Editing Electricity (section)

===Electric potential===
{{Main|Electric potential}}
{{See also|Voltage|Electric battery}}
[[File:Panasonic-oxyride.jpg|thumb|alt=Two AA batteries each have a plus sign marked at one end.|A pair of [[AA cell]]s. The +&nbsp;sign indicates the polarity of the potential difference between the battery terminals.]]

The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires [[Work (physics)|work]]. The electric potential at any point is defined as the energy required to bring a unit test charge from an infinite distance slowly to that point. It is usually measured in [[volt]]s, and one volt is the potential for which one [[joule]] of work must be expended to bring a charge of one [[coulomb]] from infinity.<ref name=uniphysics/>{{rp|494–98}} This definition of potential, while formal, has little practical application, and a more useful concept is that of [[electric potential difference]], and is the energy required to move a unit charge between two specified points. The electric field is ''[[Conservative force|conservative]]'', which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated.<ref name=uniphysics/>{{rp|494–98}} The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the term [[voltage]] sees greater everyday usage.

For practical purposes, defining a common reference point to which potentials may be expressed and compared is useful. While this could be at infinity, a much more useful reference is the [[Earth]] itself, which is assumed to be at the same potential everywhere. This reference point naturally takes the name [[Ground (electricity)|earth]] or [[Ground (electricity)|ground]]. Earth is assumed to be an infinite source of equal amounts of positive and negative charge and is therefore electrically uncharged—and unchargeable.<ref>
{{Citation
| first = Raymond A. | last = Serway
| title = Serway's College Physics
| publisher = Thomson Brooks
| page = 500
| year = 2006
| isbn =0-534-99724-4}}
</ref>

Electric potential is a [[scalar (physics)|scalar quantity]]. That is, it has only magnitude and not direction. It may be viewed as analogous to [[height]]: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall' across the voltage caused by an electric field.<ref>{{Citation
| first1 = Sue
| last1 = Saeli
| title = Using Gravitational Analogies To Introduce Elementary Electrical Field Theory Concepts
| url = http://physicsed.buffalostate.edu/pubs/PHY690/Saeli2004GEModels/older/ElectricAnalogies1Nov.doc
| access-date = 2007-12-09
| bibcode = 2007PhTea..45..104S
| last2 = MacIsaac
| first2 = Dan
| volume = 45
| year = 2007
| pages = 104
| journal = The Physics Teacher
| doi = 10.1119/1.2432088
| issue = 2
| archive-date = 2008-02-16
| archive-url = https://web.archive.org/web/20080216100859/http://physicsed.buffalostate.edu/pubs/PHY690/Saeli2004GEModels/older/ElectricAnalogies1Nov.doc
| url-status = live
}}</ref> As relief maps show [[contour line]]s marking points of equal height, a set of lines marking points of equal potential (known as [[equipotential]]s) may be drawn around an electrostatically charged object. The equipotentials cross all lines of force at right angles. They must also lie parallel to a [[electrical conductor|conductor]]'s surface, since otherwise there would be a force along the surface of the conductor that would move the charge carriers to even the potential across the surface.

The electric field was formally defined as the force exerted per unit charge, but the concept of potential allows for a more useful and equivalent definition: the electric field is the local [[gradient]] of the electric potential. Usually expressed in volts&nbsp;per&nbsp;metre, the vector direction of the field is the line of greatest slope of potential, and where the equipotentials lie closest together.<ref name=Duffin/>{{rp|60}}