by Chris Woodford. Last updated: December 1, 2016.
Zap! When a bolt of lightning leaps to the ground, we get a sudden, very vivid demonstration of the power of static electricity (electrical energy that has gathered in one place). Most of us know that static builds up when we rub things together, although that's not really a satisfying explanation. What is it about rubbing things that produces an electrical phenomenon? Although lightning is a spectacular example of static electricity, it's not something we can harness. But there are many other places where static electricity is incredibly useful; from laser printers and photocopiers to pollution-busting power plants, static can be really fantastic. So let's take a closer look at what it is and how it works!
Photo: A lightning bolt is a huge release of static electricity, in which built-up electrical potential energy shoots from the sky to the ground in a sudden, improvised, electric current. Picture by David Parsons courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
What is static electricity?
Photo: Classic static: When you rub a balloon on your pullover, you create static electricity that makes it stick. The rubbing shifts electrons from your pullover (which becomes positively charged) to the latex rubber in the balloon (which becomes negatively charged). The opposite charges make the two things stick.
We take electricity for granted: it's easy to forget that homes, offices, and factories have been powered in this clean and convenient way only since the end of the 19th century—which, in the broader run of human history, is no time at all. It was during the 19th century that pioneers such as Alessandro Volta, Michael Faraday, Joseph Henry, and Thomas Edison figured out the secrets of electricity, how to produce it, and how to make it do useful things. Before that, electricity was largely a curiosity: it was very interesting for scientists to study and play with, but there wasn't much else they could do with it. In those days, people cooked and heated their homes using wood or coal stoves and lit their rooms with candles or oil lamps; there were no such things as radios or TVs, much less cellphones or computers.
The "modern electricity" that powers everything from the phone in your pocket to the subway you ride to school or work is what we call current electricity (or electric current). It's energy that travels down a metal wire from the place where it's produced (anything from a gigantic power plant to a tiny battery) to the thing it powers (often an electric motor, heating element, or lamp). Current electricity is always on the move, carrying energy from one place to another.
Photo: Current electricity: When electricity flows around a closed loop, called a circuit, we can use it to transport energy from a source (in this case, an electricity outlet on the wall) so it does something useful (in this case, warming the heating elements on an electric fire).
Before the 19th century, the only kind of electricity people really knew about or tried to use was static electricity. The ancient Greeks understood that things could be given a static electric "charge" (a buildup of static) simply by rubbing them, but they had no idea that the same energy could be used to generate light or power machines. One of the people who helped to make the connection between static and current electricity was American statesman, publisher, and scientist Benjamin Franklin. In 1752, when Franklin tried to figure out the mysteries of electricity, he famously did so by flying a kite in a thunderstorm to catch himself some electrical energy (an extremely dangerous thing to do). A lightning bolt zapped down the kite to the ground and, had Franklin not been insulated, he might well have been killed. Franklin realized that static electricity, accumulating in the sky, became current electricity when a lightning bolt carried it down to the surface of the Earth. It was through research such as this that he developed one of his most famous inventions, the lightning rod (lightning conductor). Franklin's work paved the way for the electrical revolution of the 19th century—and the world really changed when people such as Volta and Faraday, building on Franklin's discoveries, learned how to produce electricity at will and make it do useful things.
Potential and kinetic energy
Just quickly, in passing, it's worth noting that there's another way to think of static and current electricity and to relate them to things we already know about energy. We can think of static electricity as a kind of potential energy: it's stored energy ready and waiting to do something useful for us. In a similar way, current electricity is (loosely speaking) analogous to kinetic energy: energy in movement—albeit of an electrical kind. Just as you can turn potential energy into kinetic energy (for example, by letting a bolder roll down a hill), so you can turn static electricity into current electricity (that's what a lightning bolt does) and back again (that's how a Van de Graaff generator works).
What makes static electricity?
Until a few years ago, scientists were confident that they understood static electricity and exactly how it worked. The explanation went like this...
Just like the ancient Greeks, we tend to think static electricity comes from rubbing things. So if you live in a home with nylon carpets and metal doorknobs, you'll soon learn that your body builds up a static charge when you walk across the floor, which can discharge when you touch a doorknob, giving you a tiny electric shock. In most school experiments, we also learn about static by rubbing things. You've probably tried that trick where you rub a balloon on your clothes to make it stick? You might conclude from this that static electricity is somehow connected to friction—that it's the very act of rubbing something vigorously that produces a buildup of electrical energy (in the same way that friction can produce heat and even fire).
The triboelectric effect
It's not the rubbing that's important but the fact that we're bringing two different materials into contact. Rubbing two things together vigorously simply brings them into contact again and again—and it's this that produces the static electricity through a phenomenon known as triboelectricity (or the triboelectric effect). All materials are built from atoms, which have a positive central core (the nucleus) surrounded by a kind of fuzzy "cloud" of electrons, which are the really exciting bits. Now some atoms have a more powerful pull on electrons than others; a great deal of chemistry stems from that fact. If we put two different materials in contact, and one attracts electrons more than the other, it's possible for electrons to be pulled from one of the materials to the other. When we separate the materials, the electrons effectively jump ship to the material that attracts them most strongly. As a result, one of the materials has gained some extra electrons (and becomes negatively charged) while the other material has lost some electrons (and becomes positively charged). Hey presto, we have static electricity! When we rub things together again and again, we increase the chances that more atoms will take part in this electron-swap, and that's why a static charge builds up.
Photo: How the triboelectric effect explains static electricity: 1. Ebonite (hard vulcanized rubber—shown here as a black rod) and wool (shown as gray) normally have no electric charge. 2) Put them in contact and the ebonite attracts electrons from the wool. 3) Separate them and the electrons remain on the ebonite, making it negatively charged and leaving the wool with a lack of electrons (or a positive charge). Rubbing the two substances together increases the contact between them and makes it more likely that electrons will migrate from the wool to the ebonite. The negative charge on the ebonite is exactly the same size as the positive charge on the wool; in other words, no net charge is created.
The triboelectric series
If you experiment with different materials, you find some gain positive charges when they're rubbed and some gain negative charges; some materials also gain more charge than others. It turns out that we can rank materials in order according to the charge they gain, giving us a kind of league table of materials running from positive to negative. Different books and web pages show slightly different lists, but they all broadly run from minerals (positive) through such things as wood and paper (neutral) to plastics (negative). Don't worry too much about the exact order of the list; it's going to vary for all kinds of reasons (the kind of glass or the additives in the latex, for example).
++++++++ POSITIVE ++++++++
− Hard rubber
− Saran ("cling film")
− Polyvinylchloride (PVC)
− Silicone rubber
− Ebonite (very hard vulcanized rubber)
This list is called the triboelectric series. The further apart two materials are in the series, the more static electricity will build up when you rub them together. If two materials are very close in the series, it's hard to get them to build up any charge at all no matter how hard you rub them. That would seem to confirm that static electricity isn't about rubbing, per se, but about the nature of the materials we bring into contact.