Electric charge: Difference between revisions

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==Units==
The [[International System of Units|SI]] derived unit of [[quantity]] of electric charge is the [[coulomb]] (symbol: C). The coulomb is defined as the quantity of charge that passes through the [[cross section (geometry)|cross section]] of an [[electrical conductor]] carrying one [[ampere]] for one [[second]].<ref name=CIPM1946>{{cite web |url=https://www.bipm.org/en/CIPM/db/1946/2/ |publisher=BIPM |title=CIPM, 1946: Resolution 2}}</ref> This unit was proposed in 1946 and ratified in 1948.<ref name=CIPM1946/> In modern practice, the phrase "amount of charge" is used instead of "quantity of charge".<ref name=SIBrochure>{{SIbrochure8th}}, p. 150</ref> The amount of charge in 1 electron ([[elementary charge]]) is approximately {{val|1.6|e=-19|u=C}}, and 1 coulomb corresponds to the amount of charge for about {{val|6.24|e=18|u=electrons}}. The lowercase symbol ''q'' is often used to denote a quantity of electricity or charge. The quantity of electric charge can be directly measured with an [[electrometer]], or indirectly measured with a [[galvanometer|ballistic galvanometer]].
 
After finding the [[charge quantization|quantized]] character of charge, in 1891 [[George Johnstone Stoney|George Stoney]] proposed the unit 'electron' for this fundamental unit of electrical charge. This was before the discovery of the particle by [[J. J. Thomson]] in 1897. The unit is today treated as nameless, referred to as {{em|elementary charge}}, {{em|fundamental unit of charge}}, or simply as {{em|e}}. A measure of charge should be a multiple of the elementary charge ''e'', even if at [[macroscopic scale|large scales]] charge seems to behave as a [[real number|real quantity]]. In some contexts it is meaningful to speak of fractions of a charge; for example in the charging of a [[capacitor]], or in the [[fractional quantum Hall effect]].
In 1729 [[Stephen Gray (scientist)|Stephen Gray]] was experimenting with [[static electricity]], which he generated using a glass tube. He noticed that a cork, used to protect the tube from dust and moisture, also became electrified (charged). Further experiments (e.g, extending the cork by putting thin sticks into it) showed—for the first time—that electrical effluvia (as Gray called it) could be transmitted (conducted) over a distance. Gray managed to transmit charge with twine (765 feet) and wire (865 feet).<ref name=Baigrie27>{{cite book|last=Baigrie|first=Brian |title=Electricity and magnetism: A historical perspective|year=2007|publisher=Greenwood Press|location=Westport, CT|page=27}}</ref> Through these experiments, Gray discovered the importance of different materials, which facilitated or hindered the conduction of electrical effluvia. [[John Theophilus Desaguliers]], who repeated many of Gray’s experiments, is credited with coining the terms [[electrical conductor|conductors]] and [[electrical insulation|insulators]] to refer to the effects of different materials in these experiments.<ref name=Baigrie27/> Gray also discovered electrical induction (i.e., where charge could be transmitted from one object to another without any direct physical contact). For example, he showed that by bringing a charged glass tube close to, but not touching, a lump of lead that was sustained by a thread, it was possible to make the lead become electrified (e.g., to attract and repel brass filings).<ref name=Baigrie28>{{cite book|last=Baigrie|first=Brian |title=Electricity and magnetism: A historical perspective|year=2007|publisher=Greenwood Press|location=Westport, CT|page=28}}</ref> He attempted to explain this phenomenon with the idea of electrical effluvia.<ref>{{cite book |last= Heilbron| first= J.L.|title= Electricity in the 17th and 18th Centuries: A Study of Early Modern Physics|publisher= University of California Press|year= 1979|page= 248|isbn= 978-0-520-03478-5|url= https://books.google.com/?id=UlTLRUn1sy8C&pg=PA248}}</ref>
 
Gray’s discoveries introduced an important shift in the historical development of knowledge about electric charge. The fact that electrical effluvia could be transferred from one object to another, opened the theoretical possibility that this property was not inseparably connected to the bodies that were electrified by rubbing.<ref name=Baigrie35>{{cite book|last=Baigrie|first=Brian |title=Electricity and magnetism: A historical perspective|year=2007|publisher=Greenwood Press|location=Westport, CT|page=35}}</ref> In 1733 [[Charles François de Cisternay du Fay]], inspired by Gray's work, made a series of experiments (reported in ''Mémoires de l'[[Académie Royale des Sciences]]''), showing that more or less all substances could be 'electrified' by rubbing, except for metals and fluids<ref>{{cite book |last1=Roller |first1=Duane |author-link1= |last2=Roller |first2=D.H.D.|date=1954 |title=The development of the concept of electric charge: Electricity from the Greeks to Coulomb |url=https://archive.org/details/developmentofcon0000roll|url-access=registration |location=Cambridge, MA |publisher=[[Harvard University Press]] |page=[https://archive.org/details/developmentofcon0000roll/page/40 40] |isbn=}}</ref> and proposed that electricity comes in two varieties that cancel each other, which he expressed in terms of a two-fluid theory.<ref>[http://www.sparkmuseum.com/BOOK_DUFAY.HTM Two Kinds of Electrical Fluid: Vitreous and Resinous – 1733. Charles François de Cisternay DuFay (1698–1739)] {{webarchive|url=https://web.archive.org/web/20090526211225/http://www.sparkmuseum.com/BOOK_DUFAY.HTM |date=2009-05-26}}. sparkmuseum.com</ref> When [[glass]] was rubbed with [[silk]], du Fay said that the glass was charged with ''[[glass|vitreous]] electricity'', and, when amber was rubbed with fur, the amber was charged with ''[[resin]]ous electricity''. Another important two-fluid theory from this time was proposed by [[Jean-Antoine Nollet]] (1745).<ref>{{cite book |last= Heilbron| first= J.L.|title= Electricity in the 17th and 18th Centuries: A Study of Early Modern Physics|publisher= University of California Press|year= 1979|pages= 280–289|isbn= 978-0-520-03478-5|url= https://books.google.com/?id=UlTLRUn1sy8C&pg=PA169}}</ref> In 1839, [[Michael Faraday]] showed that the apparent division between [[static electricity]], [[electric current|current electricity]], and [[bioelectricity]] was incorrect, and all were a consequence of the behavior of a single kind of [[electricity]] appearing in opposite [[electrical polarity|polarities]]. It is arbitrary which polarity is called positive and which is called negative. Positive charge can be defined as the charge left on a glass rod after being rubbed with silk.<ref>Roald K. Wangsness (1986) ''Electromagnetic Fields'' (2nd Ed.). Wiley. {{ISBN|0-471-81186-6}}.</ref>
 
Up until about 1745, the main explanation for electrical attraction and repulsion was the idea that electrified bodies gave off an effluvium.<ref>{{cite encyclopedia |last1=Heilbron |first1=John |editor-last1=Heilbron |editor-first1=John |title=Leyden jar and electrophore |date=2003 |publisher=Oxford University Press |location=New York |isbn=9780195112290 |page=459 |encyclopedia=The Oxford Companion to the History of Modern Science}}</ref>
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