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Why is a non-permanent, but long lasting, magnet called a permanent magnet?

Permanent magnets are magnets that you don't have to use energy to make them magnetic. Some types of permanent magnets, relative to the length of lives of humans, are pretty close to permanent. They decay slowly, but they do decay. When most of the magnetic domains in a material align in one direction you can call that a magnet. It helps if you can imagine a magnet as being made up of a bunch of little magnets. Each one has tiny North and South pole. If they are arranged in a random, scrambled mix, approximately the same number will be facing one way as is facing the opposite way. The fields cancel each other out. If you can make all of the little magnets line up, their magnetic fields add to each other until you have strong enough field to sense.

The length of "life" of a permanent magnet depends on many factors. When the domains are randomized again, the material will cease being magnetic, but this can be a very gradual process. This re-randomizing is affected by several things. The higher the temperature, the faster this process will happen. A sharp blow to a permanent magnet can knock its "little magnets" enough that the fields become randomized. Another magnetic field placed at some angle to the first field can rearrange those fields. This information is very useful and has many implications. Some natural materials can be magnetic. That material's field orientation, relative to the Earth's field, can be locked-in when that material formed. From this, geologists have been able to figure out that periodically the earth's magnetic poles flip around. So not even the Earth's magnet is permanent!


Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)

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