A-Level · A2 · extended response
Mapping an electromagnet without mistaking a field line for a path
A Level extension · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 6
- Topics
- 1
- Answer
- Complete
Solenoid field after background subtraction
Supplied question data. Read the labelled axes and use the observations to test the physical model.
Vertical axis (y): Flux density / mT
Horizontal axis (x): Current / A
Read the exact data as a table
| Current / A | Flux density / mT |
|---|---|
| -2 | -2 |
| -1 | -1 |
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
A long solenoid is tested at its centre. Current and magnetic flux-density measurements are supplied. A compass at the right-hand end points away from the coil when current is positive. For a long air-core solenoid use B = μ₀nI, with μ₀ = 4π×10⁻⁷ T m A⁻¹. Ignore Earth's field after background subtraction. Supplied data (Current / A; Flux density / mT): (-2, -2), (-1, -1), (0, 0), (1, 1), (2, 2).
- (a)
Identify the right-end pole for positive current, estimate turns per metre, and predict the field after reversing a 1.5 A current.
3 marks - (b)
Design a fair comparison of air and iron cores and explain a limitation of field-line maps.
3 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Identify the right-end pole for positive current, estimate turns per metre, and predict the field after reversing a 1.5 A current.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Outside a magnet, a compass's north-seeking end follows the field direction from north to south. Pointing away from the right end identifies that end as north.
- 2
The measured gradient is 1.0×10⁻³ T A⁻¹ = μ₀n, so n = 0.001/(4π×10⁻⁷)≈796 m⁻¹.
- 3
Reversing current reverses field direction. At −1.5 A the linear calibration predicts −1.5 mT; its magnitude is unchanged.
(b)
Design a fair comparison of air and iron cores and explain a limitation of field-line maps.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Control current rather than supply voltage, because resistance and heating may change. Keep probe orientation and centre position fixed, repeat measurements and subtract the ambient field.
- 2
Measure a current-up and current-down sequence to detect hysteresis or remanence in the iron core; the air-core proportional model need not describe a saturating core.
- 3
Field lines represent local directions and relative density. They are not physical wires or trajectories that isolated magnetic poles follow; a complete magnetic field map forms loops.
Complete marking guidance
Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.
- Outside a magnet, a compass's north-seeking end follows the field direction from north to south. Pointing away from the right end identifies that end as north.
- The measured gradient is 1.0×10⁻³ T A⁻¹ = μ₀n, so n = 0.001/(4π×10⁻⁷)≈796 m⁻¹.
- Reversing current reverses field direction. At −1.5 A the linear calibration predicts −1.5 mT; its magnitude is unchanged.
- Control current rather than supply voltage, because resistance and heating may change. Keep probe orientation and centre position fixed, repeat measurements and subtract the ambient field.
- Measure a current-up and current-down sequence to detect hysteresis or remanence in the iron core; the air-core proportional model need not describe a saturating core.
- Field lines represent local directions and relative density. They are not physical wires or trajectories that isolated magnetic poles follow; a complete magnetic field map forms loops.
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