Electricity

For other uses, seeElectricity (disambiguation)."Electric" redirects here. For other uses, seeElectric (disambiguation).

Lightning(pictured) andurban lightingare some of the most dramatic effects of electricity

Electricityis the set ofphysicalphenomena associated with the presence andmotionofmatterpossessing anelectric charge.Electricity is related tomagnetism,both being part of the phenomenon ofelectromagnetism,as described byMaxwell's equations.Common phenomena are related to electricity, includinglightning,static electricity,electric heating,electric dischargesand many others.

The presence of either a positive or negativeelectric chargeproduces anelectric field.The motion of electric charges is anelectric currentand produces amagnetic field.In most applications,Coulomb's lawdetermines theforceacting on an electric charge.Electric potentialis theworkdone to move an electric charge from one point to another within an electric field, typically measured involts.

Electricity plays a central role in many modern technologies, serving inelectric powerwhere electric current is used to energise equipment, and inelectronicsdealing withelectrical circuitsinvolvingactive componentssuch asvacuum tubes,transistors,diodesandintegrated circuits,and associated passive interconnection technologies.

The study of electrical phenomena dates back to antiquity, with theoretical understanding progressing slowly until, the 17th and 18th centuries. The development of the theory of electromagnetism in the 19th century marked significant progress, leading to electricity's industrial and residential application byelectrical engineersby the century's end. This rapid expansion in electrical technology at the time was the driving force behind theSecond Industrial Revolution,with electricity's versatility driving transformations in both industry and society. Electricity is integral to applications spanningtransport,heating,lighting,communications,andcomputation,making it the foundation of modern industrial society.

History

Thales,the earliest known researcher into electricity

Main articles:History of electromagnetic theoryandHistory of electrical engineering

See also:Etymology of electricity

Long before any knowledge of electricity existed, people were aware of shocks fromelectric fish.Ancient Egyptiantexts dating from2750 BCEdescribed them as the "protectors" of all other fish. Electric fish were again reported millennia later byancient Greek,RomanandArabic naturalistsandphysicians.Several ancient writers, such asPliny the ElderandScribonius Largus,attested to the numbing effect ofelectric shocksdelivered byelectric catfishandelectric rays,and knew that such shocks could travel along conducting objects.Patients with ailments such asgoutorheadachewere directed to touch electric fish in the hope that the powerful jolt might cure them.

Ancient cultures around theMediterraneanknew that certain objects, such as rods ofamber,could be rubbed with cat's fur to attract light objects like feathers.Thales of Miletusmade a series of observations onstatic electricityaround 600 BCE, from which he believed that friction rendered ambermagnetic,in contrast to minerals such asmagnetite,which needed no rubbing.Thales was incorrect in believing the attraction was due to a magnetic effect, but later science would prove a link between magnetism and electricity. According to a controversial theory, theParthiansmay have had knowledge ofelectroplating,based on the 1936 discovery of theBaghdad Battery,which resembles agalvanic cell,though it is uncertain whether the artifact was electrical in nature.

Benjamin Franklinconducted extensive research on electricity in the 18th century, as documented byJoseph Priestley(1767)History and Present Status of Electricity,with whom Franklin carried on extended correspondence.

Electricity would remain little more than an intellectual curiosity for millennia until 1600, when the English scientistWilliam GilbertwroteDe Magnete,in which he made a careful study of electricity and magnetism, distinguishing thelodestoneeffect from static electricity produced by rubbing amber.He coined theNeo-Latinwordelectricus( "of amber" or "like amber", from ἤλεκτρον,elektron,theGreekword for "amber" ) to refer to the property of attracting small objects after being rubbed.This association gave rise to the English words "electric" and "electricity", which made their first appearance in print inThomas Browne'sPseudodoxia Epidemicaof 1646.

Further work was conducted in the 17th and early 18th centuries byOtto von Guericke,Robert Boyle,Stephen GrayandC. F. du Fay.Later in the 18th century,Benjamin Franklinconducted extensive research in electricity, selling his possessions to fund his work. In June 1752 he is reputed to have attached a metal key to the bottom of a dampened kite string andflown the kite in a storm-threatened sky.A succession of sparks jumping from the key to the back of his hand showed thatlightningwas indeed electrical in nature.He also explained the apparently paradoxical behaviorof theLeyden jaras a device for storing large amounts of electrical charge in terms of electricity consisting of both positive and negative charges.

