PURPOSE: To demonstrate how an electric potential difference and current can be generated when free (conduction) electrons move through a magnetic field.

DESCRIPTION: Rotating a copper disc between the pole tips of a strong C-shaped magnet, to create a vxB force on free electrons in the copper, creates a potential difference which is picked up by brushes at the center and the outside of the disc and displayed on the galvenometer. Reverse rotation of disc to invert current (center photograph). Clicking on either of the two left-most photos above will show a short MPEG movie of the action illustrated.

SUGGESTIONS:

See Question of the Week #209 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 5H40.50)

EQUIPMENT: Barlowe's disc with rotator and brush assembly mounted on wooden plate, lecture-demonstration galvenometer.

SETUP TIME: 5 min.