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AS 91526 · 2023 paper · transformers and inductors

Question 2(b) Merit to Excellence

Practises Electromagnetic induction: eddy currents & transformers

Kate’s school has a demonstration transformer. She connects the 12 000-turn primary coil to the mains supply at 240 V RMS. The secondary has 600 turns.

N(p)12 000
V(p)240 V RMS
N(s)600

The two coils are held by a ring of laminated soft iron running through the core of each.

Explain why an AC voltage in the primary produces an AC voltage in the secondary, and why the coils are wrapped around an iron ring.

The 2023 paper, page 6 ↗ Marking schedule, page 4 ↗

Work it out

Try it on paper first. Stuck? Take one hint at a time.

  1. 1 What does an alternating voltage in the primary do to the current in it?

    It drives an alternating current, whose size and direction change all the time.

  2. 2 And what does a changing current do to the magnetic field?

    The primary’s field, and the flux it sets up, change all the time too.

  3. 3 Where does that flux go, and what does a changing flux do to the secondary?

    The iron carries it through the secondary coil. A changing flux through a coil induces an EMF in it (Faraday’s law), and because the flux keeps reversing, so does the EMF: an AC voltage.

  4. 4 Why wrap the coils round iron rather than leave them in air?

    Iron is easily magnetised, so it concentrates the flux and steers nearly all of it from the primary through the secondary. More of the primary’s flux links the secondary, so far less energy is wasted (and each coil’s inductance is higher). The iron is laminated to keep eddy currents in it small.
Answer The AC in the primary makes a flux that is always changing. The iron carries it through the secondary, and a changing flux through a coil induces an EMF there, alternating as the flux does. The iron ring concentrates and directs the flux so that almost all of it links the secondary, which makes the transformer efficient.

What the marker wanted

  • Achieved One idea: the current changes, the flux changes, the iron makes it more efficient, the iron raises the inductance, or the iron guides the flux.
  • Merit One half with its reason: the changing primary current changes the flux, which induces a voltage in the secondary; or the iron raises the efficiency, with how (guiding the flux, or cutting eddy currents); or the iron raises the inductance, with why.
  • Excellence Both halves: the changing current and flux inducing the secondary voltage, and what the iron does for efficiency, inductance or the path of the flux.

In our words, from NZQA’s 2023 marking schedule.

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Common mistake

Writing that the primary induces a current in the secondary. What is induced is a voltage; a current flows only once something is connected across it. NZQA’s 2023 report says a significant number of candidates wrote about induced or opposing currents instead of the induced voltage.

How did it go?

Got it moves it further out. Review brings it back tomorrow.