Magnetism of Permanent Magnet 20160126
Life of Magnetic Force & Lose Magnetism
How long can the magnetic force of permanent magnets retain?
To figure out this question, we shall note the following points:
1. The reason why a magnet has magnetic force is that it is actually composed of very small lattices. The characteristic of these lattices is that they have a ring current (formed by electron motion) around certain axis. And the axis of ring currents is almost the same. A loop current can be seen as a tiny magnet, and magnetically stronger magnets are just composed by a large number of these tiny magnets with same N and S poles.
2. Under natural conditions, the small lattices inside permanent magnets are in steady state, which is in the lowest energy state. Objects always tend towards the lower energy state, so circular current direction will not change. To demagnetize a magnet, the circular direction of tiny lattices must be disordered, which requires input of external energy (for example, to heat it up to a high temperature).
So under natural conditions, magnets will be in a stable state, which is permanently retaining its magnetism!
In What Case, Magnets Lose Magnetism
Commonly, magnetic performance is most sensitive to heat.
Any material has Curie temperature, T. Under the temperature higher than T, magnetization direction of the material will be the same as the outer magnetic field as you put. The material has no magnetism if you don’t put any magnetic field outside of it. This is called paramagnetism. On the other side, when Curie temperature is under temperature T, atoms of the material are inclined to form magnetism in one direction (obviously, you can guide the magnetism to point to the direction you want). It is called ferromagnetism.
Curie temperature of most materials is low. Most of them do not have spontaneous magnetism under room temperature. Human discovered that although some materials have high Curie temperature, they have ferromagnetism under room temperature. These materials can be made into common magnets.
Therefore, permanent magnet will lose magnetism when Curie temperature exceeds T. Every atom of crystal will point to a new direction. But magnetism of each crystal point to different direction, so they seem not magnetic.
However, if you put magnetic field outside a magnet if temperature decreases, crystals will be guided to point to the same direction and the magnet will get magnetism again. Magnets are always made in that way.
How long can the magnetic force of permanent magnets retain?
To figure out this question, we shall note the following points:
1. The reason why a magnet has magnetic force is that it is actually composed of very small lattices. The characteristic of these lattices is that they have a ring current (formed by electron motion) around certain axis. And the axis of ring currents is almost the same. A loop current can be seen as a tiny magnet, and magnetically stronger magnets are just composed by a large number of these tiny magnets with same N and S poles.
2. Under natural conditions, the small lattices inside permanent magnets are in steady state, which is in the lowest energy state. Objects always tend towards the lower energy state, so circular current direction will not change. To demagnetize a magnet, the circular direction of tiny lattices must be disordered, which requires input of external energy (for example, to heat it up to a high temperature).
So under natural conditions, magnets will be in a stable state, which is permanently retaining its magnetism!
In What Case, Magnets Lose Magnetism
Commonly, magnetic performance is most sensitive to heat.
Any material has Curie temperature, T. Under the temperature higher than T, magnetization direction of the material will be the same as the outer magnetic field as you put. The material has no magnetism if you don’t put any magnetic field outside of it. This is called paramagnetism. On the other side, when Curie temperature is under temperature T, atoms of the material are inclined to form magnetism in one direction (obviously, you can guide the magnetism to point to the direction you want). It is called ferromagnetism.
Curie temperature of most materials is low. Most of them do not have spontaneous magnetism under room temperature. Human discovered that although some materials have high Curie temperature, they have ferromagnetism under room temperature. These materials can be made into common magnets.
Therefore, permanent magnet will lose magnetism when Curie temperature exceeds T. Every atom of crystal will point to a new direction. But magnetism of each crystal point to different direction, so they seem not magnetic.
However, if you put magnetic field outside a magnet if temperature decreases, crystals will be guided to point to the same direction and the magnet will get magnetism again. Magnets are always made in that way.
