Editing Electricity (section)

==Concepts==
===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}}

===Electric current===
{{Main|Electric current}}

The movement of electric charge is known as an [[electric current]], the intensity of which is usually measured in [[ampere]]s. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current. Electric current can flow through some things, [[electrical conductor]]s, but will not flow through an [[electrical insulator]].<ref>{{citation|last=Al-Khalili|first=Jim|title=Shock and Awe: The Story of Electricity|work=BBC Horizon}}</ref>

By historical convention, a positive current is defined as having the same direction of flow as any positive charge it contains, or to flow from the most positive part of a circuit to the most negative part. Current defined in this manner is called [[conventional current]]. The motion of negatively charged electrons around an [[electric circuit]], one of the most familiar forms of current, is thus deemed positive in the ''opposite'' direction to that of the electrons.<ref>
{{Citation
| first = Robert | last = Ward
| title = Introduction to Electrical Engineering
| publisher = Prentice-Hall
| page = 18
| year = 1960}}
</ref> However, depending on the conditions, an electric current can consist of a flow of [[charged particle]]s in either direction or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation.

[[File:Lichtbogen 3000 Volt.jpg|thumb|left|alt=Two metal wires form an inverted V shape. A blindingly bright orange-white electric arc flows between their tips.|An [[electric arc]] provides an energetic demonstration of electric current.]]

The process by which electric current passes through a material is termed [[electrical conduction]], and its nature varies with that of the charged particles and the material through which they are travelling. Examples of electric currents include metallic conduction, where electrons flow through a [[Electrical conductor|conductor]] such as metal, and [[electrolysis]], where [[ion]]s (charged [[atom]]s) flow through liquids, or through [[plasma (physics)|plasmas]] such as electrical sparks. While the particles themselves can move quite slowly, sometimes with an average [[drift velocity]] only fractions of a millimetre per second,<ref name=Duffin/>{{rp|17}} the [[electric field]] that drives them itself propagates at close to the [[speed of light]], enabling electrical signals to pass rapidly along wires.<ref>
{{Citation
| first = L.
| last = Solymar
| title = Lectures on electromagnetic theory
| publisher = Oxford University Press
| page = [https://archive.org/details/lecturesonelectr0000soly_w5c6/page/140 140]
| year = 1984
| isbn = 0-19-856169-5
| url = https://archive.org/details/lecturesonelectr0000soly_w5c6/page/140
}}
</ref>

Current causes several observable effects, which historically were the means of recognising its presence. That water could be decomposed by the current from a voltaic pile was discovered by [[William Nicholson (chemist)|Nicholson]] and [[Anthony Carlisle|Carlisle]] in 1800, a process now known as [[electrolysis]]. Their work was greatly expanded upon by [[Michael Faraday]] in 1833. Current through a [[electrical resistance|resistance]] causes localised heating, an effect [[James Prescott Joule]] studied mathematically in 1840.<ref name=Duffin/>{{rp|23–24}} One of the most important discoveries relating to current was made accidentally by [[Hans Christian Ørsted]] in 1820, when, while preparing a lecture, he witnessed the current in a wire disturbing the needle of a magnetic compass.<ref name=berkson>
{{Citation
| first = William
| last = Berkson
| title = Fields of Force: The Development of a World View from Faraday to Einstein
| publisher = Routledge
| year = 1974
| isbn = 0-7100-7626-6
| url = https://archive.org/details/fieldsofforcedev0000berk/page/370
}}</ref>{{rp|p=370}}{{efn|Accounts differ as to whether this was before, during, or after a lecture.}} He had discovered [[electromagnetism]], a fundamental interaction between electricity and magnetics. The level of electromagnetic emissions generated by [[electric arc]]ing is high enough to produce [[electromagnetic interference]], which can be detrimental to the workings of adjacent equipment.<ref>{{cite web | title = Lab Note #105 ''EMI Reduction – Unsuppressed vs. Suppressed'' | publisher = Arc Suppression Technologies | date = April 2011 | url = http://www.arcsuppressiontechnologies.com/arc-suppression-facts/lab-app-notes/ | access-date = March 7, 2012 | archive-date = March 5, 2016 | archive-url = https://web.archive.org/web/20160305123758/http://www.arcsuppressiontechnologies.com/arc-suppression-facts/lab-app-notes/ | url-status = live}}</ref>

