15.3.2 Faraday’s Disk

The Faraday’s Disk was invented by Faraday himself. It is basically nothing more than a disk made of non-ferromagnetic material (e.g. aluminium) spinning in a (uniform) magnetic field.

The induction is motional emf in nature. We can imagine the disc as consisting of many thin slices of wires, all joined at the centre of the disc, and each extending to the circumference of the disc. As the disc spins, each “slice” will be cutting the magnetic flux.

Using FRHR. we can deduce that each slice is emf source with the positive terminal at the centre, and the negative terminal at the circumference. So the whole disc is basically many emf sources (slices) connected in parallel between the centre and the circumference of the disc . As such, to utilize the resultant emf, we must connect the external circuit between the centre and the circumference of the disc.

To obtain the magnitude of the induced emf, we note that each “slice” sweeps out an area equal to the area of the disc \displaystyle A=\pi {{r}^{2}} in one period T. So the rate of cutting is

\displaystyle \varepsilon =\frac{{d\phi }}{{dt}}=\frac{{B(\pi {{r}^{2}})}}{T}=BAf

Concept Test

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