ELECTRICITY? WHAT GOOD IS IT? WHAT GOOD IS IT? Tongues in trees, books in the running brooks, Sermons in stones, and good in everything? -quoted by Michael Faraday When I was growing up, I spent most weekends with my two cousins, Steve and Stan. Their interests were fairly different from mine, and sometimes I just stayed in their room and read or worked on some project. One project in particular was the construction of a strobe light. This was before they were integral parts of cameras, discos or even of the psychedelic scene. I had read about them and how they could seem to stop fast action. I wanted to build one and use it at night to see how moths fly. Anyway, I recall sitting on the floor trying to wire the parts together from a diagram. I was frustrated and Stan asked me to tell to him once again what I was doing. After I tried to explain it, he just looked at me and asked, “What good is it?” I must have muttered something in reply, but it was not very memorable. I tried to build the strobe because I found it fascinating. Even today, although I am often asked to justify the practical purposes of my research, I do not think about it very much. That is not why I do research. Many of the modern miracles that we all take for granted began as observations, toys, or hobbies. They then were taken by science and studied for the sake of knowledge with practical technologies flying from them along the way. No phenomena follow this description better than those of electricity and magnetism. Look around you. Observe how much your world is influenced by electricity and magnetism. A modern American home must have electricity just to operate. Its walls are filled with wires that provide energy to electrical outlets, lights, and air-conditioning systems. Porch lights, streetlights, and the occasional diabolical bug zapper will punctuate even a stroll down most streets. In the homes, we cook and prepare our meals, heat our water, and dry our hair by electricity. We entertain ourselves with electronic televisions, radios, and stereos. In fact, electricity has insinuated itself so completely into our lives, that we would have a very hard time living without it. When I was in Russia this past summer I visited a small village called Bykee. It had about 10 houses with a single, deeply rutted road that passed through it. All of the village houses got their water from a well, but they all had electricity. ELECTRICITY & MAGNETISM I feel a Want of Terms here and doubt much whether I shall be able to make this intelligible. -Benjamin Franklin The phenomenon of electricity, at least static electricity, had been known for millennia. In fact, electron is Greek for amber, one of the substances famous for producing a static charge when rubbed. Although known, static electricity was nothing more than a curiosity until the 18th Century. Benjamin Franklin (1706-1790, USA), was one of the first to explore electricity. He did so after he was given a static electricity generator by a friend in Britain. He became so immersed in the study of electricity in 1746 that almost everything else in his life took second place. He began a set of experiments that led him to write a series of papers that were well-received (after some initial jokes about a colonial yokel) at the Royal Society and elsewhere in Europe. In 1752, he conducted his most famous experiment with the kite, which helped to confirm that that lightning and electricity were the same things. Based on that, he initiated experiments that led to the practical application of the lightning rod, an invention that saved and continues to save many buildings from destruction. Those and other experiments with electricity convinced Franklin that electricity moved from one place to another as a fluid. He was not sure which way the fluid went, but he guessed that the fluid moved from an object of excess to an object of deficiency. This was his explanation for why oppositely charged objects seemed to cancel each other out. He invented the terms positive and negative charge to differentiate between the two conditions. Although little was really known about the new, mysterious force of electricity, it did exert a force (or charge) that could either attract or repel another charged object. Charles Augustin de Coulomb (1736-1806, France) examined the force between charged particles and discovered that electricity acted at a distance, and, like gravity, diminished according to the inverse square of the distance between the charges. That is, the charge of object 1 times the charge of object 2 divided by the distance between them squared can be expressed in units of force. This became known as Coulomb’s Law (see Figure 4-13). 1 Electricity works for us because we make it move in a circuit. In this sense, the movement of electricity is similar to the movement of water. In fact, the similarity does not stop there nor is it superficial. Electricity conducts through a metal wire like water moves through a pipe. If the pipe is smaller, less water gets through the pipe each second. Similarly, if a wire gets smaller, less electricity moves through it each second. Water moves downhill or as a consequence of water pressure in a pipe. Electric current moves as a consequence of electrical pressure from the power source. This is called the voltage. FIGURE 4-13. An illustration of Columb’s Law with balls of charge 1 and charge 2 repelling with a force to push them distance d apart. Volt is named after Allesandro Volta (1745-1827), the man who invented the first chemical battery. His battery (or pile as he called it) was a layer cake of different metal disks with felt soaked in a weak acid between the metal disks. Volta tried to impress Napoleon with his idea, but he was unsuccessful at first. However, Volta did impress the scientific community with a reliable source of electrical power. This led to the use of electricity for many explorations in physics, chemistry, etc. Now, the battery is a necessary component of many things in the household, including the computer on which I am writing this. The power of a battery was its ability to make positive and negative charges at the poles of the battery; the greater the difference in charge, the greater the voltage. It was the voltage that provided the electrical pressure to move an electrical current in a wire. Andre-Marie Ampere (1775-1836 France) studied this movement of electrical charges in an electrical circuit. We now refer to the flow of electrical charge as amp (short for Ampere). Throughout the initial period of study, the Franklin theory of electrical fluid remained the prevailing view. While electricity had been known for only a short time, magnetism had been studied for millennia. Magnetite, a naturally occurring magnetic ore or lodestone is relatively common and, like any other magnet, causes nails and other iron objects to jump. William Gilbert (1544-1603; England), physician to Queen Elizabeth I also studied and wrote about magnets. He concluded that all magnets have two poles. Also, like magnetic poles repel each other while opposite magnetic poles attract. It wasn’t until Hans Christian Oersted (1770-1851, Denmark) was switching a current on and off during a lecture in Denmark in 1820 that he saw evidence of the relationship between electricity and magnetism. When current flowed through a wire that was suspended over a compass, the needle in the compass moved to a position perpendicular to the wire. Ampere interpreted Oersted’s observation of the interactions between electricity and magnetism in a Newtonian way. He described, mathematically, point forces acting at a distance from each other. In what is now called Ampere’s Law, which stated that a changing electrical charge or current produced a magnetic effect. In England, however, Oersted’s observation led to a revolutionary new way of viewing electrical and magnetic phenomena by a brilliant self-taught experimental genius. 2 ELECTROMAGNETISM Several important results may be deduced from the properties of lines of force. –James Clerk Maxwell Michael Faraday (1791-1867, England) did not have proper schooling. In fact, he was self-educated. He worked in a bookbindery and read many of the books that came through his shop. One happened to be a portion of the third edition of the Encyclopedia Britannica. In particular, he read the 127-page article about electricity. At that point he became interested in science. By 1810 he began to attend lectures by the renowned chemist, Sir Humphry Davy (1778-1829, England). About a year later, Davy became temporarily blinded by an explosion in his laboratory and hired Faraday to be his eyes and his hands. Soon, Faraday was performing his own experiments in Davy’s laboratory. He discovered benzene, made the first chlorinated hydrocarbon and ethylene. He did pioneering work in the field of steel alloys and produced heavy optical glass. In 1821 Faraday heard of Oersted’s work and began to explore the strange connection between electricity and magnetism. He made a wire that circled a magnet when a current was passed through it. He also showed that a magnet would rotate around a wire carrying current. Because he did not have a background in mathematics, he did not understand Ampere’s explanation of electromagnetism. Instead, Faraday began to describe the phenomena in terms of physical models. He began to think of electricity and magnetism as fields or lines of force that extended from the wire or from the magnet.