In engineering or household applications, current is often described as being either [[direct current]] (DC) or [[alternating current]] (AC). These terms refer to how the current varies in time. Direct current, as produced by example from a [[Battery (electricity)|battery]] and required by most [[Electronics|electronic]] devices, is a unidirectional flow from the positive part of a circuit to the negative.<ref name=bird>
{{citation
| first = John | last = Bird
| title = Electrical and Electronic Principles and Technology, 3rd edition
| publisher = Newnes
| year = 2007
| isbn =978-1-4175-0543-2}}
</ref>{{rp|11}} If, as is most common, this flow is carried by electrons, they will be travelling in the opposite direction. Alternating current is any current that reverses direction repeatedly; almost always this takes the form of a [[sine wave]].<ref name=bird/>{{rp|206–07}} Alternating current thus pulses back and forth within a conductor without the charge moving any net distance over time. The time-averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the reverse. Alternating current is affected by electrical properties that are not observed under [[steady state]] direct current, such as [[inductance]] and [[capacitance]].<ref name=bird/>{{rp|223–25}} These properties however can become important when circuitry is subjected to [[transient response|transients]], such as when first energised.

===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}}

===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}}

===Electromagnets===
{{Main|Electromagnets}}
[[File:Electromagnetism.svg|thumb|left|alt=A wire carries a current towards the reader. Concentric circles representing the magnetic field circle anticlockwise around the wire, as viewed by the reader.|Magnetic field circles around a current]]

Ørsted's discovery in 1821 that a [[magnetic field]] existed around all sides of a wire carrying an electric current indicated that there was a direct relationship between electricity and magnetism. Moreover, the interaction seemed different from gravitational and electrostatic forces, the two forces of nature then known. The force on the compass needle did not direct it to or away from the current-carrying wire, but acted at right angles to it.<ref name=berkson/>{{rp|p=370}} Ørsted's words were that "the electric conflict acts in a revolving manner." The force also depended on the direction of the current, for if the flow was reversed, then the force did too.<ref>
{{Citation
| first = Silvanus P. | last = Thompson
| title = Michael Faraday: His Life and Work
| publisher = Elibron Classics
| page = 79
| year = 2004
| isbn =1-4212-7387-X}}
</ref>

Ørsted did not fully understand his discovery, but he observed the effect was reciprocal: a current exerts a force on a magnet, and a magnetic field exerts a force on a current. The phenomenon was further investigated by [[André-Marie Ampère|Ampère]], who discovered that two parallel current-carrying wires exerted a force upon each other: two wires conducting currents in the same direction are attracted to each other, while wires containing currents in opposite directions are forced apart.<ref name="elec_princ_92-93">
{{citation
| last = Morely & Hughes
| title=Principles of Electricity, Fifth edition
| pages=92–93}}</ref> The interaction is mediated by the magnetic field each current produces and forms the basis for the international [[Ampere#Definition|definition of the ampere]].<ref name="elec_princ_92-93"/>

[[File:Electric motor cycle 3.png|thumb|alt=A cut-away diagram of a small electric motor|The electric motor exploits an important effect of electromagnetism: a current through a magnetic field experiences a force at right angles to both the field and current.]]
This relationship between magnetic fields and currents is extremely important, for it led to Michael Faraday's invention of the [[electric motor]] in 1821. Faraday's [[homopolar motor]] consisted of a [[permanent magnet]] sitting in a pool of [[Mercury (element)|mercury]]. A current was allowed through a wire suspended from a pivot above the magnet and dipped into the mercury. The magnet exerted a tangential force on the wire, making it circle around the magnet for as long as the current was maintained.<ref name=iet_faraday>
{{Citation
 |last=Institution of Engineering and Technology
 |author-link=Institution of Engineering and Technology
 |title=Michael Faraday: Biography
 |url=http://www.iee.org/TheIEE/Research/Archives/Histories&Biographies/Faraday.cfm
 |access-date=2007-12-09
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070703063432/http://www.iee.org/TheIEE/Research/Archives/Histories%26Biographies/Faraday.cfm
 |archive-date=2007-07-03
}}
</ref>

Experimentation by Faraday in 1831 revealed that a wire moving perpendicular to a magnetic field developed a potential difference between its ends. Further analysis of this process, known as [[electromagnetic induction]], enabled him to state the principle, now known as [[Faraday's law of induction]], that the potential difference induced in a closed circuit is proportional to the rate of change of [[magnetic flux]] through the loop. Exploitation of this discovery enabled him to invent the first [[electrical generator]] in 1831, in which he converted the mechanical energy of a rotating copper disc to electrical energy.<ref name=iet_faraday/> [[Faraday's disc]] was inefficient and of no use as a practical generator, but it showed the possibility of generating electric power using magnetism, a possibility that would be taken up by those that followed on from his work.<ref>{{Cite book |last=Lees |first=James |url=https://books.google.com/books?id=EV4wDwAAQBAJ&pg=PT98 |title=Physics in 50 Milestone Moments: A Timeline of Scientific Landmarks |date=2017|publisher=Quad Books |isbn=978-0-85762-762-9 |language=en|at=1831: Michael Faraday creates the Faraday disc}}</ref>

