Problem
A magnet is brought close to a stationary loop; then moved away. Explain in five lines why the direction of the induced current changes between the two cases, and which fundamental conservation law lies at its root.
A magnet approaches the loop and then moves away along its axis.
Solution
Approach. The approaching magnet makes the linked flux through the loop increase. By Faraday-Neumann-Lenz’s law a current arises whose field opposes the increase: the loop behaves like a magnet that repels the approaching one.
Withdrawal. Now the flux decreases: the current reverses to oppose the decrease, and the loop attracts the magnet as it recedes.
The root cause. In both cases the induced current hinders the motion of the magnet (never assists it). If it assisted, the system would accelerate on its own, producing energy from nothing. Lenz’s minus sign is therefore the manifestation of energy conservation: moving the magnet requires work, which is recovered as electrical energy dissipated in the loop.
Collegamenti
Argomenti: Electromagnetic induction Concetti: Lenz’s law · Faraday-Neumann-Lenz law