Suspending a Magnet or Ferrous rod in a Tube

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    Magnet Rod Tube
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Discussion Overview

The discussion revolves around the feasibility of suspending a magnetic or ferrous rod within a plastic tube, specifically in the context of constructing a small version of Keith Lofstrom's Launch Loop concept. Participants explore the theoretical and practical challenges of achieving magnetic levitation without contact with the tube walls, considering both permanent magnets and alternative methods.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant is attempting to create a small version of the Launch Loop and seeks methods to achieve magnetic levitation of a rod within a tube.
  • Another participant asserts that it is impossible to achieve this with permanent magnets due to Earnshaw's theorem, suggesting the need for electromagnets or active control systems.
  • Some participants discuss the potential of using centrifugal force or spinning magnets to maintain levitation, referencing the principles of magnetic bearings.
  • A participant mentions the possibility of using solenoids with triggering circuits to push the rotor along, indicating an alternative approach to the problem.
  • There is a debate about the implications of Earnshaw's theorem, with some arguing it is not as restrictive as commonly believed, while others maintain that it applies in this scenario.
  • One participant expresses doubts about the feasibility of spinning magnets quickly enough to achieve the desired effect, indicating limitations in their capabilities.
  • Another participant challenges the idea of creating an effective monopole by positioning dipoles, asserting that it will not work in this context.

Areas of Agreement / Disagreement

Participants generally disagree on the feasibility of using permanent magnets for levitation, with some asserting it is impossible while others suggest alternative methods may exist. The discussion remains unresolved regarding the application of Earnshaw's theorem and its implications for the proposed setup.

Contextual Notes

Participants reference Earnshaw's theorem and its implications for magnetic levitation, but there are differing interpretations of its restrictions. The discussion includes assumptions about the capabilities of participants and the practicalities of constructing the proposed system.

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.
 
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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.
 
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..

 
Sketch showing fixed and movable pulley system..

Launch Loop.jpg
 

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Last edited:
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.
 
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.
 
How does spinning the magnets make it work?
 
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.
 
f95toli said:
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.
 
  • #10
rumborak said:
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)
 

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