Michael Faraday's discoveries formed the foundation of electric motor technology.

In 1775, Hugh Williamson reported a series of experiments to the Royal Society on the shocks delivered by theelectric eel;that same year the surgeon and anatomistJohn Hunterdescribed the structure of the fish'selectric organs.In 1791,Luigi Galvanipublished his discovery ofbioelectromagnetics,demonstrating that electricity was the medium by whichneuronspassed signals to the muscles.Alessandro Volta's battery, orvoltaic pile,of 1800, made from alternating layers of zinc and copper, provided scientists with a more reliable source of electrical energy than theelectrostatic machinespreviously used.The recognition ofelectromagnetism,the unity of electric and magnetic phenomena, is due toHans Christian ØrstedandAndré-Marie Ampèrein 1819–1820.Michael Faradayinvented theelectric motorin 1821, andGeorg Ohmmathematically analysed the electrical circuit in 1827.Electricity and magnetism (and light) were definitively linked byJames Clerk Maxwell,in particular in his "On Physical Lines of Force"in 1861 and 1862.

While the early 19th century had seen rapid progress in electrical science, the late 19th century would see the greatest progress inelectrical engineering.Through such people asAlexander Graham Bell,Ottó Bláthy,Thomas Edison,Galileo Ferraris,Oliver Heaviside,Ányos Jedlik,William Thomson, 1st Baron Kelvin,Charles Algernon Parsons,Werner von Siemens,Joseph Swan,Reginald Fessenden,Nikola TeslaandGeorge Westinghouse,electricity turned from a scientific curiosity into an essential tool for modern life.

In 1887,Heinrich Hertzdiscovered thatelectrodesilluminated with ultraviolet light createelectric sparksmore easily. In 1905,Albert Einsteinpublished a paper that explained experimental data from thephotoelectric effectas being the result of light energy being carried in discrete quantized packets, energising electrons. This discovery led to thequantumrevolution. Einstein was awarded theNobel Prize in Physicsin 1921 for "his discovery of the law of the photoelectric effect".The photoelectric effect is also employed inphotocellssuch as can be found insolar panels.

The firstsolid-state devicewas the "cat's-whisker detector"first used in the 1900s in radio receivers. A whisker-like wire is placed lightly in contact with a solid crystal (such as agermaniumcrystal) to detect aradiosignal by the contact junction effect.In a solid-state component, thecurrentis confined to solid elements and compounds engineered specifically to switch and amplify it. Current flow can be understood in two forms: as negatively chargedelectrons,and as positively charged electron deficiencies calledholes.These charges and holes are understood in terms of quantum physics. The building material is most often a crystallinesemiconductor.

Solid-state electronicscame into its own with the emergence oftransistortechnology. The first working transistor, agermanium-basedpoint-contact transistor,was invented byJohn BardeenandWalter Houser BrattainatBell Labsin 1947,followed by thebipolar junction transistorin 1948.

Concepts

Electric charge

Main article:Electric charge

See also:Electron,Proton,andIon

Charge on agold-leaf electroscopecauses the leaves to visibly repel each other

By modern convention, the charge carried byelectronsis defined as negative, and that byprotonsis positive.Before these particles were discovered,Benjamin Franklinhad defined a positive charge as being the charge acquired by a glass rod when it is rubbed with a silk cloth.A proton by definition carries a charge of exactly1.602176634×10⁻¹⁹coulombs.This value is also defined as theelementary 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.−1.602176634×10⁻¹⁹coulombs.Charge is possessed not just bymatter,but also byantimatter,eachantiparticlebearing an equal and opposite charge to its corresponding particle.

The presence of charge gives rise to an electrostatic force: charges exert aforceon each other, an effect that was known, though not understood, in antiquity.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 byCharles-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.

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 byCoulomb's law,which relates the force to the product of the charges and has aninverse-squarerelation to the distance between them.The electromagnetic force is very strong, second only in strength to thestrong interaction,but unlike that force it operates over all distances.In comparison with the much weakergravitational force,the electromagnetic force pushing two electrons apart is 10⁴²times that of thegravitationalattraction pulling them together.

