AS 91526 · 2024 paper · electromagnetic induction
Question 2(b) Merit
Practises Electromagnetic induction: eddy currents & transformers
Harry is experimenting with coils and magnets. Each part gives its own values.
The coil has 25 turns and an area of 1.60 × 10⁻⁴ m². It is moved into a magnetic field of flux density 1.80 Wb m⁻², with its plane perpendicular to the field lines, taking 0.300 s.
Calculate the EMF induced in the coil.
Work it out
Try it on paper first. Stuck? Take one hint at a time.
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1 How much flux passes through one turn once the coil is in the field?
The coil’s plane is at right angles to the field lines, so they all pass through it: Φ = BA = 1.80 × 1.60 × 10−4 = 2.88 × 10−4 Wb. It started outside the field with none, so ΔΦ = 2.88 × 10−4 Wb. -
2 Which law turns a change of flux into an EMF, and what do the 25 turns do?
Faraday’s law: ε = N ΔΦ/Δt. The same EMF is induced in every turn, and the turns are in series, so the EMFs add. -
3 Substitute.
ε = 25 × 2.88 × 10−4 / 0.300 = 7.20 × 10−3 / 0.300 = 0.0240 V.
What the marker wanted
- Achieved A step on the way: the flux, 2.88 × 10−4 Wb, or 7.20 × 10−3 Wb across all 25 turns, or an EMF with one error in it. The schedule lists 9.6 × 10−4 V, 150 V, 240 V and 3.85 × 10−5 V as such answers.
- Merit The induced EMF, 2.40 × 10−2 V.
In our words, from NZQA’s 2024 marking schedule.
✕Common mistake
Leaving out the 25 turns (9.6 × 10−4 V), or leaving the area in cm², which gives 240 V. The schedule credits both at Achieved only, and NZQA’s 2024 report puts the one-turn answer at Achieved and the 25-turn answer at Merit.How did it go?
Got it moves it further out. Review brings it back tomorrow.