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Eddy Currents In A Spinning Pie Plate


Consists of an aluminum pie plate balanced on the tip of a pencil. A magnet is used to induce eddy currents in the plate and spin the plate.


Demonstrate the induction of eddy currents. Demonstrate induced current/ magnetic fields. Demonstrate Lenz's law/ Faraday's law. Show that Newton's third holds, and the 'drag' experienced by the magnet (and shown by the magnet drop through copper tube demo) also translates to rotation of the disk.



  1. Tape the pencil to the top of the lab stand so that the sharpened point points upwards.
  2. Place the plate on top of the pencil. There should be a small divot in the middle so it should stay.
  3. Ensure the pie plate is balanced. You can slightly bend the edges to adjust the balance.
  4. Hold the magnet about an inch above the surface of the plate and start making circles centered on the plate.
  5. You should now see the plate slowly begin to rotate in the same direction as the magnet.


The strength of this effect is mediated by a few things:

  • The conductivity of the material. Copper would produce a stronger effect, but aluminum is still fairly conductive. A greater conductivity means the currents set up by the changing magnetic field will be stronger and therefore produce a greater force.
  • The strength of the magnetic field (the size of the magnet / the proximity of the magnet). A greater magnetic field will produce a larger change in flux and therefore a greater force.
  • The speed of the magnet. A greater speed means there'll be a larger change in flux and therefore a bigger force.

Demo room information

Location O2
Maker Unknown
Current State Working
demonstrations/5_electricity_and_magnetism/5k_electromagnetic_induction/eddy_currents_in_a_spinning_pie_plate/start.txt · Last modified: 2020/01/28 18:13 by demoroom