Physics Demo Number: 045A		Approximate Run Time: 10 min
Judy Current Pendulum (Electromagnetic Brake)
Demo Description Cleaver-shaped pieces of various flat-stock materials are suspended on a horizontal rod. The rod suspension allows the cleavers to be swung freely as physical pendula. When a given cleaver is made to swing through the poles of a strong magnet, the resulting motion depends on the construction material of the cleaver. The Aluminum cleaver is rapidly stopped in its swing through the magnet, as is one copper cleaver. Both the plastic cleaver and a second copper cleaver are not stopped.
Scientific Principles ·Â Â Â Â Â Â Â Â  Eddy Currents ·         Magnetic braking systems ·         Motional Electromagnetic Fields ·         Surface Contact Resistance
Equipment ·         Old Large Black Magnet ( Now resides around [H-6-2] if you want to still use it.) See safety issues below for old magnet. New Ceramic Magnet (in custom made aluminum and steel housing) and keeper with built in handle. ·         Pliers, Attached to the  Magnet Keeper for Old  Magnet ·         PVC stand with 4 cleaver-shaped masses (2 Copper, 1 Aluminum, 1 Plastic)
Equipment Location ·         The apparatus lives in [B-4-1].
Instructions The photo shows the white and gray PVC stand for supporting a dedicated horizontal rod and 4 cleaver- shaped masses (2 Copper, 1 Aluminum, 1 Plastic) which are attached to the rod by sleeve-bearing inserts. The masses may be pulled away from their vertical equilibrium positions and allowed to swing as physical pendula. The old black magnet may be placed in the support stand as shown after removing its cylindrical keeper ( seen in the dedicated pliers ). One should place the old  magnet on the floor and put a foot in its frame before pulling on the pliers to remove the keeper. It takes around 100 lb to pull the keeper out. Holding the keeper only by the pliers while removing or inserting it helps minimize the safety hazard of mutilation or amputation of fingers. Do not attempt to lift the PVC framework by its white posts (unless the magnet is first removed) to avoid smashed toes. It is highly recommended that one  not use the old magnet, but opt for the new one described below!!!!!! One finds that the Aluminum and one Copper pendulum will each come to a rapid halt when released to swing through the pole pieces of the magnet. The other two pendula swing freely through the magnetic field. The copper cleaver that swings freely through the magnetic field looks ( from a distance ) identical to the one that is rapidly damped. It was modified by cutting ten or so quarter inch wide strips from its bottom, after having drilled two internal holes in the cleaver perpendicular to its bottom edge. The quarter inch strips were then reattached to the cleaver with two long non-metallic screws tapped into the tops of the original drilled holes. The surface contact resistance between the strips is enough to break up the Eddy Currents in the cleaver. -------------------------------------------------------------------------------------------------------------------------------------------------- NEW  COMPONENTS A new magnet, the reworked PVC stand, and a new keeper are seen in the following photos: One may more  easily pull the keeper (seen in operational horizontal position in the final photo) from the new magnet. The free standing new keeper is on the left side of   the first of the  three photos above. The two flat steel plates on each side of  the pole pieces of the new magnet serve to intensify the field between the poles.
   Writeup created by David A. Burba Copyright © 2013, Vanderbilt University.   All Rights Reserved.