4 Questions About Lenz's Law Experiment

[Dustin]

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

 

[scottdave]

Nice questions to investigate. What properties would change as wall thickness changed (how might that affect eddy currents)?
What direction is the induced force?

One thing to add: you do not need a tube to show Lenz's Law. I take an aluminum cookie sheet and magnet (from a name badge). The magnet which you put inside your shirt and holds the namebadge on your shirt. There is a plastic part, which holds 2 or 3 of the tiny strong magnets.
If I hold the cookie sheet at a steep angle, and put the plastic part touching the sheet, the magnet just falls down at normal speed. If I flip over where the magnets are now touching the sheet, the magnet slowly slides down the sheet.

So for these magnets, that small additional distance away reduces the force.

They use combinations of magnet and a nonferrous metal in amusement park rides which want a "failsafe" way to linearly decelerate the ride. https://www.h2wtech.com/page/linear-magnetic-brakes

 

[kuruman]

Check out this video. What you want starts at 6:00 and you might find what follows interesting as well. I have the used the equipment shown, but that's not me in the video.


Aluminum will work just fine, but you need thick walls, foil will not work well if it works at all. You want increased cross section through which the current runs. The tube in the video has a wall thickness of about half an inch. If you want to stack ring magnets and drop a disk through them, you cannot use a ferromagnet because it will just stick to the walls. When you stack the ring magnets be sure to do so in repulsion separated by a gap. For that you need to find a scheme to hold the whole thing together. We used a thin plastic tube insert with plastic washers between the magnets and screwed down the two ends. The alternating directions of magnetic field create stronger eddy currents in the disk as it drops than having the magnets lined up in the same direction in which case the magnetic flux isn't changing much.

I don't think that spinning the tube will do anything. You can get stronger eddy currents by dropping its temperature which will reduce its resistivity. However, you need a serious decrease in temperature such as using liquid nitrogen which requires expertise to handle and is very dangerous if you have never handled it. Dry ice, which is more readily available, is safer.

 

[ZapperZ]

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?

It depends on the solenoid itself. If it perfectly wound, with no noticeable tilt of each turn with respect to the axis of the solenoid, then it should not matter, at least, not for the level of accuracy of your experiment. If it is loosely made (i.e. significant gaps in between each turn), and there's a tilt, then it may wobble the trajectory a bit. How this would affect the descent, I haven't figured this out.

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?

See above. If it causes a wobble in the trajectory, then the spinning may induce a torque on the magnet.

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?

This isn't straightforward. What is the field inside the tube, along the axis? Is the tube short enough that the fringe field comes into play, i.e. how much does the magnetic field along the axis varies along the length of the tube. Remember that Lenz's law depends on the rate of change of magnetic flux. A long tube may have a more uniform field along the axis, but the rate of change of the magnetic field as the copper falls through will not be as large as for a shorter tube.

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

The thickness of the walls should not matter that much for your level of experiment. But just remember that you want something that can generate a large current for a given amount of induced EMF. So something with lower resistivity will be better than something with a higher resistivity.

Zz.

 

[ZapperZ]

BTW, how about doing an experiment where you drop a magnet with a cylindrical tube geometry through a solenoid?

https://arxiv.org/pdf/0909.3880.pdf

Zz.

 

[scottdave]

I'd like to see a slow motion of that spinning copper pipe video. I am speculating that you would see the ring momentarily bouncing off the pipe. Also, the pipe may be moving some air, but I am speculating that is less of an effect.

 

[Dustin]

@kuruman - I loved that video! Thanks! Question for you: When you mention the magnet tube (which is either a chain of cylindrical magnets or stacks of ring magnets) you said to alternate their polarity as you stack them, is that right? So, something like this image below? I made this image real quick.

Green - north pole // Red - south pole // Grey washers in between

Is that what you mean? I didn't realize that would improve the effect.

@scottdave - Thanks for response! Yea, a lot of those cool Lenz's Law videos show other ways of demonstrating the effect. I do, however, have a specific project in mind that would use a 'tube' of neodymium magnets with conductive metal falling through it. (usually, its the other way around in these demonstrations).

@ZapperZ - Responding to each of your replies, in order. The copper tube would be just that. A solid, perfectly cut copper pipe that you would buy from a hardware store and stood up, vertically, as straight as possible. I should note, these questions were general, in nature, and the project in mind that I'm specifically going to pursue will actually be a 'tube' of neodymium magnets. (either stacks of ring magnets or stacked cylindrical magnets, whichever is cheaper). The 'tube' would ideally be at least 2 feet long, preferably longer. And I'd like to see the effect of a conductive metal object (non-magnetic) falling through it.

