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Jumping rings demo

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orangelearner

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So we have all seen the jumping rings demo in physics class. If you haven't, you can view it here: YouTube - Physics Demo -- Jumping Ring

What I want to know is the precisely how Lenz's law works and why AC is usually used for this demo. The textbook definition you get in most books is that the metal ring will oppose any change in magnetic flux through it. So what happens with AC?

If the AC voltage is a sine curve, then the rate of change is a cosine curve. When the rate of change is positive, there is a current induced in the metal ring and the two act like opposing magnets. But my question is: what happens when the rate of change becomes negative? Does the current in the metal loop immediately stop and go in the other direction? Is there no lag?

If there is no lag, then am I correct when I assert that the repulsive force F between the electromagnets follows this relationship:

F is directly proportional to | dΦ/dt |

In other words, as long as there is a change, no matter what direction it is in there is a repulsive force. Is that right? Because I want to work on a levitating ring demo.
 
Hi,i dont know the exact reason but it should be this-the ring taken was was behaving as resistive rather than inductive.in a resistive type of material current is directly proportional to voltage,and hence the current changes instantaneously with voltage(here induced emf).had there been a coil instead of a ring the situation would have been different
 
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