IGCSE Physics 0625: Electromagnetic Induction and Factors Affecting Induced e.m.f.

A student sets up a coil of $200$ turns of insulated wire, wound around a cardboard tube. The two ends of the coil are connected to a sensitive centre-zero meter, which can show a small current flowing in either direction. The student pushes the north pole of a bar magnet into the tube and then pulls it back out again.

(a) State what the student observes on the centre-zero meter (i) while the magnet is moving into the coil, and (ii) while the magnet is then held completely still, motionless, inside the coil. [2]

(b) Name the effect being demonstrated, and state the energy change taking place as the magnet moves. [2]

Trial 1: pushing the magnet in more slowly than in the original experiment
Trial 2: using a stronger bar magnet, pushed in at the same speed as the original
Trial 3: using a coil of $400$ turns instead of $200$ turns, with the same magnet at the same speed

For each trial, state whether the deflection on the meter would be smaller or larger than in the original experiment, and justify each answer using a factor that affects the size of an induced e.m.f. [3]

Part (a): What the meter shows

While the magnet is moving into the coil, the magnetic field linking the coil is changing, so an e.m.f. is induced and a small current flows, the pointer on the centre-zero meter deflects away from zero.

Once the magnet is held still inside the coil, the field linking the coil is no longer changing (there is no relative motion between the magnet and the coil), so no e.m.f. is induced. The pointer returns to, and stays at, zero.

Part (b): The effect and the energy change

This is a demonstration of electromagnetic induction: a changing magnetic field linking a conductor induces an e.m.f. in it.

The energy change is from the kinetic energy of the moving magnet to electrical energy in the coil circuit. The student does work pushing the magnet, and that work ends up as electrical energy driving the small induced current.

Part (c): Factors affecting the size of the induced e.m.f.

Trial 1 (magnet pushed in more slowly): the deflection would be smaller. The magnetic field through the coil changes more slowly, so a smaller e.m.f. is induced. The speed of the relative motion between the magnet and the coil is one of the factors affecting the size of the induced e.m.f.

Trial 2 (stronger magnet, same speed): the deflection would be larger. A stronger magnet produces a more concentrated field, so the field linking the coil changes by a greater amount for the same motion, inducing a larger e.m.f. The strength of the magnet is another factor.

Trial 3 (coil of $400$ turns instead of $200$ ): the deflection would be larger. Each turn of the coil has (approximately) the same e.m.f. induced in it, and the turns are connected in series, so the total e.m.f. across the coil is shared between more turns and adds up to a bigger total. The number of turns on the coil is the third factor.

Final answers

(a) (i) Pointer deflects away from zero. (ii) Pointer returns to (stays at) zero.
(b) Electromagnetic induction; kinetic energy of the magnet $\to$ electrical energy in the coil.
(c) Trial 1: smaller (slower motion). Trial 2: larger (stronger magnet). Trial 3: larger (more turns).