Charge originates from certain types ofsubatomic particles,the most familiar carriers of which are theelectronandproton.Electric charge gives rise to and interacts with theelectromagnetic force,one of the fourfundamental forcesof nature. Experiment has shown charge to be aconserved quantity,that is, the net charge within an electrically isolated system will always remain constant regardless of any changes taking place within that system.Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire.The informal termstatic electricityrefers 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 thegold-leaf electroscope,which although still in use for classroom demonstrations, has been superseded by the electronicelectrometer.

Electric current

Main article:Electric current

The movement of electric charge is known as anelectric current,the intensity of which is usually measured inamperes.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 conductors,but will not flow through anelectrical insulator.

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 calledconventional current.The motion of negatively charged electrons around anelectric circuit,one of the most familiar forms of current, is thus deemed positive in theoppositedirection to that of the electrons.However, depending on the conditions, an electric current can consist of a flow ofcharged particlesin either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation.

Anelectric arcprovides an energetic demonstration of electric current.

The process by which electric current passes through a material is termedelectrical 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 aconductorsuch as metal, andelectrolysis,whereions(chargedatoms) flow through liquids, or throughplasmassuch as electrical sparks. While the particles themselves can move quite slowly, sometimes with an averagedrift velocityonly fractions of a millimetre per second,theelectric fieldthat drives them itself propagates at close to thespeed of light,enabling electrical signals to pass rapidly along wires.

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 byNicholsonandCarlislein 1800, a process now known aselectrolysis.Their work was greatly expanded upon byMichael Faradayin 1833. Current through aresistancecauses localised heating, an effectJames Prescott Joulestudied mathematically in 1840.One of the most important discoveries relating to current was made accidentally byHans Christian Ørstedin 1820, when, while preparing a lecture, he witnessed the current in a wire disturbing the needle of a magnetic compass.He had discoveredelectromagnetism,a fundamental interaction between electricity and magnetics. The level of electromagnetic emissions generated byelectric arcingis high enough to produceelectromagnetic interference,which can be detrimental to the workings of adjacent equipment.

In engineering or household applications, current is often described as being eitherdirect current(DC) oralternating current(AC). These terms refer to how the current varies in time. Direct current, as produced by example from abatteryand required by mostelectronicdevices, is a unidirectional flow from the positive part of a circuit to the negative.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 asine wave.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 understeady statedirect current, such asinductanceandcapacitance.These properties however can become important when circuitry is subjected totransients,such as when first energised.

Electric field

Main article:Electric field

See also:Electrostatics

The concept of the electricfieldwas introduced byMichael 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 twomasses,and like it, extends towards infinity and shows an inverse square relationship with distance.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.

Field lines emanating from a positive charge above a plane conductor

An electric field generally varies in space,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.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 ofmagnetic fields.As the electric field is defined in terms offorce,and force is avector,having bothmagnitudeanddirection,it follows that an electric field is avector field.

The study of electric fields created by stationary charges is calledelectrostatics.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,whose term '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.Field lines emanating from stationary charges have several key properties: first, that they originate at positive charges and terminate at negative charges; second, that they must enter any good conductor at right angles, and third, that they may never cross nor close in on themselves.

A hollow conducting body carries all its charge on its outer surface. The field is therefore 0 at all places inside the body.This is the operating principal of theFaraday cage,a conducting metal shell which isolates its interior from outside electrical effects.

The principles of electrostatics are important when designing items ofhigh-voltageequipment. There is a finite limit to the electric field strength that may be withstood by any medium. Beyond this point,electrical breakdownoccurs and anelectric arccauses flashover between the charged parts. Air, for example, tends to arc across small gaps at electric field strengths which exceed 30 kV per centimetre. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per centimetre.The most visible natural occurrence of this islightning,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 MV and have discharge energies as great as 250 kWh.

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 thelightning conductor,the sharp spike of which acts to encourage the lightning strike to develop there, rather than to the building it serves to protect.

