Electricity

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"Electric" redirects here. For other uses, see Electric (disambiguation) and Electricity
(disambiguation).

Lightning is one of the most dramatic effects of electricity.

Electromagnetism

Electricity

Magnetism

Electrostatics[show]
Magnetostatics[show]
Electrodynamics[show]
Electrical network[show]

Covariant formulation[show]
Scientists[show]

Electricity is the set of physical phenomena associated with the presence and flow of electric
charge. Electricity gives a wide variety of well-known effects, such as lightning, static electricity,
electromagnetic induction and electric current. In addition, electricity permits the creation and
reception of electromagnetic radiation such as radio waves.
In electricity, charges produce electromagnetic fields which act on other charges. Electricity
occurs due to several types of physics:

electric charge: a property of some subatomic particles, which determines

their electromagnetic interactions. Electrically charged matter is influenced
by, and produces, electromagnetic fields, electric charges can be positive or
negative.

electric field (see electrostatics): charges are surrounded by an electric

field. The electric field produces a force on other charges. Changes in the
electric field travel at the speed of light.

electric potential: the capacity of an electric field to do work on an electric

charge, typically measured in volts.

electric current: a movement or flow of electrically charged particles,

typically measured in amperes.

electromagnets: moving charges produce a magnetic field. Electric currents

generate magnetic fields, and changing magnetic fields generate electric
currents.

In electrical engineering, electricity is used for:

electric power where electric current is used to energise equipment;

electronics which deals with electrical circuits that involve active electrical
components such as vacuum tubes, transistors, diodes and integrated
circuits, and associated passive interconnection technologies.

Electrical phenomena have been studied since antiquity, though progress in theoretical
understanding remained slow until the seventeenth and eighteenth centuries. Even then, practical
applications for electricity were few, and it would not be until the late nineteenth century that
engineers were able to put it to industrial and residential use. The rapid expansion in electrical
technology at this time transformed industry and society. Electricity's extraordinary versatility
means it can be put to an almost limitless set of applications which include transport, heating,
lighting, communications, and computation. Electrical power is now the backbone of modern
industrial society.[1]
Contents

1 History

2 Concepts

2.1 Electric charge

2.2 Electric current

2.3 Electric field

2.4 Electric potential

2.5 Electromagnets

2.6 Electrochemistry

2.7 Electric circuits

2.8 Electric power

2.9 Electronics

2.10 Electromagnetic wave

3 Production and uses

o

3.1 Generation and transmission

3.2 Applications

4 Electricity and the natural world

4.1 Physiological effects

4.2 Electrical phenomena in nature

5 Cultural perception

6 See also

7 Notes

8 References

9 External links

History

Thales, the earliest known researcher into electricity

Main articles: History of electromagnetic theory and History of electrical engineering
See also: Etymology of electricity

Long before any knowledge of electricity existed, people were aware of shocks from electric
fish. Ancient Egyptian texts dating from 2750 BCE referred to these fish as the "Thunderer of the
Nile", and described them as the "protectors" of all other fish. Electric fish were again reported
millennia later by ancient Greek, Roman and Arabic naturalists and physicians.[2] Several ancient
writers, such as Pliny the Elder and Scribonius Largus, attested to the numbing effect of electric
shocks delivered by catfish and electric rays, and knew that such shocks could travel along
conducting objects.[3] Patients suffering from ailments such as gout or headache were directed to
touch electric fish in the hope that the powerful jolt might cure them.[4] Possibly the earliest and
nearest approach to the discovery of the identity of lightning, and electricity from any other
source, is to be attributed to the Arabs, who before the 15th century had the Arabic word for
lightning (raad) applied to the electric ray.[5]

Ancient cultures around the Mediterranean knew that certain objects, such as rods of amber,
could be rubbed with cat's fur to attract light objects like feathers. Thales of Miletus made a
series of observations on static electricity around 600 BCE, from which he believed that friction
rendered amber magnetic, in contrast to minerals such as magnetite, which needed no rubbing.[6]
[7]
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, the
Parthians may have had knowledge of electroplating, based on the 1936 discovery of the
Baghdad Battery, which resembles a galvanic cell, though it is uncertain whether the artifact was
electrical in nature.[8]

