Barron's SAT 2:物理.pdf
《Barron's SAT 2:物理(第10版)》：太傻：中国最顶尖的教育服务公司，集教育服务、教育咨询、网路服务、图书出版与教育培训为一体，是中国目前服务规模最大的专注于留学服务，特别是留学信息服务与留学咨询服务的顶尖公司。旗下太傻咨询中心每年为几千位客户提供留学咨询服务，2011年度57％客户获得美国TOP50院校录取。是中国最大的出国留学网站，独立IP访问量1800万／年，网页浏览量2-2亿次／年，被留学生誉为留学DIY天堂。
作者：（美国）格维尔茨（Herman Gewirtz） （美国）沃尔夫（Jonathan S.Wolf）
In the nineteenth century, chemical experiments to explain these effects showed thepresence of molecules called ions in solution. These ions possessed similar affinities forcertain objects, such as carbon or metals, placed in the solution. These objects are calledelectrades. The experiments confirmed the existence of two types of ions, positive andnegative. The effects they produce are similar to the two types of effects produced whenebonite and glass are rubbed. Even though both substances attract small objects, theseobjects become charged oppositely when rubbed, as indicated by the behavior of thepith ball. Fllrther, chemical experiments coupled with an atomic theory demonstratedthat in solids it is the negative charges that are transferred. Additional experiments byMichael Faraday in England during the first half of the nineteenth century suggestedthe existence of a single, fundamental carrier of electric charge, which was later namedthe electron. The corresponding carrier of positive charge was termed the proton.When ebonite is rubbed with cloth, only the part of the rod in contact with the clothbecomes charged. The charge remains localized for some time （hence the name static）.For this reason, among others, rubber, along with plastic and glass, is called aninsulator. A metal rod held in your hand cannot be charged statically for two reasons.First, metals are conductorr,, that is, they allow electric charges to flow through them.Second, your body is a conductor, and any charges placed in the metal rod are con-ducted out through you （and into the earth）. This effect is called grounding. Thesilver-coated pith balls mentioned in the preceding section can become staticallycharged because they are suspended by thread, which is an insulator. They can be usedto detect the presence and sign of an electric charge, but they are not very helpful inobtaining a qualitative measurement of the magnitude of charge they possess. An instrument that is often used for qualitative measurement is the electroscope.One form of electroscope consists of two "leaves" made of gold foil （Figure 15.2a）.The leaves are vertical when the electroscope is uncharged. As a negatively chargedrod is brought near, the leaves diverge. If you recall the hypothesis that only negativecharges move in solids, you can understand that the electrons in the knob of the elec-troscope are repelled down to the leaves through the conducting stem. The knobbecomes positively charged, as can be verified with a charged pith ball, as long as therod is near but not touching （Figure 15.2b）. Upon contact, electrons are directlytransferred to the knob, stem, and leaves. The whole electroscope then becomes neg-atively charged （Figure 15.2c）. The extent to which the leaves are spread apart is anindication of how much charge is present （but only qualitatively）. If you touch theelectroscope, you will ground it and the leaves will collapse together.
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