Insulators, conductors, semiconductors, and superconductors
Every material in the world can be defined in terms of how well it conducts electricity. Certain things, such as cold glass, never conduct electricity. They're known as insulators. Materials which do conduct electricity, like copper, are called conductors. In the middle are materials known as semiconductors, which don't conduct as well as conductors, but can carry current. Last, are materials called superconductors, which when brought down to very low temperatures turn into superhighways of current -- they conduct electricity without any resistance whatsoever.
All these different materials are made of atoms that look basically alike: a nucleus with electrons circling around them. What makes them so different when it comes to conducting electricity?
The difference comes down to nothing more than how the electrons are arranged around the nucleus. The laws of quantum physics say that there are only specific bands (or tracks) in which any electron can travel. There are some interesting facts about these bands. First of all, only a very specific number of electrons can travel in each one; once it's full, it's full. Second, which track an electron is in corresponds to how much energy that electron has. And third, some of the bands are closer to each other than others.
Different atoms have different numbers of electrons, and how those electrons are arranged in the bands defines whether a material made of those atoms will conduct.
In every atom, the electrons crowd down as close to the nucleus as possible, since the bands that are closest to the nucleus are also the ones that require the least energy. That means that the outermost track might not be completely filled. If it's not filled, then it's easy for an electron to jump from one atom into an empty space in the atom next door. Ta da! Moving electrons, and therefore an electrical current. Atoms with empty spaces in the outermost electron bands are conductors.
Let's go to the next scenario, where the outermost track is completely filled. If the electrons in this track were given a little kick of energy -- say from a flash of light -- they might have enough energy to jump up to to the next, empty track. But remember, some bands are close to each other, and some aren't. In atoms where the next track is close by, an energetic electron will have no problem jumping up a track. Suddenly, this electron is in a track with empty spaces, and electrons can move from atom to atom just as described above. Since these kinds of atoms can only conduct electricity sometimes -- when given this outside jolt of energy -- they're the semiconductors. Atoms with a full outside track which is very close to the next empty track are semiconductors.
If, however, the next potential track is too far away, then an electron can't jump to it even if it's given a jolt of energy. These electrons will always stick in their assigned track, never allowed to roam to another atom -- and never forming current. Atoms with a full outside track which is far from the next empty track are insulators.
Superconductors are a whole different breed, since no material known today superconducts except at very cold temperatures. Scientists are discovering materials that do superconduct closer and closer to room temperature all the time, but no one is quite sure how that happens. However, John Bardeen, Leon Cooper, and Robert Schrieffer did come up with a theory for how the very coldest superconductors work, known as the BCS theory. In such materials, at low temperatures, the atoms vibrate in a way that forces the moving electrons closer together. Normally electrons don't like to huddle so close, since they're all electrically negative and therefore repel each other. But in superconductors, the electrons actually achieve almost an attraction for each other. The result is that as one electron moves, it pulls the next electron along right behind it. Electrons slip from atom to atom more easily than they ever do normally. Atoms which, at the right temperature, can make electrons attract instead of repel each other are superconductors.
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