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Magnet's lifespan

by Aloha
Tags: lifespan, magnet
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May17-06, 05:45 AM
P: 1
Hi guys

Does a normal stab magnet or one of those which you can stick on your refrigerator have a lifespan or are they magnetic for "ever"? If the magnet would come in contact with another oposite charged magnet in regulary sequences would that effect the "lifespan" of one or both magnets?

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May18-06, 04:31 PM
P: 20
Hi Aloha,
I think a magnet has a mind of its own. It could stay magnetic forever, it could lose it's net magnet moment, and then get it back one day. It could even abruptly change it's magnetic direction. The colder it is, the more stable it is. At a high enough temperature, all magnets will lose their [ferro] magnetic moment. If placed in a strong enough magnetic field, all magnets will surrender to the will of the field and change their magnetic moment to point with the field.

There really is no such thing as an oppositely charged magnet, since there really is no such thing as magnetic charge. Think of a magnet you hold in your hand as a bunch of little tiny microscopic magnets who like to succumb to peer pressure and point in the same direction as their neighbors, but because of thermal energy, are jiggling around and occasionally point in random directions. The higher the temperature, the harder it is for them to stay focused and work together. Sometimes gangs of tiny magnets, called regions, form and try to convince other gangs to point in their direction. All this is going on right in your hand.

You can watch the blue and the white regions duke it out on this Ising Model applet:

Play with the temperature
If white wins, it's a bar magnet pointing one way, if blue wins, the other way.
Loren Booda
May18-06, 10:02 PM
Loren Booda's Avatar
P: 3,408
As many of you know, by striking sharply an unmagnetized iron bar lined up with the Earth's magnetic field, the bar will retain enough magnetism to, e.g., pick up iron filings. Dropping a magnet generally weakens its effect.

Q: If one were to break up a magnet into its individual magnetic domains, what properties would they exhibit? Might they demonstrate enhanced magnetism?

May21-06, 07:37 PM
P: 2,292
Magnet's lifespan

In your context, magnetism, such as with a magnetized iron bar or neodymium(for example) composition DOES have a "lifespan"

A "magnetised" iron bar effects a "dissorientation" that is not "normal"
In time, the iron bar will re-orient into a non-magnetic, normal state.

Not sure about the figures, but I recall that it is somewhere around 300+ years for an "iron" magnet, and several thousand(perhaps 10's of thousands) for neodymium.
Again, I may have my figures wrong in exact detail, but nonetheless, YES... there is a "lifespan"
May22-06, 06:17 AM
P: 2,056
Why does higher temperature and mechanical stress have a negative effect on the magnet's strength? Think of a collection of atoms as representing the magnets. They all have arrows on them pointing different ways, because each atom is a little magnet that works to create a bigger one. In a more perfect magnet, all the arrows are pointing the same way. In a not-magnet, the arrows are pointing all different ways. When a ferromagnet (permanent magnet such as on your refrigerator), the arrows jumble up and point different ways. Because heat is actually the microscopic shaking of atoms and molecules, it has the same effect as the macroscopic shaking of dropping the ferromagnet. Oh look, a picture.
The main implication of the domains is that there is already a high degree of magnetization in ferromagnetic materials within individual domains, but that in the absence of external magnetic fields those domains are randomly oriented. A modest applied magnetic field can cause a larger degree of alignment of the magnetic moments with the external field, giving a large multiplication of the applied field.

These illustrations of domains are conceptual only and not meant to give an accurate scale of the size or shape of domains. The microscopic evidence about magnetization indicates that the net magnetization of ferromagnetic materials in response to an external magnetic field may actually occur more by the growth of the domains parallel to the applied field at the expense of other domains rather than the reorientation of the domains themselves as implied in the sketch.
May24-06, 03:26 AM
P: 152
can't a magnet lose it's magnetism if you hit it hard enough?
May24-06, 07:08 PM
Integral's Avatar
P: 7,318
The way you can extend the life of a magnet is to use a "keeper" this would be the metal plate you will see across the ends of a horseshoe magnet. This reduces and contains the magnetic field in the space surrounding the magnet. This will slow the inevitable decay of the magnetic field.

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