Benjamin Franklin conducted extensive research on electricity in the 18th century,

as documented by Joseph 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 scientist William Gilbert made a careful study of electricity and magnetism,
distinguishing the lodestone effect from static electricity produced by rubbing amber.[6] He
coined the New Latin word electricus ("of amber" or "like amber", from , elektron, the
Greek word for "amber") to refer to the property of attracting small objects after being rubbed.[9]
This association gave rise to the English words "electric" and "electricity", which made their first
appearance in print in Thomas Browne's Pseudodoxia Epidemica of 1646.[10]
Further work was conducted by Otto von Guericke, Robert Boyle, Stephen Gray and C. F. du
Fay. In the 18th century, Benjamin Franklin conducted 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 and flown the kite in a storm-threatened sky.[11] A succession of
sparks jumping from the key to the back of his hand showed that lightning was indeed electrical
in nature.[12] He also explained the apparently paradoxical behavior[13] of the Leyden jar as 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 1791, Luigi Galvani published his discovery of bioelectromagnetics, demonstrating that

electricity was the medium by which neurons passed signals to the muscles.[14] Alessandro Volta's
battery, or voltaic pile, of 1800, made from alternating layers of zinc and copper, provided
scientists with a more reliable source of electrical energy than the electrostatic machines
previously used.[14] The recognition of electromagnetism, the unity of electric and magnetic
phenomena, is due to Hans Christian rsted and Andr-Marie Ampre in 1819-1820; Michael
Faraday invented the electric motor in 1821, and Georg Ohm mathematically analysed the
electrical circuit in 1827.[14] Electricity and magnetism (and light) were definitively linked by
James Clerk Maxwell, in particular in his "On Physical Lines of Force" in 1861 and 1862.[15]
While the early 19th century had seen rapid progress in electrical science, the late 19th century
would see the greatest progress in electrical engineering. Through such people as Alexander
Graham Bell, Ott Blthy, Thomas Edison, Galileo Ferraris, Oliver Heaviside, nyos Jedlik,
William Thomson, 1st Baron Kelvin, Charles Algernon Parsons, Werner von Siemens, Joseph
Swan, Nikola Tesla and George Westinghouse, electricity turned from a scientific curiosity into
an essential tool for modern life, becoming a driving force of the Second Industrial Revolution.
[16]

In 1887, Heinrich Hertz[17]:843844[18] discovered that electrodes illuminated with ultraviolet light
create electric sparks more easily. In 1905 Albert Einstein published a paper that explained
experimental data from the photoelectric effect as being the result of light energy being carried in
discrete quantized packets, energising electrons. This discovery led to the quantum revolution.
Einstein was awarded the Nobel Prize in Physics in 1921 for "his discovery of the law of the
photoelectric effect".[19] The photoelectric effect is also employed in photocells such as can be
found in solar panels and this is frequently used to make electricity commercially.
The first solid-state device was 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 a

germanium crystal) in order to detect a radio signal by the contact junction effect.[20] In a solidstate component, the current is confined to solid elements and compounds engineered
specifically to switch and amplify it. Current flow can be understood in two forms: as negatively
charged electrons, and as positively charged electron deficiencies called holes. These charges and
holes are understood in terms of quantum physics. The building material is most often a
crystalline semiconductor.[21][22]
The solid-state device came into its own with the invention of the transistor in 1947. Common
solid-state devices include transistors, microprocessor chips, and RAM. A specialized type of
RAM called flash RAM is used in USB flash drives and more recently, solid-state drives to
replace mechanically rotating magnetic disc hard disk drives. Solid state devices became
prevalent in the 1950s and the 1960s, during the transition from vacuum tubes to semiconductor
diodes, transistors, integrated circuit (IC) and the light-emitting diode (LED).
Concepts
Electric charge
Main article: Electric charge
See also: electron, proton, and ion

