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

💡 Key facts

• Ohm’s law: V = I R
• Energy transferred: E = P t
• Thermistor resistance decreases when temperature increases.

🧠 What examiners want

• Correct formula first.
• Substitute values with units.
• Show rearrangement and final answer with units.

❌ Common traps

• Using the wrong version of the equation.
• Forgetting to convert minutes to seconds.
• Saying “temperature increases so current decreases” for a thermistor.

✅ Correct answer

V = I R

1 mark for selecting the correct equation.

💡 Key knowledge

This is Ohm’s law. It links voltage, current and resistance in a circuit.

❌ Common errors

• I = V R is wrong.
• R = I V is wrong.
• V = I² R is the power equation for heating, not this one.

📐 Calculation steps

1. Write the equation: V = I R
2. Substitute values: 3.9 = 0.12 × R
3. Rearrange: R = 3.9 ÷ 0.12
4. Calculate: R = 32.5 Ω

✅ Correct answer

R = 32.5 Ω

Accept 33 Ω for rounding.

3 marks: equation, substitution/rearrangement, final answer with unit.

🧠 Exam technique

• Use the numbers exactly as given: V = 3.9 , I = 0.12 .
• Always include the unit Ω .
• If you round, 33 Ω is fine.

❌ Common errors

• Dividing the wrong way round: 0.12 ÷ 3.9
• Using R = I / V
• Forgetting the unit or writing V instead of Ω

✅ Correct answer

E = P t

1 mark for the correct relationship.

💡 Key knowledge

Energy transferred = power × time.

❌ Common errors

• E = P / t is incorrect.
• P = E t is incorrect.

📐 Calculation steps

1. Use E = P t
2. Convert time to seconds: 2500 min × 60 = 150 000 s
3. Substitute: E = 0.46 × 150 000
4. Calculate: E = 69 000 J

✅ Correct answer

69 000 J

Equivalent to 6.9 × 10⁴ J .

3 marks: time conversion, substitution, final answer.

🧠 Exam technique

• Power is in watts, so time must be in seconds.
• Show the conversion clearly to gain method marks.
• Use the given power: 0.46 W .

❌ Common errors

• Using 2500 instead of converting to seconds.
• Multiplying by 2500 and getting the wrong unit.
• Writing the answer in Wh or kJ without being asked.

✅ Correct answer

Thermistor

1 mark for naming the component.

💡 Key knowledge

A thermistor is a temperature-dependent resistor, often used in sensors.

🧠 Exam technique

If you recognise a resistor symbol with a diagonal line, think “thermistor” for GCSE circuits.

✅ Correct answer

The current will increase because the resistance decreases.

2 marks: one for current increasing, one for explaining resistance decreases.

💡 Key knowledge

• For a thermistor, higher temperature means lower resistance.
• With constant potential difference, lower resistance means higher current.
• Use V = I R : if V stays the same and R goes down, I goes up.

❌ Common errors

• Saying the current decreases.
• Not linking the answer to resistance.
• Talking only about temperature without using circuit ideas.

💡 Mark pattern

• Simple recall questions: 1 mark for the exact equation or component name.
• Calculation questions: marks for correct method and correct final answer.
• Explanation questions: marks for the correct trend plus the reason.

🧠 Top-score habits

• Write the equation before numbers.
• Convert units first.
• Use short, direct science sentences in explanations.

❌ Biggest traps

• Mixing up V = I R with E = P t .
• Forgetting 60 seconds in a minute.
• Getting the thermistor direction backwards.