[droscoe (OP)]

Hello, I have four questions. As it stands now, I don't have the necessary materials to test this myself, which is how I'd prefer to learn the answer. I still plan on doing this experiment for fun once I can acquire the necessary components.

This deals with Lenz's Law. My goal is to maximize the time it takes for an object to fall through a tube.

1) Most folks demonstrate Lenz's Law by dropping a magnet down a copper pipe. From my understanding, the relative motion of the magnet, to the copper pipe, is responsible for creating Eddy currents, which, in turn, provide an opposing force on the magnet as it falls. My first question is this: Does spinning the copper tube create more Eddy currents, or opposing force? Would it slow the magnet's descent even more?

2) Regardless of the answer to my first question, if that same copper tube is spinning and you drop a magnet down into it, that magnet would spin as well, yes? (I've seen a YouTube video of ring magnets on the outside of a copper tube spinning as the copper spun) Here's my main question: If you dumped a handful of tiny magnets down the spinning copper tube, would all the tiny magnets spin individually and simply keep falling?

Will there be any centrifugal force applied onto the tiny magnets so that they start moving outward towards the walls of the copper tube as they fell? Do magnetic fields create, and impose, vortex/helical/centrifugal forces on conductive objects? Or do they simply spin and stay mostly in the center of the copper tube as they fall?

3) Now suppose the experiment in reverse. I have a 'tube' of neodymium magnets. (Probably several rings stacked up on each other, which can be expensive) and I dropped a copper object down the magnet tube. I understand that the effect will still occur; it will still fall slowly. But copper is diamagnetic. Does that dampen the opposing force created by the Eddy currents, or have little to no effect at all?It's the conductivity of copper/alumimum that is slowing it, not whether or not it's ferromagnetic or diamagnetic, right?

4) Last question. How does the copper tube's wall thickness affect this experiment? Do thicker walls generate more opposing force on the magnet, thus, it falls even slower? Or would thinner walls make it fall slower? I just assume thicker walls would make for more resistance, thus, make the object fall slower.

I hope I made sense with my questions. Copper can be...expensive, so I'll test this out soon enough. Maybe I can make an aluminum foil tube?

Thanks!
Dustin

[chiralSPO]

Hello, I have four questions. As it stands now, I don't have the necessary materials to test this myself, which is how I'd prefer to learn the answer. I still plan on doing this experiment for fun once I can acquire the necessary components.

This deals with Lenz's Law. My goal is to maximize the time it takes for an object to fall through a tube.

1) Most folks demonstrate Lenz's Law by dropping a magnet down a copper pipe. From my understanding, the relative motion of the magnet, to the copper pipe, is responsible for creating Eddy currents, which, in turn, provide an opposing force on the magnet as it falls. My first question is this: Does spinning the copper tube create more Eddy currents, or opposing force? Would it slow the magnet's descent even more?

I don't think so, but lifting the tube, and thereby increasing the relative velocity between the two, will increase the eddy currents.

2) Regardless of the answer to my first question, if that same copper tube is spinning and you drop a magnet down into it, that magnet would spin as well, yes? (I've seen a YouTube video of ring magnets on the outside of a copper tube spinning as the copper spun) Here's my main question: If you dumped a handful of tiny magnets down the spinning copper tube, would all the tiny magnets spin individually and simply keep falling?

Will there be any centrifugal force applied onto the tiny magnets so that they start moving outward towards the walls of the copper tube as they fell? Do magnetic fields create, and impose, vortex/helical/centrifugal forces on conductive objects? Or do they simply spin and stay mostly in the center of the copper tube as they fall?

I don't know, but I think it will depend on the orientation of the magnet(s) within the tube.

If the magnet is a simple dipole, and its N-S axis is aligned with the direction of falling and the axis about which the tube is spinning, I don't think there should be any difference in the interaction whether the tube it spinning or not. But if the magnet is turned 90°, such that the N-S axis is perpendicular to the axis about which the tube is spinning, then I would expect this to generate eddy currents (whether the magnet is falling or not). It all has to do with whether the applied magnetic field is changing in the conductor at a particular location.

3) Now suppose the experiment in reverse. I have a 'tube' of neodymium magnets. (Probably several rings stacked up on each other, which can be expensive) and I dropped a copper object down the magnet tube. I understand that the effect will still occur; it will still fall slowly. But copper is diamagnetic. Does that dampen the opposing force created by the Eddy currents, or have little to no effect at all?It's the conductivity of copper/alumimum that is slowing it, not whether or not it's ferromagnetic or diamagnetic, right?

Yes, it is just an issue with conductivity (which is why copper, aluminum and silver are excellent for demonstrating the effect), paramagnetic and ferromagnetic materials will have additional interactions that may (in the case of ferromagnetic for sure) dominate the overall interaction. Here is a video of a piece of aluminum being dropped through a powerful electromagnet:

I have a large neodymium magnet (a cube 5 cm on a side, where the N and S poles are in the centers of opposite faces), and I can very easily feel the drag as I move a copper plate (0.75 mm thick) across it. I can easily tell that the effect scale with relative velocity, that it falls off very dramatically with distance from the magnet, and that it depends how I move the metal with respect to the orientation of the magnet: it is strongest as I move it directly across the polar face, where the field lines are densest (doesn't matter whether N or S, and doesn't matter which way I push it, as long as it is moving perpendicular to the axis). It is slightly weaker when I move it across one of the equatorial faces parallel to the N-S axis (from N to S or S to N). I fell no drag if I move it along the equator (E to W or W to E, perpendicular to the N-S axis).

4) Last question. How does the copper tube's wall thickness affect this experiment? Do thicker walls generate more opposing force on the magnet, thus, it falls even slower? Or would thinner walls make it fall slower? I just assume thicker walls would make for more resistance, thus, make the object fall slower.

Yes, thicker walls will have less electric resistance, so will have greater currents, and therefore greater induced magnetic field, and therefore more drag. To a point. Because interaction falls off so quickly with the distance between the magnet and the conductor (I think it is proportional to distance^–6), the added thickness has less and less of an effect.

The two most important things you want to maximize the effect are: have the conductor as close as possible to the magnet, and have it be as conductive as possible (superconductors are the best for this, but are much pricier than copper, and need to be very cold...)

I hope I made sense with my questions. Copper can be...expensive, so I'll test this out soon enough. Maybe I can make an aluminum foil tube?

Thanks!
Dustin

Aluminum is much cheaper than copper, and it doesn't just have to be from foil... you can get aluminum sheets and tubes of many thicknesses fairly easily and for much, much less than similar sizes of copper.