The Sticking Power of Electromagnets
As we mentioned in the introduction, basic electromagnets aren't all that complicated; you can construct a simple version of one yourself using materials you probably have lying around the house. A conductive wire, usually insulated copper, is wound around a metal rod. The wire will get hot to the touch, which why insulation is important. The rod on which the wire is wrapped is called a solenoid, and the resulting magnetic field radiates away from this point. The strength of the magnet is directly related to the number of times the wire coils around the rod. For a stronger magnetic field, the wire should be more tightly wrapped.
OK, there’s a little more to it than that. The tighter the wire is wound around the rod, or core, the more loops the current makes around it, increasing the strength of the magnetic field. In addition to how tightly the wire is wound, the material used for the core can also control the strength of the magnet. For example, iron is a ferromagnetic metal, meaning it is highly permeable [source: Boston University]. Permeability is another way of describing how well the material can support a magnetic field. The more conductive a certain material is to a magnetic field, the higher its permeability .
All matter, including the iron rod of an electromagnet, is composed of atoms. Before the solenoid is electrified, the atoms in the metal core are arranged randomly, not pointing in any particular direction. When the current is introduced, the magnetic field penetrates the rod and realigns the atoms. With these atoms in motion, and all in the same direction, the magnetic field grows. The alignment of the atoms, small regions of magnetized atoms called domains, increases and decreases with the level of current, so by controlling the flow of electricity, you can control the strength of the magnet. There comes a point of saturation when all of the domains are in alignment, which means adding additional current will not result in increased magnetism.
By controlling the current, you can essentially turn the magnet on and off. When the current is turned off, the atoms return to their natural, random state and the rod loses its magnetism (technically, it retains some magnetic properties but not much and not for very long).
With a run-of-the-mill permanent magnet, like the ones holding the family dog’s picture to the refrigerator, the atoms are always aligned and the strength of the magnet is constant. Did you know that you can take away the sticking power of a permanent magnet by dropping it? The impact can actually cause the atoms to fall out of alignment. They can be magnetized again by rubbing a magnet on it.
The electricity to power an electromagnet has to come from somewhere, right? In the next section, we’ll explore some of the ways these magnets get their juice.