Permanent magnets and "permeance"

[hollowman]

TL;DR Summary

Permanent magnets and "permeance"

I have an indoor recumbent exerciser bike that uses a non-contact magnetic "clutch" for resistance.
There is permanent-magnetic fulcrum assembly that pivots on a hinge. And a heavy iron flywheel that spins when pedaling. The arc moves toward/away from flywheel based on resistance setting.

I've had the bike for about 10 years and have noticed that I have had to increase the resistance setting in order to achieve the SAME resistance. I added some neodymium magnets to the fulcrum, which restored resistance somewhat.

So the question is about permeance. How long do permanent magnets last in nature, given constant room temperature? Also, what about that iron flywheel. Does its proximity (errrr... magnetic "drag") have de-magnetizing effects?

 

[berkeman]

Summary:: Permanent magnets and "permeance"

So the question is about permeance. How long do permanent magnets last in nature, given constant room temperature?

I think you mean more "permanence" and not permeance. Permeance is a different concept (the inverse of magnetic reluctance):

https://en.wikipedia.org/wiki/Permeance

It looks like vibration would be the main reason that your permanent magnets lost some of their magnetization over time:

https://www.magnetassemblies.com/permanent-magnets/

Non-permanent magnets are electromagnets which require an external electric current to trigger magnetism or non-magnetism. Permanent magnets, on the other hand, retain their magnetism indefinitely or until they are demagnetized by vibration, dirt, corrosion or interfering magnetic fields. Read More…

EDIT/ADD -- Although I suppose it might be possible for the eddy currents in the flywheel to cause some demagnetization of the permanent magnets over time. Will have to do some Googling about that...

 

[anorlunda]

demagnetized by vibration, dirt

Wow. Thanks @berkeman. I didn't know that.

 

[hollowman]

I think you mean more "permanence" and not permeance. Permeance is a different concept (the inverse of magnetic reluctance):
https://en.wikipedia.org/wiki/Permeance

I meant exactly what I said: permeance
And I think it applies to permanent magnets, including magnetic recording tape.

In engineering;
https://www.arnoldmagnetics.com/res...ficient-calculations-for-rectangular-magnets/
https://product.tdk.com/en/products/magnet/pdf/e371_circuit.pdf

 

[hollowman]

Another way to think about the issue would be, all else held equal, how long to would magnetic domains remain "in static position" if the fridge magnet or audio cassette tape were isolated in some remote, inter-cluster corner of the universe?

 

[darth boozer]

In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns.

Googling "magnetic permanence" returns results about "magnetic remanence" which is more relevant to your question about the loss of magnetism.

See also: https://sciencing.com/causes-magnet-lose-its-magnetism-8229455.html

 

[hollowman]

In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns.

Googling "magnetic permanence" returns results about "magnetic remanence" which is more relevant to your question about the loss of magnetism.

And/or "Coercivity", https://en.wikipedia.org/wiki/Coercivity
" also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized"

 

[hollowman]

Another thing to note is that in engineering (especially industries related to permanent magnets/magnetism, like TDK), "permeance" is used as follows:
"in calculating a magnetic circuit, permeance P, which is the reciprocal of magnetic resistance R" ...
https://product.tdk.com/info/en/products/magnet/technote/designguide.html
"Permeance coefficient Pc is used to express the operating point of the magnet on the B-H curve. This value is defined as the ratio of magnetic flux density Bd and magnetic field strength Hd at the operating point."

 

[hollowman]

Going over some web articles, it's possible that the bike's ferrite magnets were, over time, also affected (demag'd) by the Earth's mag. field.

 

[Tom.G]

Here is a brief description of magnet weakening, it doesn't cover everything but is a start.
https://www.duramag.com/techtalk/tech-briefs/do-magnets-have-a-shelf-life/

http://www.magmamagnets.com/magnets-technical-information/permanent-magnet-stability/
(above, and many more, found with:
https://www.google.com/search?q=lifetime+of+permanent+magnet)

This thread and the link(s) in it also discuss magnet lifetime.
https://www.physicsforums.com/threads/magnet-lifetime.404696/

Cheers,
Tom

 

[hollowman]

[...]
Cheers,
Tom

Thx.
It would still be helpful if there were a more definitive or rigorous description or quantification -- beyond the somewhat roundabout terminology thrown around this thread, like "permeance". Maybe the "magnetic stuff" or "magnetic mass" of a NATURAL permanent magnet is something like the Higgs Field? Maybe "magnetic field is a vector field" or "tesla" or "webers" (as noted in resources like Wiki).
When measuring mag fields, one can use instruments such as [gauss meter]:

Also see:
https://www.magnet-sdm.com/2017/08/01/magnetic-field-strength-permanent-magnet/