# Question on Lenz's Law

1. Feb 21, 2005

### andrew410

The homopolar generator, also called the Faraday disk, is a low-voltage, high-current electric generator. It consists of a rotating conducting disk with one stationary brush (a sliding electrical contact) at its axle and another at a point on its circumference, as shown in the figure below.
Figure: http://east.ilrn.com/books/sepsp06t/pse6e.31.24p.e.jpg
A magnetic field is applied perpendicular to the plane of the disk. Assume the field is 0.935 T, the angular speed is 3180 rev/min, and the radius of the disk is 0.383 m. Find the magnitude of the emf generated between the brushes.

I know I have to use Faraday's law of induction to solve this, but there is no change in the field or the area. Any help on this would be great! Thx in advance!! :)

2. Feb 21, 2005

### Staff: Mentor

Ah, but the area does change. Imagine a radius of the disk sweeping out an area as the disk rotates.

Note that this is equivalent to calculating the "motional EMF" as the conducting disk sweeps through the magnetic field.