Charge on a gold-leaf electroscope causes the leaves to visibly repel each other

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.[17]:457 A lightweight ball suspended
from a string 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.[17]
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.[23][24]:35 The
electromagnetic force is very strong, second only in strength to the strong interaction,[25] but
unlike that force it operates over all distances.[26] In comparison with the much weaker
gravitational force, the electromagnetic force pushing two electrons apart is 1042 times that of the
gravitational attraction pulling them together.[27]
Study has shown that the origin of charge is from certain types of subatomic particles which have
the property of electric charge. Electric charge gives rise to and interacts with the
electromagnetic force, one of the four fundamental forces of nature. The most familiar carriers of
electrical charge are the electron and proton. Experiment has shown charge to be a conserved
quantity, that is, the net charge within an isolated system will always remain constant regardless
of any changes taking place within that system.[28] Within the system, charge may be transferred
between bodies, either by direct contact, or by passing along a conducting material, such as a
wire.[24]:25 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.
The charge on electrons and protons is opposite in sign, hence an amount of charge may be
expressed as being either negative or positive. By convention, the charge carried by electrons is
deemed negative, and that by protons positive, a custom that originated with the work of
Benjamin Franklin.[29] The amount of charge is usually given the symbol Q and expressed in
coulombs;[30] each electron carries the same charge of approximately 1.60221019 coulomb.
The proton has a charge that is equal and opposite, and thus +1.60221019 coulomb. Charge is
possessed not just by matter, but also by antimatter, each antiparticle bearing an equal and
opposite charge to its corresponding particle.[31]
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.[24]:25
Electric current
Main article: Electric current

The movement of electric charge is known as an electric current, the intensity of which is usually
measured in amperes. Current can consist of any moving charged particles; most commonly
these are electrons, but any charge in motion constitutes a current.
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.[32] However, depending
on the conditions, an electric current can consist of a flow of charged particles in either direction,
or even in both directions at once. The positive-to-negative convention is widely used to simplify
this situation.

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 conductor such as metal, and electrolysis, where ions (charged atoms) flow through
liquids, or through 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,
[24]:17
the electric field that drives them itself propagates at close to the speed of light, enabling
electrical signals to pass rapidly along wires.[33]
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
Nicholson and Carlisle in 1800, a process now known as electrolysis. Their work was greatly
expanded upon by Michael Faraday in 1833. Current through a resistance causes localised
heating, an effect James Prescott Joule studied mathematically in 1840.[24]:2324 One of the most
important discoveries relating to current was made accidentally by Hans Christian rsted in
1820, when, while preparing a lecture, he witnessed the current in a wire disturbing the needle of
a magnetic compass.[34] He had discovered electromagnetism, a fundamental interaction between

electricity and magnetics. The level of electromagnetic emissions generated by electric arcing is
high enough to produce electromagnetic interference, which can be detrimental to the workings
of adjacent equipment.[35]
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 and required by most electronic devices, is a
unidirectional flow from the positive part of a circuit to the negative.[36]: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.[36]:206207 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.[36]:223225 These properties however can become important when
circuitry is subjected to transients, such as when first energised.
Electric field
Main article: Electric field
See also: Electrostatics

The concept of the electric 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 masses, and like it, extends towards
infinity and shows an inverse square relationship with distance.[26] 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.[27]

Field lines emanating from a positive charge above a plane conductor

An electric field generally varies in space,[37] 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.
[17]:469470
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 fields. As the electric field is defined in terms of force, and force is a vector, so it
follows that an electric field is also a vector, having both magnitude and direction. Specifically, it
is a vector field.[17]:469470
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,[38] 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.[38] 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.[17]:479
A hollow conducting body carries all its charge on its outer surface. The field is therefore zero at
all places inside the body.[24]:88 This is the operating principal of the Faraday cage, a conducting
metal shell which isolates its interior from outside electrical effects.
The principles of electrostatics are important when designing items of 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 kV per centimetre. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per
centimetre.[39] 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 MV
and have discharge energies as great as 250 kWh.[40]
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 stroke to develop
there, rather than to the building it serves to protect[41]:155
Electric potential
Main article: Electric potential

See also: Voltage and Battery (electricity)

A pair of AA 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 requires 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 volts, and one volt is the potential for which one joule of work must be expended to
bring a charge of one coulomb from infinity.[17]:494498 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. An electric field
has the special property that it is 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.[17]:494498 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, 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 the
Earth itself, which is assumed to be at the same potential everywhere. This reference point
naturally takes the name earth or ground. Earth is assumed to be an infinite source of equal
amounts of positive and negative charge, and is therefore electrically unchargedand
unchargeable.[42]
Electric potential is a 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.[43] As relief maps show contour lines marking points of equal height, a set of lines marking
points of equal potential (known as equipotentials) 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 conductor's surface, otherwise this would produce a force that will move the charge
carriers to even the potential of 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 per metre, the vector direction of the field is
the line of greatest slope of potential, and where the equipotentials lie closest together.[24]:60
Electromagnets
Main article: Electromagnets

