Demonstrating forces between parallel wires

Abstract

A simple, highly effective demonstration of mutual repulsion (attraction) between parallel conductors is described for the classroom. The physical effects are clearly elucidated using readily available materials in conjunction with a high-current (20 A) dc power supply.

A measurement of the force between two current-carrying wires

“…Based on this formula, 1 A (ampere) is defined as the current which, when flowing within two parallel wires 1 m apart in a vacuum, gives rise to an attractive force of 2 × 10 -7 N per metre. 1 While it is not difficult to qualitatively show the effects of this force (examples are given in [1][2][3]), it is not easy to set up an experiment to provide an accurate measurement of the interaction (only a few papers [4,5] are available on the subject). In fact, from equation (1) we can infer that a simple set-up (two conductors 1 m long and separated by a distance of about 1 cm; and a current of ~5 A, generally delivered by a standard laboratory power supply) gives rise to a maximum force of the order of 5 × 10 -4 N. In order to avoid the problems connected with such a small force, we employ a large current (about 20 A) and detect attraction and repulsion by means of an electronic precision balance, whose sensitivity is 0.01 g. 2 In our set-up the maximum force is approximately 10 -2 N ~ 1 g f and the electronic balance provides a quite precise measurement.…”