Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY

A Simple and Inexpensive Apparatus to
Demonstrate The Magnetic Force on a
Current-Carrying Wire Segment

A. Tomasch, D. Gerdes, and M. Love

Department of Physics

Randall Laboratory

The University of Michigan

450 Church Street

Ann Arbor, MI 48109-1040

734-936-2959

atomasch ‘at’ umich ‘dot’ edu

Abstract:

We describe a simple apparatus to quantitatively demonstrate forces produced by a magnetic field on a current-carrying wire segment. The apparatus is based on an inexpensive digital scale with a least count of 0.01 gram, sufficiently sensitive to measure the forces produced on a small wire element. The apparatus can be used to explicitly demonstrate the dependence of the magnetic force on the cross product between the vector current element and the magnetic field, yielding a force proportional to the sine of the angle between them. Students can also verify that the magnetic force is proportional to the current in the wire and explicitly verify the right hand rule for the vector cross product of the current element and the magnetic field.

Description:

This is an exceptionally simple apparatus to construct. A small table is constructed from a commercial plastic cutting board and 1” Lucite rod support legs. Two banana binding posts are mounted to the side of this small support table. The wire segment is then attached to the binding posts. The two high field magnets are separated with foam board spacers, producing an approximately uniform magnetic field of ~0.2 T between the poles. The magnet rests on a pedestal (small plastic cup) to keep the fringe fields away from the digital scale. The pedestal is in turn attached to a simple cardstock compass rose and pointer assembly which measures the angle between the wire segment and the magnetic field in the magnet gap. Photos of the apparatus appear below: