Electricity

Electricity is the set of physical phenomena associated with

the presence and motion of matter possessing an electric
charge. Electricity is related to magnetism, both being part
of the phenomenon of electromagnetism, as described by
Maxwell's equations. Common phenomena are related to
electricity, including lightning, static electricity, electric
heating, electric discharges and many others.

The presence of either a positive or negative electric charge

produces an electric field. The motion of electric charges is
an electric current and produces a magnetic field. In most
applications, Coulomb's law determines the force acting on
an electric charge. Electric potential is the work done to
move an electric charge from one point to another within an
electric field, typically measured in volts.
Lightning (pictured) and urban lighting are
Electricity plays a central role in many modern some of the most dramatic effects of
technologies, serving in electric power where electric electricity
current is used to energise equipment, and in electronics
dealing with electrical circuits involving active components
such as vacuum tubes, transistors, diodes and integrated 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 by
electrical engineers by the century's end. This rapid expansion in electrical technology at the time was the
driving force behind the Second Industrial Revolution, with electricity's versatility driving
transformations in both industry and society. Electricity is integral to applications spanning transport,
heating, lighting, communications, and computation, making it the foundation of modern industrial
society.[1]

History
Long before any knowledge of electricity existed, people were aware of shocks from electric fish.
Ancient Egyptian texts dating from 2750 BCE 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 electric catfish and electric
rays, and knew that such shocks could travel along conducting objects.[3]
Patients with ailments such as gout or headache were directed to touch
electric fish in the hope that the powerful jolt might cure them.[4]

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.[5][6][7][8] Thales was incorrect in believing the attraction was
due to a magnetic effect, but later science would prove a link between Thales, the earliest known
magnetism and electricity. According to a controversial theory, the researcher into electricity
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 artefact was electrical in nature.[9]

Electricity would remain little more than an intellectual curiosity for

millennia until 1600, when the English scientist William Gilbert wrote
De Magnete, in which he made a careful study of electricity and
magnetism, distinguishing the lodestone effect from static electricity
produced by rubbing amber.[5] He coined the Neo-Latin word electricus
("of amber" or "like amber", from ἤλεκτρον, elektron, the Greek word for
"amber") to refer to the property of attracting small objects after being
rubbed.[10] 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.[11] Isaac Newton made early
investigations into electricity,[12] with an idea of his written down in his
Benjamin Franklin book Opticks arguably the beginning of the field theory of the electric
conducted extensive force.[13]
research on electricity in the
18th century, as Further work was conducted in the 17th and early 18th centuries by Otto
documented by Joseph von Guericke, Robert Boyle, Stephen Gray and C. F. du Fay.[14] Later in
Priestley (1767) History and the 18th century, Benjamin Franklin conducted extensive research in
Present Status of electricity, selling his possessions to fund his work. In June 1752 he is
Electricity, with whom
reputed to have attached a metal key to the bottom of a dampened kite
Franklin carried on
extended correspondence.
string and flown the kite in a storm-threatened sky.[15] A succession of
sparks jumping from the key to the back of his hand showed that lightning
was indeed electrical in nature.[16] He also explained the apparently
paradoxical behavior[17] 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.[14]

In 1775, Hugh Williamson reported a series of experiments to the Royal Society on the shocks delivered
by the electric eel;[18] that same year the surgeon and anatomist John Hunter described the structure of
the fish's electric organs.[19][20] In 1791, Luigi Galvani published his discovery of bioelectromagnetics,
demonstrating that electricity was the medium by which neurons passed signals to the muscles.[21][22][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.[21][22] The recognition of
electromagnetism, the unity of electric and magnetic phenomena, is due to
Hans Christian Ørsted and André-Marie Ampère in 1819–1820. Michael
Faraday invented the electric motor in 1821, and Georg Ohm
mathematically analysed the electrical circuit in 1827.[22] 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.[23]: 148

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, Ottó Bláthy, Michael Faraday's
Thomas Edison, Galileo Ferraris, Oliver Heaviside, Ányos Jedlik, William discoveries formed the
Thomson, 1st Baron Kelvin, Charles Algernon Parsons, Werner von foundation of electric motor
Siemens, Joseph Swan, Reginald Fessenden, Nikola Tesla and George technology.