===Electric circuits===
{{Main|Electric circuit}}
[[File:Ohms law voltage source.svg|thumb|A basic [[electric circuit]]. The [[voltage source]] ''V'' on the left drives a [[Current (electricity)|current]] ''I'' around the circuit, delivering [[electrical energy]] into the [[resistor]] ''R''. From the resistor, the current returns to the source, completing the circuit.|alt=refer to caption]]

An electric circuit is an interconnection of electric components such that electric charge is made to flow along a closed path (a circuit), usually to perform some useful task.<ref>{{cite book|chapter-url=https://openstax.org/books/physics/pages/19-2-series-circuits |title=Physics |publisher=OpenStax |first1=Paul Peter |last1=Urone |display-authors=etal |year=2023 |isbn=978-1-951693-21-3 |page=612 |chapter=19.2: Series Circuits}}</ref>

The components in an electric circuit can take many forms, which can include elements such as [[resistor]]s, [[capacitor]]s, [[switch]]es, [[transformer]]s and [[electronics]]. [[Electronic circuit]]s contain [[Active device|active component]]s, usually [[semiconductor]]s, and typically exhibit [[non-linear]] behaviour, requiring complex analysis. The simplest electric components are those that are termed [[passivity (engineering)|passive]] and [[linear]]: while they may temporarily store energy, they contain no sources of it, and exhibit linear responses to stimuli.<ref name=Alexander>{{Citation | last1 = Alexander | first1 = Charles | last2 = Sadiku | first2 = Matthew | title = Fundamentals of Electric Circuits | publisher = McGraw-Hill | year = 2006 | edition = 3, revised |isbn =978-0-07-330115-0}}</ref>{{rp|15–16}}

The [[resistor]] is perhaps the simplest of passive circuit elements: as its name suggests, it [[Electrical resistance|resists]] the current through it, dissipating its energy as heat. The resistance is a consequence of the motion of charge through a conductor: in metals, for example, resistance is primarily due to collisions between electrons and ions. [[Ohm's law]] is a basic law of [[circuit theory]], stating that the current passing through a resistance is directly proportional to the potential difference across it. The resistance of most materials is relatively constant over a range of temperatures and currents; materials under these conditions are known as 'ohmic'. The [[ohm]], the unit of resistance, was named in honour of [[Georg Ohm]], and is symbolised by the Greek letter Ω. 1&nbsp;Ω is the resistance that will produce a potential difference of one volt in response to a current of one amp.<ref name=Alexander/>{{rp|30–35}}

The [[capacitor]] is a development of the Leyden jar and is a device that can store charge, and thereby storing electrical energy in the resulting field. It consists of two conducting plates separated by a thin [[Insulator (electricity)|insulating]] [[dielectric]] layer; in practice, thin metal foils are coiled together, increasing the surface area per unit volume and therefore the [[capacitance]]. The unit of capacitance is the [[farad]], named after [[Michael Faraday]], and given the symbol ''F'': one farad is the capacitance that develops a potential difference of one volt when it stores a charge of one coulomb. A capacitor connected to a voltage supply initially causes a current as it accumulates charge; this current will however decay in time as the capacitor fills, eventually falling to zero. A capacitor will therefore not permit a [[steady state]] current, but instead blocks it.<ref name=Alexander/>{{rp|216–20}}

The [[inductor]] is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the current through it. When the current changes, the magnetic field does too, [[electromagnetic induction|inducing]] a voltage between the ends of the conductor. The induced voltage is proportional to the [[Time derivative|time rate of change]] of the current. The constant of proportionality is termed the [[inductance]]. The unit of inductance is the [[Henry (unit)|henry]], named after [[Joseph Henry]], a contemporary of Faraday. One henry is the inductance that will induce a potential difference of one volt if the current through it changes at a rate of one ampere per second. The inductor's behaviour is in some regards converse to that of the capacitor: it will freely allow an unchanging current but opposes a rapidly changing one.<ref name=Alexander/>{{rp|226–29}}

===Electric power===
{{main|electric power}}
Electric power is the rate at which [[electric energy]] is transferred by an [[electric circuit]]. The [[SI]] unit of [[power (physics)|power]] is the [[watt (unit)|watt]], one [[joule]] per [[second]].

