Modeling the induced voltage of a disk magnet in free fall

Abstract

We drop a circular disk magnet through a thin coil of wire, record the induced voltage, and compare the results to an analytic model based on the dipole approximation and Faraday's law, which predicts that the difference between the voltage peak magnitudes corresponding to the entry and exit of the magnet should be in proportion to $z_0^{-1/2}$, where $z_0$ is the initial height of the magnet above the center of the coil. Agreement between the model and experimental data are excellent. This easily-reproduced experiment provides an opportunity for students at a range of levels to quantitatively explore the effects of magnetic induction.