23 - Electric Fields - Electricity and Magnetism W PA R T 4...

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Electricity and Magnetism e now study the branch of physics concerned with electric and magnetic phe- nomena. The laws of electricity and magnetism have a central role in the oper- ation of such devices as radios, televisions, electric motors, computers, high-energy accelerators, and other electronic devices. More fundamentally, the in- teratomic and intermolecular forces responsible for the formation of solids and liq- uids are electric in origin. Furthermore, such forces as the pushes and pulls between objects and the elastic force in a spring arise from electric forces at the atomic level. Evidence in Chinese documents suggests that magnetism was observed as early . The ancient Greeks observed electric and magnetic phenomena possi- B . C . They found that a piece of amber, when rubbed, becomes electrified and attracts pieces of straw or feathers. The Greeks knew about magnetic forces from observations that the naturally occurring stone magnetite (Fe 3 O 4 ) is at- tracted to iron. (The word electric comes from elecktron , the Greek word for “amber.” comes from Magnesia , the name of the district of Greece where magnetite was first found.) In 1600, the Englishman William Gilbert discovered that electrification is not limited to amber but rather is a general phenomenon. In the years following this discovery, scientists electrified a variety of objects. Experiments by Charles Coulomb in 1785 confirmed the inverse-square law for electric forces. It was not until the early part of the nineteenth century that scientists established that electricity and magnetism are related phenomena. In 1819, Hans Oersted dis- covered that a compass needle is deflected when placed near a circuit carrying an electric current. In 1831, Michael Faraday and, almost simultaneously, Joseph Henry showed that when a wire is moved near a magnet (or, equivalently, when a magnet is moved near a wire), an electric current is established in the wire. In 1873, James Clerk Maxwell used these observations and other experimental facts as a basis for formulating the laws of electromagnetism as we know them today. ( Electromagnet- is a name given to the combined study of electricity and magnetism.) Shortly thereafter (around 1888), Heinrich Hertz verified Maxwell’s predictions by producing electromagnetic waves in the laboratory. This achievement led to such practical de- velopments as radio and television. Maxwell’s contributions to the field of electromagnetism were especially signifi- cant because the laws he formulated are basic to all forms of electromagnetic phe- nomena. His work is as important as Newton’s work on the laws of motion and the W P A R T 4 Lightning is a dramatic example of electrical phenomena occurring in nature. While we are most familiar with lightning originating from thunderclouds, it can occur in other situations, such as in a volcanic eruption (here, the Sakurajima volcano, Japan). (M. Zhilin/ M. Newman/Photo Researchers, Inc.) 705
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Chapter 23 Electric Fields CHAPTE R OUTLI N E 23.1 Properties of Electric Charges 23.2 Charging Objects By Induction 23.3
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