Westinghouse, electricity turned from a scientific curiosity into an

essential tool for modern life.[24]

In 1887, Heinrich Hertz[25]: 843–44 [26] 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".[27] The
photoelectric effect is also employed in photocells such as can be found in solar panels.

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) to detect a
radio signal by the contact junction effect.[28] In a solid-state 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.[29][30]

Solid-state electronics came into its own with the emergence of transistor technology. The first working
transistor, a germanium-based point-contact transistor, was invented by John Bardeen and Walter Houser
Brattain at Bell Labs in 1947,[31] followed by the bipolar junction transistor in 1948.[32]

Concepts

Electric charge
By modern convention, the charge carried by electrons is defined as negative, and that by protons is
positive.[33] Before these particles were discovered, Benjamin Franklin had defined a positive charge as
being the charge acquired by a glass rod when it is rubbed with a silk cloth.[34] A proton by definition
carries a charge of exactly 1.602 176 634 × 10−19 coulombs. This value is also defined as the elementary
charge. No object can have a charge smaller than the elementary charge, and any amount of charge an
object may carry is a multiple of the elementary charge. An electron has
an equal negative charge, i.e. −1.602 176 634 × 10−19 coulombs. Charge is
possessed not just by matter, but also by antimatter, each antiparticle
bearing an equal and opposite charge to its corresponding particle.[35]

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.[25]: 457 A lightweight ball suspended by a fine thread can be
charged by touching it with a glass rod that has itself been charged by
rubbing with a cloth. If a similar ball is charged by the same glass rod, it is
found to repel the first: the charge acts to force the two balls apart. Two
balls that are charged with a rubbed amber rod also repel each other.
Charge on a gold-leaf
However, if one ball is charged by the glass rod, and the other by an electroscope causes the
amber rod, the two balls are found to attract each other. These phenomena leaves to visibly repel each
were investigated in the late eighteenth century by Charles-Augustin de other
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.[25]

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.[36][37]: 35 The electromagnetic force is very strong,
second only in strength to the strong interaction,[38] but unlike that force it operates over all distances.[39]
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.[40]

Charge originates from certain types of subatomic particles, the most familiar carriers of which are the
electron and proton. Electric charge gives rise to and interacts with the electromagnetic force, one of the
four fundamental forces of nature. Experiment has shown charge to be a conserved quantity, that is, the
net charge within an electrically isolated system will always remain constant regardless of any changes
taking place within that system.[41] Within the system, charge may be transferred between bodies, either
by direct contact or by passing along a conducting material, such as a wire.[37]: 2–5 The informal term
static electricity refers to the net presence (or 'imbalance') of charge on a body, usually caused when
dissimilar materials are rubbed together, transferring charge from one to the other.

Charge can be measured by a number of means, an early instrument being the gold-leaf electroscope,
which although still in use for classroom demonstrations, has been superseded by the electronic
electrometer.[37]: 2–5

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. Electric current can flow through some things,
electrical conductors, but will not flow through an electrical insulator.[42]
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.[43] 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.

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,[37]: 17 the electric field that drives them itself propagates at
An electric arc provides an energetic
close to the speed of light, enabling electrical signals to pass
demonstration of electric current.
rapidly along wires.[44]

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.[37]: 23–24 One of the most important discoveries relating to current was
made accidentally by Hans Christian Ørsted in 1820, when, while preparing a lecture, he witnessed the
current in a wire disturbing the needle of a magnetic compass.[23]: 370 [a] 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.[45]

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.[46]: 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.[46]: 206–07 Alternating current thus pulses
back and forth within a conductor without the charge moving any net distance over time. The time-
averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the
reverse. Alternating current is affected by electrical properties that are not observed under steady state
direct current, such as inductance and capacitance.[46]: 223–25 These properties however can become
important when circuitry is subjected to transients, such as when first energised.

Electric field
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.[39] 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.[40]

An electric field generally varies in space,[b] 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.[25]: 469–70 The conceptual charge, termed a 'test charge',
must be vanishingly small to prevent its own electric field
disturbing the main field and must also be stationary to prevent the
effect of magnetic fields. As the electric field is defined in terms of
force, and force is a vector, having both magnitude and direction,
it follows that an electric field is a vector field.[25]: 469–70
Field lines emanating from a
The study of electric fields created by stationary charges is called positive charge above a plane
electrostatics. The field may be visualised by a set of imaginary conductor
lines whose direction at any point is the same as that of the field.
This concept was introduced by Faraday,[47] 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.[47] Field lines emanating
from stationary charges have several key properties: first, they originate at positive charges and terminate
at negative charges; second, they must enter any good conductor at right angles, and third, they may
never cross nor close in on themselves.[25]: 479

A hollow conducting body carries all its charge on its outer surface. The field is therefore 0 at all places
inside the body.[37]: 88 This is the operating principle of the Faraday cage, a conducting metal shell that
isolates its interior from outside electrical effects.

