EnergyAQA GCSE Physics (8463)

Conservation and Dissipation of Energy

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

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Master the topic

Worked examples5 Key formulae3 Definitions9 Common mistakes6

Step-by-step worked examples

01.Reducing wasted energy when cycling
Foundation
Question
A cyclist wants to go faster without pedalling harder. Describe TWO things she could do to reduce wasted energy, naming the dissipation method she is reducing.
Solution
1. 1
  Reduce friction.
  $\text{Lubricate the chain and gears} \to \text{less friction} \to \text{less thermal dissipation in the moving parts.}$
2. 2
  Reduce air resistance.
  $\text{Wear tight-fitting cycling clothes / a streamlined helmet} \to \text{less air resistance} \to \text{less thermal dissipation in the surrounding air.}$
Answer
(i) Lubricate moving parts to reduce friction. (ii) Wear streamlined clothing/helmet to reduce air resistance.
02.Comparing two insulators (RP2 analysis)
Higher
Question
A student investigated thermal insulators. After 10 minutes, water wrapped in bubble wrap fell from 80 °C to 65 °C; water wrapped in cotton wool fell from 80 °C to 71 °C. Which is the better insulator and why?
Solution
1. 1
  Calculate the temperature drop for each.
  $\Delta\theta_{\text{bubble}} = 80 - 65 = 15\,\text{°C};\quad \Delta\theta_{\text{cotton}} = 80 - 71 = 9\,\text{°C}$
2. 2
  Compare.
  $\text{Cotton wool drops less, so it transferred less energy by heating in the same time.}$
3. 3
  Justify.
  $\text{Cotton wool has more trapped air pockets, giving lower thermal conductivity.}$
Answer
Cotton wool is the better insulator — temperature drop was 9 °C vs 15 °C for bubble wrap. Cotton wool traps more air, which has very low thermal conductivity.
Examiner note
Top-band answer compares the NUMBERS, names the dissipation process (heating/conduction), and links to low thermal conductivity of trapped air.
03.Petrol engine efficiency
Foundation
Question
A petrol engine receives 2000 J of chemical energy per cycle. It delivers 500 J of useful kinetic energy. Calculate the engine's efficiency as a percentage.
Solution
1. 1
  Use the formula.
  $\text{efficiency} = \frac{\text{useful}}{\text{total}}$
2. 2
  Substitute.
  $\text{efficiency} = \frac{500}{2000} = 0.25$
3. 3
  Convert to %.
  $0.25 \times 100 = 25\%$
Answer
25 %.
04.LED useful output
Higher
Question
A 9 W LED bulb has an efficiency of 80 %. Calculate the useful (light) power it produces and the wasted (thermal) power.
Solution
1. 1
  Useful power = efficiency × input.
  $P_{\text{useful}} = 0.80 \times 9 = 7.2\,\text{W}$
2. 2
  Wasted = total − useful.
  $P_{\text{wasted}} = 9 - 7.2 = 1.8\,\text{W}$
Answer
7.2 W useful (light); 1.8 W wasted (thermal/IR).
05.Reading a Sankey diagram
Higher
Question
A car receives 5000 J of chemical energy from petrol per cycle. The Sankey diagram shows 1200 J going to the useful kinetic store and the rest dissipated. Calculate (a) the energy wasted and (b) the efficiency.
Solution
1. 1
  Wasted = input − useful.
  $E_{\text{wasted}} = 5000 - 1200 = 3800\,\text{J}$
2. 2
  Efficiency = useful / total.
  $\text{efficiency} = 1200 / 5000 = 0.24 = 24\%$
Answer
(a) 3800 J wasted. (b) Efficiency = 24 %.

Key formulae

Conservation of energy
$E_{\text{input}} = E_{\text{useful}} + E_{\text{wasted}}$
- $E_{\text{input}}$
  — Total energy supplied (J)
- $E_{\text{useful}}$
  — Energy in the intended store (J)
- $E_{\text{wasted}}$
  — Energy dissipated to surroundings (J)
When to use
Whenever an energy-flow diagram or Sankey is described or drawn.
Efficiency (energy)
$\text{efficiency} = \dfrac{E_{\text{useful}}}{E_{\text{total}}}$
- $E_{\text{useful}}$
  — Useful energy output (J)
- $E_{\text{total}}$
  — Total energy input (J)
When to use
Use when given energy values. Decimal output; ×100 for %.
Efficiency (power)
$\text{efficiency} = \dfrac{P_{\text{useful}}}{P_{\text{total}}}$
- $P_{\text{useful}}$
  — Useful power output (W)
- $P_{\text{total}}$
  — Total power input (W)
When to use
Use when given power values instead of energy.

