# How does a copper ring levitates above a AC supply primary coil?

Doesn't AC current gives decreasing current and so decreasing magnetic fields or flux. This should then attract the copper ring above primary coil. When magnitude of current increases in AC then due to Lentz law as we know ring will get repelled. But what about the decrease of current. Dosen't current decreases like it increases in AC. What is the reason of that lift. Even when anyone blocks ring at a place it still try to lift. Why??? I am very confused on that topic. I am trying to find it hard from everywhere and thinking myself too. But it seems like how?why? Why only lift above?

I was wrong in my first answer. It is slightly more subtle than I thought.

Perhaps it has to do with the shape of the exciting field and its oscillation frequency.

If the field diminishes quickly (spatially) with the distance to the exciting coil (e.g. if the field diverges strongly), the second coil could be in the air during the attracting phase, so that it might not see said "attractive phase" as strongly as he felt the repulsive phase.

If the frequency of the oscillator, the shape of the field, and the mass of the second coil (the ring), are right, and the attractive phase always coincides with the ring being in the air, then it could work.

So, if I have got it right this time, it means that it is not a purely electromagnetic problem, but that is also has a mechanical aspect.

One can picture it naively as the exciting field "joggling" with the second coil (the way a ping-pong player bounced a ball with a paddle).

• @Serge_Hulne For the first i also thought that it would be the reason(if get your text right in my head) but i already considered another situation that i holded that ring above the primary coil so that for both increasing and decreasing magnetic field, the height remains same therefore giving equal amount of force for repulsion and attraction. Do we then feel that it is vibrating there or only pushed above? As much i have seen , it forces only upward to move no attraction seems to be there( i actually don't know cause i had not tried this by myself except levitating the ring) Apr 20, 2021 at 22:44
• I'm not sure what you mean. If English is difficult for you, you could try to formulate your question in another language and then translate it using Google Translate, for instance. Apr 20, 2021 at 23:07
• Here is a detailed explanation, arxiv.org/pdf/1404.6024.pdf , see conclusion : "VI. DISCUSSION AND CONCLUSIONS Unveiling the physics behind the flight of the conductive jumping ring in the Thomson experiment is not a straightforward task because the electromagnetic variables of the ring-electromagnet system implicitly depend on the ring motion itself. This motion takes place in a divergent magnetic field, and thus we faced a true electro-dynamics problem." Apr 21, 2021 at 6:54

Besides the bad English, I think what you're referring to is the jumping ring experiment, which is a demonstration of Faraday's laws of electromagnetic induction and also of Lenz's law. You can read more about it here.

• Actually I have not read about these equations before. In equation they told about the total force acting in a cycle of ac current. But if we want to talk about the main reason for such behaviour, what is the important thing. I want if you could think it simply like when i have holden the ring with my hand at certain height, the change in flux at there would be same for increasing and decreasing current. Now everything is similar but the once it should repel and then attract the magnet(source). Apr 20, 2021 at 20:37