The principles of electrostatics are important when designing items of high-voltage 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.[48]: 2 The most visible
natural occurrence of this is lightning, caused when charge becomes separated in the clouds by rising
columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of a
large lightning cloud may be as high as 100 MV and have discharge energies as great as
250 kWh.[48]: 201–02
The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is
forced to curve around sharply pointed objects. This principle is exploited in the lightning conductor, the
sharp spike of which acts to encourage the lightning strike to develop there, rather than to the building it
serves to protect.[49]: 155

Electric potential
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.[25]: 494–98 This definition of potential, while formal, has
little practical application, and a more useful concept is that of
electric potential difference, and is the energy required to move a
unit charge between two specified points. The electric field is
conservative, which means that the path taken by the test charge is
irrelevant: all paths between two specified points expend the same A pair of AA cells. The + sign
energy, and thus a unique value for potential difference may be indicates the polarity of the potential
stated.[25]: 494–98 The volt is so strongly identified as the unit of difference between the battery
choice for measurement and description of electric potential terminals.

difference that the term voltage sees greater everyday usage.

For practical purposes, defining a common reference point to which potentials may be expressed and
compared is useful. While this could be at infinity, a much more useful reference is the Earth itself, which
is assumed to be at the same potential everywhere. This reference point naturally takes the name earth or
ground. Earth is assumed to be an infinite source of equal amounts of positive and negative charge and is
therefore electrically uncharged—and unchargeable.[50]

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.[51] 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, since otherwise there
would be a force along the surface of the conductor that would move the charge carriers to even the
potential across the surface.
The electric field was formally defined as the force exerted per unit charge, but the concept of potential
allows for a more useful and equivalent definition: the electric field is the local gradient of the electric
potential. Usually expressed in volts per metre, the vector direction of the field is the line of greatest slope
of potential, and where the equipotentials lie closest together.[37]: 60

Electromagnets
Ørsted's discovery in 1821 that a magnetic field existed around all
sides of a wire carrying an electric current indicated that there was
a direct relationship between electricity and magnetism. Moreover,
the interaction seemed different from gravitational and
electrostatic forces, the two forces of nature then known. The
force on the compass needle did not direct it to or away from the
current-carrying wire, but acted at right angles to it.[23]: 370
Ørsted's words were that "the electric conflict acts in a revolving
manner." The force also depended on the direction of the current,
for if the flow was reversed, then the force did too.[52]

Ørsted did not fully understand his discovery, but he observed the
Magnetic field circles around a effect was reciprocal: a current exerts a force on a magnet, and a
current magnetic field exerts a force on a current. The phenomenon was
further investigated by Ampère, who discovered that two parallel
current-carrying wires exerted a force upon each other: two wires conducting currents in the same
direction are attracted to each other, while wires containing currents in opposite directions are forced
apart.[53] The interaction is mediated by the magnetic field each current produces and forms the basis for
the international definition of the ampere.[53]

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.[54]

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 The electric motor exploits an
electromagnetic induction, enabled him to state the principle, now important effect of
known as Faraday's law of induction, that the potential difference electromagnetism: a current through
induced in a closed circuit is proportional to the rate of change of a magnetic field experiences a force
at right angles to both the field and
magnetic flux through the loop. Exploitation of this discovery
current.
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.[54] 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.[55]

Electric circuits
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.[56]

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 A basic electric circuit. The voltage
components are those that are termed passive and linear: while source V on the left drives a current
they may temporarily store energy, they contain no sources of it, I around the circuit, delivering
and exhibit linear responses to stimuli.[57]: 15–16 electrical energy into the resistor R.
From the resistor, the current
The resistor is perhaps the simplest of passive circuit elements: as returns to the source, completing
the circuit.
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 Ω. 1 Ω is the resistance that will produce a potential difference of one volt in response to a
current of one amp.[57]: 30–35

