E LECTRICITY AND M AGNETISM

 Electricity - a basic feature of matter that makes up everything in the universe. When people hear the word electricity, they think of lights, electronic devices, etc. but electricity is much more important than that.  Electricity and magnetism together make up a force called electromagnetism, one of the fundamental forces of the universe.

One of the most important properties of electricity is electrical energy. This source of energy had so many practical applications that it greatly changed the way people lived. The demand for electric energy grew steadily during the 1990’s. today, most people cannot imagine life without electricity.

E-D ISCOVERY

W E W ELCOME G REEKS B EARING E-G IFTS Several thousand years ago, the ancient Greeks observed that a substance called amber attracted bits of lightweight material, after it was rubbed with cloth. Amber is a good insulator, so it easily holds electric charge. The Greek word for amber is elektron. The English word electricity and electron come from this word. Other people, including ancient Greeks and Chinese, knew another substance that could attract things. It was a black rock called lodestone or magnetite. Lodestone attracts iron objects, which tend to be heavy. It contrast, amber attracts only light things, like straw.

E ARLY E In 1551, the Italian mathematician Girolamo Cardano a.k.a. Jerome Cardano, realized that the attracting effects of amber and magnetite must be different. Cardano was the first to note the difference between electricity and magnetism. 1600, the English physician William Gilbert reported that such materials as glass, sulfur, and wax behaved like amber. He called these materials electrics. Today, we know that what he called electrics are materials that are good insulator.

In 1730’s, the French scientist Charles Dufay, found that charged pieces of glass attracted amberlike substances but repelled other glasslike substances. He decided that there must be two kinds of electricity. He called one vitreous (for glasslike substances) and the other resinous (for amberlike substances). Dufay found negative and positive elecrtic charge, he thought of them as two kinds of “electric fluid”.

E-A TTRACT AND E-R EPEL Benjamin Franklin began to experiment with electricityin1746. he thought that there was only one kind of electric fluid. If an object with an excess of fluid touched an object deficient in fluid, the fluid would not be shared. Franklin’s idea explained how opposite charges cancel each other when they come out. Franklin used the term positive for what he thought was an excess of electric fluid. He used the term negative for a deficiency fluid. Franklin did not know that electricity is not a fluid.

In 1752, Franklin performed his famous experiment of flying a kite during thunderstorm. When the kite and string became electrically charged, Franklin concluded that the storm clouds were themselves charged. He became convinced that lightning was a huge electric spark. Fortunately, lightning did not strike his kite. If it had, he would probably have been killed.

In 1767, the English scientist Joseph Priestly described the mathematical law that shows how attraction weakens as the distance between oppositely charged object increases. In 1785, the French scientist Charles Augustin de Coulomb confirmed Priestly’s law. Coulomb showed that the law also held true for repulsive force between objects with the same charge. Today, the principle is known as Coulomb’s law.

In 1771, Luigi Galvani, an Italian anatomy professor, found that the leg of a recently killed frog would twitch when touched with two different metals at the same time. Galvani’s work attracted much attention. In the late 1790’s Alessandro Volta, an Italian physicist, offered explanation. Volta showed that chemical action occurs in a moist material in contact with two different metals. The chemical reaction results in an electric current. The flow of current had made Galvani’s frog twitch.

Volta gathered pairs of disks, consisting of one silver and one zinc disk. He separated the pairs with paper or cloth moistened with salt water. By piling up a stack of such disks, Volta constructed the first battery, called a voltaic pile. And so, many experiments with Volta’s battery and electric circuits followed.