Physics Dept. Demonstration Manual

DEM44.1 - Eddy Currents Induced by a Linearly-Moving Magnet

Objective:

To show the effects of a moving magnetic field opposing Eddy Currents induced in a hollow copper pipe.

 
Apparatus:  
  • Copper pipe
  • Brass cylinder
  • Neodymium magnet

Method:

  • The copper tube is held vertically, lightly with two fingers, very near the top.

  • An assistant holds a hand under the bottom of the pipe.

  • The brass cylinder is dropped through the tube, with no effect.

  • The Neodymium magnet is then dropped through the tube.

Observe that,

  1. The tube feels 'heavier' as the magnet passes down it.

  2. The magnet slows down, arriving at the end after a noticeably longer time than with the Brass cylinder.

The moving magnet has induced circulating currents whose own magnetic field opposes that of the magnet.

Note:

Before proceeding, demonstrate that the brass and copper are indeed non-magnetic.