Faraday paradox

The Faraday paradox is an experiment that was first described by Michael Faraday and appears at first glance to be contrary to its law of induction.

The test arrangement comprises a cylindrical permanent magnet and an adjacent conductive disc, which are both rotatably mounted on an axle. The axis of symmetry of the magnet and the disc coincide with the axis of rotation, the magnet has its polarization in the axial direction ( that is, the poles are located on the axis). On the disk, the electric voltage is measured between the axis and its periphery; to be mounted on the outside and close to the axis of sliding contacts.

Added to the disc in rotation during the magnet rests, the result is a voltage at the terminals. This can be described by the Lorentz force and the flow rule ( unipolar induction ). The terminal voltage also occurs when the disc and the magnet are mechanically connected to each other and are moved together. However, if only the magnet moves, and resting the disc, occurs no clamping voltage. This was amazing for Faraday, because he assumed that it only important for the emergence of the voltage that the plate moves toward the magnet.

In fact, the magnetic field of the permanent magnet of its rotation (largely) independent. Therefore, it makes no difference whether or not it rotates. On the other hand acts (for a stationary observer ) on the electrons in the disk, the Lorentz force as soon as they are moved in a magnetic field. Therefore, a voltage is measured between the stationary sliding contacts exactly when the disk rotates.

When the experiment was not viewed from the viewpoint of the observer at rest, rather than a disk together with the moving ( rotating about the axis ) observer, it will always measure a voltage of zero between the center and the edge of the disc; the magnetic field is so regardless of any rotation of the magnet. In contrast, one will find an induced voltage in the circuit between the rotating ( for the observer ) sliding contacts, because it is a rotating magnetic field in the conductor in this circuit.

This is not easy to understand effect led to many misunderstandings and so, for example, to various experiments, based on the unipolar induction a kind of perpetual motion machine to build the so-called N- machine.