AQA GCSE Combined Science: Trilogy Physics Paper 2 (Higher), 2020: Question 7

💡 Key knowledge

• A current in a wire creates a magnetic field around the wire.
• In a magnetic field, the wire experiences a force.
• The force on the wire and the force on the balance are equal and opposite.
• Use F = BIL when the current is at right angles to the magnetic field.

🧠 Exam technique

• Link the balance reading to a change in force.
• Use the graph gradient, not just one point, for part (c).
• Show unit conversions clearly: g → kg, then use weight = mass × g .
• Give the final value to a sensible number of significant figures.

✅ Correct answer

The downward force on the balance increased, so the wire must have experienced an equal and opposite upward force.

💡 Key knowledge

• When the switch is closed, current flows in the wire.
• The wire in the magnetic field experiences a magnetic force.
• By Newton’s third law, the force on the wire and the force on the balance are equal and opposite.

🧠 How marks are awarded

• 1 mark for saying the balance reading went up because the downward force on the balance increased.
• 1 mark for stating the wire had an equal and opposite upward force.

❌ Common errors

• Just saying “the wire was pushed up” without linking to the balance.
• Saying “the magnet pushed the wire up” without explaining the equal and opposite force.
• Talking about “weight” instead of the force change caused by the current.

📐 Calculations

1. Find the difference in mass: 254.8 g − 252.3 g = 2.5 g
2. Convert to kilograms: 2.5 g = 0.0025 kg
3. Calculate the force using F = mg
4. F = 0.0025 × 9.8 = 0.0245 N

Accept: 0.02375 N if using 2.5/1000 × 9.8, depending on rounding.

✅ Correct answer

Work out the difference between the two masses, convert it to kg, then multiply by gravitational field strength to get the force.

🧠 Exam technique

• Use the change in mass, not the total mass.
• State the equation F = mg .
• Include units: g, kg, N.

❌ Common errors

• Using 254.8 g on its own instead of the difference.
• Forgetting to convert grams to kilograms.
• Writing the answer in grams, not newtons.

📐 Calculations: step-by-step

1. Find the gradient of the force-current graph.
2. Use two well-separated points on the straight line, for example:
   (0.02 A, 0.0 N) and (0.70 A, 0.0210 N)
3. Calculate gradient:
   (0.0210 − 0.0) ÷ (0.70 − 0.02) = 0.031
4. Use F = BIL so gradient = BL
5. Substitute 0.031 = B × 0.125
6. Rearrange:
   B = 0.031 ÷ 0.125 = 0.248 T
7. Final answer: B = 0.25 T

✅ Correct answer

Magnetic flux density = 0.25 T

💡 Key knowledge

• The graph is a straight line through the origin, showing force is directly proportional to current.
• For a wire at right angles to the field: F = BIL
• The gradient of an F – I graph is BL .
• Given L = 0.125 m , you can find B .

🧠 Exam technique

• Use the gradient of the best-fit line, not a single plotted point.
• Choose points far apart to reduce error.
• Write the substitution clearly so the examiner can follow the method.
• Round only at the end, and keep units throughout.

❌ Common errors

• Using a pair of data points instead of the gradient: this can limit marks.
• Forgetting that gradient = force ÷ current.
• Using 0.125 as the gradient by mistake.
• Giving the answer in the wrong unit or with too many premature rounding steps.

Part (a)

The downward force on the balance increased, so the wire must have experienced an equal and opposite upward force.

Part (b)

Find the difference in mass, convert it to kg, then multiply by gravitational field strength to get the force.

Part (c)

Gradient = 0.031 , then 0.031 = B × 0.125 , so B = 0.25 T .