Is there such thing as "magnetic friction" and what controls it?
Microcontrolled
Posts: 2,461
Assume you have a large disk magnet acting as a table, and a smaller magnet that would be configured in a way that it would float effortlessly on top of the magnetic disk. Assume now that you could float the magnet and then spin it around the disk with a swipe of your hand, much like the "Quantum magnets" video posted on this forum a while back, which I'll post a link to shortly.
It was while watching that video that I had a thought, assuming you put the whole setup in a vacuum and gave it a spin, just how long would it spin before coming to a stop? And what force acting upon the smaller magnet would allow it to come to a stop? Wouldn't there have to be some sort of "magnetic friction" that would keep it from spinning indefinably?
I figured the forum could find some answers here.
Thanks,
Microcontrolled
It was while watching that video that I had a thought, assuming you put the whole setup in a vacuum and gave it a spin, just how long would it spin before coming to a stop? And what force acting upon the smaller magnet would allow it to come to a stop? Wouldn't there have to be some sort of "magnetic friction" that would keep it from spinning indefinably?
I figured the forum could find some answers here.
Thanks,
Microcontrolled
Comments
I may be not fully correct on this, but you would essentially have the equivalent of a linear motor. How long it would spin would be determined by the magnitude of eddy currents induced in the opposite magnet as spinning results in moving magnetic fields. The magnetic fields resulting from the eddy currents would be in opposition to the fields of the fixed and spinning magnets. The magnitude of the eddy currents would depend on a lot of factors. Otherwise the rules are likely still the same as all motors. Dynamic braking could be another term for magnetic friction.
Frank
http://en.wikipedia.org/wiki/Eddy_current_brake
But in the case you describe, the magnetic fields would repel each other and therefore, I guess, the magnetic field of each disk would be prevented from entering the opposing disk. So, as long as the fields are perfect and don't have some kind of distortion, I think they could, in theory, spin forever.
But I'm just guessing on that.
No it's not. I'm still thinking about it. And I can assure you, if there's a vacuum anywhere in any of the farthest bowls of space, it's in my thought processes.
I'm guessing it depends on whether or not the bowl of space is in free fall. And how deep that bowl might be.
'
Some really cool stuff on magnets
I haven't seen an answer to your question yet.
I'm not aware of any arrangement of magnets that will let you do this.
Now if you want to talk about superconductors, that's different. The eddie currents others have mentioned is what allows the superconductor to levitate. The eddie currents in a superconductor don't cause any heat. There isn't any "magnetic friction". Just like there isn't "gravity friction". Planets seem content to keep moving around the sun for a very long time. Keeping something moving doesn't require energy.
If you could move the magnetic track and superconductor into a vacuum, I'm pretty sure the superconductor would continue around the track as long as it remained in its superconducting state.
Magnetic damping and eddie current brakes don't apply to superconductors the same as they do to normal conductors.
Superconductors are strange, not just because they don't resist the flow of electricity. The way they interact with magnets isn't easily understood.
I was hoping some expert on superconductors would enlighten us. I'm no expert. My knowledge of superconductors comes from a group presentation/paper I did in my modern physics class. I learned enough to know I'm not the only non-expert replying here.
Duane
I'm thinking of something like this, though I didn't describe it very well initially.
Red is north, grey is south. Sorry for the rough sketch.
Tell me though, would this setup work, or not?
In your case both ends of the system are magnets but of course they are also both condutors.
This is your "magnetic friction". As an experiment try sliding a strong magnet accross the surface of an aluminium sheet. Or get hold of an old mechanical speedometer take it apart and see how that works.
Have you ever played with magnets? That configuration in your picture would never be stable. As far as I know there is no such stable configuration.
You can have ringed magnets floating above one another, but it requires something to hold the top magnet in place. I've seen ring magnets with a dowel through the center of them. The dowel would cause significant friction.
I only purely magnetic levitation (without electronics to continually monitor and adjust the field) I know of is with a top. The top was very difficult to adjust and would only spin in place. I still don't see a magnetic track arrangement without electronics or some sort of physical stabilization (which would cause friction).
If only we had magnetic monopoles, all sort of interesting things could be done.
Duane
Edit: I hadn't seen Heater's post before I posted this reply. I agree with what he said.
[video=youtube_share;Ws6AAhTw7RA]
'
Great video RON
Yes, amazing video.
All demos I had seen (and performed) would levitate a small magnet over a superconducting disk.
I assumed Microcontrolled was referring to this video in the OP.
Very cool.
Duane
@Duane Degn: Yes, I was assuming both the top and bottom magnet where secured by a wooden structure of some sort, but I didn't want it to obscure the rendering so I left it out.
Google "bismuth levitation" to see some examples.
See yet another reason to like the element of Bismuth.
You might also fall in love with pyrolytic carbon/graphite.
Check it out on youtube, etc.
http://www.youtube.com/watch?v=k9Zwc5U2kHE
Superconductors
The ability of superconductors to exactly counter magnetic fields. That's about the limit of my knownledge on the subject.
I'm not sure if it's clear or not but the cold disk is the superconductor. The track is made up of magnets.
High temperature superconductors (like the one in the video) are some sort of ceramic material. I think many have the element Yttrium in them. (I remember seeing the Y symbol in a formula.)
Now, what if the copper tube were a superconducting tube?
I think a superconducting tube would completely stop the magnet from falling.
Duane
The disk in the video is a wafer of sapphire, and covered with a 1 micro-meter thin layer of so-called "high-temperature" yttrium-barium copper oxide superconductor (meaning that it superconducts when cooled with liquid nitrogen, iirc). Magnetic fields don't normally penetrate superconductors but because the layer is so thin some magnetic field lines will penetrate here and there and it's those that keep the wafer in place.
So it's very far from trying to levitate two magnetic disks above each other - the magnetic attraction/repulsion isn't really the force involved here.
All in my layman terms & understanding, of course - I'm not a physicist.
-Tor
EDIT: I found a great explanation from the guys behind this: http://www.quantumlevitation.com/levitation/The_physics.html
There's a drawing at the bottom of the page which describes how the wafer is held in place.
Duane