The capacitor is a development of the Leyden jar and is a device that can store charge, and thereby
storing electrical energy in the resulting field. It consists of two conducting plates separated by a thin
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.[57]: 216–20

The inductor is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the
current through it. When the current changes, the magnetic field does too, 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.[57]: 226–29

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

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.[58]

Electronics
Electronics deals with electrical circuits that involve active
electrical components such as vacuum tubes, transistors, diodes,
sensors and integrated circuits, and associated passive
interconnection technologies.[59]: 1–5, 71 The nonlinear behaviour
of active components and their ability to control electron flows
makes digital switching possible,[59]: 75 and electronics is widely
used in information processing, telecommunications, and signal
processing. Interconnection technologies such as circuit boards,
electronics packaging technology, and other varied forms of Surface-mount electronic
communication infrastructure complete circuit functionality and components
transform the mixed components into a regular working system.
Today, most electronic devices use semiconductor components to perform electron control. The
underlying principles that explain how semiconductors work are studied in solid state physics,[60]
whereas the design and construction of electronic circuits to solve practical problems are part of
electronics engineering.[61]

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.[25]: 696–700 These variations are an electromagnetic wave. Electromagnetic
waves were analysed theoretically by James Clerk Maxwell in 1864. Maxwell developed a set of
equations that could unambiguously describe the interrelationship between electric field, magnetic field,
electric charge, and electric current. He could moreover prove that in a vacuum such a wave would travel
at the speed of light, and thus light itself was a form of electromagnetic radiation. Maxwell's equations,
which unify light, fields, and charge are one of the great milestones of theoretical physics.[25]: 696–700

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.[62]

Production, storage and uses

Generation and transmission

In the 6th century BC the Greek philosopher Thales of
Miletus experimented with amber rods: these were the
first studies into the production of electricity. While
this method, now known as the triboelectric effect,
can lift light objects and generate sparks, it is
extremely inefficient.[63] 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 electricity.[63]

Electrical power is usually generated by electro-

mechanical generators. These can be driven by steam Early 20th-century alternator made in Budapest,
produced from fossil fuel combustion or the heat Hungary, in the power generating hall of a
released from nuclear reactions, but also more directly hydroelectric station (photograph by Prokudin-
from the kinetic energy of wind or flowing water. The Gorsky, 1905–1915).
steam turbine invented by Sir Charles Parsons in 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.[64] Electricity generated by solar panels rely on a different mechanism: solar radiation is converted
directly into electricity using the photovoltaic effect.[65]

Demand for electricity grows with great rapidity as a nation

modernises and its economy develops.[66] The United States
showed a 12% increase in demand during each year of the first
three decades of the twentieth century,[67] a rate of growth that is
now being experienced by emerging economies such as those of
India or China.[68][69]

Environmental concerns with electricity generation, in specific the

contribution of fossil fuel burning to climate change, have led to
Wind power is of increasing
an increased focus on generation from renewable sources. In the
importance in many countries.
power sector, wind and solar have become cost effective, speeding
up an energy transition away from fossil fuels.[70]

Transmission and storage

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.[71][72]

Normally, demand for electricity must match the supply, as storage of electricity is difficult.[71] A certain
amount of generation must always be held in reserve to cushion an electrical grid against inevitable
disturbances and losses.[73] With increasing levels of variable 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 of
technology readiness: batteries (electrochemical storage), chemical storage such as hydrogen, thermal or
mechanical (such as pumped hydropower).[74]

Applications
Electricity is a very convenient way to transfer energy, and it has been adapted to a huge, and growing,
number of uses.[75] 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.[76] 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.[77]

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.[78] A number of countries, such
as Denmark, have issued legislation restricting or banning the use of resistive electric heating in new
buildings.[79] Electricity is however still a highly practical energy source for heating and refrigeration,[80]
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.[81][82] Electrification is expected to play a major role in the
decarbonisation of sectors that rely on direct fossil fuel burning, such as
transport (using electric vehicles) and heating (using heat pumps).[83][84]

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. Electrically
powered vehicles are used in public transportation, such as electric buses
and trains,[85] and an increasing number of battery-powered electric cars
in private ownership.
The incandescent light bulb,
an early application of Electricity is used within telecommunications, and indeed the electrical
electricity, operates by telegraph, demonstrated commercially in 1837 by Cooke and
Joule heating: the passage Wheatstone,[86] was one of its earliest applications. With the construction
of current through of first transcontinental, and then transatlantic, telegraph systems in the
resistance generating heat.
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.

