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Name: Robin
Status: other
Age: N/A
Location: FL
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Date: N/A


Question:
Why doesn't an electromagnet become a permanent magnet over time? It is magnetized by electricity so what makes it not become a permanent magnet?



Replies:
Robin,

A moving electrical charge produces a magnetic field and a moving magnetic field produces an electrical field. An electromagnet works by coiling a bunch of wire and spinning a couple of magnets around that wire at high speeds. When this occurs the magnets induce an electric current in the wire and hence the electricity production. Once the magnets stop spinning, the induced electrical field dissipates and the current stops flowing through the wire. There is no contact between the magnets and the wire and there should be no reason that the magnets will induce the wire they are spinning around to become magnetic. In order to understand what causes magnetic properties in the first place, I suggest going to Wikipedia and reading the section on magnetism. Here is the link: http://en.wikipedia.org/wiki/Magnets

Matt Voss


Suppose you start your electromagnet as a copper coil with no iron core, it is not as strong, but it is an electromagnet, and there are no ferro-magnetic elements in, say, plastic and copper. No iron, it cannot be permanently magnetized.

Then you insert an iron core. Depending on small amounts of "crud" in the iron, it can be magneticly soft or magneticly hard, or in between. Pure iron, and especially nickel-iron alloys, are magneticly soft. The added elements in Neodymium-Iron-Boron or AlNiCo makes them both mechanically brittle and magnetically hard. Plain old carbon-steel is somewhere in between hard and soft.

If soft it gets magneticly polarized inside quite easily when an external field is applied, but then demagnetizes equally easily, spontaneously, when the field is removed. This is how we typically remember iron, as a passively magnetic material.

Hard iron has "magnetic internal friction" inside it. It resists magnetization up to some strength of applied field, then gets polarized fairly suddenly with a stronger field. After the external field is removed, some of the internal polarization is stuck in there, to a strength just a little less than the field required to coerce it to polarization in the first place. Now it is a permanent magnet. Materials with a higher "coercivity" (threshold of magnetization) are harder to magnetize, but they are stronger magnets afterwards.

If the coercivity is low but not zero, the material is in-between, slightly hard. You can magnetize it by stroking it with a permanent magnet, but its residual magnetism will be notably weak, and may decay later. You can use it as the core of an electromagnet, but it will not obediently demagnetize all the way when you turn off the current, and it will require some energy each time you reverse the current. If you use it in a transformer, it will waste lots of energy and get hot.

A hobbyist, so far, does not have a real easy time finding just the right material. Most steels you typically find will have low but noticeable coercivity. For making magnetic devices, it is neither high nor very low, so it is not great.

Cannibalizing transformers or motors might be your best bet for finding magnetically soft iron. The thin multiple layers they are made with are only helpful, provided the direction of your magnetic lines are in the plane of those layers.

As for magnetically hard materials, they are used for nothing but permanent magnets. So you will have to use a magnet as raw material to make a magnet. Try wrapping a fat pile of copper wire around some small metal bar magnet, and connect the N and S poles, going around the outside of the wire, using iron pipe-strap. They give it a short (1 second) pulse of DC high current in one direction or the other. If the copper winding is just right (draws high current, makes strong field, probably gets hot, but not so much current it heats to melting), maybe you can re-magnetize the bar in the opposite direction. That's pretty much the same as making a magnet. (This will not work easily with ceramic or rare-earth magnets. Look for a medium-strength metal magnet.) Yet another thing I wish I had ever actually tried before recommending it. But it is pretty much what all the magnet manufacturers do. So there you have an electromagnet that _does_ become permanently magnetized, in the very first moment you turn it on, no less.

Jim Swenson


The core of an electromagnet operates on the principle of aligning "more or less" free electrons in the core by the electric winding. When the current is stopped the alignment is removed and the electrons in the core are once again randomly oriented, and hence the metal core is no longer magnetic. In some cases, depending upon the metal, the magnetic domains (groups of magnetized atoms) do not lose their orientation when the electric current is turned off and the core remains magnetized, at least partially. In some cases this residual magnetism can be "un-done" by heating or by hitting the core with a hammer -- that is by mechanically stressing the core metal.

Vince Calder



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