Practice with flashcards

Card 1 of 90 known · 0 to review

Key definitions and keywords

Conservation of energyExaminer keyword: The total energy of a closed system stays constant. Energy can move between stores but cannot be created or destroyed.
DissipationExaminer keyword: The spreading of energy to the surroundings, usually as thermal energy, where it can no longer do useful work.
Thermal conductivity: A measure of how easily a material allows energy to transfer through it by conduction (W/m·K). Low conductivity = good insulator.
Lubrication: Using oil or grease to reduce friction between moving surfaces.
Streamlining: Shaping an object so that fluid (air or water) flows smoothly around it, reducing drag.
EfficiencyExaminer keyword: The fraction (or percentage) of input energy that becomes useful output.
Useful energyExaminer keyword: Energy transferred to the intended store (the purpose of the device).
Wasted energyExaminer keyword: Energy dissipated to unintended stores, usually the thermal store of the surroundings.
Sankey diagram: A diagram showing energy flows with arrow widths proportional to the energy each represents.

Common mistakes and misconceptions

Mistake
Saying 'energy is lost' (full stop).
Why it happens
Conservation rule feels like a slogan; students forget energy goes somewhere.
How to avoid it
Always name the destination store, e.g. 'thermal store of the surroundings'.
Source: AQA Paper 1 Examiner Report 2023.
Mistake
Saying streamlining 'increases the engine's power'.
Why it happens
Cause and effect confused.
How to avoid it
Streamlining REDUCES wasted energy. The engine's power is unchanged, so MORE of that power becomes useful kinetic energy.
Mistake
Ignoring thickness when discussing insulation.
Why it happens
Focus on the named material only.
How to avoid it
Two factors: thermal conductivity AND thickness. Doubling thickness roughly halves heat loss.
Mistake
Reporting efficiency over 100 %.
Why it happens
Useful and total swapped in the formula.
How to avoid it
Useful is always SMALLER than total in real devices. If your answer exceeds 100 %, you've inverted.
Mistake
Quoting decimal answers (e.g. 0.4) when % is asked, or vice versa.
Why it happens
Forgetting the ×100 step (or doing it twice).
How to avoid it
Re-read the question. ALWAYS state '%' or 'as a decimal' explicitly.
Mistake
Saying 'make the input bigger' to improve efficiency.
Why it happens
Confusing input with output.
How to avoid it
Efficiency improvements reduce WASTE: lubricate, insulate, streamline.

EnergyAQA GCSE Physics (8463)

Conservation and Dissipation of Energy

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

Previous Next

Master the topic

Worked examples5 Key formulae3 Definitions9 Common mistakes6

Step-by-step worked examples

01.Reducing wasted energy when cycling
Foundation
Question
A cyclist wants to go faster without pedalling harder. Describe TWO things she could do to reduce wasted energy, naming the dissipation method she is reducing.
Solution
1. 1
  Reduce friction.
  $\text{Lubricate the chain and gears} \to \text{less friction} \to \text{less thermal dissipation in the moving parts.}$
2. 2
  Reduce air resistance.
  $\text{Wear tight-fitting cycling clothes / a streamlined helmet} \to \text{less air resistance} \to \text{less thermal dissipation in the surrounding air.}$
Answer
(i) Lubricate moving parts to reduce friction. (ii) Wear streamlined clothing/helmet to reduce air resistance.
02.Comparing two insulators (RP2 analysis)
Higher
Question
A student investigated thermal insulators. After 10 minutes, water wrapped in bubble wrap fell from 80 °C to 65 °C; water wrapped in cotton wool fell from 80 °C to 71 °C. Which is the better insulator and why?
Solution
1. 1
  Calculate the temperature drop for each.
  $\Delta\theta_{\text{bubble}} = 80 - 65 = 15\,\text{°C};\quad \Delta\theta_{\text{cotton}} = 80 - 71 = 9\,\text{°C}$
2. 2
  Compare.
  $\text{Cotton wool drops less, so it transferred less energy by heating in the same time.}$
3. 3
  Justify.
  $\text{Cotton wool has more trapped air pockets, giving lower thermal conductivity.}$
Answer
Cotton wool is the better insulator — temperature drop was 9 °C vs 15 °C for bubble wrap. Cotton wool traps more air, which has very low thermal conductivity.
Examiner note
Top-band answer compares the NUMBERS, names the dissipation process (heating/conduction), and links to low thermal conductivity of trapped air.
03.Petrol engine efficiency
Foundation
Question
A petrol engine receives 2000 J of chemical energy per cycle. It delivers 500 J of useful kinetic energy. Calculate the engine's efficiency as a percentage.
Solution
1. 1
  Use the formula.
  $\text{efficiency} = \frac{\text{useful}}{\text{total}}$
2. 2
  Substitute.
  $\text{efficiency} = \frac{500}{2000} = 0.25$
3. 3
  Convert to %.
  $0.25 \times 100 = 25\%$
Answer
25 %.
04.LED useful output
Higher
Question
A 9 W LED bulb has an efficiency of 80 %. Calculate the useful (light) power it produces and the wasted (thermal) power.
Solution
1. 1
  Useful power = efficiency × input.
  $P_{\text{useful}} = 0.80 \times 9 = 7.2\,\text{W}$
2. 2
  Wasted = total − useful.
  $P_{\text{wasted}} = 9 - 7.2 = 1.8\,\text{W}$
Answer
7.2 W useful (light); 1.8 W wasted (thermal/IR).
05.Reading a Sankey diagram
Higher
Question
A car receives 5000 J of chemical energy from petrol per cycle. The Sankey diagram shows 1200 J going to the useful kinetic store and the rest dissipated. Calculate (a) the energy wasted and (b) the efficiency.
Solution
1. 1
  Wasted = input − useful.
  $E_{\text{wasted}} = 5000 - 1200 = 3800\,\text{J}$
2. 2
  Efficiency = useful / total.
  $\text{efficiency} = 1200 / 5000 = 0.24 = 24\%$
Answer
(a) 3800 J wasted. (b) Efficiency = 24 %.