I only ask about the spinning because recycling plants use spinning magnetic rotors to generate eddy currents, which then propels conductive metals (like aluminum) off of a conveyor belt. I just thought a vertically rotating magnet tube might induce something similar. K&J Magnetics actually created two videos of homemade, DIY magnetic seperators using a spinning tube of magnets. It flung aluminum of their makeshift conveyor belt very well. Obviously, the aluminum is moving OVER the outside of the spinning magnets, and I'm curious what would happen to conductive metal INSIDE a spinning magnet rotor as it fell vertical. Would it experience more opposing force?

With regards to my last question, I did see a youtube video (from someone who clearly understands magnetism) where they timed a falling magnet through a copper tube. They then put an aluminum tube that had a slightly larger inner-diameter around that copper tube, and then another copper tube with an even larger inner-diameter around that aluminum tube (so 3 conductive metal tubes) and the magnet took much longer to fall. A LOT longer. I just wasn't sure if there was something else going on that I didn't understand. It seemed clear to me that a thicker wall in the copper pipe provided more opposing force to the falling magnet.

 

[ZapperZ]

I only ask about the spinning because recycling plants use spinning magnetic rotors to generate eddy currents, which then propels conductive metals (like aluminum) off of a conveyor belt. I just thought a vertically rotating magnet tube might induce something similar.

Yeah, but look at how the coils are wound in that case. It is done in such a way to maximize the rate of change in magnetic flux. It is not your simple solenoid-type of coil. The details here are important.

Zz.

 

[Dustin]

Yeah, but look at how the coils are wound in that case. It is done in such a way to maximize the rate of change in magnetic flux. It is not your simple solenoid-type of coil. The details here are important.

Zz.

I apologize, but I may be misunderstanding you. I'm a layman on the subject of magnetism. The magnetic rotors that they were using in their aluminum separators were literally made of block magnets glued to a pvc pipe and they were just spun around. There are no wound coils. Even product videos of industrial separators show a simple cylinder of magnets. There are no 'coils'.

Video 1 - demonstration
Video 2 - how they built it

Again, I may misunderstand. But this is what I'm envisioning, a cylindrical stack of magnets (forming a tube) or, if I have to, several stacks of ring magnets. They alternate the magnets' poles. I'm really curious what would happen if this was stood up, vertically, and conductive metal fell through it from top to bottom. These horizontal seperators must spin faster than the conveyor belt, as to generate powerful enough Eddy currents for the 'propulsion'. Surely, that would make more opposing magnetic force INSIDE the rotor as well?

 

[ZapperZ]

I apologize, but I may be misunderstanding you. I'm a layman on the subject of magnetism. The magnetic rotors that they were using in their aluminum separators were literally made of block magnets glued to a pvc pipe and they were just spun around. There are no wound coils. Even product videos of industrial separators show a simple cylinder of magnets. There are no 'coils'.

Video 1 - demonstration
Video 2 - how they built it

Again, I may misunderstand. But this is what I'm envisioning, a cylindrical stack of magnets (forming a tube) or, if I have to, several stacks of ring magnets. They alternate the magnets' poles. I'm really curious what would happen if this was stood up, vertically, and conductive metal fell through it from top to bottom. These horizontal seperators must spin faster than the conveyor belt, as to generate powerful enough Eddy currents for the 'propulsion'. Surely, that would make more opposing magnetic force INSIDE the rotor as well?

Sorry. In my very quick reading of your post, I read magnetic MOTORS, instead of magnetic ROTORS.

Zz.

 

[kuruman]

Is that what you mean? I didn't realize that would improve the effect.

That's exactly what I mean. This causes the polarity of the field to alternate with distance which translates to higher flux change as you switch from "B-up" to "B-down" and back over a short distance as the disk drops. The more change in magnetic flux, the stronger the eddy currents.

I just wasn't sure if there was something else going on that I didn't understand. It seemed clear to me that a thicker wall in the copper pipe provided more opposing force to the falling magnet.

The only thing you need to understand is that you need metal in order to have a current running in it. You can't have a current running in air because, unlike a metal, air does not have free charge carriers. The falling magnet creates a circulating electric field in the space around it. If you put metal in that region, the electric field will push the free electrons in the metal as long as the metal forms a closed loop surrounding the falling magnet. You get a current loop that forms an "electromagnet" with a polarity that will always oppose the polarity of the falling magnet. A magnet falling through tube can be considered as generating a stacked pile of induced current loops. The ones below the magnet exert a net force that tend to push the falling magnet up while the ones above it exert a net force that tends to pull it up.

