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 = il × 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.

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As a consequence of the vertical motion of the wire, the moving carriers charge q in it has a vertical velocity component vvert as well as a horizontal drift velocity vd (along the wire). Then the work Wmagt due to the magnetic force may be written as
the first term of which represents the work to lift the wire, while the second one represents a work done against the battery (the external emf), and such that when divided by the charge q it gives the εmot.
8.
Note that if one fits a straight line to the experimental trace in Fig. 4, the voltage difference VfV0 coincides exactly with the motional emf and the maximum difference is practically zero.
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