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= Electricity =
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Introduction
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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.
Electricity plays a central role in many modern technologies, serving
in electric power where electric 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.
History
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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. 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.
Patients with ailments such as gout or headache were directed to
touch electric fish in the hope that the powerful jolt might cure
them.
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.
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 artefact was electrical in nature.
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. 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.
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.
Isaac Newton made early investigations into electricity, with an idea
of his written down in his book 'Opticks' arguably the beginning of
the field theory of the electric force.
Further work was conducted in the 17th and early 18th centuries by
Otto von Guericke, Robert Boyle, Stephen Gray and C. F. du Fay. Later
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.
. It is uncertain if Franklin personally carried out this experiment,
but it is popularly attributed to him. A succession of sparks jumping
from the key to the back of his hand showed that lightning was indeed
electrical in nature. He also explained the apparently paradoxical
behavior 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.
In 1775, Hugh Williamson reported a series of experiments to the Royal
Society on the shocks delivered by the electric eel; that same year
the surgeon and anatomist John Hunter described the structure of the
fish's electric organs. In 1791, Luigi Galvani published his discovery
of bioelectromagnetics, demonstrating that electricity was the medium
by which neurons passed signals to the muscles.
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. 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. 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.
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, Thomas Edison, Galileo Ferraris, Oliver Heaviside, Ányos
Jedlik, William Thomson, 1st Baron Kelvin, Charles Algernon Parsons,
Werner von Siemens, Joseph Swan, Reginald Fessenden, Nikola Tesla and
George Westinghouse, electricity turned from a scientific curiosity
into an essential tool for modern life.
In 1887, Heinrich Hertz 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". 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. 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.
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, followed by the bipolar junction
transistor in 1948.
Electric charge
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By modern convention, the charge carried by electrons is defined as
negative, and that by protons is positive. 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. A proton by definition carries a charge of exactly . 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. . Charge is possessed not just by matter,
but also by antimatter, each antiparticle bearing an equal and
opposite charge to its corresponding particle.
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.
A lightweight ball suspended by a fine thread can be charged by
touching it with a glass rod that has itself been charged by rubbing
with a cloth. If a similar ball is charged by the same glass rod, it
is found to repel the first: the charge acts to force the two balls
apart. Two balls that are charged with a rubbed amber rod also repel
each other. However, if one ball is charged by the glass rod, and the
other by an amber rod, the two balls are found to attract each other.
These phenomena were investigated in the late eighteenth century by
Charles-Augustin de Coulomb, who deduced that charge manifests itself
in two opposing forms. This discovery led to the well-known axiom:
'like-charged objects repel and opposite-charged objects attract'.
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.
The electromagnetic force is very strong, second only in strength to
the strong interaction,
but unlike that force it operates over all distances.
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.
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.
Within the system, charge may be transferred between bodies, either
by direct contact or by passing along a conducting material, such as a
wire. The informal 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.
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.
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.
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, the electric field that drives them itself
propagates at close to the speed of light, enabling electrical signals
to pass rapidly along wires.
Current causes several observable effects, which historically were the
means of recognising its presence. That water could be decomposed by
the current from a voltaic pile was discovered 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. 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.
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.
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.
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. 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. These properties however can become important when
circuitry is subjected to transients, such as when first energised.
Electric field
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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. 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.
An electric field generally varies in space, and its strength at any
one point is defined as the force (per unit charge) that would be felt
by a stationary, negligible charge if placed at that point. The
conceptual charge, termed a 'test charge', must be vanishingly small
to prevent its own electric field disturbing the main field and must
also be stationary to prevent the effect 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.
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,
whose term 'lines of force' still sometimes sees use. The field lines
are the paths that a point positive charge would seek to make as it
was forced to move within the field; they are however an imaginary
concept with no physical existence, and the field permeates all the
intervening space between the lines. Field lines emanating from
stationary charges have several key properties: first, 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.
A hollow conducting body carries all its charge on its outer surface.
The field is therefore 0 at all places inside the body. This is the
operating 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.
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.
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.
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. 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 energy, and thus a unique
value for potential difference may be stated. The volt is so strongly
identified as the unit of choice for measurement and description of
electric potential difference that the 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.
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. 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.
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. Ørsted's words were that "the
electric conflict acts in a revolving manner." The force also depended
on the direction of the current, for if the flow was reversed, then
the force did too.
Ørsted did not fully understand his discovery, but he observed the
effect was reciprocal: a current exerts a force on a magnet, and a
magnetic field exerts a force on a current. The phenomenon was further
investigated 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.
The interaction is mediated by the magnetic field each current
produces and forms the basis for the international definition of the
ampere.
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.
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. 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.
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.
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.
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 Ω.
1 Ω is the resistance that will produce a potential difference of one
volt in response to a current of one amp.
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.
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.
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.
