Nobody Loves A Transformer, Part 1: Wire
by Brian McGinty
I can't think of a more boring electronic component than the transformer. They are coiled wire and iron, so mechanically rugged that they never break. They are big and heavy in a world that values youth and beauty - oops, smaller and lighter. Yet most of us would agree that wire is even less interesting than that.
Transformers are made out of wire, and wire is made out of copper. The copper is almost always pure, one of the few metals that is used in industry in its pure (unalloyed) form. It conducts electrons well since they travel an unusually long way-about 100 atomic diameters-between collisions. Far and away most materials are insulators, where the electrons quickly run into atoms ("ion cores" in physics' parlance.) This will make the discussion of wire easier, since the only purpose of the coatings on them is to prevent metal from touching other metal. Enamel paint works fine. This ends the electrical influence of the insulator.
Electrons in a wire, much like a big herd of cattle, don't move anywhere on their own initiative. They move in response to the electric field, for reasons unknown. It is an inate property of electrons, decided before we got hired. Each electron has the same charge (a fixed, small amount of charge that can be looked up elsewhere.) This is really a measure of how much electrons repel each other via the electric field, which again is an inate property of unknown provenance.
This electric potential moves the electrons, but the energy is carried in two ways, as a current inside the wire, and as a magnetic field outside the wire. Magnetic fields (called eddy currents) move within the wire as well, which we can ignore for now. The magnetic field extends away from the wire, falling away smoothly with distance. It falls away pretty quickly, as a cube power: double the distance and the strength drops to one-eighth. The field also has a direction (look up the "right hand rule"), which is a bit misleading. The field's only property is that its strength drops off with distance, but all electrons entering the field are always diverted the same direction, hence the "direction" of the magnetic field.
It has a direction due to another form you must sign on your first day of work, "spin". Everybody agrees on one point: This property should not have been called spin, almost any other name would have been better. The electron is not spinning (actually it is, which is why everyone wishes "spin" was called something else.) The magnetic field is not spinning. However, the path an electron takes will be a straight line until it nears a magnetic field, then it curves. Much like a cue ball struck with some "English," which is the origin of the term "spin", if memory serves.
Yet another misleading term used with magnetic fields are "lines of force," which do exist but are not inherent. Most people will recall seeing these from science class, from the demonstration with a magnet, piece of paper, and iron filings. In a perfect world iron cooled from a molten state will harden into a single crystal, but in practice it hardens as a shattered crystal. There are thousands of randomly oriented tiny crystal regions per square inch. The size and number of lines of force are determined by these crystal regions, and in fact "kilolines per square inch" has been used as a measure of magnetic field strength. Cast iron peaks at about 60 kilolines per square inch (with 200 ampere/turns per inch of magnetizing force.)
With enough electric potential we can make them move down any wire against their wishes. The number of collisions with atoms (resistance) increases with the number of electrons, creating heat. We can force current down the wire until it melts, or (flash of insight) use a larger diameter wire.
We've come up to a straight piece of wire with a current flowing through
it in one direction (dc, direct current.) The wire has a magnetic field
extending away from it that always kicks electrons in the same direction.
Next time we'll take a closer look at the magnetic field.
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