AQA GCSE Physics Physics Paper 1 (Higher), November 2020: Question 7

9 marks · Standard Demand difficulty · Short Answer

Analyze temperature changes in an electric kettle, determine the mass of water using specific heat capacity and graph data, and explain how to calculate useful power output from a temperature-time graph.

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Question

Figure 10 is a line graph showing temperature in degrees Celsius on the y-axis against time after the kettle was switched on in seconds on the x-axis. The temperature starts at around 22 °C, stays nearly constant for the first 10 seconds, then increases steadily in a straight line before curving slightly as it approaches 100 °C at 100 seconds. Three sub-questions follow: 07.1 asks why temperature does not immediately increase (1 mark); 07.2 asks to determine the mass of water given energy transferred is 155000 J and specific heat capacity is 4200 J/kg °C, giving the answer to 2 significant figures (5 marks); 07.3 asks to explain how the straight section of Figure 10 can be used to calculate useful power output (3 marks).
Question text

07 An electric kettle was switched on.

Figure 10 shows how the temperature of the water inside the kettle changed.

Figure 10

07.1 When the kettle was switched on the temperature of the water did not immediately

start to increase.

Suggest one reason why.

[1 mark]

07.2 The energy transferred to the water in 100 seconds was 155 000 J.

specific heat capacity of water = 4200 J/kg °C

Determine the mass of water in the kettle.

Use Figure 10.

Give your answer to 2 significant figures.

[5 marks]

Mass of water (2 significant figures) = kg

07.3 The straight section of the line in Figure 10 can be used to calculate the useful power

output of the kettle.

Explain how.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme details answers for questions 07.1, 07.2, and 07.3. For 07.1, it accepts that the heating element takes time to heat up (1 mark). For 07.2, it shows working to find temperature change Delta Theta = 78 °C from the graph, substitute into energy equals mass times specific heat capacity times temperature change, rearrange for mass, calculate 0.4731 kg, and round to 0.47 kg (5 marks). For 07.3, it awards marks for identifying gradient as rate of temperature increase or Delta Theta over t, stating power equals energy over time or Pt = mc Delta Theta, and showing power equals gradient times mc (3 marks).

Question 7

AO / Spec.

Question Answers Extra information Mark

Ref.

07.1 the heating element of the allow the kettle takes time to 1 AO3

kettle takes time to heat up heat up 4.1.1.3

07.2 Δϴ = 78 (°C) 1 AO2

4.1.1.3

155 000 = m × 4200 × 78 allow a correct substitution using 1 4.3.2.2

an incorrect value of Δϴ

155 000 allow a correct rearrangement 1

m = using an incorrect value of Δϴ

4200 ×78

m = 0.4731 (kg) allow a correct calculation of 1

mass using an incorrect value of

Δϴ

m = 0.47 (kg) 1

07.3 ∆θ allow gradient = rate of 1 AO1

Gradient = temperature increase 4.1.1.3

t

allow calculation of gradient 4.3.2.2

Pt = mcΔϴ 1 4.1.1.4

P = gradient × mc 1

Total 9

How to answer it

Electric Kettle Heating Analysis

What this question tests: This question assesses your ability to interpret temperature-time graphs, apply the specific heat capacity equation ( E = m × c × Δθ ), rearrange formulas, work with significant figures, and link graphical gradients to physical rates of energy transfer.
Question 07.1

Initial Temperature Delay

When the kettle was switched on, the temperature of the water did not immediately start to increase. Suggest one reason why. (1 mark)

✅ Correct Answer

The heating element of the kettle takes time to heat up (or the kettle takes time to heat up).

💡 Key Knowledge

Energy isn't transferred directly to the water instantly. The metal heating element must absorb thermal energy and reach a higher temperature than the water before heat conduction can efficiently begin.

Question 07.2

Calculating Mass from Specific Heat Capacity

The energy transferred to the water in 100 seconds was 155,000 J. Specific heat capacity of water = 4,200 J/kg °C. Determine the mass of water in the kettle using Figure 10. Give your answer to 2 significant figures. (5 marks)

📐 Step-by-Step Calculation

  1. Find initial and final temperature from Figure 10:
    Initial temperature at t = 0 s is 22 °C .
    Final temperature at t = 100 s is 100 °C .
  2. Calculate temperature change ( Δθ ):
    Δθ = 100 - 22 = 78 °C
  3. Substitute values into the specific heat capacity formula:
    155000 = m × 4200 × 78
  4. Rearrange to solve for mass ( m ):
    m = 155000 / (4200 × 78) = 0.4731 kg
  5. Round to 2 significant figures:
    m = 0.47 kg

❌ Common Errors

  • Reading the initial temperature as 0 °C instead of 22 °C where the curve actually starts.
  • Forgetting to round to the requested 2 significant figures, losing the final accuracy mark.

🧠 Exam Technique

Always write out the formula in full before substituting numbers. Even if your final calculation goes wrong, method marks are awarded for correct substitution and rearrangement!

Question 07.3

Relating Graph Gradients to Power

The straight section of the line in Figure 10 can be used to calculate the useful power output of the kettle. Explain how. (3 marks)

✅ Correct Answer (Mark Scheme Breakdown)

  • Mark 1: Gradient = Δθ / t (or rate of temperature increase).
  • Mark 2: Power P = E / t combined with E = mcΔθ gives Pt = mcΔθ .
  • Mark 3: Therefore, P = gradient × mc .

💡 Examiner Insight

Top-level responses immediately recognised that the linear (straight) section represents a constant rate of heating. By linking the gradient of the temperature-time graph to the power equation, students unlocked full marks by showing that power depends directly on how steep the temperature rise is relative to the mass and specific heat capacity.

Topics

Physics · Required Practicals · P1: Energy · P3: Particle Model of Matter · Required Practicals

Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 1 (Higher), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.