiency
interpret a Sankey diagram
complete missing values on a Sankey diagram
compare two devices using efficiency evidence
define power as the rate of energy transfer
explain that watts measure joules transferred each second
compare renewable and non-renewable energy resources
explain advantages and disadvantages of energy resources
evaluate a suitable resource for a UK location
identify independent, dependent, and control variables in an energy investigation
explain fair testing, repeatability, reliability, accuracy, and precision
identify anomalies and suggest improvements
answer longer questions using evidence, calculations, and scientific vocabulary

Final Quick Review

Energy is measured in joules and can be stored in different stores. It is transferred by pathways such as heating, radiation, electrical transfer, sound, and mechanical work. Energy is conserved, so it cannot be created or destroyed. When energy is wasted, it has not vanished; it has been transferred in a way that is not useful, often to thermal stores in the surroundings. Efficiency compares useful output energy with total input energy. Sankey diagrams show energy transfers with arrow widths. Renewable and non-renewable resources both have advantages and disadvantages, and good scientific evaluations use evidence rather than one-sided claims.