Magnet repulsive force -- how long does it last?

[cel123456]

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?

 

[anuttarasammyak]

Two magnets repel and repel force which depends on distance keeps the same forever. Motion of magnets would die due to other factors e.g., friction.

 

[cel123456]

Two magnets repel and repel force which depends on distance keeps the same forever. Motion of magnets would die due to other factors e.g., friction.

Normally how long this repel process will stop?

 

[Drakkith]

Normally how long this repel process will stop?

The repulsive force will not stop until the magnets are destroyed/demagnetized (for a permanent magnet) or until the electrical current stops (for an electromagnet).

 

[sophiecentaur]

Normally how long this repel process will stop?

The magnetic force behaves like the gravitational force. The force of attraction of a mass to the Earth (its weight) stays the same as long as other things don't change. Two permanent magnets will experience the same force between them until 'something' changes. They don't 'get tired', if that's what you are suggesting.

 

[anuttarasammyak]

Normally how long this repel process will stop?

Damped oscillation as lectured here might be of your interest.

 

[sophiecentaur]

Damped oscillation as lectured here might be of your interest.

Not really in this discussion. The Energy in the Oscillation will decay but the force pulling the pendulum bob towards Earth at the equilibrium position won't.

 

[Keith_McClary]

common sense, how long magnet will stop repel each other?

See Paleomagnetism

The oldest rocks on the ocean floor are 200 mya – very young when compared with the oldest continental rocks, which date from 3.8 billion years ago. In order to collect paleomagnetic data dating beyond 200 mya, scientists turn to magnetite-bearing samples on land to reconstruct the Earth's ancient field orientation.

 

[Klystron]

{snip}Two permanent magnets will experience the same force between them until 'something' changes. They don't 'get tired', if that's what you are suggesting.

My late wife usually deferred to my EM knowledge except for animism and 'tired magnet' theory. One day a garnet decorated refrigerator magnet popped off the 'fridge and rolled across the kitchen floor.

"Aha", she exclaimed, "It got tired and fell off!", validating her theory.

I countered that only the glue holding the garnet had failed. The flat magnet was still firmly attached to the refrigerator. We literally had to pry the magnet loose when we sold the fridge.

 

[Keith_McClary]

Here's some info from a magnet company, but bear in mind that they would like to sell you new magnets.
https://www.duramag.com/techtalk/tech-briefs/do-magnets-have-a-shelf-life/
https://www.duramag.com/techtalk/te...ure-and-demagnetization-from-external-fields/

 

[anuttarasammyak]

The Energy in the Oscillation will decay but the force pulling the pendulum bob towards Earth at the equilibrium position won't.

I see your point. @cel123456 do you share it with @sophiecentaur?
Say your concern comes from sustainability of magnetic force itself not motion around the equillibrium, because loop current generates magnetic force, the loop current is lost if power to the circuit is lost. Superconducting magnet does not need power supply to keep current but it is fragile and tends to do phase transition to normal mode. Loop current of an electron spin is as stable as its charge but order of collective electron spins which make permanent magnet is disturbed by heat or magnetic field coming from environment. All these factors and more would make generated magnetic force unstable.

 

[etotheipi]

Not really in this discussion. The Energy in the Oscillation will decay but the force pulling the pendulum bob towards Earth at the equilibrium position won't.

Pedantic point, but couldn't help myself: the force on the pendulum bob at the equilibrium position is by definition zero, but I guess you mean it's the sum of two oppositely directed forces (weight & magnetic) whose respective [equal] magnitudes at the [fixed] position of equilibrium are the same each time the bob passes through that point.

 

[sophiecentaur]

Haha if you are going to be pedantic then explain, if the force goes to zero, why the bob doesn’t just float away. It’s the RESTORING force that goes to zero. I wrote what I wrote with some care.

 

[etotheipi]

Haha if you are going to be pedantic then explain, if the force goes to zero, why the bob doesn’t just float away. It’s the RESTORING force that goes to zero. I wrote what I wrote with some care.

Unfortunately here you are being imprecise [e.g. you just say "force", without specifying if you mean a single force or the resultant], so it's hard to tell if what you wrote is correct or incorrect.

The hovering bar magnet is, at any moment, subject to a downward weight force and an upward magnetic force. At the equilibrium position, the weight exactly balances the magnetic force. Displacing the hovering magnet slightly from its equilibrium position results in the magnetic force either slightly increasing or slightly decreasing, so that in either case the resultant force (weight + magnetic) points toward the equilibrium position. [i.e. this resultant force is what you called the restoring force]

Basically I'm saying your original statement is correct only if by "force" you mean the weight only and not the resultant, and I suspect this is what you actually mean.

