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AS 91526 · 2024 paper · electromagnetic induction

Question 2(c) Merit to Excellence

Practises Electromagnetic induction: eddy currents & transformers

Harry is experimenting with coils and magnets. Each part gives its own values.

Harry drops a bar magnet through a metal ring.

Explain why its acceleration is not equal to g = 9.81 m s⁻². Include what happens as the magnet enters the ring and as it leaves.

The 2024 paper, page 4 ↗ Marking schedule, page 3 ↗

Work it out

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

  1. 1 As the magnet falls towards the ring, what happens to the flux through the ring?

    It grows: the nearer the magnet, the more of its field passes through the ring.

  2. 2 What does a changing flux do in a metal ring?

    It induces an EMF (Faraday’s law). The ring is a closed conductor, so a current flows round it.

  3. 3 Which way does that current’s own magnetic field act?

    Against the change that made it (Lenz’s law). The ring’s field repels the approaching magnet: an upward force as it enters.

  4. 4 Once the magnet is through, what happens to the flux?

    It shrinks as the magnet moves away. The induced current now flows the other way round, and its field pulls the magnet back: an upward force again, this time opposing it leaving.

  5. 5 So what does that do to the acceleration?

    Going in and coming out there is an upward magnetic force as well as the weight, so the net downward force is less than mg and the acceleration is less than 9.81 m s−2. The magnet still speeds up, just more slowly than in free fall.
Answer Entering: the flux through the ring increases, inducing a current whose field repels the magnet. Leaving: the flux decreases, inducing a current the other way whose field attracts it back. Either way the induced field opposes the motion (Lenz’s law), so the acceleration is less than g.

What the marker wanted

  • Achieved One correct statement: the acceleration is below 9.81 m s−2, the flux through the ring changes, a voltage or current is induced, an opposing field or flux forms, or the magnet is slowed.
  • Merit Explains that the changing flux through the ring induces a current whose field opposes the magnet’s motion, or that the acceleration is below 9.81 m s−2 because of that opposing field.
  • Excellence Explains both halves: the magnet entering the ring and the magnet leaving it.

In our words, from NZQA’s 2024 marking schedule.

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Common mistake

Explaining only the magnet on its way in. NZQA’s 2024 report puts explaining the approach at Merit, and the approach and the leaving together at Excellence.

How did it go?

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