Suspending a Magnet or Ferrous rod in a Tube

[CockatooDude]

Let us start with a bit of background information. I am working on seeing if I can construct a very small version of Keith Lofstrom's Launch Loop concept, to see how well I can get it to work. To do this, I am using a larger clear plastic tube as the sheath, and a smaller plastic tube with either magnets or bits of iron at regular intervals as the rotor. inside of the sheath. And I have run into a bit of a problem, how do I get, say a magnetic rod, or steel rod, to magnetically "hover" in the middle of the sheath, so it doesn't touch the walls as it moves through the tube. I bought some small disk magnets to experiment with, and I can get them to repel sideways, but only if they at the right position from each other. Maybe if I placed a ring magnet around the sheath and put an iron rod in the middle, it would repel it from all sides? At any rate, the most important question here is how one could get a magnetic or iron/steel rod to not touch the walls of a plastic sheath, can I do it with permanent magnets, and how would I place the magnets? Thanks in advance for any answers.

 

[f95toli]

No, its impossible (literally) to do this with permanent magnets. See Earnshaw's theorem.
You either need an electromagnet where the field is changing, and/or some sort of active controller.

This is a frequent question here on PF so if you search you will find several threads discussing this.

 

[CWatters]

As I understand it Keith Lofstrom's Launch Loop uses the centrifugal force of the moving rotor cable to lift up the sheath. Perhaps you can do something similar without the sheath? I was reminded of this..

 

[CWatters]

Sketch showing fixed and movable pulley system..

 

[CWatters]

Sorry if this is obvious but (in addition to the problem f95toLi mentions) you can't use permanent magnets to push the rotor along either.

 

[CockatooDude]

Thank you all for the replies, yes, I have since discovered Earnshaw's theorem. However if the magnets on the outside of the tube, or the entire inside tube was spun quickly then it would work just fine. Although I don't have the time to actually go through with building that. As for pushing the inner tube, I was planning on using solenoids with triggering circuits similar to that of a coilgun.

 

[CWatters]

How does spinning the magnets make it work?

 

[CockatooDude]

The same way magnetic bearings work. Earshaw's theorem simply states that a collection of point charges, in this case magnets, cannot be maintained in a position of equilibrium solely by their static electrostatic or magnetic forces. In my proposed setup, both sets of magnets would face each other, and have the north poles of the inner magnets pointing radially outwards, and the north poles of the outer magnets pointing radially inwards. If one of the sets, either the outer or the inner set, was spun quickly, there wouldn't be enough time at anyone position of the inner magnets relative to the outer magnets for the system to destabilize. However, for my capabilities and for the amount of time I have, spinning one of the sets of magnets very quickly is too much for me to accomplish. And I have since abandoned the project. Maybe I'll return to it later on my own time.

 

[rumborak]

No, its impossible (literally) to do this with permanent magnets. See Earnshaw's theorem.
You either need an electromagnet where the field is changing, and/or some sort of active controller.

This is a frequent question here on PF so if you search you will find several threads discussing this.

Given the frequency with which the question comes up, I think we should be a bit more discerning in our answer. Earnshaw's theorem is not as restrictive as most people assume. For example, you can very easily levitate a block of Bismuth in a static magnetic field because it is diamagnetic.

 

[Vanadium 50]

Earnshaw's theorem is not as restrictive as most people assume.

True, but in this case it will not work. You cannot make an effective monopole by positioning dipoles north end out. (True for a sphere, and true for a tube)