Fig. 19.1 shows a bar magnet being pushed towards a coil of 200 turns connected to a sensitive ammeter. The north pole of the magnet faces end X of the coil. As the magnet approaches, a current is detected in the circuit. Lenz's law states that the direction of the induced e.m.f. always opposes the change producing it. The ammeter can measure current in both directions. The coil is wound on a hollow plastic former. The bar magnet moves at a steady speed of 0.50 m/s towards the coil. The connecting wires have negligible resistance.
(a) State the polarity that end X of the coil must become as the north pole approaches. [1]
(b) Explain, using Lenz's law, why end X must have this polarity. [2]
(c) State what happens to the current direction in the coil when the magnet is pulled away from end X. [1]
(d) Explain why the magnet would require a force to push it into the coil, even though the coil has no battery. [2]
(e) State the energy transfer that occurs when the magnet is pushed into the coil. [1]
(a) End X of the coil must become a north pole as the north pole of the magnet approaches.
(b) By Lenz's law, the induced e.m.f. (and hence the induced current) must flow in a direction that opposes the change producing it. The change here is the north pole of the magnet approaching end X. To oppose this approach, end X must repel the incoming north pole. Since like poles repel, end X must itself become a north pole. This creates a repulsive force that tends to push the magnet away, opposing the motion that caused the induction.
(c) When the magnet is pulled away from end X, the current reverses direction. End X now becomes a south pole to attract the retreating north pole, opposing the withdrawal of the magnet (again consistent with Lenz's law).
(d) The induced current in the coil creates its own magnetic field. This field opposes the magnet's motion (Lenz's law), producing a repulsive force between the coil and the approaching magnet. To continue pushing the magnet into the coil, the person must do work against this repulsive magnetic force. This is essential for energy conservation: the kinetic energy of the person's hand is the source of the electrical energy generated in the coil. Without this opposing force, energy would be created from nothing, violating conservation of energy.
(e) The energy transfer is: kinetic energy of the magnet (and the person's hand) is transferred to electrical energy in the circuit. If the circuit contains resistance (which all real circuits do), this electrical energy is ultimately dissipated as thermal energy (heat).
