In a classroom demo, a 200-turn coil is connected to a galvanometer. A bar magnet is moved along the coil's axis: first pushed into the coil, then held still inside it, then pulled out. During which part(s) will the galvanometer show a nonzero deflection (induced current)?
- Only while the magnet is moving into the coil and while it is moving out of the coil (correct answer)
- Only while the magnet is held stationary inside the coil
- While the magnet is moving and also while it is held stationary inside the coil
- Never, because a permanent magnet cannot induce an EMF without a battery
Explanation: This question tests understanding of electromagnetic induction, specifically Faraday's law (changing magnetic flux induces EMF) and the requirement that flux must change for induction to occur. Faraday's law states that a changing magnetic flux through a conductor induces an electromotive force (EMF): ε = -N(ΔΦ/Δt), where N is the number of turns in the coil, Φ is the magnetic flux (Φ = BA cos(θ)), and the rate of change ΔΦ/Δt determines the magnitude of induced EMF—the key requirement is that flux must be changing with time; if the flux is constant (magnet stationary, steady field, constant orientation), then ΔΦ/Δt = 0 and no EMF is induced. When the magnet moves into the coil, the magnetic field strength B at the location of the coil increases, causing the magnetic flux Φ = BA through the coil to change—this changing flux (ΔΦ/Δt ≠ 0) induces an EMF in the coil according to Faraday's law, which drives a current through the circuit if the circuit is complete (galvanometer provides complete circuit); when the magnet is held stationary inside the coil, the flux is constant (ΔΦ/Δt = 0), so no EMF is induced and the galvanometer shows zero current; when the magnet is pulled out, the flux decreases, inducing EMF again but in the opposite direction by Lenz's law. Choice A is correct because it accurately identifies that induction occurs when magnetic flux is changing, not when it's constant. Choice B incorrectly claims current is induced when the magnet is stationary inside the coil, but Faraday's law requires changing flux (ΔΦ/Δt ≠ 0)—when the magnet isn't moving, the flux through the coil is constant, so ΔΦ/Δt = 0 and no EMF is induced, which is why the galvanometer shows zero. To analyze electromagnetic induction scenarios, follow these steps: (1) identify what is changing (magnet position, coil orientation, field strength, loop area), (2) determine if this change affects the magnetic flux Φ through the conductor (Φ = BA cos(θ)), (3) if Φ is changing, then ΔΦ/Δt ≠ 0 and EMF is induced by Faraday's law, (4) the magnitude of induced EMF increases with faster change, more coil turns, and complete circuit allows current I = ε/R, (5) use Lenz's law to predict current direction: induced field opposes the flux change (if flux increasing, induced field points opposite; if flux decreasing, induced field points same direction). Key insight: it's not the mere presence of a magnetic field that induces current, but rather the change in flux—this is why moving a magnet toward a coil induces current (flux increasing), holding it stationary produces no current (flux constant), and moving it away again induces current in the opposite direction (flux decreasing), and why generators work through continuous rotation (flux continuously changing) while a coil sitting in a steady field produces no power.