AQA GCSE Physics Physics Paper 2 (Higher), June 2022: Question 7

14 marks · Standard Demand difficulty · Short Answer

Identify transformer components, calculate output potential difference, explain transformer operation, determine magnetic force direction, and calculate cable length using the motor effect equation.

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Question

Figure 12 shows a transformer consisting of a primary coil A with 200 turns and input pd, a secondary coil B with 1200 turns and output pd, wound around a central core C. Parts 07.1 to 07.3 ask for the identification of parts A, B, and C, calculation of output pd from an input of 230 V, and an explanation of why an alternating current is induced in the output. Figure 13 shows an overhead electrical cable carrying a current of 50 A from pole A to pole B across Earth's horizontal magnetic field directed to the right. Parts 07.4 to 07.6 ask for the direction of the magnetic force, a calculation of the cable length given a force of 0.045 N and magnetic flux density of 60 microteslas, and an assumption made.
Question text

07 The National Grid uses transformers to change potential difference (pd).

Figure 12 shows a transformer.

Figure 12

07.1 Identify the parts of the transformer labelled in Figure 12.

[2 marks]

A

B

C

07.2 There is an alternating input pd of 230 V.

Determine the output pd.

Use the Physics Equations Sheet.

[3 marks]

25 Output pd =

V

07.3 The input pd causes an alternating current.

Explain why there is an alternating current in the output when the transformer is

*24* connected to a circuit.

[3 marks]

Figure 13 shows a large cable supported by two wooden poles. The cable is

connected to an electricity supply.

Figure 13

07.4 There is a force on the cable due to the Earth’s magnetic field when the current is in

the direction A to B.

What is the direction of this force?

[1 mark]

Tick ( ) one box.

Down

Left

Right

Up 27

07.5 The cable experiences a force of 0.045 N due to the Earth’s magnetic field.

magnetic flux density = 60 μT

current = 50 A

Calculate the length of the cable between A and B.

Use the Physics Equations Sheet.

[4 marks]

Length = m

07.6 State one assumption you made in your calculation.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for Question 7: 07.1 awards 1 mark for A primary coil and B secondary coil, and 1 mark for C iron core. 07.2 shows 230/Vs = 200/1200, rearranged to Vs = (1200 * 230)/200, giving 1380 V (3 marks). 07.3 awards 3 marks for: alternating current causes changing magnetic field in primary coil, creates changing magnetic field in core, induces alternating potential difference/current across secondary coil. 07.4 gives 1 mark for 'down'. 07.5 gives 4 marks: converting 60 microteslas to 60 x 10^-6 T, substituting into F = BIl, rearranging l = 0.045 / (60 x 10^-6 * 50), giving 15 m. 07.6 gives 1 mark for the assumption that the wire is at right angles to the magnetic field, cable is straight, or field/current is uniform. Total: 14 marks.

Question 7

AO /

Question Answers Extra information Mark

Spec. Ref.

07.1 A primary coil 1 AO1

and 4.7.3.4

B secondary coil

C iron core 1

AO /

Spec. Ref.

07.2 230 200 1 AO2

= 4.7.3.4

Vs 1200

1200 × 230 1

Vs =

Vs = 1380 (V) 1

AO /

Spec. Ref.

07.3 (the alternating current causes) 1 AO2

a changing magnetic field 4.7.3.4

around the primary (coil)

creates magnetic field that allow creates a changing 1

changes direction in the core magnetic field in the core

this induces an alternating 1

potential difference across the

secondary (coil causing an

alternating current)

AO /

Spec. Ref.

07.4 down 1 AO2

4.7.2.2

AO /

Spec. Ref.

07.5 B = 60 × 10-6 (T) 1 AO2

4.7.2.2

0.045 = 60 × 10-6 × 50 × l allow correct substitution of 1

incorrectly / not converted value

of B

0.045 allow correct rearrangement 1

l = -6 using an incorrectly / not

60 × 10 × 50

converted value of B

l = 15 (m) allow a correct calculation using 1

an incorrectly / not converted

value of B

AO /

Spec. Ref.

07.6 the wire / force is at right angles allow the current is constant 1 AO3

to the magnetic field allow the cable is straight 4.7.2.2

allow the field is uniform

allow the force is constant

Total Question 7 14

How to answer it

Transformers & Magnetic Forces in Cables

OVERVIEW & SPEC CHECK

What this question tests

This question assesses core understanding of electromagnetism: identifying the parts and working principles of a transformer, applying the transformer turns ratio equation, using Fleming’s left-hand rule to find the direction of magnetic force on a current-carrying wire, rearranging the force equation F = B × I × l with metric prefix conversions (microteslas, μT), and evaluating physics modelling assumptions.

PART 07.1 • 2 MARKS

Transformer Structure

Identify the parts of the transformer labelled in Figure 12

✅ Correct Answer

  • A: primary coil
  • B: secondary coil [1 mark for both A & B]
  • C: iron core [1 mark]

🧠 Exam Technique

Notice how marks are bundled! To get the first mark, you must have both primary and secondary coils correct. Identifying just one coil earns zero marks.

Always specify iron core, not just "metal core", as iron is easily magnetised and demagnetised.

PART 07.2 • 3 MARKS

The Transformer Equation

Determine the output potential difference (pd)

📐 Step-by-Step Calculation

  1. State the formula:
    Vp / Vs = Np / Ns
  2. Substitute values:
    230 / Vs = 200 / 1200 [1 mark]
  3. Rearrange to make Vs the subject:
    Vs = (1200 × 230) / 200 [1 mark]
  4. Calculate final value:
    Vs = 1380 V [1 mark]

❌ Common Errors

  • Inverting the ratio: Mixing up Np and Ns leads to dividing by 6 instead of multiplying by 6.
  • Sanity check: This is a step-up transformer because it has more turns on the secondary coil ( 1200 > 200 ). Therefore, the output pd must be greater than the input pd ( 1380 V > 230 V ).
PART 07.3 • 3 MARKS

How a Transformer Works

Explain why there is an alternating current in the output circuit

✅ Model 3-Step Explanation

  1. The alternating current creates a changing magnetic field around the primary coil. [1 mark]
  2. This creates a changing magnetic field in the iron core (which constantly changes direction). [1 mark]
  3. This magnetic field passes through the secondary coil and induces an alternating potential difference (and current) across the secondary coil. [1 mark]

💡 Key Knowledge

Transformers only work with alternating current (a.c.), never direct current (d.c.).

  • A constant direct current creates a steady, unchanging magnetic field.
  • Without a changing magnetic field cutting through the secondary coil, no pd can ever be induced!
  • Remember the magic word: induces.
PART 07.4 • 1 MARK

Fleming's Left-Hand Rule

Determine the direction of the force on the cable

✅ Correct Box to Tick

[ ✓ ] Down

[1 mark]

🧠 Exam Technique: Three Finger Rule

  • First Finger = Field (pointing to the right, in direction of Earth's field lines).
  • seCond Finger = Current (pointing into the page / along the wire from A to B).
  • THumb = THrust / Force (points Down).
PART 07.5 • 4 MARKS

Force on a Conductor Calculation

Calculate the length of the cable between A and B

📐 Step-by-Step Calculation

  1. Convert microteslas (μT) to teslas (T):
    B = 60 μT = 60 × 10−6 T [1 mark]
  2. Substitute into F = B × I × l :
    0.045 = (60 × 10−6) × 50 × l [1 mark]
  3. Rearrange to solve for length ( l ):
    l = 0.045 / (60 × 10−6 × 50)
    l = 0.045 / 0.003 [1 mark]
  4. Calculate final answer:
    l = 15 m [1 mark]

❌ Common Unit Trap

Prefix μ (micro): Many students forget that μ = 10−6 and either leave it as 60 or multiply by 10−3 (milli).

If you forgot to convert the unit, the mark scheme still awards compensation marks for correct substitution and rearrangement, giving an answer of 1.5 × 10−5 m (max 3/4 marks).

PART 07.6 • 1 MARK

Modelling Assumptions

State one assumption you made in your calculation

✅ Any One Acceptable Answer

  • The wire / force is at right angles (90°) to the magnetic field.
  • The cable is straight.
  • The magnetic field is uniform.
  • The current is constant.
[1 mark]

💡 Why is this an assumption?

The equation F = B × I × l only applies directly when the conductor is perpendicular (90°) to the magnetic field lines. In reality, power cables sag in a curve (catenary), meaning the angle varies along the wire!

Topics

Physics · P7: Magnetism and Electromagnetism

Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 2 (Higher), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.