5K20.25 - Lenz's Law - Tubes and Magnets

Code Number:
5K20.25
Demo Title:
Lenz's Law - Tubes and Magnets
Condition:
Excellent
Principle:
Eddy Currents, Magnetic Induction
Area of Study:
Electricity & Magnetism

Disclaimer

Equipment:
Copper and Aluminum Tubes, Brass, Iron, and Aluminum Slugs, Set of Kitchen Magnets, and Set of Neodymium Magnets.
Procedure:

Set the tube up so there is approximately 3" between the bottom of the tube and the drop pad.  Drop any of the wood, copper, or aluminum slugs down the tube and show that there is no significant damping.  The ceramic magnet will show some damping but will still drop quickly.  Drop the neodymium magnet stack and observe that it takes up to 24 seconds for them to travel down the 1 meter tube and 12 seconds for the 1/2 meter tube.

The neodymium magnets now come in a ring version.  These can be used with a copper rod to show eddy currents, and may be more visible than the tube and magnet version.

If you don't wish to use a tube with the slot cut into it you may use the copper and aluminum tubes that have the magnetic detecting film taped to them.  You will be able to observe where the magnet is in the tube as the film will change color when influenced by the poles of the magnet stack.

 

References:
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  • David Simmons and Robert Speers, "Magnetic Damping of a Mass-Spring Vertical Accelerometer", TPT, Vol. 35, #1, Jan. 1997, p. 49.
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  • John A. Pelesko, Michael Cesky, and Sharon Huertas, "Lenz's Law and Dimensional Analysis", AJP, Vol. 73, #1, Jan. 2005, p. 37.
  • Kenneth D. Hahn et. al., "Eddy Current Damping of a Magnet Moving Through a Pipe", AJP, Vol. 66, #12, Dec. 1998, p. 1066.
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  • "B-280. Pump Ring - Cow Magnet", DICK and RAE Physics Demo Notebook, 1993.
  • Bob Devantery, "Lenz's Law in Slow Motion", PIRA Newsletter, Vol. 3, #8, Dec. 1988, p. 9.
  • Robert Ehrlich, "N.5. Dropping a Magnet Down a Pipe", Turning the World Inside Out and 174 Other Simple Physics Demonstrations, p. 167 - 169.
  • Tik L. Liem, "The Dropping Race", Invitations to Science Inquiry - Supplement to 1st and 2nd Ed. p. 84.
PDF icon University of Maryland Physics Department QOTW Question #126
PDF icon University of Maryland Physics Department QOTW Answer #126




Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.