Several times in class, we caused an aluminum ring to jump away from a solenoid

Question

Several times in class, we caused an aluminum ring to jump away from a solenoid that was plugged into a wall socket. The wall voltage has an amplitude of 120 volts and oscillates at a frequency of 60 Hertz. Using the force equation and Maxwell's equations of electromagnetism, explain why the ring jumps when power is applied to the solenoid? Why does the ring get hot when you hold it down over the end of the solenoid? How would the experimental results change if we used lead (Pb) rings rather than aluminum?

Explanation / Answer

The alternating current from the wall causes a changing flux in the ring. This changing flux induces a current which creates an induced magnetic field that opposes the change in flux. The interaction of these fields is what causes the ring to jump. When the ring is held down and now allowed to jump, there is a very large current being induced in it, and the ohmic power loss is what causes its temperature to increase. ( P = I^2*R) The results with a lead ring would not be the same because it is not a good conductor.

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