Magnet repulsive force -- how long does it last?

[tech99]

The permanent magnets in old earphones slowly lose power over many years. It is also normal practice to use a "keeper" on a permanent horse shoe magnet to preserve its strength. The external field of a magnet is in a direction to slowly demagnetise the magnet.

 

[sophiecentaur]

The permanent magnets in old earphones slowly lose power over many years. It is also normal practice to use a "keeper" on a permanent horse shoe magnet to preserve its strength. The external field of a magnet is in a direction to slowly demagnetise the magnet.

This is true but it takes us from the 'theoretical idea; to the practical reality. It's not the Field that gets 'tired' in old headphones; it's the energy levels associated with the dipole orientation in old fashioned magnetic materials. At room temperature, there is a finite probability of the occasional flip to a lower energy state. All materials can be expected to change over time - even when that change corresponds to longer than the possible life of the Universe.

The OP asks "how long will it last?". Well - how long is a piece of string?

 

[snorkack]

Iron heated to its Curie temperature of 770 degrees immediately loses all magnetism and does not recover it on cooling.
Is there any way to predict speed of magnetic creep - like, a magnet will keep half its initial strength after so much time heating at 700 Celsius, or after so much (a longer time) heating at 600 degrees, et cetera?

 

[mitochan]

The law of entropy increase in statisc or thermal mechanics would work here. The ordered spins of molecules tend to be randomized which leads zero magnetic field. The ordered motion of charges e.g. electric currents in wire would be dissipated which also leads to zero magnetic field.

 

[sophiecentaur]

The law of entropy increase in statisc or thermal mechanics would work here. The ordered spins of molecules tend to be randomized which leads zero magnetic field. The ordered motion of charges e.g. electric currents in wire would be dissipated which also leads to zero magnetic field.

Yes. Although there can hardly be a "formula" for this as the decay rate will depend on the elements involved and the detailed structure of the magnet. I think you'd need to do measurements involving an oven to produce a curve for each particular material.

 

[JackCatDaily]

As we know the magnet will stop to repel each other after some time, is there any formulae to calculate when it will stop? From common sense, how long magnet will stop repel each other? 1years?

Sounds like you are asking if there is an energy drain in the magnets that will eventually lead to a demagnetized state of the magnets. Magnets are perpetual—never dying—unless some outside influence changes them.

 

[snorkack]

Sounds like you are asking if there is an energy drain in the magnets that will eventually lead to a demagnetized state of the magnets. Magnets are perpetual—never dying—unless some outside influence changes them.

The point is, there is an energy drain. Magnetization is a store of energy. There are processes which convert magnetization energy into heat. So the question is, are there any quantitative expressions for the speed of spontaneous demagnetization?

 

[mitochan]

So the question is, are there any quantitative expressions for the speed of spontaneous demagnetization?

Statistical mechanics say it is the factor of
e^{\frac{-E_g}{k_BT}}
where E_g is an energy gap to overcome potential peaks to other states.

 

[paradisePhysicist]

Also, if you point the magnets in the same direction as the Earths magnetic field, they should (theoretically) last longer. The website Keith_Mclary posted said magnets should be stored in the same direction as nearby magnets.

 

[Keith_McClary]

The website Keith_Mclary posted said magnets should be stored in the same direction as nearby magnets.

I think it means the opposite:

Keep the magnets attracting in a row, and where the rows are attracting

 

[sophiecentaur]

Also, if you point the magnets in the same direction as the Earths magnetic field, they should (theoretically) last longer.

You need to be realistic about this. The 'rules' for looking after permanent magnets were formulated when the best we could do involved using a suitable steel alloy and a suitable shape (such as a horseshoe. Keepers and proper storage boxes were important. Nowadays, we have fantastically strong PMs, made from fancy alloys and they can be used for decades (centuries?) for simple jobs like door catches, without needing special storage with keepers

But all this depends on what a magnet is to be used for. If a permanent magnet is ever to be used in a measurement process then some calibration could be needed. (Analogue meters for instance depend on the field inside to be unchanging.) As with al Engineering, the numbers count and you'd need to do much better than use a word like "last".

I trawled around for some hard facts about this. Most manufacturers are a bit vague but I did find this link which says 5% loss in 100 years for a neodymium magnet. If my old Avometer was 5% out after 100 years, I wouldn't feel too bad about it (but it won't have a neodymium magnet in it, of course).

Feel free to trawl for your own information.

 

[paradisePhysicist]

You need to be realistic about this. The 'rules' for looking after permanent magnets were formulated when the best we could do involved using a suitable steel alloy and a suitable shape (such as a horseshoe. Keepers and proper storage boxes were important. Nowadays, we have fantastically strong PMs, made from fancy alloys and they can be used for decades (centuries?) for simple jobs like door catches, without needing special storage with keepers

Every little thing helps. For instance, the solar panels are said to be good for 25 years. If we could increase that to 27 years that is a worthwhile improvement.

But all this depends on what a magnet is to be used for. If a permanent magnet is ever to be used in a measurement process then some calibration could be needed. (Analogue meters for instance depend on the field inside to be unchanging.) As with al Engineering, the numbers count and you'd need to do much better than use a word like "last".

I trawled around for some hard facts about this. Most manufacturers are a bit vague but I did find this link which says 5% loss in 100 years for a neodymium magnet. If my old Avometer was 5% out after 100 years, I wouldn't feel too bad about it (but it won't have a neodymium magnet in it, of course).

Feel free to trawl for your own information.

I guess my question is, did they actually store a neodymium magnet for 100 years or just measured it for a few years and assumed a linear movement? There are a lot of variables influencing the age of magnet such as North pole shifting, random solar bursts of radiation and other things. I have no idea how much the North pole effects magnets, I guess you would have to sit a bunch of magnets (8 per direction, and then 4 different types of magnets) in the same room as other magnets in different directions (20 id say, in total 640 magnets) then measure the magnetism after 100 years, as well as making sure the room is/was temperature uniform throughout all areas.

I think it means the opposite:

Not sure I understand, the website says to keep the magnets attracting, which is what I suggested.

 

[sophiecentaur]

Not sure I understand, the website says to keep the magnets attracting, which is what I suggested.

Whatever you meant in your description, you implied they would 'all' be side by side, pointing in the same direction. That's where the 'wrong' comment came from. Diagrams are always a good idea, even if they're a pain to insert into a post. " NSNSNSNS" could have made it clear. You suggested they should all point to the North Pole, which is NNNNNNN. SSSSSSSS.

 

[snorkack]

The thing is that the hysteresis loop with its features like remanence and coercivity is a short term one - it will shrink over time spontaneously.
If you put a magnetized magnet into no magnetic field, its magnetization is a store of energy and dissipates over time. If you put the magnet into the original magnetizing field then the magnetization is the lowest energy state and stays forever, or builds up if missing. If you put it in field weaker than the original magnetizing field but in the same direction, the magnetization dissipates slower, and to the value fitting the external field, not zero.