Creating a Poleless Magnet Ring

[mrspeedybob]

mrspeedybob,

Oh, dear, I thought that this had been settled. Your reasoning is very clever and insightful, and I certainly did not consider the resultant vectors. The field lines of the iron atoms seem analogous to those around the turns of a toroidal electromagnet. In that case, the external magnetic field is severely attenuated by symmetry. I cannot find a flaw in your analysis, probably because I cannot visualize the field sufficiently, but since I know that the effect in the toroidal electromagnet is real, I must conclude that somehow your analysis is incorrect. Perhaps someone else can shed more light on this matter.

Mike

The reason the field is significantly stronger within the material is that the fields produced from opposite sides of the material (or from opposite sides of the coil) add together at points in between them and partially cancel for points not in between them. They do not however completely cancel because one side of the coil or material is always closer then the other side. If you actually construct this magnet you will find that it does indeed have an external field,

 

[MS La Moreaux]

In analyzing Faraday's disk dynamo (the case with a uniform magnetic field through the disk), I have come up with the following. The force on a free electron in the disk is radial. Its path in the disk is therefore along a radius, relative to the disk. The drift speed of an electron is on the order of microns per second. Therefore, it will be dwarfed by the rotation speed of the disk and will be dragged along with the disk as the disk rotates. If we consider the drift path of a single electron as the circuit, it will be a very tight spiral of millions of turns between the axle and the rim, as viewed in space rather than relative to the disk. This spiral will be stationary in space, not rotating with the disk. Thus, there will be no change with time of the flux linking the circuit.

Mike

 

[MS La Moreaux]

mrspeedybob,

It has occurred to me that the external magnetic field of a ring magnet would only be due to the atoms at the surface and would thus be negligible. In the interior, the atoms would be like little bar magnets lined up head to tail. Thus, the flux would flow from one to another all the way around the ring, keeping their flux entirely confined to the material of the ring.

My physics textbook states that an infinitely long electromagnet would have no external magnetic field. If a finite electromagnet is bent around so that its ends are joined seamlessly, it would also have no external field.

Mike

 

[thehacker3]

mrspeedybob,

It has occurred to me that the external magnetic field of a ring magnet would only be due to the atoms at the surface and would thus be negligible. In the interior, the atoms would be like little bar magnets lined up head to tail. Thus, the flux would flow from one to another all the way around the ring, keeping their flux entirely confined to the material of the ring.

My physics textbook states that an infinitely long electromagnet would have no external magnetic field. If a finite electromagnet is bent around so that its ends are joined seamlessly, it would also have no external field.

Mike

But ring magnets DO have a decently strong magnetic field - so what gives?

 

[MS La Moreaux]

thehacker3,

Are you referring to the internal or external field?

Mike

 

[thehacker3]

thehacker3,

Are you referring to the internal or external field?

Mike

External

 

[MS La Moreaux]

thehacker3,

Where do you get the idea that ring magnets have a decently strong external field? Do you understand that the ring magnet is magnetized circularly so that there are no poles? Toroidal transformers are used because they have severely reduced external fields.

Mike