Problem
An infinite, perfectly conducting horizontal metal plane replaces the floor. You drop a small magnet above it. Discuss qualitatively the trajectory of the magnet.
Solution
As the magnet approaches, the magnetic flux through the plane tends to change. In a perfect conductor this change is prevented: surface currents are generated (eddy currents, or “image currents”) that oppose the entry of the flux, as required by Lenz’s law. The effect is equivalent to a mirror image of the magnet repelling it. The magnet slows down as it approaches the plane, because the repulsive force grows as the distance decreases. In a perfect conductor (superconductor) the currents do not dissipate, so the repulsion is persistent: the magnet never touches the plane but comes to rest at an equilibrium height where the repulsive force exactly balances its weight, remaining in stationary levitation.
Links
Topics: Induzione elettromagnetica Concepts: Legge di Lenz · Correnti di Foucault Skills: Analisi di casi limite e fantafisica