AQA GCSE Combined Science: Trilogy Physics Paper 2 (Higher), June 2025: Question 5

✅ Correct Answer

Random error (or random)

💡 Key Knowledge

Pure melting ice is known to be at 0.0 °C.

• Random errors cause readings to fluctuate unpredictably above and below the true value.
• Systematic errors (e.g. zero errors) cause readings to differ from the true value by a consistent amount in the same direction every time.

❌ Common Errors

• Writing "systematic error" or "zero error" — incorrect because the values spread both above (+0.5 °C) and below (-0.5 °C) the true value.
• Writing vague terms like "human error" (never accepted in AQA mark schemes).

🧠 Exam Technique

Look at the pattern: if the values are scattered around the true value (both + and -), it is random. If every reading was off by exactly +0.5 °C, it would be systematic.

📐 Step-by-Step Calculation

1. Identify Max & Min values:
   Maximum = +0.5 °C
   Minimum = -0.5 °C
2. Calculate Range:
   Range = Max - Min = 0.5 - (-0.5) = 1.0 °C
3. Calculate Uncertainty:
   Uncertainty = Range ÷ 2 = 1.0 ÷ 2 = ±0.5 °C

❌ Common Calculation Traps

• Sign Error: Calculating 0.5 - 0.5 = 0 instead of subtracting a negative: 0.5 - (-0.5) = 1.0 .
• Forgetting to divide by 2: Giving the full range (1.0 °C) instead of the uncertainty.
• Alternative method: Calculating the mean (0.0 °C) and finding the maximum deviation ( 0.5 - 0.0 = 0.5 °C ) is also accepted by the examiner.

✅ Mark Scheme Requirements

• Scale (1 mark): Linear scale on y-axis of 10 °C per large square (1 cm = 10 °C), starting from 0 or sensible value, using over 50% of grid.
• Plotting (2 marks): All points plotted correctly within ±½ small square. (1 mark if 3 or 4 points correct).
• Line of Best Fit (1 mark): A single smooth curve that levels off at around 76 °C, clearly ignoring the anomaly at (8 min, 65 °C).

🧠 Exam Technique: Spotting Anomalies

Always inspect the trend in the table before plotting:

18 → 55 → 68 → 73 → 65 → 76 → 76

Notice the temperature steadily rises, drops suddenly at 8 mins (65 °C), then climbs to 76 °C. 8 minutes is an anomalous result! Do not force your curve through it.

❌ Common Errors

• "Dot-to-dot" ruler lines: Never join the points with straight segments when data shows a continuous heating curve.
• Including the anomaly: Drawing a dip or wave to hit (8, 65) forfeits the line mark.
• Awkward scales: Choosing increments of 3, 7, or 15 makes plotting accurately almost impossible and loses the scale mark.

💡 Diagram Description for Graph

The y-axis should go from 0 to 80 (or 10 to 80). The plotted points form a steep initial gradient that gradually flattens out (asymptotes) towards 76 °C. The point at (8, 65) sits well below the curve and is circled or ignored by the best-fit line.

📐 Step-by-Step Calculation

1. Convert time units to seconds:
   Target unit is °C/s .
   Time = 2.0 × 60 = 120 s
2. Calculate temperature change (Δθ):
   Change = 63 - 18 = 45 °C
3. Calculate rate of change:
   Rate = Δθ ÷ time = 45 ÷ 120 = 0.375 °C/s

❌ Common Calculation Traps

• Missing the unit conversion: Dividing by 2 instead of 120 gives 22.5 °C/min , losing 2 marks if uncorrected. Always check whether the question specifies /s or /min!
• Wrong starting value: Using 0 °C instead of initial room temperature (18 °C). Change is 63 - 18 = 45 , not 63.

✅ Model Answer (4 Distinct Marking Points)

1. Black surface absorption: Black surfaces are good absorbers of infrared radiation (better than white).
2. White surface absorption: White surfaces are poor absorbers (or good reflectors) of infrared radiation.
3. Increasing emission: As the temperature of the cubes increases, they emit infrared radiation at a greater rate.
4. Thermal equilibrium: At constant temperature, the cubes emit infrared radiation at the same rate that they absorb it.

💡 Core Physics Principles

• Matt Black: Best emitter, best absorber, worst reflector.
• Shiny White / Silver: Worst emitter, worst absorber, best reflector.
• Net Energy Transfer:
  • Rate absorbed > Rate emitted → Temperature rises.
  • Rate absorbed = Rate emitted → Temperature remains constant (Dynamic Equilibrium).

❌ Examiner Warning: Misconceptions

• "Heat" vs "Infrared": The mark scheme explicitly states: "ignore references to heat throughout". Always write infrared or radiation.
• "Black attracts heat": Surfaces do not "attract" radiation; they absorb it.
• Confusing emission with reflection: White surfaces reflect incoming radiation; black surfaces absorb incoming radiation and emit when hot.
• Incomplete equilibrium explanation: Saying "the cube runs out of heat" or "it reaches room temperature" scores 0. You must compare the rate of emission with the rate of absorption.

🧠 Exam Technique: Two-Part Prompts

The prompt gives two observations to explain:

• Observation 1 (initial rates) → addressed by points 1 & 2 (colour properties).
• Observation 2 (stopped changing temperature) → addressed by points 3 & 4 (temperature-dependent emission and balanced rates).

Structuring your answer with two sub-headings ensures you hit all 4 marks!