Editing Electric field (section)

== Description ==
[[File:VFPt image charge plane horizontal.svg|thumb|250px|Electric field of a positive point [[electric charge]] suspended over an infinite sheet of conducting material. The field is depicted by [[field line|electric field lines]], lines which follow the direction of the electric field in space.  The induced charge distribution in the sheet is not shown.]]
The electric field is defined at each point in space as the force that would be experienced by an [[infinitesimally small]] stationary [[test charge]] at that point divided by the charge.<ref name=uniphysics>{{citation| first = Francis | last = Sears| title = University Physics |edition=6th| publisher = Addison Wesley| year = 1982| isbn = 0-201-07199-1|display-authors=etal}}</ref>{{rp|469–70}} The electric field is defined in terms of [[force]], and force is a [[Euclidean vector|vector]] (i.e. having both [[Magnitude (mathematics)|magnitude]] and [[Direction (geometry)|direction]]), so it follows that an electric field may be described by a [[vector field]].<ref name=uniphysics/>{{rp|469–70}} The electric field acts between two charges similarly to the way that the [[gravitational field]] acts between two [[mass]]es, as they both obey an [[inverse-square law]] with distance.<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> This is the basis for [[Coulomb's law]], which states that, for stationary charges, the electric field varies with the source charge and varies inversely with the square of the distance from the source. This means that if the source charge were doubled, the electric field would double, and if you move twice as far away from the source, the field at that point would be only one-quarter its original strength.

The electric field can be visualized with a set of [[field line|lines]] whose direction at each point is the same as those of the field, a concept introduced by [[Michael Faraday]],<ref name="elec_princ_p73">{{citation |last = Morely & Hughes |title = Principles of Electricity |edition=5th |year = 1970 |page = 73 |publisher = Longman |isbn = 0-582-42629-4 }}</ref> whose term '[[Line of force|lines of force]]' is still sometimes used. This illustration has the useful property that, when drawn so that each line represents the same amount of [[flux]], the [[Field strength|strength of the field]] is proportional to the density of the lines.<ref name="Tou">{{cite book |last1 = Tou |first1 = Stephen |title = Visualization of Fields and Applications in Engineering |publisher = John Wiley and Sons |date = 2011 |pages = 64 |url = https://books.google.com/books?id=4kjP1ALPTfUC&q=%22field+line%22+vector+field+tangent&pg=PT64 |isbn = 9780470978467 }}</ref> Field lines due to stationary charges have several important properties, including that they always originate from positive charges and terminate at negative charges, they enter all good conductors at right angles, and they never cross or close in on themselves.<ref name=uniphysics/>{{rp|479}} The field lines are a representative concept; the field actually permeates all the intervening space between the lines. More or fewer lines may be drawn depending on the precision to which it is desired to represent the field.<ref name="elec_princ_p73"/> The study of electric fields created by stationary charges is called [[electrostatics]].

[[Faraday's law of induction|Faraday's law]] describes the relationship between a time-varying magnetic field and the electric field. One way of stating Faraday's law is that the [[Curl (mathematics)|curl]] of the electric field is equal to the negative [[time derivative]] of the magnetic field.<ref name=griffiths>{{cite book| last=Griffiths| first=David J. |title=Introduction to electrodynamics |date=1999 |publisher=Prentice Hall |isbn=0-13-805326-X |edition=3rd |location=Upper Saddle River, NJ |oclc=40251748 }}</ref>{{rp|327}} In the absence of time-varying magnetic field, the electric field is therefore called [[conservative vector field|conservative]] (i.e. curl-free).<ref name=griffiths />{{rp|24,90–91}} This implies there are two kinds of electric fields: electrostatic fields and fields arising from time-varying magnetic fields.<ref name=griffiths/>{{rp|305–307}} While the curl-free nature of the static electric field allows for a simpler treatment using electrostatics, time-varying magnetic fields are generally treated as a component of a unified [[electromagnetic field]]. The study of magnetic and electric fields that change over time is called [[electrodynamics]].