chapter  5
13 Pages

Electricity

Some 2,500 years ago, the Greek philosopher Thales of Miletus observed that rubbing a piece of amber would give it the ability to attract light objects. This phenomenon we now call static electricity. It’s the ‘static cling’ of clothes as they come out of the dryer. It’s the spark or shock from reaching for a metal object after shuffling across a carpet. It’s the maddening tendency of Styrofoam packing ‘peanuts’ to stick to just about anything. Like many ideas and observations of the ancient Greeks, little or nothing was done to develop the practical consequences. The study of electrical phenomena languished for the next two millennia. Seventeenth-century investigators in Britain and in Europe began

to make experimental observations on static electricity. William Gilbert developed the term ‘electrics’ to describe substances that would generate the attractive forces when rubbed; such a substance was said to be ‘electrified.’ The electrified substance was thought to have gained some sort of electrical fluid that-once actually in the substance-remained stationary, hence the term static electricity. The scientists who studied electricity came to be called ‘electricians’—a word with a totally different connotation today. Otto von Guericke built a machine that consisted of a large ball

of sulfur that could be rotated at high speeds by means of a crank. Electricity was produced by friction by applying a cloth pad, or

even his hand. Von Guericke was able to transmit electricity a meter or more through a moistened string. He had no way of realizing it at the time, but he had constructed the first electric generator. He not only generated electricity in greater amounts than ever before, but he also showed that it could be transmitted over distances. Benjamin Franklin, the first American scientist to be recognized

as ‘world-class,’ proposed in the mid-eighteenth century the concept that electricity is a fluid. Stephen Gray, an English ‘electrician,’ recognized that the electric fluid is not necessarily static, but can move through some kinds of objects. He and other scientists recognized that substances could be divided into two classes: those that readily pass the electric fluid, which nowadays we call conductors; and those that retain the electric fluid, which we now call insulators. As a rule, good conductors are metals. Wood, rubber, and plastics are examples of insulators. Following Gray’s lead, the French ‘electrician’ Charles Du Fay observed that there seemed to be two ‘kinds’ of electricity. One is generated by rubbing a piece of glass; the other, from rubbing a piece of resin. He referred to these as vitreous electricity and resinous electricity. He showed that the same kinds of electricity repelled each other, but opposite kinds attracted. Franklin realized that the concept of two kinds of electricity-

vitreous and resinous-was not necessary. The presence of one ensured the absence of the other. Franklin refined Du Fay’s concept to develop the idea of positive and negative electricity. Matter under ordinary conditions was not electrified. If electrical charge were accumulated on an object, it would be said to be positively charged. If charge were removed, that object would be said to be negatively charged. Franklin had the insight that electricity was not created by rubbing a glass tube; it was only being transferred. When an ‘un-electrified’ object was rubbed, it could either gain electric fluid and become positive, or it could lose electric fluid and become negative. When a positively charged body is brought into contact with an uncharged or negatively charged body, the excess electric fluid in the positively charged body would flow into the uncharged or negatively charged one. Similarly, when an uncharged object is brought into contact with a negatively charged object (i.e., one that has a deficit of electric fluid), some electric fluid would flow from

the uncharged to the negatively charged object. Despite Franklin’s unquestioned brilliance, not much was truly understood about electricity by the early nineteenth century. Franklin extended his work by proposing that lightning is elec-

tric. He proved it in 1752, in one of the best-known scientific experiments of history: the kite-in-a-thunderstorm experiment. Remarkably, the experiment did not kill him. The next person to try the experiment, a Russian ‘electrician’ in St. Petersburg, was struck dead. With his gift for turning scientific observations into practical inventions, Franklin immediately turned his kite experiment into the first practical-indeed, life-saving-application of the new understanding of electricity, the lightning rod. Franklin’s hypothesis about the flow of electric fluid suggests the

analogy of a flow of water downhill. Something that flows must do so because of a difference in potential. It is possible to use that analogy to describe an electrical system. Electrical potential energy, or electrical potential, is measured in volts, in honor of the Italian ‘electrician’ Alessandro Volta, and is given the symbol V. (Volta’s work will be discussed in more detail later.) In keeping with the analogy of a waterfall, the low-potential side of an electrical system is sometimes referred to as the ‘ground.’ In a waterfall, water moves from high to low gravitational

potential energy. In the more general case of a gravitational system, the ‘thing’ that is moving is matter. Water is simply one specific kind of matter. Both the difference in potential and the amount or quantity of matter that flows are important. A single drop of water falling the height of the world’s tallest waterfall will be not nearly as effective in doing work as many thousands of liters of water flowing down just a few meters in a working waterwheel. The same is true in a thermal system. Pouring a teacup of boiling water onto an icy sidewalk isn’t going to melt much ice. In a river or waterfall, the current can be measured as the volume

of water flowing in a unit of time, e.g., cubic meters per hour. In an electrical system, the motion of electrical charge is called an electric current. Although the flow is the electric charge, it is more common and more convenient to express flow as current, the amount of charge flowing per unit time. Units of electric current are named in honor of a French ‘electrician,’ André Ampère, the ampère, or, as often called, the amp.