Magnetic field circles around a current

rsted'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.[34] rsted's slightly obscure 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.[44]
rsted 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 Ampre, 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.[45] The
interaction is mediated by the magnetic field each current produces and forms the basis for the
international definition of the ampere.[45]

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. 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.[46]
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.[46] 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.
Electrochemistry

Italian physicist Alessandro Volta showing his "battery" to French emperor Napoleon
Bonaparte in the early 19th century.
Main article: Electrochemistry

The ability of chemical reactions to produce electricity, and conversely the ability of electricity to
drive chemical reactions has a wide array of uses.
Electrochemistry has always been an important part of electricity. From the initial invention of
the Voltaic pile, electrochemical cells have evolved into the many different types of batteries,
electroplating and electrolysis cells. Aluminium is produced in vast quantities this way, and
many portable devices are electrically powered using rechargeable cells.
Electric circuits
Main article: Electric circuit

A basic electric circuit. The voltage source V on the left drives a current I around the
circuit, delivering electrical energy into the resistor R. 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 as
resistors, capacitors, switches, transformers and electronics. Electronic circuits contain active
components, usually semiconductors, and typically exhibit non-linear behaviour, requiring
complex analysis. The simplest electric components are those that are termed passive and linear:
while they may temporarily store energy, they contain no sources of it, and exhibit linear
responses to stimuli.[47]:1516
The resistor is perhaps the simplest of passive circuit elements: as its name suggests, it 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 . 1 is the resistance
that will produce a potential difference of one volt in response to a current of one amp.[47]:3035
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 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.[47]:216220
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,
inducing a voltage between the ends of the conductor. The induced voltage is proportional to the
time rate of change of the current. The constant of proportionality is termed the inductance. The
unit of inductance is the 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.[47]:226229
Electric power
Main article: electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit
of power is the watt, one joule per second.
Electric power, like mechanical power, is the rate of doing work, measured in watts, and
represented by the letter P. The term wattage is used colloquially to mean "electric power in
watts." The electric power in watts 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
where
Q is electric charge in coulombs
t is time in seconds
I is electric current in amperes
V is electric potential or voltage in volts

Electricity generation is often done with electric generators, but can also be supplied by chemical
sources such as electric batteries or by other means from a wide variety of sources of energy.
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
meters, 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.[48]
Electronics
Main article: electronics

Surface mount electronic components

Electronics deals with electrical circuits that involve active electrical components such as
vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection
technologies. The nonlinear behaviour of active components and their ability to control electron

flows makes amplification of weak signals possible and electronics is widely used in information
processing, telecommunications, and signal processing. The ability of electronic devices to act as
switches makes digital information processing possible. Interconnection technologies such as
circuit boards, electronics packaging technology, and other varied forms of communication
infrastructure complete circuit functionality and transform the mixed components into a regular
working system.
Today, most electronic devices use semiconductor components to perform electron control. The
study of semiconductor devices and related technology is considered a branch of solid state
physics, whereas the design and construction of electronic circuits to solve practical problems
come under electronics engineering.
Electromagnetic wave
Main article: Electromagnetic wave

Faraday's and Ampre's work showed that a time-varying magnetic field acted as a source of an
electric field, and a time-varying electric field was a source of a magnetic field. Thus, when
either field is changing in time, then a field of the other is necessarily induced.[17]:696700 Such a
phenomenon has the properties of a wave, and is naturally referred to as 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 such a wave would necessarily travel at the speed of light, and thus light itself was a
form of electromagnetic radiation. Maxwell's Laws, which unify light, fields, and charge are one
of the great milestones of theoretical physics.[17]:696700
Thus, the work of many researchers enabled the use of electronics to convert signals into 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.
Production and uses
Generation and transmission
Main article: Electricity generation
See also: Electric power transmission and Mains electricity

Early 20th-century alternator made in Budapest, Hungary, in the power generating

hall of a hydroelectric station (photograph by Prokudin-Gorsky, 19051915).

