Register to reply

Ring Magnet

by MS La Moreaux
Tags: magnet, ring
Share this thread:
MS La Moreaux
#1
Sep19-10, 06:21 PM
P: 79
Has anyone heard of a permanent magnet in the shape of a ring or toroid with no poles? I believe that one could be made by winding a ring of magnetic material, such as steel, with a wire winding, like one winding of a toroidal transformer. DC current could then be passed through the winding for a sufficient length of time and then the winding removed. One would then have a magnet with no poles, the flux circulating around the ring. I believe that if the ring were uniform and symmetrical, there would be no magnetic field external to the ring. I cannot think of a practical use for such a magnet but just am interested in the principle. Comments?

Mike
Phys.Org News Partner Physics news on Phys.org
A new, tunable device for spintronics
Watching the structure of glass under pressure
New imaging technique shows how cocaine shuts down blood flow in mouse brains
Drakkith
#2
Sep19-10, 10:28 PM
Mentor
Drakkith's Avatar
P: 11,877
Is this material a hollow torus or a solid?
pallidin
#3
Sep20-10, 04:38 PM
P: 2,292
Remember those old horseshoe shape magnets with a soft iron "keeper" connecting the poles?
There you have it.
Note that there is still some external magnetism, however.

MS La Moreaux
#4
Sep20-10, 06:46 PM
P: 79
Ring Magnet

Drakkith,

I was thinking of solid, but I do not believe that it would make any difference.


pallidin,

Yes, it is similar, but the maximum symmetry of the torus is necessary to eliminate any external field.

Mike
Bob S
#5
Sep20-10, 08:20 PM
P: 4,663
Quote Quote by MS La Moreaux View Post
Has anyone heard of a permanent magnet in the shape of a ring or toroid with no poles? I believe that one could be made by winding a ring of magnetic material, such as steel, with a wire winding, like one winding of a toroidal transformer. DC current could then be passed through the winding for a sufficient length of time and then the winding removed. One would then have a magnet with no poles, the flux circulating around the ring. I believe that if the ring were uniform and symmetrical, there would be no magnetic field external to the ring.
Binside is everywhere parallel to the boundary between air and the magnetic material. If H-tangential is continuous across this boundary, then the tangential Bair = Binside/μ, where μ is the relative permeability.
I cannot think of a practical use for such a magnet but just am interested in the principle. Comments?
Would there be an Aharanov-Bohm effect

http://en.wikipedia.org/wiki/Aharono...%93Bohm_effect

on any charged particle going through the hole in the ring? Is there a practical use?

Bob S
granpa
#6
Sep20-10, 08:33 PM
P: 2,258
Aharonov–Bohm effect:

Schematic of double-slit experiment in which Aharonov–Bohm effect can be observed: electrons pass through two slits, interfering at an observation screen, with the interference pattern shifted when a magnetic field B is turned on in the cylindrical solenoid.

http://en.wikipedia.org/wiki/Aharono...%93Bohm_effect
stevenb
#7
Sep20-10, 11:24 PM
P: 697
Quote Quote by MS La Moreaux View Post
pallidin,

Yes, it is similar, but the maximum symmetry of the torus is necessary to eliminate any external field.

Mike
As pointed out by Bob S, even toroidal symmetry will not eliminate the external field if you only have a permanent magnet.

You could encase the torus in a superconductor to shield the magnetic field, and truly confine it.

A tightly wound toroidal coil with DC current is a cheaper way to have field inside and essentially zero field outside.

Quote Quote by MS La Moreaux
... DC current could then be passed through the winding for a sufficient length of time and then the winding removed ...
In other words, don't remove the windings from the ring, and don't turn off the current. Basically, the field from the current will cancel the field from the core, on the outside of the ring. (you can also think of this in terms of the boundary condition mentioned by Bob S, but now include the sheet linear current density K: i.e. Ht1-Ht2=K, which allows Ht2=0 on the outside)
MS La Moreaux
#8
Sep22-10, 02:08 PM
P: 79
The lack of an external field is the result of symmetry, not the particular source of the field. The lack of poles implies no external field. If there were an external field, there would be poles. If there is an external field, what does it look like?

