Advanced Lab or Lecture Demo Competition.

Apparatus Title: Homopolar Motor/Generator

Abstract: Current runs from the center of a vertical metal disk (which is free to rotate) to its lower edge which rests in a conductive liquid. The lower edge of the disk is also sandwiched by a horseshoe magnet. The right-hand rule can be used to explain the direction in which the disk will spin.

Description: A metal disk is free to rotate on an axle. One terminal of a DC power source is connected to the axle. The lower edge of the disk rests in a shallow pool of conductive liquid. The pool itself is made of metal and is connected to the other end of the power supply. The current's path therefore runs radially in the disk, from the bottom of the disk to the axle. A horseshoe magnet causes a magnetic field to permeate the lower edge of the disk. Use the right-hand rule to show the direction of the current and the magnetic field and predict the direction in which the disk will turn. F = qV x B.

Equipment List: ·

• 0-20 V, 40-A, DC power source ·
• bus strip with resistive load wire ·
• momentary contact switch ·
• 2-mm thick copper disk, 15-cm diameter, on a mounted axle ·
• glass liquid trough that holds 2 ml mercury ·
• horseshoe magnet ·
• 5 banana plug wires