Answer to Question #170999 in Physics for Jennie J

Question

In an experiment, metallic rings are placed on a pole above a coil of wire. An a.c. current is fed to the coil and the rings are observed to rise and levitate above the coil. 

Explain why the rings levitate, and explain why they reach a stable height above the coil.

Answer 1

The rings levitate because of Eddy currents: as we know, according to Faraday's law, the induced EMF creates a current that creates a magnetic field that opposes the field that induced the EMF.

Since the current in the coil is changing rapidly according to sine or cosine law, the force that makes the rings levitate is sinusoidal or cosinusoidal manner as well. Since the phases of the inducing and induced force are shifted by 90°, the resultant force has a constant term, which lifts the ring upward. As it reaches equilibrium with the force of gravity, the rings acquire stable position.