Mike
Bob S
#9
Sep22-10, 02:27 PM
P: 4,663
Quote Quote by MS La Moreaux View Post
The lack of an external field is the result of symmetry, not the particular source of the field. The lack of poles implies no external field. If there were an external field, there would be poles. If there is an external field, what does it look like?
If there are no surface currents (coils), Curl H is continuous across the boundary between air and the ring (magnetic material), meaning Curl H = 0, or tangential Hair = Hring.. So Bair = μ0Hair = Bring/μ, where μ is the relative permeability of the ring material at field Bring.

Bob S
MS La Moreaux
#10
Sep22-10, 02:36 PM
P: 79
Bob S,

So what does the field look like? What is its shape?

Mike
Phrak
#11
Sep22-10, 02:50 PM
P: 4,512
Quote Quote by stevenb View Post
As pointed out by Bob S, even toroidal symmetry will not eliminate the external field if you only have a permanent magnet.

You could encase the torus in a superconductor to shield the magnetic field, and truly confine it.
Now, that's an interesting proposal. But will the counter-currents in the superconductor, required for cancelation of external fields, also cancel all internal fields?
thehacker3
#12
Sep22-10, 10:48 PM
P: 69
http://unitednuclear.com/index.php?m...roducts_id=290
pallidin
#13
Sep23-10, 12:27 AM
P: 2,292
Most commercial ring magnets actually have the poles on the "flat" sides.
The same is true for the above link, but harder to envision.
thehacker3
#14
Sep23-10, 10:55 AM
P: 69
Quote Quote by pallidin View Post
Most commercial ring magnets actually have the poles on the "flat" sides.
The same is true for the above link, but harder to envision.
Well then I'm puzzled by the question - what defines a pole? Like what makes the north pole of a magnet a pole, and not the middle of it?
stevenb
#15
Sep23-10, 04:56 PM
P: 697
Quote Quote by Phrak View Post
Now, that's an interesting proposal. But will the counter-currents in the superconductor, required for cancelation of external fields, also cancel all internal fields?
That's an interesting question about an "interesting proposal". Keep in mind that the proposal is not my invention. I've just read discussions on the Aharonov–Bohm effect that talk about using superconductors to remove any doubt that the electron is traveling in a field free region.

I'm not very knowledgable about superconductor theory so I hesitate to answer definitively. My best guess is that generally the internal fields won't cancel, and that the internal field will always be strengthened. But, I wouldn't be overly surprised if an expert comes here and tells us that it is possible to configure a superconducting shield with a particular core material in a way that cancels both internal and external fields. It seems counter-intuitive to me, but my intuition has failed me often enough that I'm not surprised when it happens.
stevenb
#16
Sep23-10, 05:10 PM
P: 697
Quote Quote by thehacker3 View Post
Well then I'm puzzled by the question - what defines a pole? Like what makes the north pole of a magnet a pole, and not the middle of it?
That's actually a good question. A magnetic "pole" is one of those things that everyone talks about and assumes is straightforward, until they try to define it. I like to think of a pole as the part of a magnet that has the most concentrated field that is typically coming mostly perpendicularly to the surface.

For example, a typical bar magnet or a spherical magnet (like the earth) are like dipoles, and the dipole field pattern (you can look it up in any EM book) has clear poles. Magnetic field lines are always closed, so the north poles field lines circulate back to the south pole.

The OPs example is interesting because there is no clearly identifiable pole, due to the symmetry. The field lines internal to the toroid just circulate back on themselves always. The external field lines also circulate back on themselves.

However, typical ring magnets aren't magnetized in this way. They can be magnetized so that the top and bottom (think heads and tails of a coil-like object) are the north and south poles. Also, they can be magetized so that one side of the circular cross section is the north pole and the oposite side is the south pole. Think about a two dimensional version of the earth (circle rather than sphere) with a north and south pole.
MS La Moreaux
#18
Sep25-10, 02:57 PM
P: 79
All magnetic field lines of a permanent magnet must pass through the magnetic material, since the iron atoms are the source of the field lines, and therefore the lines must pass through the iron atoms. Any external field lines must therefore pass through the surface of the torus. There is no way for this to happen without violating symmetry. Thus, there can be no external field.

Mike


Register to reply

Related Discussions
Ampere's law for a closed ring bar magnet Introductory Physics Homework 1
Finding the B-field at a point outside ring of current IN Plane of ring Introductory Physics Homework 1
Would running an electrical current through a magnet destroy the magnet? General Physics 4
What make the magnet to be magnet with magnetic field? General Physics 23
Ring versus Disk Magnet Classical Physics 3