Eddy current damping of a magnet moving through a pipe

A harmonic oscillator consisting of a neodymium magnet attached inside a sp ring is driven through resonance. Eddy currents, induced in pipes surroundi ng the magnet, result in a damping of the motion of the magnet. Our experim ent makes precise measurements of the motion and damping of the magnet as w e vary pipe composition, length, thickness, radius, and position. A theoret ical analysis combining the standard differential equation for a damped, dr iven harmonic oscillator and Faraday's law of electromagnetic induction giv es excellent agreement with the experimental results. This analysis allows for the calculation of electromagnetic damping for any pipe configuration w hich is coaxial with the magnet's motion and provides strong evidence for t he eddy current damping analysis of the common demonstration of dropping a magnet down a conducting tube. (C) 1998 American Association of Physics Tea chers.