AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2024: Question 6

💡 Key knowledge

• Energy can be stored as compressed air.
• When demand is low, extra electricity is used to compress air.
• When demand is high, compressed air is released to turn a turbine and generator.

🧠 Exam technique

• Use exact physics words: efficiency, kinetic energy, collisions, energy transferred.
• For calculations, show substitution and units clearly.
• For 4-mark explanations, write linked steps in order.

✅ Correct answer

Efficiency = 0.72 × 0.86 = 0.6192

Efficiency = 0.62 (2 significant figures)

📐 Calculation steps

1. Identify the two process efficiencies: 0.72 and 0.86 .
2. Multiply them: 0.72 × 0.86 = 0.6192 .
3. Round to 2 significant figures: 0.62 .

❌ Common errors

• Adding the efficiencies instead of multiplying them.
• Giving a percentage when a decimal is expected.
• Rounding too early and losing accuracy.

💡 Key knowledge

When processes happen one after another, the overall efficiency is the product of the individual efficiencies.

🧠 How marks were awarded

• 1 mark for multiplying the two efficiencies.
• 1 mark for the correct decimal value.
• 1 mark for the final 2 s.f. answer.

✅ Correct answer

At higher temperature the average speed of the air particles is greater (or their average kinetic energy is greater). This means the particles collide with the turbine more frequently and/or each collision has greater force. Therefore more energy is transferred each second, so the power transferred to the turbine increases.

💡 Key knowledge

• Temperature is related to the average kinetic energy of particles.
• Faster particles mean more collisions with the turbine.
• More frequent or more forceful collisions transfer more energy per second.

🧠 Exam technique

• Use a chain of cause and effect: temperature → particle speed/KE → collisions → energy transfer.
• For full marks, include both motion and energy transfer.
• Link to power as “energy transferred each second.”

❌ Common errors

• Saying particles “vibrate more” — this is not the right idea for gas particles here.
• Only saying “temperature increases” without explaining how that affects collisions.
• Forgetting to mention energy transferred each second / power.

✅ What top answers did

• Used the phrase “average kinetic energy”.
• Explained collisions clearly.
• Finished by linking to more energy transferred each second.

✅ Correct answer

Energy transferred = 3.96 × 10¹¹ J

📐 Calculation steps

1. Find the mass of air using density:
   density = mass / volume
   48 = mass / (5.0 × 10⁵)
2. Rearrange:
   mass = 48 × 5.0 × 10⁵ = 2.4 × 10⁷ kg
3. Find the temperature change:
   Δθ = 27 - 12 = 15 °C
4. Use the energy equation:
   E = mcΔθ
   E = 2.4 × 10⁷ × 1100 × 15
5. Calculate:
   E = 3.96 × 10¹¹ J

❌ Common calculation traps

• Not using the density equation first.
• Forgetting to convert the answer into standard form.
• Using the wrong temperature change, e.g. 27 + 12 instead of 27 - 12.
• Mixing up units: density in kg/m³, temperature change in °C, energy in J.

💡 Key knowledge

• density = mass / volume
• E = mcΔθ
• For temperature change, the size of 1 °C is the same as 1 K, so °C is acceptable here.

🧠 How marks were awarded

• 1 mark for using density to find mass.
• 1 mark for the correct mass value.
• 1 mark for substituting into E = mcΔθ .
• 1 mark for a correct calculated energy value.
• 1 mark for the answer in standard form.

✅ Correct answer

Efficient energy storage means less energy is wasted. This means less fossil fuel needs to be burned to generate the same amount of useful electricity, so less carbon dioxide is released. It also means surplus energy from renewable sources can be stored and used later, helping to meet demand when renewables are not generating all the time.

💡 Key knowledge

• Burning fossil fuels releases carbon dioxide.
• Efficient storage reduces waste, so less extra generation is needed.
• Stored renewable energy can be used when demand is high or when the wind/ الشمس? No — use GCSE wording: when renewable output is low.

🧠 Exam technique

• To get full marks, make a clear chain: efficient storage → less wasted energy → less fossil fuel burned → less CO₂.
• Then add why storage helps with renewables: they are not always available all the time.

❌ Common errors

• Only saying “it is better for the environment” without explaining why.
• Not mentioning fossil fuels or carbon dioxide.
• Not linking energy storage to matching supply and demand.

✅ What top answers did

• Explained that less wasted energy means less fuel needed.
• Linked stored energy to unreliable renewable generation.
• Made the final link to reduced CO₂ emissions and meeting demand.

💡 Remember

• Multiply efficiencies for linked processes.
• Warmer gas particles move faster and collide more effectively.
• Use density = mass / volume before E = mcΔθ .
• Efficient storage helps reduce fossil fuel use and CO₂ emissions.

🧠 Best exam habits

• Show steps in calculations.
• Use precise physics words.
• Keep explanations linked and logical.
• Check final units and significant figures.