AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2021: Question 6

✅ Correct answer

The particles gain kinetic energy, so the internal energy increases.

For full marks you can also say the particles’ potential energy increases, because internal energy is the total of kinetic and potential energy.

💡 Key knowledge

• Internal energy = total kinetic energy + total potential energy of particles.
• When temperature rises, particles move faster, so kinetic energy increases.
• At GCSE level, linking temperature rise to faster particles is often enough for the first mark.

🧠 Exam technique

• Use the phrase internal energy directly.
• Explain the link: temperature up → particles move faster → kinetic energy up → internal energy up.
• If you mention potential energy too, that is fine and can strengthen the answer.

❌ Common errors

• Saying only “temperature increases” without explaining internal energy.
• Confusing internal energy with heat.
• Writing that particles “gain heat” instead of gaining kinetic energy.

✅ Correct answer

Latent heat of vaporisation

Accept: specific latent heat of vaporisation

💡 Key knowledge

• Latent heat is the energy needed for a change of state.
• Vaporisation means liquid changing to gas.
• In this question, the coolant evaporates, so the correct term is about vaporisation.

❌ Common errors

• Writing just “heat” or “energy” — too vague.
• Mixing up specific heat capacity with latent heat.
• Writing “latent heat of fusion” — that is melting/freezing, not evaporating.

✅ Correct answer

Stays the same

💡 Key knowledge

• In a closed system, mass is conserved.
• Changing state does not change the amount of substance.
• The particles are still there; they are just further apart in a gas.

🧠 Exam technique

• For multiple-choice style questions, check whether the question asks about mass, not volume or density.
• Evaporation changes arrangement and spacing, not total mass.

❌ Common errors

• Saying it decreases because the gas “spreads out”.
• Confusing mass with density.
• Thinking some particles disappear during a state change.

✅ Correct answer

The gas particles move faster, so they collide with the walls more often and with a greater force.

This increases the pressure in the pipe.

💡 Key knowledge

• Gas pressure is caused by particles colliding with the walls of the container.
• Higher temperature means higher kinetic energy and faster-moving particles.
• Faster particles give more frequent and stronger collisions.

🧠 Exam technique

• Use both ideas for 2 marks: more collisions and greater force.
• Write in cause-and-effect order: temperature up → particle speed up → pressure up.

❌ Common errors

• Just saying “particles move faster” without linking to pressure.
• Talking about density instead of pressure.
• Saying pressure increases because particles “take up more space” — not the key reason.

📐 Calculations: step-by-step

1. Find the useful energy transferred: 87.5% efficiency means the useful output energy is
   0.875 × 1 560 000 = 1 365 000 J
2. Write the specific heat capacity equation:
   E = m × c × ΔT
3. Calculate the temperature change:
   ΔT = 22.1 - 11.6 = 10.5 °C
4. Substitute values:
   1 365 000 = 125 × c × 10.5
5. Rearrange:
   c = 1 365 000 ÷ (125 × 10.5)
6. Calculate:
   c = 1040 J/kg °C
7. Answer in standard form:
   1.04 × 10³ J/kg °C

✅ Correct answer

c = 1.04 × 10³ J/kg °C

Equivalent form: 1040 J/kg °C

💡 Key knowledge

• Specific heat capacity tells you how much energy is needed to raise 1 kg of a substance by 1 °C.
• Use the equation: E = m × c × ΔT
• Efficiency = useful output energy ÷ total input energy
• Temperature change must be found using subtraction, not the final temperature.

🧠 Exam technique

• Write the equation before substituting numbers.
• Convert 87.5% into 0.875 .
• Use the correct unit for c: J/kg °C .
• Give your final answer in standard form when asked.

❌ Common errors

• Using 1560 instead of 1 560 000 J.
• Forgetting to convert efficiency from % to a decimal.
• Using 22.1 + 11.6 instead of subtraction.
• Dropping the units or writing the wrong units.
• Not giving the final answer in standard form.

✅ Correct answer

The advertisement ignores the energy taken from the surrounding air.

The total energy input is greater than the electrical energy supplied, so the system is not getting 400% efficiency from electricity alone.

Efficiency must be less than 100%.

💡 Key knowledge

• A heat pump transfers energy from the outside air into the building.
• The useful output can be bigger than the electrical input because extra energy comes from the surroundings.
• But efficiency is still based on input and output of the whole system, so it cannot be more than 100%.

🧠 Exam technique

• Make sure your answer mentions surrounding air or outside air.
• Say clearly that the electricity is not the only input.
• Finish with the key conclusion: efficiency must be less than 100%.

❌ Common errors

• Thinking the heat pump creates energy.
• Saying “400% efficiency is impossible” without explaining why.
• Forgetting the energy gained from the outside air.
• Mixing up useful output energy with total input energy.

✅ Top-level responses

• Used precise physics language: particles, kinetic energy, internal energy, pressure, efficiency.
• Showed working clearly in the calculation.
• Used the correct equation and rearranged it properly.
• Explained the heat pump idea: extra energy comes from the air, not just the mains electricity.

❌ Where students lost marks

• Vague explanations with no physics vocabulary.
• Confusing latent heat with specific heat capacity.
• Leaving out units in the calculation.
• Claiming efficiency can be above 100% without explaining the external energy source.

✅ You should know

• Internal energy = kinetic + potential energy of particles.
• Latent heat of vaporisation is the energy for liquid → gas.
• Gas pressure comes from collisions with the walls.
• E = m × c × ΔT
• efficiency = useful output ÷ total input

🧠 Final exam tip

If you see a heat pump question, always ask: Where does the energy come from? The answer is usually from both the electricity supply and the surrounding air.