Electric potential

Main article:Electric potential

See also:VoltageandElectric battery

A pair ofAA cells.The + 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 requireswork.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 involts,and one volt is the potential for which onejouleof work must be expended to bring a charge of onecoulombfrom infinity.This definition of potential, while formal, has little practical application, and a more useful concept is that ofelectric potential difference,and is the energy required to move a unit charge between two specified points. An electric field has the special property that it isconservative,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.The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the termvoltagesees greater everyday usage.

For practical purposes, it is useful to define a common reference point to which potentials may be expressed and compared. While this could be at infinity, a much more useful reference is theEarthitself, which is assumed to be at the same potential everywhere. This reference point naturally takes the nameearthorground.Earth is assumed to be an infinite source of equal amounts of positive and negative charge, and is therefore electrically uncharged—and unchargeable.

Electric potential is ascalar quantity,that is, it has only magnitude and not direction. It may be viewed as analogous toheight: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.As relief maps showcontour linesmarking points of equal height, a set of lines marking points of equal potential (known asequipotentials) may be drawn around an electrostatically charged object. The equipotentials cross all lines of force at right angles. They must also lie parallel to aconductor'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 localgradientof the electric potential. Usually expressed in volts per metre, the vector direction of the field is the line of greatest slope of potential, and where the equipotentials lie closest together.

Electromagnets

Main article:Electromagnets

Magnetic field circles around a current

Ørsted's discovery in 1821 that amagnetic fieldexisted 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.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.

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 byAmpère,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.The interaction is mediated by the magnetic field each current produces and forms the basis for the internationaldefinition of the ampere.

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 theelectric motorin 1821. Faraday'shomopolar motorconsisted of apermanent magnetsitting in a pool ofmercury.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.

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 aselectromagnetic induction,enabled him to state the principle, now known asFaraday's law of induction,that the potential difference induced in a closed circuit is proportional to the rate of change ofmagnetic fluxthrough the loop. Exploitation of this discovery enabled him to invent the firstelectrical generatorin 1831, in which he converted the mechanical energy of a rotating copper disc to electrical energy.Faraday's discwas 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.

Electric circuits

Main article:Electric circuit

A basicelectric circuit.Thevoltage sourceVon the left drives acurrentIaround the circuit, deliveringelectrical energyinto theresistorR.From the resistor, the current returns to the source, completing the circuit.

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.

The components in an electric circuit can take many forms, which can include elements such asresistors,capacitors,switches,transformersandelectronics.Electronic circuitscontainactive components,usuallysemiconductors,and typically exhibitnon-linearbehaviour, requiring complex analysis. The simplest electric components are those that are termedpassiveandlinear:while they may temporarily store energy, they contain no sources of it, and exhibit linear responses to stimuli.

Theresistoris perhaps the simplest of passive circuit elements: as its name suggests, itresiststhe 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 lawis a basic law ofcircuit 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'. Theohm,the unit of resistance, was named in honour ofGeorg Ohm,and is symbolised by the Greek letter Ω. 1 Ω is the resistance that will produce a potential difference of one volt in response to a current of one amp.

Thecapacitoris 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 thininsulatingdielectriclayer; in practice, thin metal foils are coiled together, increasing the surface area per unit volume and therefore thecapacitance.The unit of capacitance is thefarad,named afterMichael Faraday,and given the symbolF: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 asteady statecurrent, but instead blocks it.

Theinductoris 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,inducinga voltage between the ends of the conductor. The induced voltage is proportional to thetime rate of changeof the current. The constant of proportionality is termed theinductance.The unit of inductance is thehenry,named afterJoseph 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.

Electric power

Main article:electric power

Electric power is the rate at whichelectric energyis transferred by anelectric circuit.TheSIunit ofpoweris thewatt,onejoulepersecond.

Electric power, likemechanical power,is the rate of doingwork,measured inwatts,and represented by the letterP.The termwattageis used colloquially to mean "electric power in watts." The electric power inwattsproduced by an electric currentIconsisting of a charge ofQcoulombs everytseconds passing through anelectric potential(voltage) difference ofVis

$P={\text{work done per unit time}}={\frac {QV}{t}}=IV\,$

where

Qis electric charge incoulombs

tis time in seconds

Iis electric current inamperes

Vis electric potential or voltage involts

Electric power is generally supplied to businesses and homes by theelectric power industry.Electricity is usually sold by thekilowatt hour(3.6 MJ) which is the product of power in kilowatts multiplied by running time in hours. Electric utilities measure power usingelectricity meters,which keep a running total of the electric energy delivered to a customer. Unlikefossil fuels,electricity is a lowentropyform of energy and can be converted into motion or many other forms of energy with high efficiency.

