Suspending a Magnet or Ferrous rod in a Tube

In summary, the conversation discusses the concept of constructing a small version of Keith Lofstrom's Launch Loop using a clear plastic tube as the sheath and a smaller plastic tube with magnets or iron pieces as the rotor. The main question is how to prevent the rotor from touching the walls of the sheath as it moves through. The participants suggest using an electromagnet or an active controller, but one person points out that Earnshaw's theorem makes it impossible to achieve this with permanent magnets. The conversation also touches on the idea of spinning the magnets or using diamagnetic materials, but the project has since been abandoned due to time and resource constraints.
  • #1
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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  • #2
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.
 
  • #3
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..

 
  • #4
Sketch showing fixed and movable pulley system..

Launch Loop.jpg
 

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  • #5
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.
 
  • #6
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.
 
  • #7
How does spinning the magnets make it work?
 
  • #8
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.
 
  • #9
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)
 

1. How does suspending a magnet or ferrous rod in a tube work?

Suspending a magnet or ferrous rod in a tube works by utilizing the principles of magnetic force and gravity. The magnet or ferrous rod is attracted to the walls of the tube, and gravity pulls it downwards, resulting in a balanced equilibrium. This allows the magnet or rod to remain suspended in the center of the tube.

2. What factors affect the stability of a magnet or ferrous rod suspended in a tube?

The stability of a suspended magnet or ferrous rod can be affected by several factors, including the strength of the magnet, the diameter and length of the tube, and the viscosity of the fluid inside the tube (if any). A stronger magnet, shorter tube, and thicker fluid will result in a more stable suspension.

3. Can any type of magnet or ferrous rod be suspended in a tube?

Yes, as long as the magnet or rod is ferromagnetic (attracted to a magnetic field) and has a strong enough magnetic force to overcome the force of gravity, it can be suspended in a tube. However, the size and shape of the magnet or rod may affect its stability in the tube.

4. What applications does suspending a magnet or ferrous rod in a tube have?

Suspending a magnet or ferrous rod in a tube has various applications in the fields of science, engineering, and education. It can be used to demonstrate the principles of magnetism and gravity, to measure the strength of a magnet, and to create stable magnetic levitation systems for transportation or energy storage.

5. Are there any safety precautions to consider when suspending a magnet or ferrous rod in a tube?

Yes, it is important to handle strong magnets with caution as they can cause serious injuries if mishandled. It is also important to keep the suspended magnet or rod away from electronic devices and credit cards, as the magnetic field can interfere with their functioning. Additionally, make sure the tube is securely attached and the magnet or rod is stable before conducting any experiments or demonstrations.

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