Editing Electricity (section)

===Electric charge===
{{Main|Electric charge}}
{{See also|Electron|Proton|Ion}}

[[File:Electroscope.svg|thumb|upright|alt=A clear glass dome has an external electrode that connects through the glass to a pair of gold leaves. A charged rod touches the external electrode and makes the leaves repel.|Charge on a [[gold-leaf electroscope]] causes the leaves to visibly repel each other]]

By modern convention, the charge carried by [[electron]]s is defined as negative, and that by [[proton]]s is positive.<ref>International Electrotechnical Commission, [https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=113-02-13 IEV ref 113-02-13]</ref> Before these particles were discovered, [[Benjamin Franklin]] had defined a positive charge as being the charge acquired by a glass rod when it is rubbed with a silk cloth.<ref>Lawrence S. Lerner (1997). ''Physics for Scientists and Engineers'', Volume 2, p. 636</ref> A proton by definition carries a charge of exactly {{val|1.602176634|e=-19|u=coulombs}}. This value is also defined as the [[elementary charge]]. No object can have a charge smaller than the elementary charge, and any amount of charge an object may carry is a multiple of the elementary charge. An electron has an equal negative charge, i.e. {{val|-1.602176634|e=-19|u=coulombs}}. Charge is possessed not just by [[matter]], but also by [[antimatter]], each [[antiparticle]] bearing an equal and opposite charge to its corresponding particle.<ref>
{{Citation
| first = Frank | last = Close
| title = The New Cosmic Onion: Quarks and the Nature of the Universe
| publisher = CRC Press
| page = 51
| year = 2007
| isbn = 978-1-58488-798-0}}
</ref>

The presence of charge gives rise to an electrostatic force: charges exert a [[force]] on each other, an effect that was known, though not understood, in antiquity.<ref name=uniphysics>
{{Citation
| first = Francis | last = Sears
| title = University Physics, Sixth Edition
| publisher = Addison Wesley
| year = 1982
| isbn = 0-201-07199-1|display-authors=etal}}
</ref>{{rp|457}} A lightweight ball suspended by a fine thread can be charged by touching it with a glass rod that has itself been charged by rubbing with a cloth. If a similar ball is charged by the same glass rod, it is found to repel the first: the charge acts to force the two balls apart. Two balls that are charged with a rubbed amber rod also repel each other. However, if one ball is charged by the glass rod, and the other by an amber rod, the two balls are found to attract each other. These phenomena were investigated in the late eighteenth century by [[Charles-Augustin de Coulomb]], who deduced that charge manifests itself in two opposing forms. This discovery led to the well-known axiom: ''like-charged objects repel and opposite-charged objects attract''.<ref name=uniphysics/>

The force acts on the charged particles themselves, hence charge has a tendency to spread itself as evenly as possible over a conducting surface. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by [[Coulomb's law]], which relates the force to the product of the charges and has an [[inverse-square]] relation to the distance between them.<ref>{{citation|last=Coulomb|first=Charles-Augustin de|year=1785|title=Histoire de l'Academie Royal des Sciences|location=Paris|quote=The repulsive force between two small spheres charged with the same type of electricity is inversely proportional to the square of the distance between the centres of the two spheres.}}</ref><ref name=Duffin>
{{Citation
| first = W.J.
| last = Duffin
| title = Electricity and Magnetism, 3rd edition
| publisher = McGraw-Hill
| year = 1980
| isbn = 0-07-084111-X
| url = https://archive.org/details/electricitymagn00duff
}}
</ref>{{RP|35}} The electromagnetic force is very strong, second only in strength to the [[strong interaction]],<ref>
{{citation
| last = National Research Council
| title = Physics Through the 1990s
| pages = 215–16
| year = 1998
| publisher = National Academies Press
| isbn =0-309-03576-7}}
</ref> but unlike that force it operates over all distances.<ref name=Umashankar>
{{citation
| first = Korada | last = Umashankar
| title = Introduction to Engineering Electromagnetic Fields
| pages = 77–79
| year = 1989
| publisher = World Scientific
| isbn =9971-5-0921-0}}
</ref> In comparison with the much weaker [[gravitational force]], the electromagnetic force pushing two electrons apart is 10<sup>42</sup> times that of the [[gravitation]]al attraction pulling them together.<ref name=hawking>
{{Citation
| first = Stephen | last = Hawking
| title = A Brief History of Time
| publisher = Bantam Press
| page = 77
| year = 1988
| isbn =0-553-17521-1}}</ref>

Charge originates from certain types of [[subatomic particle]]s, the most familiar carriers of which are the [[electron]] and [[proton]]. Electric charge gives rise to and interacts with the [[electromagnetic force]], one of the four [[fundamental force]]s of nature. Experiment has shown charge to be a [[conserved quantity]], that is, the net charge within an electrically isolated system will always remain constant regardless of any changes taking place within that system.<ref>
{{Citation
| first = James
| last = Trefil
| title = The Nature of Science: An A–Z Guide to the Laws and Principles Governing Our Universe
| publisher = Houghton Mifflin Books
| page = [https://archive.org/details/natureofsciencea00tref/page/74 74]
| year = 2003
| isbn = 0-618-31938-7
| url = https://archive.org/details/natureofsciencea00tref/page/74
}}
</ref> Within the system, charge may be transferred between bodies, either by direct contact or by passing along a conducting material, such as a wire.<ref name=Duffin/>{{rp|2–5}} The informal term [[static electricity]] refers to the net presence (or 'imbalance') of charge on a body, usually caused when dissimilar materials are rubbed together, transferring charge from one to the other.

Charge can be measured by a number of means, an early instrument being the [[gold-leaf electroscope]], which although still in use for classroom demonstrations, has been superseded by the electronic [[electrometer]].<ref name=Duffin/>{{rp|2–5}}