Electronics

Main article:electronics

Surface-mountelectronic components

Electronics deals withelectrical circuitsthat involveactive electrical componentssuch asvacuum tubes,transistors,diodes,sensorsandintegrated circuits,and associated passive interconnection technologies.Thenonlinearbehaviour of active components and their ability to control electron flows makes digitalswitchingpossible,and electronics is widely used ininformation processing,telecommunications,andsignal processing.Interconnection technologies such ascircuit boards,electronics packaging technology, and other varied forms of communication infrastructure complete circuit functionality and transform the mixed components into a regular workingsystem.

Today, most electronic devices usesemiconductorcomponents to perform electron control. The underlying principles that explain how semiconductors work are studied insolid state physics,whereas the design and construction ofelectronic circuitsto solve practical problems are part ofelectronics engineering.

Electromagnetic wave

Main article: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.These variations are anelectromagnetic wave.Electromagnetic waves were analysed theoretically byJames Clerk Maxwellin 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 thespeed 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.

The work of many researchers enabled the use of electronics to convert signals intohigh frequencyoscillating currents and, via suitably shaped conductors, electricity permits the transmission and reception of these signals via radio waves over very long distances.

Production, storage and uses

Generation and transmission

Main article:Electricity generation

See also:Electric power transmissionandMains electricity

Early 20th-centuryalternatormade inBudapest,Hungary,in the power generating hall of ahydroelectricstation (photograph byProkudin-Gorsky,1905–1915).

In the 6th century BC the Greek philosopherThales of Miletusexperimented with amber rods: these were the first studies into the production of electricity. While this method, now known as thetriboelectric effect,can lift light objects and generate sparks, it is extremely inefficient.It was not until the invention of thevoltaic pilein the eighteenth century that a viable source of electricity became available. The voltaic pile, and its modern descendant, theelectrical battery,store energy chemically and make it available on demand in the form of electricity.

Electrical power is usually generated by electro-mechanicalgenerators.These can be driven bysteamproduced fromfossil fuelcombustion or the heat released from nuclear reactions, but also more directly from thekinetic energyof wind or flowing water. Thesteam turbineinvented bySir Charles Parsonsin 1884 is still used to convert the thermal energy of steam into a rotary motion that can be used by electro-mechanical generators. Such generators bear no resemblance to Faraday's homopolar disc generator of 1831, but they still rely on his electromagnetic principle that a conductor linking a changing magnetic field induces a potential difference across its ends.Electricity generated bysolar panelsrely on a different mechanism:solar radiationis converted directly into electricity using thephotovoltaic effect.

Wind poweris of increasing importance in many countries.

Demand for electricity grows with great rapidity as a nation modernises and its economy develops.The United States showed a 12% increase in demand during each year of the first three decades of the twentieth century,a rate of growth that is now being experienced by emerging economies such as those of India or China.

Environmental concerns with electricity generation,in specific the contribution of fossil fuel burning toclimate change,have led to an increased focus on generation fromrenewable sources.In the power sector,windandsolarhave become cost effective, speeding up anenergy transitionaway from fossil fuels.

Transmission and storage

The invention in the late nineteenth century of thetransformermeant that electrical power could be transmitted more efficiently at a higher voltage but lower current. Efficientelectrical transmissionmeant in turn that electricity could be generated at centralisedpower stations,where it benefited fromeconomies of scale,and then be despatched relatively long distances to where it was needed.

Normally, demand of electricity must match the supply, as storage of electricity is difficult.A certain amount of generation must always be held inreserveto cushion an electrical grid against inevitable disturbances and losses.With increasing levels ofvariable renewable energy(wind and solar energy) in the grid, it has become more challenging to match supply and demand. Storage plays an increasing role in bridging that gap. There are four types of energy storage technologies, each in varying states oftechnology readiness:batteries(electrochemical storage), chemical storage such ashydrogen,thermal or mechanical (such aspumped hydropower).

