The induced-emf amplitude is 0.840 V when the search-coil normal is parallel to the field. Predict the amplitude when the normal is at 60.0° to the field.
[2 marks]
Total for this question: 2
A-level Physics required practicals
Search coil — flux linkage and angle
AQA 3.7.5.3 · RP11
A-level Physics (7408) · Required practical 11 — 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
Use a search coil and oscilloscope to investigate how the induced-emf amplitude, as a proxy for changing flux linkage, depends on the angle between the coil normal and a magnetic field.
Apparatus
- Large circular field coil and low-voltage sinusoidal supply or signal generator
- Small search coil with rigid leads and an angular pointer
- Oscilloscope and connecting leads
- Protractor or calibrated rotary mount
- Clamp stands and a ruler for centring and alignment
Apparatus & techniques (AT)
AT a · analogue measurement
Measure coil angle and central alignment using a protractor, pointer and ruler.
AT b · digital instruments
Read induced-emf amplitude and drive settings from digital oscilloscope or signal-generator displays.
AT f · circuit construction
Construct and check the field-coil drive and search-coil measurement circuits before energising them.
AT h · signal generator/oscilloscope
Use a signal generator to produce the alternating field and an oscilloscope to measure the search-coil signal.
Safety
Hazard
The driven field coil can heat if the current is too large.
Control
Use a low-voltage supply, current limit within the coil rating, switch off between runs and stop if the coil warms appreciably.
Hazard
Trailing leads and unstable clamps can pull apparatus over.
Control
Secure both coils, route leads away from the bench edge and rotate only the intended search-coil mount.
Method
- 1Connect the field coil to a fixed-frequency sinusoidal supply or signal generator and check that the current is within the coil rating.
- 2Clamp the search coil at the centre of the field coil and connect it to the oscilloscope. Define θ as the angle between the search-coil normal and the field direction.
- 3Set θ = 0° with the coil normal parallel to the field. Adjust the oscilloscope so the sinusoidal trace is stable and record one consistent amplitude measure.
- 4Rotate the search coil through a range of measured angles without shifting its centre. Record induced-emf amplitude at each angle while keeping drive frequency and amplitude unchanged.
- 5Repeat angle and amplitude readings, including orientations near the expected maximum and zero, and record whether reversing the normal changes phase rather than amplitude magnitude.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 2: Editorial focus: define the angle convention, preserve coil centring and choose oscilloscope settings that resolve the signal.
- CPAC 3: Editorial focus: operate the driven coil within its current rating and secure coils and leads.
- CPAC 4: Editorial focus: record angle and one consistent amplitude measure across a sufficient angular range.
- CPAC 5: Editorial focus: apply cosine linearisation and diagnose systematic angular or pickup offsets.
Variables
Independent
Angle between the search-coil normal and the magnetic field
Dependent
Induced-emf amplitude displayed on the oscilloscope
Control
- Field-coil drive frequency and amplitude
- Search coil, field coil and their separation/centres
- Oscilloscope voltage scale and the chosen amplitude definition
- Nearby magnetic materials and lead arrangement
Results & processing
- Calculate cos(θ) for each orientation and plot induced-emf amplitude against cos(θ). A straight line supports the flux-linkage dependence NΦ = NBA cos(θ) for fixed sinusoidal field amplitude and frequency.
- The amplitude magnitude is greatest when the coil normal is parallel to the field and approaches zero when the coil plane is parallel to the field.
- A non-zero amplitude at the nominal 90° position can indicate angular zero error, imperfect centring or pickup in the leads.
Analysis skills
- Transform angle data using cos(θ) and test for a linear induced-emf-amplitude relationship.
- Relate maximum and zero orientations to the coil normal rather than the coil plane.
- Use intercept and residual patterns to diagnose systematic angular offset or stray pickup.
Uncertainty
Sources
- Protractor resolution and finite thickness of the coil or pointer
- Oscilloscope voltage resolution, trace thickness and electrical noise
- Off-centre rotation and field non-uniformity
- Drift in signal-generator amplitude or frequency
Calculations
- Use repeated amplitude half-range as a random uncertainty estimate where appropriate.
- Propagate angular uncertainty into cos(θ), or show it as horizontal error bars where significant.
- Use maximum and minimum acceptable gradients to estimate uncertainty in the proportionality constant.
Interpretation
- Angular uncertainty has little effect on cos(θ) near 0° but produces a comparatively important absolute cos(θ) uncertainty near 90°.
- Repeats reduce random scope noise but cannot remove a fixed angular-zero or off-centre alignment error.
Exam angles
- Identify the correct angle convention from a diagram of the coil plane and normal.
- Predict scope amplitude and phase behaviour at 0°, 90° and after reversing the coil normal.
- Justify plotting induced-emf amplitude against cos(θ).
- Explain how an angular zero error changes the graph and how the setup can reveal it.
Where students lose marks
Reading the angle between the coil plane and the field but using it as the angle to the coil normal.
Fix: Draw and label the normal, define θ before taking data, and use that same angle in cos(θ).
- Examiner report: P3-23 · PDF p. 5
Mixing peak amplitude and peak-to-peak voltage between readings.
Fix: Choose one amplitude measure, label it in the results table and retain the same oscilloscope scale and reading convention throughout.
Rotating the search coil about an off-centre point so both angle and field strength change.
Fix: Use a rotary mount or clamps that keep the search coil centred while it turns.
Improve the method
- Read the protractor from two well-separated pointer boundaries or use a rigid pointer to reduce angular judgement uncertainty.
- Take smaller angle steps near 90°, where a zero offset is easiest to detect.
- Twist and secure connecting leads to reduce stray pickup and keep their position fixed.
Source references
- Specification: AQA 7408 specification §3.7.5.3 · PDF p. 39
Try it — exam-style
Easy
ORIGINAL
Medium
ORIGINAL
The smallest signal occurs at a protractor reading of 94°. The true minimum should occur when the coil normal is at 90° to the field. State the angular zero offset and explain why repeating readings will not remove it.
[3 marks]
Total for this question: 3
Easy
ORIGINAL
A student records the angle between the coil plane and the magnetic field as 25°. Calculate the angle θ between the coil normal and the field for use in NΦ = NBA cos(θ).
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on search coil — flux linkage and angle?
Define the angle before touching the cosine — that one habit prevents most search-coil errors.