 

[sophiecentaur]

the force pulling the pendulum bob towards Earth at the equilibrium position won't.

Basically I'm saying your original statement is correct only if by "force" you mean the weight only and not the resultant, and I suspect this is what you actually mean.

 

[etotheipi]

Okay, good! I just thought the original wording was ambiguous and wanted to eliminate any possible doubt for anyone who stumbles across this thread.

 

[sophiecentaur]

The same force pulls the bob towards the Earth at all times; its weight. If I had meant to say 'restoring force' then I would have said it'. If you were really fussy, then you could say that, as the height of the bob above ground does actually vary and that the field above the ground is not strictly uniform but I doubt that the model in your head would include that factor.
Imo you were trying to play the pedantry game (great fun, of course) and chose the wrong playing field.

 

[etotheipi]

Imo you were trying to play the pedantry game (great fun, of course) and chose the wrong playing field.

This is a natural feature of a written-format scientific discussion forum! If there is something vaguely ambiguous then it's good practice to ask for clarification, especially with things like in this thread where it's easy to make conceptual mistakes.

 

[rude man]

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?

Assuming you are thinking of permanent magnets, the answer is that the magnet holds electrons spinning about their own axes. If the spin axes are aligned
the electrons produce "amperian currents" along the magnet's surface which by themselves never reduce in intensity. They effectively see zero resistance.
(A much lesser amount of magnetic field is also produced by the electrons' motion about the nucleus.)

But the axes can be partially misaligned due to temperature, mechanical impact, etc. and then the electron spins don't add so much, producing lower net current and therefore magnetic field.

So, bottom line, if the permanent magnet is kept at low temeperature and not subject to shock etc. then the magnet theoretically almost never loses magnetic strength.

 

[weirdoguy]

electrons spinning about their own axes.

Um, how?

 

[rude man]

cf. Fermi-Dirac statistics.
Also C. Kittel ch. 12, Ferromagnetism and Antiferromagnetism

 

[sophiecentaur]

Um, how?

Electrons have 'spin' and they are charged. That causes them to behave like small electromagnets with a Moment Dipole Moment. But the 'current' in these electromagnets cannot decay. That's a consequence of Quantum Mechanics.
In most materials, most of the time, the total of electrons with one spin equals the total with the other spin so most materials cannot be permanently magnetised.

 

[rude man]

Electrons have 'spin' and they are charged. That causes them to behave like small electromagnets with a Moment Dipole Moment. But the 'current' in these electromagnets cannot decay. That's a consequence of Quantum Mechanics.
In most materials, most of the time, the total of electrons with one spin equals the total with the other spin so most materials cannot be permanently magnetised.

As I understand it, only the spinning electrons at the material's surface produce the magnetic field. Interior electrons cancel each other.

 

[sophiecentaur]

As I understand it, only the spinning electrons at the material's surface produce the magnetic field. Interior electrons cancel each other.

Can that be correct? Where have you read that?
If you take a permanent magnet and break it half way, there will be two new poles. Also there is an internal field all the way through.

 

[rude man]

Can that be correct? Where have you read that?
If you take a permanent magnet and break it half way, there will be two new poles. Also there is an internal field all the way through.

What you point out does not negate the idea of surface currents.
Yes there is an internal field; that's how a solenoid works ... I see no way in which my description differs in essence from a solenoid.

 

[weirdoguy]

Electrons have 'spin'

Well, yes, they have spin, but since when this means that they are spinning around their own axes?

 

[rude man]

Well, yes, they have spin, but since when this means that they are spinning around their own axes?

If I may respond to @sophiecentaur 's post:
Since about 13 billion years ago, the estimate age of the universe.

 

[russ_watters]

Several posts deleted to clean up the thread. Thanks everyone who tried to contribute positively to that.

 

[Keith_McClary]

Magnetic Fields of Asteroids Lasted Hundreds of Millions of Years

 

[sophiecentaur]

Magnetic Fields of Asteroids Lasted Hundreds of Millions of Years

And so do the bands of alternating magnetisation in the widening gaps between some tectonic plates. These alternations are due to the new ferrous material coming to the surface and cooling down, aligned with the Earth's magnetic field at the time and then remaining magnetised. The magnetic stripes are evidence of when the Earth's field flips (every couple of hundred thousand years) and the records go back many cycles.

 

[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.