AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2019: Question 5

✅ Full-mark focus

• Explain heating using electron collisions and kinetic energy.
• Choose a sensible control variable.
• Link graph shape to energy transfer and efficiency.
• Use E = Pt and E = mcΔT correctly.

🧠 Examiner tip

• Marks are often split into separate ideas.
• Use correct scientific words: electrons, collisions, kinetic energy, efficiency.
• In calculations, convert kW to W and grams to kilograms.

❌ Common traps

• Saying “the kettle gets hotter because electricity is added” without explaining how.
• Not stating a control variable clearly.
• Forgetting that the graph does not go through zero.
• Using the wrong temperature change or units in the SHC calculation.

✅ Correct answer

Electrons collide with particles in the heating element, which increases the kinetic energy of the particles in the heating element.

💡 Key knowledge

• The mains supply provides a current through the element.
• Electrical energy is transferred to thermal energy.
• Particles in the metal vibrate more rapidly, so temperature rises.

🧠 Exam technique

For 2 marks, you usually need two linked points:

• one about electrons/collisions/current
• one about energy/temperature/particle motion

❌ Common errors

• “Electricity makes it hot” — too vague for full marks.
• Saying particles “gain heat” instead of kinetic energy.
• Talking about water heating rather than the metal element.

✅ Correct answer

The starting temperature of the water.

💡 Key knowledge

In a fair test, control variables keep the investigation valid. Here, changing only the mass of water helps show its effect on time taken to boil.

🧠 Examiner technique

State a variable that can actually be kept the same, such as:

• starting temperature of the water
• same kettle / same heating element
• same room temperature

But the mark scheme specifically credits the starting temperature of the water.

❌ Common errors

• “Keep everything the same” — not specific enough.
• Choosing the mass of water, which is the independent variable.
• Choosing “time” as a control variable, even though it is what is measured.

✅ Correct answer

The heating element of the kettle took time to heat up.

💡 Key knowledge

At 0 g of water, the kettle still needs time to warm the element and transfer energy before boiling can occur.

🧠 Exam technique

For a 1-mark graph explanation, give a short physical reason. The examiners wanted a cause: the kettle is not instantly at full heating power at the start.

❌ Common errors

• “Because the graph is wrong” — not an explanation.
• “Because there is no water” — that doesn’t explain the intercept.
• Confusing the origin with the y-axis intercept.

✅ Correct answer

The rate of energy transfer per kg of water to the surroundings decreases as the mass of water increases.

or

The efficiency of the kettle increases as the mass of water increases.

💡 Key knowledge

A non-linear graph means the relationship is not a constant proportion. As more water is heated, the amount of energy lost to the surroundings changes relative to the mass of water.

🧠 Examiner insight

Top answers linked the shape of the graph to changing energy loss or changing efficiency. Simply saying “it’s not a straight line” is not enough.

❌ Common errors

• Saying “because the kettle is bigger/heavier” — irrelevant.
• “Because the mass changes” — too obvious and not explanatory.
• Talking about boiling point changing with mass; it does not.

📐 Step-by-step calculation

1. Use E = Pt
   Power = 2.6 kW = 2600 W
   Time = 120 s
   So, E = 2600 × 120 = 312000 J
2. Use E = mcΔT
   Mass = 0.80 kg
   Temperature change = 100 - 18 = 82 °C
   So, 312000 = 0.80 × c × 82
3. Rearrange:
   c = 312000 ÷ (0.80 × 82)
4. Calculate:
   c = 4756.1...
5. Round to 2 significant figures:
   c = 4800 J/kg °C

✅ Correct answer

4800 J/kg °C

This is the expected 2 s.f. answer.

💡 Key knowledge

• E = Pt gives the energy transferred by the kettle.
• E = mcΔT links energy, mass, specific heat capacity and temperature change.
• ΔT means temperature change, not final temperature.
• Specific heat capacity units: J/kg °C

🧠 How the marks are awarded

• 1 mark for using E = Pt .
• 1 mark for getting 312000 J .
• 1 mark for setting up 312000 = 0.80 × c × 82 .
• 1 mark for rearranging correctly.
• 1 mark for calculating 4756 .
• 1 mark for rounding to 4800 J/kg °C.

❌ Common calculation traps

• Using 2.6 instead of 2600 in the power equation.
• Using 80 kg instead of 0.80 kg.
• Using 18 as the temperature change instead of 82.
• Forgetting units or giving the answer in J/g °C.
• Rounding too early before finishing the calculation.

✅ Top-level features

• Used precise physics vocabulary.
• Gave a complete explanation, not just a statement.
• Showed all calculation steps clearly.
• Used correct unit conversions and 2 significant figures.

🧠 Quick revision checklist

• Current in the element → electrons collide with particles.
• Energy transfer → particles gain kinetic energy.
• Control variable → starting temperature of water.
• Graph shape → efficiency/energy loss changes.
• SHC calc → E = Pt , then E = mcΔT .