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. The nonlinear behaviour of active
components and their ability to control electron flows makes digital
switching possible, 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 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 underlying principles that explain how
semiconductors work are studied in solid state physics, whereas the
design and construction of electronic circuits to solve practical
problems are part of electronics engineering.
Electromagnetic wave
======================
Faraday's and Ampère's work showed that a time-varying magnetic field
created an electric field, and a time-varying electric field created a
magnetic field. Thus, when either field is changing in time, a field
of the other is always induced. These variations are 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.
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.
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.
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.
Electrical power is usually generated by electro-mechanical
generators. These can be driven by steam produced from fossil fuel
combustion or the heat released from nuclear reactions, but also more
directly from the kinetic energy of wind or flowing water. The 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.
Electricity generated by solar panels rely on a different mechanism:
solar radiation is converted directly into electricity using the
photovoltaic effect.
Demand for electricity grows with great rapidity as a nation
modernises and its economy develops. The United States showed a 12%
increase in demand during each year of the first three decades of the
twentieth century, a rate of growth that is now being experienced by
emerging economies such as those of India or China.
Environmental concerns with electricity generation, in specific the
contribution of fossil fuel burning to climate change, have led to an
increased focus on generation from renewable sources. In the power
sector, wind and solar have become cost effective, speeding up an
energy transition away from fossil fuels.
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.
Normally, demand for electricity must match the supply, as storage of
electricity is difficult. A certain amount of generation must always
be held in reserve to cushion an electrical grid against inevitable
disturbances and losses. 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).
Applications
==============
Electricity is a very convenient way to transfer energy, and it has
been adapted to a huge, and growing, number of uses. The invention of
a 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.
Public utilities were set up in many cities targeting the burgeoning
market for electrical lighting. In the late 20th century and in modern
times, the trend has started to flow in the direction of deregulation
in the electrical power sector.
The 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.
A number of countries, such as Denmark, have issued legislation
restricting or banning the use of resistive electric heating in new
buildings.
Electricity is however still a highly practical energy source for
heating and refrigeration,
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.
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).
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, and an increasing
number of battery-powered electric cars in private ownership.
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 transcontinental, 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.
Electronic devices make use of the transistor, perhaps one of the most
important inventions of the twentieth century,
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.
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.
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.
If the current is sufficiently high, it will cause muscle
contraction, fibrillation of the heart, and tissue burns. The lack of
any visible sign that a conductor is electrified makes electricity a
particular hazard. The pain caused by an electric shock can be
intense, leading electricity at times to be employed as a method of
torture.
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.
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.
Certain crystals, such as quartz, or even sugar, generate a potential
difference across their faces when pressed.
This phenomenon is known as piezoelectricity, from the Greek
'piezein' (πιέζειν), meaning to press, and was discovered in 1880 by
Pierre and Jacques Curie. The effect is reciprocal: when a
piezoelectric material is subjected to an electric field it changes
size slightly.
Some organisms, such as sharks, are able to detect and respond to
changes in electric fields, an ability known as electroreception,
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. 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. 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.
An electric shock stimulates this system and causes muscles to
contract. Action potentials are also responsible for coordinating
activities in certain plants.
Cultural perception
======================================================================
It is said that in the 1850s, British politician William Ewart
Gladstone asked the scientist Michael Faraday why electricity was
valuable. Faraday answered, "One day sir, you may tax it." 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."
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. 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, 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'. Electrically powered vehicles of every
sort featured large in adventure stories such as those of Jules Verne
and the 'Tom Swift' books. 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.
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,
an event that usually signals disaster. The people who 'keep' it
flowing, such as the nameless hero of Jimmy Webb's song "Wichita
Lineman" (1968), are still often cast as heroic, wizard-like figures.
See also
======================================================================
* Ampère's circuital law, connects the direction of an electric
current and its associated magnetic currents.
* Electric potential energy, the potential energy of a system of
charges
* Electricity market, the sale of electrical energy
*Etymology 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
*
External links
======================================================================
* [
http://www.ibiblio.org/kuphaldt/electricCircuits/DC/DC_1.html
'Basic Concepts of Electricity'] chapter from
[
http://www.ibiblio.org/kuphaldt/electricCircuits/DC/index.html
'Lessons In Electric Circuits Vol 1 DC'] book and
[
http://www.ibiblio.org/kuphaldt/electricCircuits/ series].
* [
https://books.google.com/books?id=n-MDAAAAMBAJ&pg=PA772
"One-Hundred Years of Electricity", May 1931, Popular Mechanics]
* [
https://www.worldstandards.eu/electricity/plugs-and-sockets/ Socket
and plug standards]
* [
http://amasci.com/miscon/elect.html Electricity Misconceptions]
*
[
https://web.archive.org/web/20151201064159/http://www.micro.magnet.fsu.edu/electromag/java/diode/index.html
Electricity and Magnetism]
* [
http://steverose.com/Articles/UnderstandingBasicElectri.html
Understanding Electricity and Electronics in about 10 Minutes]
License
=========
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License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Electricity
