History Lecture for Unit 50
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We begin our detailed investigation of electricity and magnetism by looking at the phenomena associated with static (unmoving) charges. Any object which has an excess of positive or negative charges will have an electrical charge and react to other charged objects.
These phenomena are all around us--every time you "zap" yourself by walking across a dry rug and touching a metal doorknob, or try to get your hair to lie down while combing it on a dry day, or when you watch a lightning bolt flash across the sky, you are looking at static electricity in action. From ancient times we have many records of observations of electrical phenomena, from lightning and St. Elmo's fire to electrical eels, and the attractive power of rubbed amber. Our old friend Thales of Miletus is reported to have talked about how rubbing amber stone with cat fur made it spark. The amber didn't have to be in direct contact with the fur after it had been "charged" by rubbing; just bringing the fur close to the amber would make it glow. Obviously, something was influencing the behavior over the intervening space.
Plato made a list of such phenomena in his Timaeus:
Moreover, as to the flowing of water, the fall of the thunderbolt, and the marvels that are observed about the attraction of amber and the Herculean stones, — in none of these cases is there any attraction; but he who investigates rightly, will find that such wonderful phenomena are attributable to the combination of certain conditions — the non-existence of a vacuum, the fact that objects push one another round, and that they change places, passing severally into their proper positions as they are divided or combined.
Although he was mistaken about the attraction (there is an attractive force involved), his list suggests that not only were people aware of electrical phenomena, but that they also recognized similarities between this class of events.
There are also ancient and medieval records of magnetic effects, which were often interpreted as magical or at the very least, partaking of the deepest mysteries of nature. St. Augustine in Book XXI of The City of God gives an excellent description of the attraction of lodestone for iron, and the ability of diamonds to block this attraction, based on his own observations. His purpose is to show how phenomena that cannot be accounted for by reason may nevertheless be true, but the account demonstrates that certain magnetic and electrical phenomena were well known, however mysterious. Magnetism was the subject of investigation by the English scientist William Gilbert, who published his book De Magnete in 1600. Gilbert discussed both the behavior of magnets and different electrical phenomena, even listing a number of objects subject to static charges like the amber and cat fur. But along many of his contemporaries, he believed that magnetism and electricity were unrelated phenomena, and his theory influenced philosophers for the next two centuries.
In the late seventeenth century, the German Otto von Guericke used a more mechanical approach to investigating static electrical phenomena. Guericke invented a vacuum pump, but his research was not confined to the properties of vacua and air pressure. He build a machine to generate electricity (a ball of sulfur which rotated against a rough surface, absorbing charge from it), and showed that the resulting charges could repel as well as attract different objects. You can duplicate von Guericke's experiment with an inflated balloon: just rub it against your hair or a wool sweater on a dry day, and you'll create enough charge on the balloon that electrical attraction will hold it to a wall and keep it from falling under the force of gravity.
Fifty years later, Stephen Grey sent a paper to the Royal Society in which he explained how he transmitted charge from a generator like von Guericke's over 650 feet in metal wires suspended by silk thread. He discovered that metal suspension wires wouldn't work, since they acted as grounds and transmitted the flowing charge back to earth before it could reach the end of the wire. He thus gave the first descriptions of materials as insulators (non-conductors) and conductors of electricity. He also discovered that he could induce a charge into an object merely by bringing another charged object close to the first, then removing the first from contact with the ground. Grey's report is the first evidence of someone inducing electrical charge.
Charles du Fay, a French scientist, repeated Grey's experiments with the same results—a valuable confirmation, since du Fay's experiments were done in a different country with different equipment. Du Fay also proposed that there were two different electricities, vitreous and resinous. Vitreous electricity in one object repelled vitreous electricity in another object, but attracted resinous electricity.
Among others (the invention is usually attributed to Pieter van Musschenbroek in 1745), du Fay learned how to "store" charge in a Leyden jar, a glass container with metal inserts which did not quite touch. The Leyden jar was the first capacitor. This was an important invention, because it allowed experimenters to keep charge around for experimenting at their own convenience; they no longer had to wait for a nice dry day when collection would be easy.
Another major step was taken by the American colonial Benjamin Franklin, who became fascinated with electrical phenomena and turned his considerable intelligence and disciplined industry on the problem. He came to the conclusion that lightning is electrical in nature, which he attempted to prove by flying a kite during an electrical storm. Just being in the thundercloud was enough; the kite collected charge, sent it down the wet string and into the Leyden jar which Franklin was using. Contrary to some versions of the story, Franklin's kite was not hit by lightning, but that of a young man who tried to repeat Franklin's experiment was, and the man was killed [so do not try this experiment yourself]. Franklin described his research in a series of letters which he sent to Peter Colinson, then president of the Royal Society, who arranged for their publication in England.
Read the description of Franklin's "lightning rod" at the Franklin Institute, the science museum in Philadelphia which Franklin started.
Franklin's studies led him to the conclusion that electrification of an object did not create charge in the object, but moved it there from somewhere else. He was the first to voice a law of the conservation of electrical charge, which joined Lavoisier's rule about the conservation of matter as two of the three great conservation laws of physical science (the third is the conservation of energy).
Joseph Priestley (whom we met investigating phlogiston) also studied the affects of electric attraction over distance and realized that the attraction of electricity was subject to the same distance law as gravity, that is, the force exerted by an electrical charge varied as 1/r2.
The man who put all these observations together was the French scientist Charles-Augustin de Coulomb. He continued Priestley's experiments, and in 1785, published the rule that
where q1 and q2 where two different sources of charge and r was the distance between them. He knew that k was a constant of proportionality, like the factor G in Newton's equation for the force of gravity, and eventually it was determined to be 8.99 * 109 N*m2 /C2. N stands for Newtons, the force unit, m for meters, and C is Coulombs, the units of charge.
At the same time that Coulomb was performing his experiments, the Italian Luigi Galvani was experimenting with the effects of electrical stimuli on frogs. He discovered that a charged object would make the frog's leg jump when the object touched the frog's nerves, confirming his suspicions that nerve impulses were electrical in nature. He also discovered that when two different metals were in contact with the frog's nerves, the same thing happened—the legs moved—even though neither metal was charged. He called this animal electricity, but he didn't have a good explanation for why it occurred.
His compatriot, Allesandro Volta, was more astute. He had already invented the electrophorus, a device which he used to charge his Leyden jars, and he had ranked a number of materials according to their ability to respond to charge. He took Galvani's observations, and built what he called a pile (we now call it a Voltaic pile). He stacked pairs of copper and zinc plates between non-conduction materials which he soaked in a weak acid (which is a weak conductor). This was the first battery.
Read the description of Volta's life and accomplishments at the Yaeger Center for Electrochemical Sciences.
Sir Humphrey Davy, the head of the Royal Institute in London, built an enormous Voltaic pile for experimentation, but it was the Frenchman Siméon-Denis Poisson who established a relationship for the power in a Voltaic pile: voltage = electrical energy/charge, or V = E/q.
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