Electronic devices make use of the transistor, perhaps one of the most important inventions of the
twentieth century,[87] and a fundamental building block of all modern circuitry. A modern integrated
circuit may contain many billions of miniaturised transistors in a region only a few centimetres square.[88]

Electricity and the natural world

Physiological effects
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.[89] 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.[90] If the current is sufficiently high, it will cause muscle contraction, fibrillation
of the heart, and tissue burns.[89] 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.[91] Death caused by an electric shock—electrocution—is
still used for judicial execution in some US states, though its use had become very rare by the end of the
20th century.[92]

Electrical phenomena in nature

Electricity is not a human invention, and may be observed in
several forms in nature, notably 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 due to the
natural dynamo of circulating currents in the planet's core.[93]
Certain crystals, such as quartz, or even sugar, generate a potential
difference across their faces when pressed.[94] This phenomenon is
known as piezoelectricity, from the Greek piezein (πιέζειν), The electric eel, Electrophorus
meaning to press, and was discovered in 1880 by Pierre and electricus
Jacques Curie. The effect is reciprocal: when a piezoelectric
material is subjected to an electric field it changes size slightly.[94]

Some organisms, such as sharks, are able to detect and respond to changes in electric fields, an ability
known as electroreception,[95] while others, termed electrogenic, are able to generate voltages themselves
to serve as a predatory or defensive weapon; these are electric fish in different orders.[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.[96] An electric shock stimulates this
system and causes muscles to contract.[97] Action potentials are also responsible for coordinating
activities in certain plants.[96]

Cultural perception
It is said that in the 1850s, British politician William Ewart Gladstone asked the scientist Michael
Faraday why electricity was valuable. Faraday answered, "One day sir, you may tax it."[98][99][100]
However, according to Snopes.com "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."[101]

In the 19th and early 20th centuries, 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 depicted it as a
mysterious, quasi-magical force that can slay the living, revive the dead or otherwise bend the laws of
nature.[102]: 69 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 public familiarity with electricity as the lifeblood of the Second Industrial Revolution grew, its
wielders were more often cast in a positive light,[102]: 71 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.[102]: 71 Electrically powered vehicles of every sort featured large in adventure stories such as
those of Jules Verne and the Tom Swift books.[102]: 71 The masters of electricity, whether fictional or real
—including scientists such as Thomas Edison, Charles Steinmetz or Nikola Tesla—were popularly
conceived of as having wizard-like powers.[102]: 71

With electricity ceasing to be a novelty and becoming a necessity of everyday life in the later half of the
20th century, it acquired particular attention by popular culture only when it stops flowing,[102]: 71 an
event that usually signals disaster.[102]: 71 The people who keep it flowing, such as the nameless hero of
Jimmy Webb's song "Wichita Lineman" (1968),[102]: 71 are still often cast as heroic, wizard-like
figures.[102]: 71

See also
Energy portal

Electronics portal

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 of electricity, the origin of the word electricity and its current different usages
Hydraulic analogy, an analogy between the flow of water and electric current
Developmental bioelectricity – Electric current produced in living cells

Notes
a. Accounts differ as to whether this was before, during, or after a lecture.
b. Almost all electric fields vary in space. An exception is the electric field surrounding a planar
conductor of infinite extent, the field of which is uniform.
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External links
Basic Concepts of Electricity (http://www.ibiblio.org/kuphaldt/electricCircuits/DC/DC_1.html)
chapter from Lessons In Electric Circuits Vol 1 DC (http://www.ibiblio.org/kuphaldt/electricCir
cuits/DC/index.html) book and series (http://www.ibiblio.org/kuphaldt/electricCircuits/).
"One-Hundred Years of Electricity", May 1931, Popular Mechanics (https://books.google.co
m/books?id=n-MDAAAAMBAJ&pg=PA772)
Socket and plug standards (https://www.worldstandards.eu/electricity/plugs-and-sockets/)
Electricity Misconceptions (http://amasci.com/miscon/elect.html)
Electricity and Magnetism (https://web.archive.org/web/20151201064159/http://www.micro.
magnet.fsu.edu/electromag/java/diode/index.html)
Understanding Electricity and Electronics in about 10 Minutes (http://steverose.com/Articles/
UnderstandingBasicElectri.html)
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