Electricity
From Wikipedia, the free encyclopedia
Lightning is one of the most dramatic effects of electricity.
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of
electric charge. These include many easily recognizable phenomena, such as
lightning,
static electricity, and the flow of
electrical current in an electrical wire. In addition, electricity encompasses less familiar concepts such as the
electromagnetic field and
electromagnetic induction.
The word is from the
New Latin ēlectricus, "amber-like"
[a], coined in the year 1600 from the Greek
ήλεκτρον (electron) meaning
amber (hardened plant resin), because electrical effects were produced classically by rubbing amber.
In general usage, the word "electricity" adequately refers to a number of physical effects. In a scientific context, however, the term is vague, and these related, but distinct, concepts are better identified by more precise terms:
The most common use of the word "electricity" is less precise. It refers to:
Electrical phenomena have been studied since antiquity, though advances in the science were not made until the seventeenth and eighteenth centuries. Practical applications for electricity however remained 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 as a source of energy means it can be put to an almost limitless set of applications which include
transport,
heating,
lighting,
communications, and
computation. Electrical power is the backbone of modern industrial society, and is expected to remain so for the foreseeable future.
[1]
History
Thales, the earliest researcher into electricity
Long before any knowledge of electricity existed people were aware of shocks from
electric fish.
Ancient Egyptian texts dating from
2750 BC 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
torpedo 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 Miletos made a series of observations on
static electricity around 600 BC, 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
ήλεκτρον [
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]
In 1791,
Luigi Galvani published his discovery of
bioelectricity, demonstrating that electricity was the medium by which
nerve cells passed signals to the muscles.
[13] 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.
[13] 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.
[13] 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.
[14]
While it had been the early 19th century that had seen rapid progress in electrical science, the late 19th century would see the greatest progress in
electrical engineering. Through such people as
Nikola Tesla,
Galileo Ferraris,
Thomas Edison,
Ottó Bláthy,
Ányos Jedlik,
Sir Charles Parsons,
Joseph Swan,
George Westinghouse,
Ernst Werner von Siemens,
Alexander Graham Bell and
Lord Kelvin, electricity was turned from a scientific curiosity into an essential tool for modern life, becoming a driving force for the
Second Industrial Revolution.
[15]
Concepts
Electric charge
Electric charge is a property of certain
subatomic particles, which gives rise to and interacts with the
electromagnetic force, one of the four
fundamental forces of nature. Charge originates in the
atom, in which its most familiar carriers are the
electron and
proton. It is 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.
[16] Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire.
[17] 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 presence of charge gives rise to the electromagnetic force: charges exert a
force on each other, an effect that was known, though not understood, in antiquity.
[18] 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.
[18]
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.
[19][20] The electromagnetic force is very strong, second only in strength to the
strong interaction,
[21] but unlike that force it operates over all distances.
[22] In comparison with the much weaker
gravitational force, the electromagnetic force pushing two electrons apart is 10
42 times that of the
gravitational attraction pulling them together.
[23]
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.
[24] The amount of charge is usually given the symbol
Q and expressed in
coulombs;
[25] each electron carries the same charge of approximately −1.6022×10
−19 coulomb. The proton has a charge that is equal and opposite, and thus +1.6022×10
−19 coulomb. Charge is possessed not just by
matter, but also by
antimatter, each
antiparticle bearing an equal and opposite charge to its corresponding particle.
[26]
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.
[17]
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.
[27] 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. While the particles themselves can move quite slowly, sometimes with an average
drift velocity only fractions of a millimetre per second,
[17] the
electric field that drives them itself propagates at close to the
speed of light, enabling electrical signals to pass rapidly along wires.
[28]
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.
[29] Current through a
resistance causes localised heating, an effect
James Prescott Joule studied mathematically in 1840.
[29] 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.
[30] He had discovered
electromagnetism, a fundamental interaction between electricity and magnetics.
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.
[31] 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
sinusoidal wave.
[32] 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.
[33] These properties however can become important when circuitry is subjected to
transients, such as when first energised.
Electric field
Main article:
Electric fieldThe 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.
[22] 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.
[23]
Field lines emanating from a positive charge above a plane conductor
An electric field generally varies in space,
[34] 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.
[35] 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.
[35]
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,
[36] 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.
[36] 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.
[37]
A hollow conducting body carries all its charge on its outer surface. The field is therefore zero at all places inside the body.
[38] 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.
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