The Expected but Well-Hidden EMF in the Jumping Wire Experiment and its Unexpected Role

Abstract

As presented in physics textbooks, as well as in a few papers, the typical example of an induced motional electromotive force εmot = Blv consists of a conductive rod of length l frictionlessly sliding with speed v along parallel wires within an orthogonal and constant uniform magnetic field of magnitude B. End-of-chapter problems, variants of that sliding rod, are additionally posed in those textbooks. The horizontal jumping wire of length l carrying a current i that is tossed upward by a magnetic force, in a transverse horizontal magnetic field B, is an important variant, as evidenced by its popularity as a demonstration experiment (with numerous videos and blogs posted on the web). A simplistic account of the wire-jump experiment is often given exclusively in terms of the well-known magnetic force F = i l × B that propels the wire upward. The jumping wire is an eye-catching demonstration for both students and the public, and it indeed deserves to be analyzed in greater detail, since as shown below it hides fundamental and interesting physics, certainly much more than what the eye catches.