Sudden polarity reversal in opposing permanent magnets?

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Summary:

Opposing permanent ring magnets, when forced close together in alignment, suddenly attract. Need help finding resources explaining why.

Main Question or Discussion Point

Hi all,

I'm working with a pair of multi-pole neodymium ring magnets (rated ~119lbF) and finding that if the opposing poles are forced together, with the magnets in alignment, that at very close range (may be a millimeter or two apart?) the two magnets will suddenly snap together as if one has reversed polarity. There is NO electric current involved, but the only resources I am finding about this phenomenom are related to electromagnets and induced magnetism.

Can anyone point me to a source explaining what's happening?

Thank you!
 
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Answers and Replies

  • #2
berkeman
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Summary:: Opposing permanent ring magnets, when forced close together in alignment, suddenly attract. Need help finding resources explaining why.

Hi all,

I'm working with a pair of multi-pole neodymium ring magnets (~55lbF) and finding that if the opposing poles are forced together, with the magnets in alignment, that at very close range (may be a millimeter or two apart?) the two magnets will suddenly snap together as if one has reversed polarity. There is NO electric current involved, but the only resources I am finding about this phenomenom are related to electromagnets and induced magnetism.

Can anyone point me to a source explaining what's happening?

Thank you!
Welcome to the PF. :smile:

What's a "ring" magnet?

Can you post a link to the datasheet or more information about these magnets? Thanks.
 
  • #4
berkeman
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https://www.mcmaster.com/2494N17

Ring magnet is a magnet with a hole in the middle, like a donut.
Hmm, that is confusing to me. They show discrete poles on the top (and also "N" on the bottom?), which implies that there are S pole areas between them on the top surface. It looks like they are designed for high attachment force in contact with a ferrous metal, not for interaction with other magnets. Do the two magnets feel like they twist at all when you say the force turns attractive between them?

1590177486398.png


A multipole cylindrical magnet would seem to have this kind of magnetic field distribution...

https://www.hsmagnets.com/wp-content/uploads/2015/11/D_plot_of_multipole_ring_magnets.jpg

1590177787558.png
 
  • #5
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Yeah, McMaster detail drawings aren't always quite right, though, so I don't know how trustworthy that is. They do have clearly marked sides, with matching-marks being VERY attracted to each other and non-marked sides repelling. No, they don't twist, which I also initially thought might be happening, but it's not.

I'm going to try to repeat this with "standard" magnets after I get off a work call.
 
  • #6
berkeman
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I'm going to try to repeat this with "standard" magnets after I get off a work call.
Keep in mind that when you are pressing magnets together in opposition, you probably are causing a partial loss of magnetization. So the magnets will not be as strong after you do this a few times. How much is lost depends on the kind of the magnet.

http://www.mceproducts.com/Knowledge_Base/Articles/Demagnetization_of_Permanent_Magnets.htm

https://www.duramag.com/techtalk/te...ure-and-demagnetization-from-external-fields/

Demagnetizing from External Fields

Magnet alloy is magnetized with a sufficiently intense magnetic field that is established in the same direction as the magnet’s orientation. When a magnetized magnet is exposed to a strong magnetic field that is established in opposition to the magnet’s magnetization, part of the magnet may be demagnetized. This demagnetization essentially reduces the effective field of the magnet and the magnet’s performance will therefore degrade.


External demagnetizing fields can originate from fields created by electromagnets/coils or other neighboring permanent magnets. An example of a coil would be a motor application where the dynamic fields created by a coil set interact with fields from permanent magnets to create motion. An example of a static case of demagnetizing fields from neighboring permanent magnets could be a Halbach Array where some internal magnets have a poor L/D and low Hci and are demagnetized from other array magnets. A dynamic case could be a permanent magnetic toque coupler which “slips” and like magnet poles rotate over one another.
 
  • #7
I noticed a similar phenomenon in a magnetic drive I made for a magnetic Tesla Turbine. With a certain spacing, there seems to be a sweet zone where the fields interlock and provide attraction, from a normally repelling mode.
Here is my demo of the "Magic Magnetic Coupler" where the effect is shown:

 

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