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# 4 Questions About Lenz's Law Experiment

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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? Recycling companies spin a magnet rotor very fast in able to propel aluminum (which is conductive) off of conveyor belts...

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

Edited by droscoe
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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? Recycling companies spin a magnet rotor very fast in able to propel aluminum (which is conductive) off of conveyor belts...

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

I'll need to go very slowly and carefully, as I am very rusty on my old A Level electromagnetism.

I'll have a go at (1), at least. My recollection of Lenz's Law is that the induced current will be such as to "oppose the change to which it is due". That would seem to say that spinning the tube would have no effect on the rate of fall, as the induced current would flow in a direction that  would not slow its descent.

Does that seem reasonable?

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Well, I was thinking this:

Imagine a vertical 'tube' of magnets. The magnets are rectangular bars, positioned vertically (lengthwise), with the poles being on the left/ride sides. It would be much easier to spin the whole thing very fast, create more magnetic flux change, than a stationary tube with the magnets' poles on the up/down sides of horizontally placed magnets.

Does this make sense? Recycling companies use these kinds of rotors (lined with magnets) to propel aluminum off of a horizontally moving conveyor belt. If you simply took that same magnetic rotor, stood it up vertically, surely something falling down that tube would experience the same amount of change in magnetic flux.

Again, I'm just trying to maximize change in magnetic flux, thus, a slow descent of an object.

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Well, I was thinking this:

Imagine a vertical 'tube' of magnets. The magnets are rectangular bars, positioned vertically (lengthwise), with the poles being on the left/ride sides. It would be much easier to spin the whole thing very fast, create more magnetic flux change, than a stationary tube with the magnets' poles on the up/down sides of horizontally placed magnets.

Does this make sense? Recycling companies use these kinds of rotors (lined with magnets) to propel aluminum off of a horizontally moving conveyor belt. If you simply took that same magnetic rotor, stood it up vertically, surely something falling down that tube would experience the same amount of change in magnetic flux.

Again, I'm just trying to maximize change in magnetic flux, thus, a slow descent of an object.

Yes but you have to resolve the component vectors of the rate of change in magnetic flux. What I am suggesting is that whatever component of flux change is due to spinning will not induce a current that impedes the rate of fall.  The orientation of the magnets won't make a difference to this, I don't think.

But I'm not familiar with your aluminium conveyor belt example: can you provide a link to something that describes this, so I can see how it works?

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Allow me to further clarify my thoughts here. First image I've created below here is of a cylinder of ring magnets stacked on top of each other, with alternating poles up/down, and gray washers between each one. (This strengthens the effect as a conductive object falls through). If THIS configuration were to spin, then I think you'd be correct. The magnetic orientations are the same even if it's spinning. So there'd be no added benefit.

The configuration that I'm trying to describe is doesn't involve ring magnets stacked on top of each other, but instead thin bar magnets (or magnet wedges) their poles alternated horizontally instead (left/right) and positioned around to form a ring. Look at this product here, and imagine a stack of those on top of each other. Spin that cylinder like how recylcing companies rotate their magnetic rotors, and you have the same systems but on a smaller scale. So it would look like this below:

K&J Magnetics actually shows how to build a DIY eddy current seperator. Obviously, their pvc cylinder is positioned horizontal and the conveyor belt moved over the top of it.

The conductive metals are experiencing the opposing force from the rapid change in magnetic flux and are propelled slightly off the belt. In industrial applications, MUCH larger rotors are used and are spun much faster than this (I'm sure).

Here's a video demonstrating the one kind of seperator, for industrial applications.

I was imagining, instead, flipping that same configuration vertically, and dropping something down the tube. I'm certainly only a novice on this topic, but I would assume the greater RPM of the cylinder would increase change rate in magnetic flux, and thus, stronger opposing force from eddy currents.

If the key component is change in magnetic flux as a conductive metals fall down the cylinder, and rate of that change can be manipulated by a high RPM rotation and properly alternated/positioned magnets, I don't see how a vertical configuration wouldn't achieve the same results of inducing strong opposing forces to a descending object.

