Science teaching festival
21 - 25 November 2005
CERN, Geneva

Faraday's Law: A Paradox Experiment

A paradox experiment on inducing a current in a cobber plate with a circular magnet.

Faraday’s disk or homopolar generator:
The device is constructed from a brass disk that can rotate in front of a circular magnet and we measure the induction force between the center of the disk and a point in its surface (with the aid of a brush).

1st experiment:
We rotate the disk and keep the magnet still. We notice a DC current induced in concordance with Faraday’s law.

2nd experiment:
We keep the disk still and rotate the magnet. We expect an induction force to appear but the voltometer dissapoints us.

This result seems logical as the symmetry of the magnetic field with respect to the disk’s rotation axis does not alter anything, that is the magnet rotates in a frozen field and so no induction voltage is measured.

3rd experiment:
We attach the disk to the magnet and rotate bothd with any relative motion of the disk with respect to the magnet. We expect not to measure an EMF but the voltometer does.

It is obvious that as in the second experiment, the magnetic field is still while the disk rotates inside the field so the appearance of induction force seems perfectly logical the same as in the first experiment.

Contact:

Ioannis Kitsos

Institute:

Eniaio Lykeio, Greece

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