Key formulae

Conservation of energy
$E_{\text{input}} = E_{\text{useful}} + E_{\text{wasted}}$
- $E_{\text{input}}$
  — Total energy supplied (J)
- $E_{\text{useful}}$
  — Energy in the intended store (J)
- $E_{\text{wasted}}$
  — Energy dissipated to surroundings (J)
When to use
Whenever an energy-flow diagram or Sankey is described or drawn.
Efficiency (energy)
$\text{efficiency} = \dfrac{E_{\text{useful}}}{E_{\text{total}}}$
- $E_{\text{useful}}$
  — Useful energy output (J)
- $E_{\text{total}}$
  — Total energy input (J)
When to use
Use when given energy values. Decimal output; ×100 for %.
Efficiency (power)
$\text{efficiency} = \dfrac{P_{\text{useful}}}{P_{\text{total}}}$
- $P_{\text{useful}}$
  — Useful power output (W)
- $P_{\text{total}}$
  — Total power input (W)
When to use
Use when given power values instead of energy.

Practice with flashcards

Card 1 of 90 known · 0 to review

Key definitions and keywords

Conservation of energyExaminer keyword: The total energy of a closed system stays constant. Energy can move between stores but cannot be created or destroyed.
DissipationExaminer keyword: The spreading of energy to the surroundings, usually as thermal energy, where it can no longer do useful work.
Thermal conductivity: A measure of how easily a material allows energy to transfer through it by conduction (W/m·K). Low conductivity = good insulator.
Lubrication: Using oil or grease to reduce friction between moving surfaces.
Streamlining: Shaping an object so that fluid (air or water) flows smoothly around it, reducing drag.
EfficiencyExaminer keyword: The fraction (or percentage) of input energy that becomes useful output.
Useful energyExaminer keyword: Energy transferred to the intended store (the purpose of the device).
Wasted energyExaminer keyword: Energy dissipated to unintended stores, usually the thermal store of the surroundings.
Sankey diagram: A diagram showing energy flows with arrow widths proportional to the energy each represents.

Common mistakes and misconceptions

Mistake
Saying 'energy is lost' (full stop).
Why it happens
Conservation rule feels like a slogan; students forget energy goes somewhere.
How to avoid it
Always name the destination store, e.g. 'thermal store of the surroundings'.
Source: AQA Paper 1 Examiner Report 2023.
Mistake
Saying streamlining 'increases the engine's power'.
Why it happens
Cause and effect confused.
How to avoid it
Streamlining REDUCES wasted energy. The engine's power is unchanged, so MORE of that power becomes useful kinetic energy.
Mistake
Ignoring thickness when discussing insulation.
Why it happens
Focus on the named material only.
How to avoid it
Two factors: thermal conductivity AND thickness. Doubling thickness roughly halves heat loss.
Mistake
Reporting efficiency over 100 %.
Why it happens
Useful and total swapped in the formula.
How to avoid it
Useful is always SMALLER than total in real devices. If your answer exceeds 100 %, you've inverted.
Mistake
Quoting decimal answers (e.g. 0.4) when % is asked, or vice versa.
Why it happens
Forgetting the ×100 step (or doing it twice).
How to avoid it
Re-read the question. ALWAYS state '%' or 'as a decimal' explicitly.
Mistake
Saying 'make the input bigger' to improve efficiency.
Why it happens
Confusing input with output.
How to avoid it
Efficiency improvements reduce WASTE: lubricate, insulate, streamline.

Conservation and Dissipation of Energy

Master the topic

Step-by-step worked examples

01.Reducing wasted energy when cycling

02.Comparing two insulators (RP2 analysis)

03.Petrol engine efficiency

04.LED useful output

05.Reading a Sankey diagram

Key formulae

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions

Conservation and Dissipation of Energy

Master the topic

Step-by-step worked examples

01.Reducing wasted energy when cycling

02.Comparing two insulators (RP2 analysis)

03.Petrol engine efficiency

04.LED useful output

05.Reading a Sankey diagram

Key formulae

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions