AQA GCSE Combined Science: Trilogy Physics Paper 2 (Higher), 2019: Question 4

✅ Correct answer

The force is downwards.

On the options shown, tick the down arrow.

💡 Key knowledge

• A current-carrying wire in a magnetic field experiences a force.
• This is called the motor effect.
• Use Fleming’s left-hand rule to find the direction.

🧠 Exam technique

• Always work from the magnetic field direction and current direction shown in the diagram.
• For a 1-mark multiple-choice question, you only need the correct tick.
• If asked to explain, name the rule or the motor effect.

❌ Common errors

• Choosing the force direction as if it were the current direction.
• Confusing the direction of the magnetic field with the force.
• Using Fleming’s right-hand rule instead of the left-hand rule.

✅ Correct answers

Any two of the following:

1. Reverse the direction of the current.
2. Reverse the direction of the magnetic field.

💡 Key knowledge

The direction of the force depends on:

• the direction of the current, and
• the direction of the magnetic field.

If either one changes direction, the force changes direction too.

🧠 Exam technique

• You need two separate ways, worth 1 mark each.
• Use clear phrasing: “reverse the current” or “reverse the magnetic field”.
• Don’t write vague answers like “change it” or “turn it around” unless you say what is being changed.

❌ Common errors

• Saying “increase the current” — this changes the size of the force, not its direction.
• Saying “change the wire length” — also changes size, not direction.
• Writing the same idea twice in different words. That only gains one mark.

📐 Calculations: step-by-step

1. Write down the equation: F = BIL
2. Convert the magnetic flux density: 360 mT = 0.360 T
3. Substitute the values:
   0.072 = 0.360 × I × 0.050
4. Rearrange:
   I = 0.072 ÷ (0.360 × 0.050)
5. Calculate:
   I = 4.0 A

✅ Correct answer

Current = 4.0 A

Marks are awarded for:
1 mark for converting 360 mT to 0.360 T
1 mark for correct substitution
1 mark for correct rearrangement
1 mark for final answer

💡 Key knowledge

• F = force in newtons (N)
• B = magnetic flux density in tesla (T)
• I = current in amps (A)
• L = length of wire in metres (m)

🧠 Exam technique

• Always convert mT to T before calculating.
• Show rearrangement clearly to access method marks.
• Include the unit A in your final answer.
• The answer 4.0 A should be given to a sensible level of precision.

❌ Common calculation traps

• Using 360 instead of 0.360 for B.
• Forgetting to divide by both B and L .
• Writing the final answer without units.
• Not showing working, which can lose method marks.

✅ Correct answer

A full-mark answer should include these ideas:

1. There is a magnetic field due to the permanent magnet.
2. A current in the coil creates a magnetic field and so the coil experiences a force.
3. The current is in opposite directions in the two sides of the coil, so the forces act in opposite directions.
4. This produces a turning effect so the coil rotates.
5. The split-ring commutator reverses the current every half-turn so the coil keeps turning in the same direction.

💡 Key knowledge

• Opposite forces on either side of the coil make it rotate.
• The split-ring commutator is essential because it keeps the motor turning continuously.
• Carbon brushes maintain electrical contact with the rotating commutator.

🧠 Exam technique

• Link the ideas in a logical order: magnetic field → current → forces → rotation → commutator.
• Use the words opposite directions and turning effect or moment.
• To describe a motor diagram, say what each part does, not just name the parts.

❌ Common errors

• Only naming parts like “magnet, coil, brushes” without explaining their function.
• Saying the coil rotates “because of electricity” without mentioning magnetic forces.
• Forgetting that the current is opposite in the two sides of the coil.
• Not mentioning the commutator, which is needed for continuous rotation.

Motor effect

A current-carrying wire in a magnetic field experiences a force.

Direction changes

Reverse the current or reverse the magnetic field to reverse the force.

Equation

F = BIL

Simple motor

Opposite forces on the coil cause rotation; the commutator keeps it turning the same way.