Edited by droscoe
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Allow me to further clarify my thoughts here. First image I've created below here is of a cylinder of ring magnets stacked on top of each other, with alternating poles up/down, and gray washers between each one. (This strengthens the effect as a conductive object falls through). If THIS configuration were to spin, then I think you'd be correct. The magnetic orientations are the same even if it's spinning. So there'd be no added benefit.

The configuration that I'm trying to describe is doesn't involve ring magnets stacked on top of each other, but instead thin bar magnets (or magnet wedges) their poles alternated horizontally instead (left/right) and positioned around to form a ring. Look at this product here, and imagine a stack of those on top of each other. Spin that cylinder like how recylcing companies rotate their magnetic rotors, and you have the same systems but on a smaller scale. So it would look like this below:

K&J Magnetics actually shows how to build a DIY eddy current seperator. Obviously, their pvc cylinder is positioned horizontal and the conveyor belt moved over the top of it.

The conductive metals are experiencing the opposing force from the rapid change in magnetic flux and are propelled slightly off the belt. In industrial applications, MUCH larger rotors are used and are spun much faster than this (I'm sure).

Here's a video demonstrating the one kind of seperator, for industrial applications.

I was imagining, instead, flipping that same configuration vertically, and dropping something down the tube. I'm certainly only a novice on this topic, but I would assume the greater RPM of the cylinder would increase change rate in magnetic flux, and thus, stronger opposing force from eddy currents.

If the key component is change in magnetic flux as a conductive metals fall down the cylinder, and rate of that change can be manipulated by a high RPM rotation and properly alternated/positioned magnets, I don't see how a vertical configuration wouldn't achieve the same results of inducing strong opposing forces to a descending object.

I don't think the arrangement matters, because however you arrange the magnets it will only be the component of the field perpendicular to the direction of fall of the conductor that contributes to eddy currrents with the right orientation to oppose the falling.

The field will form curving loops between N and S poles of each magnet, which will be intercepted and "cut" by the conductor as it falls. But only the horizontal component of the field flux lines at any point will be cut in this way. Motion parallel to the field lines does not cut them, so the vertical component of the field does not contribute any effect.

Spinning the tube of magnets will make the conductor cut the vertical component of the field, and the induced eddy currents will oppose the thing that is causing the cutting of the vertical component of magnetic flux, which is the relative motion between the two. So the conductor will start to spin in the same direction as the spinning of the tube.  But making it spin will not contribute to slowing down the rate of fall.

In my opinion you cannot afford to talk merely in terms of the total rate of cutting magnetic flux and the total magnitude of the eddy currents generated: you need to analyse them as vectors, and consider the horizontal and vertical components  of each. The horizontal effect will not affect the vertical motion, nor will the vertical affect the horizontal.

Thanks for the video by the way. What this shows is that the radial component of the flux lines from the spinning magnets sweeps across the metal objects on the conveyor, inducing eddy currents that try to minimise the relative motion between the metal object and the spinning magnets, i.e. by ejecting them in the direction of spin. But note that there is no force moving them sideways, perpendicular to the conveyor.

Edited by exchemist
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I suppose something like this is what I'm talking about building for myself.

Vertical Eddy Current Seperator

Actually, I think the magnetic rotor is tilted so that the conductive objects can ride a perfectly horizontal conveyor belt. The conductive objects actually hit the thing on the outside.

I'm just curious what would happen if dropped INSIDE of it. Does it still descend slowly through it like all those copper pipe and magnet experiments you see?

Edited by droscoe
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I suppose something like this is what I'm talking about building for myself.

Vertical Eddy Current Seperator

Actually, I think the magnetic rotor is tilted so that the conductive objects can ride a perfectly horizontal conveyor belt. The conductive objects actually hit the thing on the outside.

I'm just curious what would happen if dropped INSIDE of it. Does it still descend slowly through it like all those copper pipe and magnet experiments you see?

I think they will spin, as we have been discussing, and no doubt will have their rate of fall reduced. However, as I say, I think the two effects would be independent of one another.

Edited by exchemist

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