Examiners want you to describe energy transfers using the correct stores and pathways. Study the examples, answer the 20 questions, then move on to Sankey diagrams. Write your answers first, then tap to check. ← Energy topic
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How to describe an energy transfer:
1. Which store does the energy START in?
2. HOW is it transferred? (work done by a force · electrically · heating · radiation)
3. Which store does it END in?
4. Where is energy WASTED? (usually dissipated to the thermal store of the surroundings)
Part 1: Real-life energy transfers
| Situation | Energy transfer | Wasted energy |
|---|---|---|
| Electric kettle | Transferred electrically from the mains to the heating element, then by heating to the thermal store of the water | Thermal store of the kettle and surrounding air; sound |
| Car braking | Work done by friction transfers energy from the kinetic store of the car to the thermal store of the brakes | Sound; thermal store of the surroundings |
| Ball thrown upwards | Kinetic store decreases and gravitational potential store increases as it rises (work done against gravity) | Thermal store of the air (work done against air resistance) |
| Catapult or bow firing | Elastic potential store of the band or string → kinetic store of the stone or arrow | Thermal store of the band; sound |
| Battery-powered torch | Chemical store of the battery → transferred electrically → carried away by light (radiation) | Thermal store of the bulb and surroundings |
| Car engine | Chemical store of the fuel → kinetic store of the car | Thermal store of the engine and surroundings; sound |
| Solar panel | Energy carried by radiation from the Sun → transferred electrically | Thermal store of the panel |
| Wind turbine | Kinetic store of the wind → kinetic store of the blades → transferred electrically | Thermal store of the generator; sound |
| Bungee jumper falling | Gravitational potential store → kinetic store → elastic potential store of the rope as it stretches | Thermal store of the air and rope |
| Charging a phone | Transferred electrically from the mains → chemical store of the battery | Thermal store of the charger and phone |
| Electric motor lifting a load | Transferred electrically → kinetic store of the motor → gravitational potential store of the load | Thermal store of the motor (friction); sound |
| Swinging pendulum | Gravitational potential store ↔ kinetic store, over and over | Thermal store of the air and pivot – so it slowly stops |
Watch out: “energy is used up” is never right. Energy is always transferred – the wasted energy is dissipated, usually heating the surroundings.
Part 2: 20 energy transfer questions
Use g = 9.8 N/kg. Questions 1–10 are about describing transfers; 11–20 include calculations.
Q1 A ball is thrown straight up. Describe the energy transfer as it rises.
Show answer
Energy is transferred from the ball’s kinetic store to its gravitational potential store, by work done against gravity. A little is wasted in the thermal store of the air due to air resistance.
Q2 Describe the energy transfer in a battery-powered torch.
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Energy in the chemical store of the battery is transferred electrically to the bulb. It is carried away by light (useful) and by heating, to the thermal store of the surroundings (wasted).
Q3 A cyclist brakes and stops. Which store does the energy start in, which store does it end in, and how is it transferred?
Show answer
Starts in the kinetic store of the bike and cyclist; ends in the thermal store of the brakes (and surroundings); transferred by work done by friction.
Q4 A stretched catapult is released. Describe the energy transfer.
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Energy is transferred from the elastic potential store of the catapult to the kinetic store of the stone, by work done by the elastic force.
Q5 Give the useful energy transfer and two wasted energy transfers for an electric kettle.
Show answer
Useful: to the thermal store of the water. Wasted: heating the kettle itself; heating the surrounding air; sound.
Q6 What is the useful energy transfer in a hairdryer? What is wasted?
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Useful: the thermal store and kinetic store of the air blown out. Wasted: sound, and heating the hairdryer’s casing.
Q7 Describe the energy transfer in a solar-powered calculator.
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Energy carried by light (radiation) is absorbed by the solar cell and transferred electrically to the calculator’s circuit. Some is wasted heating the cell and circuit.
Q8 A bungee jumper is at the lowest point of their jump and is momentarily still. Which store is most of the energy in now?
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The elastic potential store of the stretched rope. (Their kinetic store is zero because they’re momentarily still; some energy has been dissipated to the thermal store of the air.)
Q9 A dropped ball never bounces back up to the height it started from. Explain why.
Show answer
Some energy is dissipated on each bounce and during the flight – to the thermal store of the ball, the ground and the air, and as sound. So less energy is in the gravitational potential store at the top of each bounce.
Q10 Explain why oiling a bicycle chain makes the bicycle more efficient.
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Lubrication reduces friction between the moving parts, so less work is done against friction and less energy is wasted in the thermal store of the chain. More of the input energy goes to the useful kinetic store.
Q11 Calculate the kinetic energy of a 0.50 kg ball moving at 4.0 m/s.
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Ek = ½mv² = 0.5 × 0.50 × 4.0² = 4.0 J
Q12 A 60 kg student climbs a 3.0 m flight of stairs. Calculate the gain in their gravitational potential energy.
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Ep = mgh = 60 × 9.8 × 3.0 = 1764 J
Q13 A spring with a spring constant of 200 N/m is stretched by 0.10 m. Calculate the elastic potential energy stored.
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Ee = ½ke² = 0.5 × 200 × 0.10² = 1.0 J
Q14 A 2000 W kettle is switched on for 90 s. How much energy is transferred?
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E = Pt = 2000 × 90 = 180 000 J
Q15 An electric motor is supplied with 800 J and transfers 600 J usefully. Calculate its efficiency and the energy wasted.
Show answer
Efficiency = 600 ÷ 800 = 0.75 (75%). Wasted = 800 − 600 = 200 J.
