30.5: Faraday Disk Dynamo
A Faraday disk dynamo is a DC generator, producing an emf that is constant in time. It consists of a conducting disk that rotates with a constant angular velocity in the magnetic field, perpendicular to the disk's plane. The rotation of the disk causes a change in magnetic flux, which induces an emf, causing opposite charges to develop on the rim and in the center of the disk. The polarity of the induced emf can be determined by the direction of the magnetic field and the direction of the rotation of the disk. If the rotating disk is connected to an external circuit using brushes, then an induced current flows.
According to Lenz's law, the induced emf always opposes the cause producing it. For the Faraday disk dynamo, the induced emf opposes the rotation of the disk. Therefore, external power must be supplied to keep the disk rotating at a constant angular velocity.
The induced emf in such a dynamo can range from a few millivolts to a few volts. The magnitude of the induced emf can be further enhanced by connecting multiple generators in the series or by increasing the magnetic field's strength, the disk's radius, and the disk's angular velocity.