Applications

Theincandescent light bulb,an early application of electricity, operates byJoule heating:the passage ofcurrentthroughresistancegenerating heat.

Electricity is a very convenient way to transfer energy, and it has been adapted to a huge, and growing, number of uses.The invention of a practicalincandescent light bulbin the 1870s led tolightingbecoming one of the first publicly available applications of electrical power. Although electrification brought with it its own dangers, replacing the naked flames of gas lighting greatly reduced fire hazards within homes and factories.Public utilities were set up in many cities targeting the burgeoning market for electrical lighting. In the late 20th century and in modern times, the trend has started to flow in the direction of deregulation in the electrical power sector.

The resistiveJoule heatingeffect employed in filament light bulbs also sees more direct use inelectric heating.While this is versatile and controllable, it can be seen as wasteful, since most electrical generation has already required the production of heat at a power station.A number of countries, such as Denmark, have issued legislation restricting or banning the use of resistive electric heating in new buildings.Electricity is however still a highly practical energy source for heating andrefrigeration,withair conditioning/heat pumpsrepresenting a growing sector for electricity demand for heating and cooling, the effects of which electricity utilities are increasingly obliged to accommodate.Electrification is expected to play a major role in thedecarbonisationof sectors that rely on direct fossil fuel burning, such as transport (usingelectric vehicles) and heating (usingheat pumps).

The effects of electromagnetism are most visibly employed in theelectric motor,which provides a clean and efficient means of motive power. A stationary motor such as awinchis easily provided with a supply of power, but a motor that moves with its application, such as anelectric vehicle,is obliged to either carry along a power source such as a battery, or to collect current from a sliding contact such as apantograph.Electrically powered vehicles are used in public transportation, such as electric buses and trains,and an increasing number of battery-poweredelectric carsin private ownership.

Electricity is used withintelecommunications,and indeed theelectrical telegraph,demonstrated commercially in 1837 byCookeandWheatstone,was one of its earliest applications. With the construction of firsttranscontinental,and thentransatlantic,telegraph systems in the 1860s, electricity had enabled communications in minutes across the globe.Optical fibreandsatellite communicationhave taken a share of the market for communications systems, but electricity can be expected to remain an essential part of the process.

Electronic devices make use of thetransistor,perhaps one of the most important inventions of the twentieth century,and a fundamental building block of all modern circuitry. A modernintegrated circuitmay contain many billions of miniaturised transistors in a region only a few centimetres square.

Electricity and the natural world

Physiological effects

Main article:Electrical injury

A voltage applied to a human body causes an electric current through the tissues, and although the relationship is non-linear, the greater the voltage, the greater the current.The threshold for perception varies with the supply frequency and with the path of the current, but is about 0.1 mA to 1 mA for mains-frequency electricity, though a current as low as a microamp can be detected as anelectrovibrationeffect under certain conditions.If the current is sufficiently high, it will cause muscle contraction,fibrillationof the heart, andtissue burns.The lack of any visible sign that a conductor is electrified makes electricity a particular hazard. The pain caused by an electric shock can be intense, leading electricity at times to be employed as a method oftorture.Death caused by an electric shock—electrocution—is still used forjudicial executionin some US states, though its use had become very rare by the end of the 20th century.

Electrical phenomena in nature

Main article:Electrical phenomena

Theelectric eel,Electrophorus electricus

Electricity is not a human invention, and may be observed in several forms in nature, notablylightning.Many interactions familiar at the macroscopic level, such astouch,frictionorchemical bonding,are due to interactions between electric fields on the atomic scale. TheEarth's magnetic fieldis due to thenatural dynamoof circulating currents in the planet's core.Certain crystals, such asquartz,or evensugar,generate a potential difference across their faces when pressed.This phenomenon is known aspiezoelectricity,from theGreekpiezein(πιέζειν), meaning to press, and was discovered in 1880 byPierreandJacques Curie.The effect is reciprocal: when a piezoelectric material is subjected to an electric field it changes size slightly.

