The “jumping ring” demonstration. If you wind a solenoidal coil around an iron core (the iron is there to beef up the magnetic field), place a metal ring on top, and plug it in, the ring will jump several feet in the air(Fig. 7.24). Why?

Question

The “jumping ring” demonstration. If you wind a solenoidal coil around an iron core (the iron is there to beef up the magnetic field), place a metal ring on top, and plug it in, the ring will jump several feet in the air(Fig. 7.24). Why?

Accepted Answer

Before you turned on the current, the flux through the ring was zero. Afterward a flux appeared (upward, in the diagram), and the emf generated in the ring led to a current (in the ring) which, according to Lenz’s law, was in such a direction that its field tended to cancel this new flux. This means that the current in the loop is opposite to the current in the solenoid. And opposite currents repel, so the ring flies off.[latex]^{11}[/latex]

[latex]^{11}[/latex] For further discussion of the jumping ring (and the related “floating ring”), see C. S. Schneider and J. P. Ertel, Am. J. Phys. 66, 686 (1998); P. J. H. Tjossem and E. C. Brost, Am. J. Phys. 79, 353 (2011). 12For a discussion of this amazing demonstration see K. D. Hahn et al., Am. J. Phys. 66, 1066 (1998) and G. Donoso, C. L. Ladera, and P. Martin, Am. J. Phys. 79, 193 (2011).

Question 7.6: The “jumping ring” demonstration. If you wind a solenoidal c...

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