AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2019: Question 4

✅ Correct answer

Less than 20 Ω

Mark: 1 mark for ticking less than 20 Ω .

💡 Key knowledge

• The lamp is in series with the rest of the circuit.
• The two resistors are in parallel.
• Two 10 Ω resistors in parallel have a resistance less than 10 Ω.
• So the total resistance is 10 Ω + less than 10 Ω, which is less than 20 Ω.

🧠 Exam technique

• Look at the circuit layout first: components on the same path are in series; components on branches are in parallel.
• You do not need to calculate the exact value here because the options are ranges.

❌ Common errors

• Thinking parallel resistors add to 20 Ω — they do not.
• Forgetting that the lamp must be added to the parallel combination.
• Choosing “exactly 20 Ω” because of the two 10 Ω resistors; this ignores the parallel effect.

✅ Correct answer

The resistance of the lamp is added to the total resistance of the resistors in parallel. The resistors in parallel have a total resistance of less than 10 Ω, so the total circuit resistance is less than 20 Ω.

Mark breakdown: 2 marks available — one for saying the lamp resistance is added in series, and one for stating the parallel part is less than 10 Ω.

💡 Key knowledge

• In a series part of a circuit, resistances add up.
• In a parallel part, the total resistance is smaller than the smallest resistor.
• That means two 10 Ω resistors in parallel give a resistance below 10 Ω.

🧠 Exam technique

• Use clear science words: series, parallel, total resistance.
• A full-mark explanation links the circuit structure to the resistance result.

❌ Common errors

• Saying only “the resistors add” — that is incomplete because the resistors are in parallel.
• Saying “parallel means bigger resistance” — this is wrong.
• Writing an answer that does not mention the lamp at all.

✅ Correct answer

Draw the thermistor symbol: a resistor symbol with a diagonal line through it and a small bar across the diagonal, as shown in the mark scheme.

Examiner note: 1 mark was awarded for the correct symbol only.

💡 Key knowledge

• A thermistor is a resistor whose resistance changes with temperature.
• The GCSE symbol is usually a resistor rectangle with a diagonal line and a small mark across it.

🧠 Exam technique

• If you are asked to draw a symbol, make it neat and recognisable.
• Use the standard circuit symbol, not a written label like “thermistor”.

❌ Common errors

• Drawing a normal resistor symbol instead of a thermistor.
• Adding extra details that make the symbol unclear.
• Labelling the component but not drawing the symbol.

✅ Correct answer

The current increased because the resistance of the thermistor decreased.

Mark breakdown: 2 marks — one for saying the current increased, and one for saying the resistance decreased.

💡 Key knowledge

• For an NTC thermistor, when temperature increases, resistance decreases.
• With the same battery voltage, lower resistance means higher current.
• Use Ohm’s law idea: if voltage stays the same and resistance goes down, current goes up.

🧠 Exam technique

• The best answers give both the change and the reason.
• Example: “The current increased because the thermistor resistance decreased.”

❌ Common errors

• Saying the resistance increases with temperature — for the thermistor in this question, it decreases.
• Just writing “because temperature increased” without linking to resistance and current.
• Mixing up current and potential difference.

✅ Correct answer

The resistor: the potential difference across the resistor becomes 0 V because there is a short circuit across it.

The lamp: the potential difference across the lamp increases because the current increases.

Mark breakdown: 4 marks total — 2 for the resistor explanation and 2 for the lamp explanation.

💡 Key knowledge

• Closing the switch creates an alternative low-resistance route.
• That means the resistor is bypassed, so the potential difference across it becomes 0 V.
• With less total resistance in the circuit, the current increases.
• The lamp then gets a larger share of the battery’s potential difference, so its p.d. increases.

📐 Step-by-step reasoning

1. The switch closes and makes a path with very low resistance.
2. Current takes the easier route through the switch.
3. No current goes through the resistor branch, so p.d. across the resistor is 0 V.
4. Total circuit resistance decreases.
5. Current in the circuit increases.
6. The lamp’s p.d. increases, so the lamp would get brighter.

🧠 Exam technique

• Answer both parts separately: resistor then lamp.
• Use causal language: because, so, therefore.
• For full marks, explain the link between resistance, current, and potential difference.

❌ Common errors

• Trap: saying the resistor p.d. just “decreases” without explaining that it becomes 0 V.
• Forgetting to mention the short circuit or alternative route.
• Saying the lamp p.d. decreases — the mark scheme says it increases.
• Not linking the lamp’s change to the increase in current.

💡 What to remember

• Series resistances add.
• Parallel resistance is lower than the smallest resistor.
• A thermistor’s resistance decreases when temperature increases.
• Lower resistance at the same voltage means higher current.
• A switch can create a short circuit, making the p.d. across another component 0 V.

🧠 How marks were awarded

• 04.1: 1 mark for the correct option only.
• 04.2: 2 separate explanation marks.
• 04.3: 1 mark for the correct thermistor symbol.
• 04.4: 2 marks for current change + resistance reason.
• 04.5: 4 marks for both components, with correct direction of change and reasoning.