Switching on a current of 3.00 A changes the balance reading by 2.80 g. The active wire length is 0.120 m and the wire is perpendicular to the field. Calculate the flux density. Use g = 9.81 N kg−1.
[3 marks]
Total for this question: 3
A-level Physics required practicals
Force on a current-carrying wire — top-pan balance
AQA 3.7.5.1 · RP10
A-level Physics (7408) · Required practical 10 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.
Board and spec code confirmed against AQA 7408 · registry checked 2026-07-11How this checking works
Investigate how magnetic force depends on current, flux density and active wire length by measuring the corresponding change in a top-pan balance reading.
Apparatus
- Top-pan balance and a stable magnet assembly that produces an approximately uniform field
- Straight rigid wire or conducting rod held horizontally in the field
- Low-voltage dc supply, switch, ammeter and variable resistor
- Metre rule, set square and clamps
- Field probe or different magnet spacings where flux density is varied
Apparatus & techniques (AT)
AT a · analogue measurement
Measure active wire length and alignment from metre-rule and set-square readings at the magnet gap.
AT b · digital instruments
Use the digital balance and ammeter to record small mass changes and current values.
AT f · circuit construction
Construct a current-limited series circuit containing the wire, ammeter, variable resistor and switch.
Safety
Hazard
A high current can heat the wire and resistor.
Control
Use a current-limiting resistor, stay within component ratings, close the switch only for readings and allow cooling between runs.
Hazard
Strong magnets can trap fingers, attract loose steel objects or upset the balance.
Control
Move magnets with controlled grips, keep ferromagnetic objects away and secure the assembly before zeroing.
Method
- 1Place the magnet assembly centrally on the balance and zero the balance. Clamp the straight wire so its active section is horizontal, perpendicular to the field and clear of the magnet.
- 2Construct the series circuit with the ammeter, variable resistor, switch and wire. Check the current limit and the expected force direction before closing the switch.
- 3Record the balance reading with no current, then pass a measured current briefly and record the new stable reading. Convert the change in indicated mass to force using F = Δmg.
- 4Repeat for several currents while keeping flux density, active length and angle fixed. Reverse the current as a direction check if the apparatus permits it safely.
- 5Repeat the investigation by varying active length or flux density one at a time, re-zeroing the balance and measuring the field-region boundaries rather than the full wire length.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 2: Editorial focus: select a measurable current range, define active length and control the wire–field geometry.
- CPAC 3: Editorial focus: limit current and secure the magnet, wire and balance against heating or movement.
- CPAC 4: Editorial focus: record paired balance readings, current and active length with appropriate resolution.
- CPAC 5: Editorial focus: convert mass change to force, extract B from a gradient and evaluate field non-uniformity.
Variables
Independent
Current, flux density or active wire length, varied one at a time
Dependent
Magnetic force inferred from the change in balance reading
Control
- The two quantities from I, B and L that are not being varied
- Wire orientation at 90° to the field and its vertical position in the gap
- The same balance zero and magnet assembly
- Short current-on time to limit heating
Results & processing
- Plot force F against the chosen independent variable I, B or L. A straight line through the origin supports F = BIL when the wire is perpendicular to the field.
- For an F–I graph, gradient = BL; divide by the measured active length to determine B.
- Use the sign of the balance change and Fleming's left-hand rule to check that the inferred force direction is physically consistent.
Analysis skills
- Convert balance mass change to force using F = Δmg with mass in kilograms.
- Use an F–I gradient to determine B from gradient/L, or test proportionality against B or L.
- Use force-direction reasoning to distinguish the force on the wire from the equal and opposite force on the magnet and balance.
Uncertainty
Sources
- Balance resolution, zero drift and vibration
- Ammeter resolution and current fluctuations caused by heating
- Uncertain active-length boundary and non-uniform fringe field
- Wire not exactly perpendicular to the magnetic field
Calculations
- Convert the balance resolution to a force uncertainty by multiplying its kilogram value by g.
- Combine percentage uncertainties in gradient and active length when calculating B.
- Use maximum and minimum acceptable graph gradients where error bars justify them.
Interpretation
- Repeating paired readings reduces random balance scatter but does not correct a consistently misidentified active length.
- A non-zero intercept can indicate balance offset, lead forces or misalignment rather than a failure of proportionality alone.
Exam angles
- Convert a signed balance change into magnetic-force magnitude and direction.
- Determine flux density from an F–I graph rather than a single reading.
- Explain why field uniformity and active-length boundaries matter.
- Use Fleming's left-hand rule consistently for current, field and force directions.
Where students lose marks
Using the full wire length instead of the section inside the effective magnetic field.
Fix: Measure the active length between the field-region boundaries and acknowledge that fringe field makes the boundary uncertain.
- Examiner report: P3-24 · PDF p. 5
Treating a balance change in grams as though it were already a force in newtons.
Fix: Convert grams to kilograms and calculate F = Δmg before plotting or using F = BIL.
Choosing a current direction that produces the opposite balance change from the stated force direction.
Fix: Apply Fleming's left-hand rule to the conventional current and field directions before energising the circuit, then use current reversal as a check.
Improve the method
- Take paired current-off and current-on readings at each setting to correct for slow balance drift.
- Use both current directions and compare equal-magnitude balance changes to expose a zero offset.
- Map or measure flux density across the active region and keep the wire central and level.
Source references
- Specification: AQA 7408 specification §3.7.5.1 · PDF p. 39
Try it — exam-style
Medium
ORIGINAL
Easy
ORIGINAL
The gradient of a graph of force against current is 0.0108 N A−1. The active wire length is 0.150 m. Determine the flux density.
[2 marks]
Total for this question: 2
Medium
ORIGINAL
An active length is measured as 120 ± 1 mm. Calculate its percentage uncertainty and state why using the full 300 mm wire length would be invalid.
[3 marks]
Total for this question: 3
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on force on a current-carrying wire — top-pan balance?
Balance reading, force direction and graph gradient form one chain — practise keeping every link explicit.