Some organisms, such assharks,are able to detect and respond to changes in electric fields, an ability known aselectroreception,while others, termedelectrogenic,are able to generate voltages themselves to serve as a predatory or defensive weapon; these areelectric fishin different orders.The orderGymnotiformes,of which the best known example is theelectric eel,detect or stun their prey via high voltages generated from modified muscle cells calledelectrocytes.All animals transmit information along their cell membranes with voltage pulses calledaction potentials,whose functions include communication by the nervous system betweenneuronsandmuscles.An electric shock stimulates this system, and causes muscles to contract.Action potentials are also responsible for coordinating activities in certain plants.

Cultural perception

It is said that in the 1850s, British politicianWilliam Ewart Gladstoneasked the scientistMichael Faradaywhy electricity was valuable. Faraday answered, "One day sir, you may tax it."However, according to Snopes "the anecdote should be considered apocryphal because it isn't mentioned in any accounts by Faraday or his contemporaries (letters, newspapers, or biographies) and only popped up well after Faraday's death."

In the 19th and early 20th century, electricity was not part of the everyday life of many people, even in the industrialisedWestern world.Thepopular cultureof the time accordingly often depicted it as a mysterious, quasi-magical force that can slay the living, revive the dead or otherwise bend the laws of nature.This attitude began with the 1771 experiments ofLuigi Galvaniin which the legs of dead frogs were shown to twitch on application ofanimal electricity."Revitalization" or resuscitation of apparently dead or drowned persons was reported in the medical literature shortly after Galvani's work. These results were known toMary Shelleywhen she authoredFrankenstein(1819), although she does not name the method of revitalization of the monster. The revitalization of monsters with electricity later became a stock theme in horror films.

As the public familiarity with electricity as the lifeblood of theSecond Industrial Revolutiongrew, its wielders were more often cast in a positive light,such as the workers who "finger death at their gloves' end as they piece and repiece the living wires" inRudyard Kipling's 1907 poemSons of Martha.Electrically powered vehicles of every sort featured large in adventure stories such as those ofJules Verneand theTom Swiftbooks.The masters of electricity, whether fictional or real—including scientists such asThomas Edison,Charles SteinmetzorNikola Tesla—were popularly conceived of as having wizard-like powers.

With electricity ceasing to be a novelty and becoming a necessity of everyday life in the later half of the 20th century, it required particular attention by popular culture only when itstopsflowing,an event that usually signals disaster.The people whokeepit flowing, such as the nameless hero ofJimmy Webb's song "Wichita Lineman"(1968),are still often cast as heroic, wizard-like figures.

See also

• Ampère's circuital law,connects the direction of an electric current and its associated magnetic currents.
• Electric potential energy,the potential energy of a system of charges
• Electricity market,the sale of electrical energy
• Etymology ofelectricity,the origin of the wordelectricityand its current different usages
• Hydraulic analogy,an analogy between the flow of water and electric current
• Developmental bioelectricity– Electric current produced in living cells

References

• Benjamin, Park (1898),A history of electricity: (The intellectual rise in electricity) from antiquity to the days of Benjamin Franklin,New York: J. Wiley & Sons
• Hammond, Percy (1981),"Electromagnetism for Engineers",Nature,168(4262), Pergamon:4–5,Bibcode:1951Natur.168....4G,doi:10.1038/168004b0,ISBN0-08-022104-1,S2CID27576009
• Morely, A.; Hughes, E. (1994),Principles of Electricity(5th ed.), Longman,ISBN0-582-22874-3
• Nahvi, Mahmood; Joseph, Edminister (1965),Electric Circuits,McGraw-Hill,ISBN978-0071422413
• Naidu, M.S.; Kamataru, V. (1982),High Voltage Engineering,Tata McGraw-Hill,ISBN0-07-451786-4
• Nilsson, James; Riedel, Susan (2007),Electric Circuits,Prentice Hall,ISBN978-0-13-198925-2
• Patterson, Walter C. (1999),Transforming Electricity: The Coming Generation of Change,Earthscan,ISBN1-85383-341-X

External links

• Basic Concepts of Electricitychapter fromLessons In Electric Circuits Vol 1 DCbook andseries.
• "One-Hundred Years of Electricity", May 1931, Popular Mechanics
• Illustrated view of how an American home's electrical system works
• Socket and plug standards
• Electricity Misconceptions
• Electricity and Magnetism
• Understanding Electricity and Electronics in about 10 Minutes