Q16 A lamp has an efficiency of 0.20. It is supplied with 60 J of energy every second. How much energy is transferred as light each second?
Show answer
Useful output = efficiency × input = 0.20 × 60 = 12 J each second.
Q17 A 1200 kg car travelling at 20 m/s brakes to a stop. How much energy is transferred to the thermal store of the brakes (assume all of it)?
Show answer
Ek = ½ × 1200 × 20² = 240 000 J
Q18 A 0.20 kg ball is dropped from a height of 5.0 m. Ignoring air resistance, calculate its speed just before it hits the ground.
Show answer
Ep lost = 0.20 × 9.8 × 5.0 = 9.8 J = Ek gained.
9.8 = ½ × 0.20 × v², so v² = 98 and v = 9.9 m/s
Q19 A 500 kg roller coaster car starts at rest at the top of a 30 m drop. Ignoring friction, calculate its maximum speed at the bottom. Why is the real speed lower?
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Ep = 500 × 9.8 × 30 = 147 000 J = Ek.
v² = 2 × 147 000 ÷ 500 = 588, so v = 24 m/s.
In reality, work is done against friction and air resistance, so some energy is dissipated to thermal stores and less ends up in the kinetic store.
Q20 A crane motor lifts a 400 kg load through 12 m in 20 s. Calculate the useful power output of the motor.
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Ep = 400 × 9.8 × 12 = 47 040 J
P = E ÷ t = 47 040 ÷ 20 = 2352 W (about 2.4 kW)
✅ Score out of 20.
Part 3: Sankey diagrams
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A Sankey diagram shows where the energy supplied to a device ends up.
• The WIDTH of each arrow is proportional to the amount of energy.
• The input arrow is on the left. Useful energy goes straight on; wasted energy bends away (usually downwards).
• Total input = total useful + total wasted (energy is conserved).
• Efficiency = useful output ÷ total input.
Diagram A – a filament lamp
| Arrow | Energy | Direction on the diagram |
|---|---|---|
| Input – electrical | 100 J | Enters from the left |
| Useful – light | 10 J | Goes straight on (thin arrow) |
| Wasted – thermal | 90 J | Bends downwards (wide arrow) |
S1 Using Diagram A, state the total energy input and the useful energy output of the lamp.
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Input = 100 J (electrical). Useful output = 10 J (light).
S2 Calculate the efficiency of the filament lamp.
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10 ÷ 100 = 0.10 (10%)
S3 Why is the thermal arrow drawn much wider than the light arrow?
Show answer
The width of each arrow is proportional to the energy it represents. 90 J is transferred by heating but only 10 J as light, so the thermal arrow is 9 times wider.
Diagram B – an electric motor
| Arrow | Energy | Direction on the diagram |
|---|---|---|
| Input – electrical | 400 J | Enters from the left |
| Useful – kinetic | 280 J | Goes straight on |
| Wasted – thermal | 80 J | Bends downwards |
| Wasted – sound | 40 J | Bends downwards |
S4 Using Diagram B, how much energy does the motor waste in total?
Show answer
80 J (thermal) + 40 J (sound) = 120 J
S5 Calculate the efficiency of the motor as a percentage.
Show answer
280 ÷ 400 = 0.70 → 70%
S6 Show that Diagram B obeys the law of conservation of energy.
Show answer
Total output = 280 + 80 + 40 = 400 J, which equals the input of 400 J. No energy has been created or destroyed.
S7 A television is supplied with 200 J of energy. It transfers 150 J by heating and 20 J as sound. The rest is transferred as light. (a) How much energy is transferred as light? (b) If light and sound are both useful, what is the efficiency?
Show answer
(a) 200 − 150 − 20 = 30 J
(b) Useful = 30 + 20 = 50 J; efficiency = 50 ÷ 200 = 0.25 (25%)
Drawing Sankey diagrams
Use squared paper and a ruler. Choose a scale first, e.g. 1 square = 10 J. Draw the input arrow on the left, the useful arrow going straight on to the right, and the wasted arrows bending downwards. Label every arrow with its energy and type.
S8 An LED lamp is supplied with 100 J. It transfers 80 J as light and 20 J by heating. Draw a Sankey diagram using a scale of 1 square = 10 J.
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Input arrow: 10 squares wide, labelled “Electrical 100 J”. Useful arrow going straight on: 8 squares wide, labelled “Light 80 J”. Wasted arrow bending down: 2 squares wide, labelled “Thermal 20 J”. Check: 8 + 2 = 10 squares.
S9 A car engine transfers 1000 J from the chemical store of its fuel. 250 J goes to the kinetic store of the car, 650 J is wasted by heating and 100 J is wasted as sound. (a) Draw a Sankey diagram using a scale of 1 square = 50 J. (b) Calculate the efficiency.
Show answer
(a) Input: 20 squares (“Chemical 1000 J”). Useful, straight on: 5 squares (“Kinetic 250 J”). Wasted, bending down: 13 squares (“Thermal 650 J”) and 2 squares (“Sound 100 J”). Check: 5 + 13 + 2 = 20.
(b) 250 ÷ 1000 = 0.25 (25%)
S10 Compare your Sankey diagram for the LED lamp (S8) with Diagram A for the filament lamp. Which lamp should a family choose, and why?
Show answer
The LED lamp. It is 80% efficient compared with 10% for the filament lamp – its useful (light) arrow is much wider and its wasted arrow much narrower. It wastes less energy heating the surroundings, so it needs less input energy to give the same light, which means lower electricity bills.
✅ Score out of 10. Want more? Try the Save the Kettle Company lesson or the Energy fill the gaps page.
