Editing Electricity (section)

===Electric field===
{{Main|Electric field}}
{{See also|Electrostatics}}

The concept of the electric [[Field (physics)|field]] was introduced by [[Michael Faraday]]. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two [[mass]]es, and like it, extends towards infinity and shows an inverse square relationship with distance.<ref name=Umashankar/> However, there is an important difference. Gravity always acts in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Since large bodies such as planets generally carry no net charge, the electric field at a distance is usually zero. Thus gravity is the dominant force at distance in the universe, despite being much weaker.<ref name=hawking/>

[[File:VFPt image charge plane horizontal.svg|thumb|Field lines emanating from a positive charge above a plane conductor]]

An electric field generally varies in space,{{efn|Almost all electric fields vary in space. An exception is the electric field surrounding a planar conductor of infinite extent, the field of which is uniform.}} and its strength at any one point is defined as the force (per unit charge) that would be felt by a stationary, negligible charge if placed at that point.<ref name=uniphysics/>{{rp|469–70}} The conceptual charge, termed a '[[test charge]]', must be vanishingly small to prevent its own electric field disturbing the main field and must also be stationary to prevent the effect of [[magnetic field]]s. As the electric field is defined in terms of [[force]], and force is a [[Euclidean vector|vector]], having both [[Magnitude (mathematics)|magnitude]] and [[Direction (geometry)|direction]], it follows that an electric field is a [[vector field]].<ref name=uniphysics/>{{rp|469–70}}

The study of electric fields created by stationary charges is called [[electrostatics]]. The field may be visualised by a set of imaginary lines whose direction at any point is the same as that of the field. This concept was introduced by Faraday,<ref name="elec_princ_p73">
{{citation
| last = Morely & Hughes
| title = Principles of Electricity, Fifth edition
| year = 1970
| page = 73
| publisher = Longman
| isbn = 0-582-42629-4}}</ref> whose term '[[Line of force|lines of force]]' still sometimes sees use. The field lines are the paths that a point positive charge would seek to make as it was forced to move within the field; they are however an imaginary concept with no physical existence, and the field permeates all the intervening space between the lines.<ref name="elec_princ_p73"/> Field lines emanating from stationary charges have several key properties: first, they originate at positive charges and terminate at negative charges; second, they must enter any good conductor at right angles, and third, they may never cross nor close in on themselves.<ref name=uniphysics/>{{rp|479}}

A hollow conducting body carries all its charge on its outer surface. The field is therefore 0 at all places inside the body.<ref name=Duffin/>{{rp|88}} This is the operating principle of the [[Faraday cage]], a conducting metal shell that isolates its interior from outside electrical effects.

The principles of electrostatics are important when designing items of [[high voltage|high-voltage]] equipment. There is a finite limit to the electric field strength that may be withstood by any medium. Beyond this point, [[electrical breakdown]] occurs and an [[electric arc]] causes flashover between the charged parts. Air, for example, tends to arc across small gaps at electric field strengths which exceed 30&nbsp;kV per centimetre. Over larger gaps, its breakdown strength is weaker, perhaps 1&nbsp;kV per centimetre.<ref name=hv_eng>
{{Citation
| first1 = M.S.| last1 = Naidu
| first2 = V.| last2 = Kamataru
| title = High Voltage Engineering
| publisher = Tata McGraw-Hill
| page = 
| year = 1982
| isbn =0-07-451786-4}}
</ref>{{rp|p=2}} The most visible natural occurrence of this is [[lightning]], caused when charge becomes separated in the clouds by rising columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of a large lightning cloud may be as high as 100&nbsp;MV and have discharge energies as great as 250&nbsp;kWh.<ref name=hv_eng/>{{rp|pp=201–02}}

The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principle is exploited in the [[lightning conductor]], the sharp spike of which acts to encourage the lightning strike to develop there, rather than to the building it serves to protect.<ref name="Nahin2002">{{citation|author=Paul J. Nahin|author-link=Paul J. Nahin|title=Oliver Heaviside: The Life, Work, and Times of an Electrical Genius of the Victorian Age|date=9 October 2002|publisher=JHU Press|isbn=978-0-8018-6909-9}}</ref>{{rp|155}}