In the 6th century BC, the Greek philosopher Thales of Miletus experimented with amber rods
and these experiments were the first studies into the production of electrical energy. While this
method, now known as the triboelectric effect, can lift light objects and generate sparks, it is
extremely inefficient.[49] It was not until the invention of the voltaic pile in the eighteenth century
that a viable source of electricity became available. The voltaic pile, and its modern descendant,
the electrical battery, store energy chemically and make it available on demand in the form of
electrical energy.[49] The battery is a versatile and very common power source which is ideally
suited to many applications, but its energy storage is finite, and once discharged it must be
disposed of or recharged. For large electrical demands electrical energy must be generated and
transmitted continuously over conductive transmission lines.
Electrical power is usually generated by electro-mechanical generators driven by steam produced
from fossil fuel combustion, or the heat released from nuclear reactions; or from other sources
such as kinetic energy extracted from wind or flowing water. The modern steam turbine invented
by Sir Charles Parsons in 1884 today generates about 80 percent of the electric power in the
world using a variety of heat sources. 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.[50] The
invention in the late nineteenth century of the transformer meant that electrical power could be
transmitted more efficiently at a higher voltage but lower current. Efficient electrical
transmission meant in turn that electricity could be generated at centralised power stations, where
it benefited from economies of scale, and then be despatched relatively long distances to where it
was needed.[51][52]

Wind power is of increasing importance in many countries

Since electrical energy cannot easily be stored in quantities large enough to meet demands on a
national scale, at all times exactly as much must be produced as is required.[51] This requires
electricity utilities to make careful predictions of their electrical loads, and maintain constant coordination with their power stations. A certain amount of generation must always be held in
reserve to cushion an electrical grid against inevitable disturbances and losses.
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,[53] a rate of growth that is now being experienced by emerging
economies such as those of India or China.[54][55] Historically, the growth rate for electricity
demand has outstripped that for other forms of energy.[56]:16
Environmental concerns with electricity generation have led to an increased focus on generation
from renewable sources, in particular from wind and hydropower. While debate can be expected
to continue over the environmental impact of different means of electricity production, its final
form is relatively clean[56]:89
Applications

The light bulb, an early application of electricity, operates by Joule heating: the
passage of current through resistance generating heat

Electricity is a very convenient way to transfer energy, and it has been adapted to a huge, and
growing, number of uses.[57] The invention of a practical incandescent light bulb in the 1870s led
to lighting becoming 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.[58] Public utilities were set up in many cities
targeting the burgeoning market for electrical lighting.
The resistive Joule heating effect employed in filament light bulbs also sees more direct use in
electric 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.[59] A number
of countries, such as Denmark, have issued legislation restricting or banning the use of resistive
electric heating in new buildings.[60] Electricity is however still a highly practical energy source
for heating and refrigeration,[61] with air conditioning/heat pumps representing a growing sector
for electricity demand for heating and cooling, the effects of which electricity utilities are
increasingly obliged to accommodate.[62]
Electricity is used within telecommunications, and indeed the electrical telegraph, demonstrated
commercially in 1837 by Cooke and Wheatstone, was one of its earliest applications. With the
construction of first intercontinental, and then transatlantic, telegraph systems in the 1860s,
electricity had enabled communications in minutes across the globe. Optical fibre and satellite
communication have taken a share of the market for communications systems, but electricity can
be expected to remain an essential part of the process.

The effects of electromagnetism are most visibly employed in the electric motor, which provides
a clean and efficient means of motive power. A stationary motor such as a winch is easily
provided with a supply of power, but a motor that moves with its application, such as an electric
vehicle, is obliged to either carry along a power source such as a battery, or to collect current
from a sliding contact such as a pantograph.
Electronic devices make use of the transistor, perhaps one of the most important inventions of
the twentieth century,[63] and a fundamental building block of all modern circuitry. A modern
integrated circuit may contain several billion miniaturised transistors in a region only a few
centimetres square.[64]
Electricity is also used to fuel public transportation, including electric buses and trains. [65]
Electricity and the natural world
Physiological effects
Main article: Electric shock

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.[66] 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 an electrovibration effect under certain conditions.[67] If the current is sufficiently high, it will
cause muscle contraction, fibrillation of the heart, and tissue burns.[66] 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 of torture.
Death caused by an electric shock is referred to as electrocution. Electrocution is still the means
of judicial execution in some jurisdictions, though its use has become rarer in recent times.[68]
Electrical phenomena in nature