Electric power, like [[power (physics)|mechanical power]], is the rate of doing [[work (electrical)|work]], measured in [[watt]]s, and represented by the letter ''P''. The term ''wattage'' is used colloquially to mean "electric power in watts." The electric power in [[watt]]s produced by an electric current ''I'' consisting of a charge of ''Q'' coulombs every ''t'' seconds passing through an [[electric potential]] ([[voltage]]) difference of ''V'' is
:<math>P = \text{work done per unit time} = \frac {QV}{t} = IV \,</math>
where
:''Q'' is electric charge in [[coulomb]]s
:''t'' is time in seconds
:''I'' is electric current in [[ampere]]s
:''V'' is electric potential or voltage in [[volt]]s

Electric power is generally supplied to businesses and homes by the [[electric power industry]]. Electricity is usually sold by the [[kilowatt hour]] (3.6 MJ) which is the product of power in kilowatts multiplied by running time in hours. Electric utilities measure power using [[electricity meter]]s, which keep a running total of the electric energy delivered to a customer. Unlike [[fossil fuels]], electricity is a low [[entropy]] form of energy and can be converted into motion or many other forms of energy with high efficiency.<ref>{{citation|last=Smith|first=Clare|year=2001|title=Environmental Physics}}</ref>

===Electronics===
{{main|electronics}}
[[File:Arduino ftdi chip-1.jpg|thumb|[[Surface-mount]] electronic components]]

Electronics deals with [[electrical circuit]]s that involve active electrical components such as [[vacuum tube]]s, [[transistor]]s, [[diode]]s, [[sensor]]s and [[integrated circuit]]s, and associated passive interconnection technologies.<ref name="HH2015">{{cite book|title=The Art of Electronics |edition=3rd |first1=Paul |last1=Horowitz |first2=Winfield |last2=Hill |publisher=Cambridge University Press |year=2015 |isbn=978-0-521-80926-9}}</ref>{{rp|1–5,71}} The [[Nonlinear element|nonlinear]] behaviour of active components and their ability to control electron flows makes digital [[switch]]ing possible,<ref name="HH2015"/>{{rp|75}} and electronics is widely used in [[Data processing|information processing]], [[telecommunications]], and [[signal processing]]. Interconnection technologies such as [[circuit board]]s, electronics packaging technology, and other varied forms of communication infrastructure complete circuit functionality and transform the mixed components into a regular working [[system]].<ref name="HH2015"/>

Today, most electronic devices use [[semiconductor]] components to perform electron control. The underlying principles that explain how semiconductors work are studied in [[solid state physics]],<ref>{{Cite book |last=Singleton |first=John |url=https://books.google.com/books?id=9SLSBAAAQBAJ |title=Band Theory and Electronic Properties of Solids |date=2001-08-30 |publisher=Oxford University Press |isbn=978-0-19-105746-5 |pages=49 |language=en}}</ref> whereas the design and construction of [[electronic circuit]]s to solve practical problems are part of [[electronics engineering]].<ref>{{Cite book |last1=Agarwal |first1=Anant |url=https://books.google.com/books?id=lGgP7FDEv3AC |title=Foundations of Analog and Digital Electronic Circuits |last2=Lang |first2=Jeffrey |date=2005-07-01 |publisher=Elsevier |isbn=978-0-08-050681-4 |language=en}}</ref>

===Electromagnetic wave===
{{main|Electromagnetic wave}}
Faraday's and Ampère's work showed that a time-varying magnetic field created an electric field, and a time-varying electric field created a magnetic field. Thus, when either field is changing in time, a field of the other is always induced.<ref name=uniphysics/>{{rp|696–700}} These variations are an [[electromagnetic wave]]. Electromagnetic waves were analysed theoretically by [[James Clerk Maxwell]] in 1864. Maxwell developed a set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. He could moreover prove that in a vacuum such a wave would travel at the [[speed of light]], and thus light itself was a form of electromagnetic radiation. [[Maxwell's equations]], which unify light, fields, and charge are one of the great milestones of theoretical physics.<ref name=uniphysics/>{{rp|696–700}}

The work of many researchers enabled the use of electronics to convert signals into [[radio frequency|high frequency]] oscillating currents and, via suitably shaped conductors, electricity permits the transmission and reception of these signals via radio waves over very long distances.<ref>{{cite book |url=https://books.google.com/books?id=XCaOycMZ6_0C |page=17 |title=Wireless Telegraphy |author=[[Charles LeGeyt Fortescue]] |year=1913 |publisher=[[Cambridge University Press]]|isbn=9781107605909 }}</ref>