Recently there’s been quite a lot of talk about new high-temperature superconductors and how they may revolutionize technology. (In fact, some of that talk was mine, since I wrote a column on superconductivity a while back.)
Superconductors are materials that transmit electricity perfectly–in other words, materials in which, once electrons start to flow, they never stop. These superconductors could eventually lead to everything from much more efficient power lines (which would mean less use of fossil fuels and hence less pollution) to more powerful computers.
Yet despite the well-deserved excitement superconductors are generating (if you’ll pardon the term), when it comes right down to it, every bit as important as materials that conduct electricity freely are those that put up a little resistance.
Electrical resistance is just what it sounds like: the resistance in an electrical circuit to the flow of electrons. Resistance arises because the moving electrons keep running into other particles. The number of collisions, and hence the amount of resistance, varies from substance to substance.
In some substances, such as rubber, electrons won’t flow at all. Those materials are called insulators. In good conductors, such as gold, electrons flow very freely. In superconductors, as I mentioned, they flow without any resistance at all. However, there are a lot of other materials, called resistors, that, while they do conduct electricity, do so reluctantly, and get pretty hot under the collar when asked.
That heat comes about because the collisions between the electrons and the molecules of the resistor cause the resistor’s molecules to vibrate faster and faster. Since heat is molecular vibration, the substance heats up. In effect, the resistor transforms electrical energy into heat energy. The process is analogous to friction in hydraulic or mechanical systems, in which kinetic energy is transformed into heat.
This transformation of electricity into heat can be a problem at times. In power transmission, even though good conductors are used, there is still a huge loss of electrical energy because over the great distances involved even what little resistance there is adds up. The heat caused by resistance is also a problem in today’s supercomputers, where so many circuits are packed so tightly (to minimize the distance electricity has to flow and thus maximize speed) that they have to be cooled with liquid nitrogen (at -197 degrees Celsius) just to keep them from fusing.
Speaking of fusing, the fuses in your house are there because if you try to draw too much power through your house’s wiring, you also increase the amount of heat produced by the wiring’s natural resistance. A fuse is just a piece of wiring with more resistance than the rest. As a result, if the circuit is overloaded the fuse heats up more and burns out first, breaking the circuit and halting the flow of current.
Without fuses, overloaded wiring would eventually get so hot it would set fire to its own insulation, and probably the whole building. (As well, the resistance of most metals increases with temperature, so as the wire heats up, it conducts electricity even more poorly and thus heats up even faster, creating a kind of feedback loop. That’s why wires, when overloaded past a certain point, burn through almost instantly.)
It’s these kinds of problems that superconductors might someday abolish. But what’s a problem in one setting is a benefit in another. When it comes time to put electricity to work, we need good resistors as much as we need good conductors–maybe more.
A light bulb, for instance, uses resistance to create light; the filament is a resistor, and when electricity flows through it it gets so hot it glows. (It doesn’t burn up because the bulb contains almost no oxygen; break the bulb, though, and it burns out instantly.)
A toaster is another device in which we put resistance to good use. The metal coils in the toaster conduct electricity reluctantly, and as a result get very hot–hot enough to cook bread. An electric stove works exactly the same way. So do electric heaters.
Sometimes you want only a little bit of current flowing through a device, and sometimes you want a lot. To control current flow, you use a variable resistors. A radio or television volume control knob is a kind of variable resistor called a potentiometer; when you want lots of volume, you turn it so resistance is minimal, and when you want very little volume, you turn it so there is a lot of resistance. Light-dimming switches use another kind of variable resistor called a rheostat. Again, lots of resistance means very little light, while little resistance means lots of light.
When you think about it, as important as it is for us to be able to transmit electricity freely, once we’re ready to use it, it’s even more important that we be able to control it. That’s where resistors come in.
Superconductors may someday enable you to ride to work in a magnetically levitated train and give you a desktop computer with the power of today’s supercomputers–but when you get up in the morning to make your toast and coffee, it’s the resistors you’re going to be grateful to.
