AQA GCSE Physics Physics Paper 2 (Higher), June 2025: Question 9

8 marks · Standard Demand difficulty · Short Answer

Answer questions about the structure, calculations, and operating principles of an electrical transformer.

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Question

Question 9 consists of three parts. Part 09.1 shows Figure 11, a schematic diagram of a transformer with an iron core, primary coil labeled Input, and secondary coil labeled Output, asking why the core is made from iron. Part 09.2 asks to calculate the primary potential difference in volts for a transformer with 345 primary turns, 6000 secondary turns, and a secondary potential difference of 400 kV. Part 09.3 asks to explain why an alternating current in the primary coil produces an alternating potential difference across the secondary coil.

Mark scheme

Show the mark scheme Mark scheme for Question 9. For 09.1: iron is easily magnetised (and demagnetised) or magnetically soft (1 mark). For 09.2: converts 400 kV to 400,000 V (1 mark), substitutes into equation Vp / 400000 = 345 / 6000 (1 mark), rearranges to Vp = 0.0575 * 400000 (1 mark), calculates answer 23,000 V (1 mark). For 09.3: ac in primary causes a changing magnetic field in core (1 mark), secondary coil cuts through changing magnetic field (1 mark), inducing a potential difference across secondary coil (1 mark). Total 8 marks.

How to answer it

Transformer Structure, Calculations & Induction

📋 What this question tests

This question evaluates your understanding of electromagnetism and transformers (AQA Specification 4.7.3.4):

  • Magnetic Materials: Properties of the soft iron core in a transformer.
  • Transformer Equation: Unit conversion (kV to V) and rearranging Vp / Vs = np / ns to calculate primary potential difference.
  • Electromagnetic Induction: Explaining step-by-step how an alternating current in the primary coil induces an alternating potential difference in the secondary coil.

Question 09.1

Material Properties of the Transformer Core (1 Mark)

✅ Mark Scheme Answer

Iron is easily magnetised (and demagnetised).

Also acceptable: "It is magnetically soft".

❌ Common Errors & Misconceptions

  • Writing "it conducts electricity": Electricity does not flow through the core from primary to secondary. Electricity conducting in the core actually causes wasted energy (eddy currents).
  • Just saying "it is magnetic": Too vague. Steel is also magnetic, but it stays permanently magnetised, making it useless for a transformer core.

Question 09.2

Transformer Equation Calculation (4 Marks)

📐 Step-by-Step Calculation

  1. Convert prefix units to standard units:
    Secondary potential difference ( Vs ) = 400 kV = 400 000 V [Mark 1]
  2. Recall and substitute into the transformer equation:
    Vp / Vs = np / ns
    Vp / 400 000 = 345 / 6000 [Mark 2]
  3. Rearrange to solve for Vp:
    Vp = (345 / 6000) × 400 000
    Vp = 0.0575 × 400 000 [Mark 3]
  4. State final value with correct units:
    Vp = 23 000 V (or 23 kV, but the question explicitly asked for volts) [Mark 4]

🧠 Exam Technique: Sanity Check

The primary has 345 turns and the secondary has 6000 turns. This is a step-up transformer, meaning:

  • Secondary voltage must be greater than primary voltage.
  • Since Vs = 400 000 V , Vp must be much smaller. 23 000 V makes complete sense!

❌ Calculation Traps

  • Forgetting the kilo prefix: Using 400 instead of 400 000 gives 23 V. You lose the conversion mark, although error-carried-forward (ECF) allows partial credit.
  • Inverting the ratio: Writing 6000 / 345 instead of 345 / 6000 will incorrectly give a larger primary voltage.

Question 09.3

Explaining Electromagnetic Induction (3 Marks)

✅ Model Answer (3/3 Marks)

A full-mark answer requires a logical chain of three physical steps:

  1. The alternating current (a.c.) in the primary coil creates a changing magnetic field in the iron core. [Mark 1]
  2. The secondary coil cuts through this changing magnetic field. [Mark 2]
  3. This induces an alternating potential difference across the ends of the secondary coil. [Mark 3]

💡 Key Knowledge: The Chain of Induction

Remember the sequence: Current → Changing Field → Cutting Field → Induced pd.

  • Transformers do not work with direct current (d.c.) because a steady d.c. produces a constant, stationary magnetic field that cannot induce a voltage.

❌ Examiner Pitfalls

  • Saying current "flows through the iron core": The coils are electrically insulated from each other and the core. The transfer of energy is entirely magnetic.
  • Missing the word "changing": Just saying "there is a magnetic field" loses mark 1. It must be alternating or changing.
  • Saying "creates" instead of "induces": Always use the technical term induced potential difference.

Topics

Physics · P7: Magnetism and Electromagnetism

Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.