The electric eel, Electrophorus electricus

Main article: Electrical phenomena

Electricity is not a human invention, and may be observed in several forms in nature, a
prominent manifestation of which is lightning. Many interactions familiar at the macroscopic
level, such as touch, friction or chemical bonding, are due to interactions between electric fields
on the atomic scale. The Earth's magnetic field is thought to arise from a natural dynamo of
circulating currents in the planet's core.[69] Certain crystals, such as quartz, or even sugar,
generate a potential difference across their faces when subjected to external pressure.[70] This
phenomenon is known as piezoelectricity, from the Greek piezein (), meaning to press,
and was discovered in 1880 by Pierre and Jacques Curie. The effect is reciprocal, and when a
piezoelectric material is subjected to an electric field, a small change in physical dimensions
takes place.[70]
Some organisms, such as sharks, are able to detect and respond to changes in electric fields, an
ability known as electroreception,[71] while others, termed electrogenic, are able to generate
voltages themselves to serve as a predatory or defensive weapon.[3] The order Gymnotiformes, of
which the best known example is the electric eel, detect or stun their prey via high voltages
generated from modified muscle cells called electrocytes.[3][4] All animals transmit information
along their cell membranes with voltage pulses called action potentials, whose functions include
communication by the nervous system between neurons and muscles.[72] An electric shock
stimulates this system, and causes muscles to contract.[73] Action potentials are also responsible
for coordinating activities in certain plants.[72]
Cultural perception

In 1850, William Gladstone asked the scientist Michael Faraday why electricity was valuable.
Faraday answered, One day sir, you may tax it.[74]
In the 19th and early 20th century, electricity was not part of the everyday life of many people,
even in the industrialised Western world. The popular culture of the time accordingly often
depicts it as a mysterious, quasi-magical force that can slay the living, revive the dead or
otherwise bend the laws of nature.[75] This attitude began with the 1771 experiments of Luigi
Galvani in which the legs of dead frogs were shown to twitch on application of animal
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 to Mary Shelley
when she authored Frankenstein (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 the Second Industrial Revolution
grew, its wielders were more often cast in a positive light,[76] such as the workers who "finger
death at their gloves' end as they piece and repiece the living wires" in Rudyard Kipling's 1907
poem Sons of Martha.[76] Electrically powered vehicles of every sort featured large in adventure

stories such as those of Jules Verne and the Tom Swift books.[76] The masters of electricity,
whether fictional or realincluding scientists such as Thomas Edison, Charles Steinmetz or
Nikola Teslawere popularly conceived of as having wizard-like powers.[76]
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 it stops
flowing,[76] an event that usually signals disaster.[76] The people who keep it flowing, such as the
nameless hero of Jimmy Webbs song "Wichita Lineman" (1968),[76] are still often cast as heroic,
wizard-like figures.[76]
See also

Energy portal

Electronics portal

Ampre'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

Hydraulic analogy, an analogy between the flow of water and electric current

Notes
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Morris, Simon C. (2003), Life's Solution: Inevitable Humans in a Lonely
Universe, Cambridge University Press, pp. 182185, ISBN 0-521-82704-3
The Encyclopedia Americana; a library of universal knowledge (1918), New
York: Encyclopedia Americana Corp

 Stewart, Joseph (2001), Intermediate Electromagnetic Theory, World

Scientific, p. 50, ISBN 981-02-4471-1
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Uman, Martin (1987), All About Lightning (PDF), Dover Publications, ISBN 0486-25237-X
Riskin, Jessica (1998), Poor Richards Leyden Jar: Electricity and economy in
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 Richard C. Jaeger, Travis N. Blalock, Microelectronic circuit design, pp.46-47,

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Universe, CRC Press, p. 51, ISBN 1-58488-798-2
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1.
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External links

Media related to Electricity at Wikimedia Commons

Look up electricity in Wiktionary, the free dictionary.

Basic Concepts of Electricity chapter from Lessons In Electric Circuits Vol 1

DC book and series.

"One-Hundred Years of Electricity", May 1931, Popular Mechanics

Illustrated view of how an American home's electrical system works

Electricity around the world

Electricity Misconceptions

Electricity and Magnetism

Understanding Electricity and Electronics in about 10 Minutes

World Bank report on Water, Electricity and Utility subsidies

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