 

[Dustin]

I really appreciate all these constructive responses.

I suppose I have one follow up question. Is there any information, or any 'law' that I can look up that deals with the relationship between the strength of the Eddy currents as it relates to the mass of the conductive metal?

If instead of an aluminum object, or ball, what if...aluminum flakes or pellets fall down the magnetic tube?

The only thing I've learned from YouTube videos is that the distance from the object's outer edges to the inner walls of the tube have a significant effect on the strength of Eddy currents generated, thus, the speed at which is descends down the tube. It seems like the smaller the conductive object is, the less magnetic field it would induce. I guess I could always think about making a smaller diameter magnet tube?

Sorry, lots of questions on my mind!

 

[kuruman]

If the point is to increase magnetic flux, by alternating polarities as I stacked them veritcally, then doing so horizontally as well will further strengthen those eddy currents if the 'tube' is also spinning...yes? Am I thinking about this correctly? (keep in mind that a conductive metal object falling vertically down the tube)

You want the flux to alternate in the same direction as the velocity of the falling disk. Besides, how are you going to do this? The configuration calls for disk magnets that look like washers with one face the N pole and the other the S pole.

 

[Dustin]

You want the flux to alternate in the same direction as the velocity of the falling disk. Besides, how are you going to do this? The configuration calls for disk magnets that look like washers with one face the N pole and the other the S pole.

Sorry, I realized how silly and overkill this would be for my needs. (and expensive).

I rewrote the statement to reflect what I'm really wanting to know.

 

[kuruman]

If instead of an aluminum object, or ball, what if...aluminum flakes fall down the magnetic tube?

If you have aluminum flakes falling down inside the magnetic tube, you will have to show that they fall slower inside than outside. This might be difficult because of the air resistance that might cause the flakes to reach terminal velocity rather quickly. Besides, flakes being, well, flaky are thin and you not much eddy currents will be induced. Remember, the more metal you have, the more charge carriers you have the stronger the eddy currents.

 

[Jon Abel]

Nice questions to investigate. What properties would change as wall thickness changed (how might that affect eddy currents)?
What direction is the induced force?

One thing to add: you do not need a tube to show Lenz's Law. I take an aluminum cookie sheet and magnet (from a name badge). The magnet which you put inside your shirt and holds the namebadge on your shirt. There is a plastic part, which holds 2 or 3 of the tiny strong magnets.
If I hold the cookie sheet at a steep angle, and put the plastic part touching the sheet, the magnet just falls down at normal speed. If I flip over where the magnets are now touching the sheet, the magnet slowly slides down the sheet.

So for these magnets, that small additional distance away reduces the force.

They use combinations of magnet and a nonferrous metal in amusement park rides which want a "failsafe" way to linearly decelerate the ride. https://www.h2wtech.com/page/linear-magnetic-brakes

If you are trying to avoid the Lenz Force - you might want to try 304 or 316 stainless steel tubes instead of copper. Austentic Stainless Steel is both non-magnetic and non-conductive - at least on paper. You may need to heat treat the SS to 2950 F if you did any threading, bending or cutting - since these cold-work processes magnetize the steel. Heat treatments will again de-magnetize them.

 

[berkeman]

If you are trying to avoid the Lenz Force - you might want to try 304 or 316 stainless steel tubes instead of copper. Austentic Stainless Steel is both non-magnetic and non-conductive - at least on paper. You may need to heat treat the SS to 2950 F if you did any threading, bending or cutting - since these cold-work processes magnetize the steel. Heat treatments will again de-magnetize them.

Non-conductive? Can you post a link to a source about that? Is it because of some coating or something?

 

[Jon Abel]

My wording was incorrect. "poor conductor" is the correct wording - not "non-conductive".

You can see below how it compares to aluminum, copper, and gold, etc.
Resistivity is calculated as (1 / conductivity).

http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/

 

[Jon Abel]

But, short of purchasing large tubes of carbon fiber, or titanium alloy - 316 stainless steel is probably your best choice. I am using it to build Tubular Linear Induction Motors for my project.

 

[kuruman]

To @Jon Abel : You do realize that continuing this thread will likely go unnoticed by the OP, @Dustin, who joined PF and posted it on April 15, 2018, and "was last seen" on April 20, 2018.

 

[Jon Abel]

Sure, whatever. Maybe he'll see it in his email. Either way, I don't see a lot of other current questions or examples about Lenz's Law - so I shared what I've been working with.