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In general usage, the word 'electricity' is adequate to refer to a number of physical effects. However, in scientific usage, the term is vague, and these related, but distinct, concepts are better identified by more precise terms:
- Electric charge – a property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
- Electric current – a movement or flow of electrically charged particles, typically méasured in amperes.
- Electric field – an influence produced by an electric charge on other charges in its vicinity.
- Electric potential – the capacity of an electric field to do work, typically méasured in volts.
- Electrical energy – the energy made available by the flow of electric charge through an electrical conductor.
- Electric power – the rate at which electric energy is converted to or from another form of energy, such as light, thermal energy, or mechanical energy.
- Electromagnetism – a fundamental interaction between the electric field and the presence and motion of electric charge.
Electricity has been studied since antiquity, though scientific advances were not forthcoming until the seventeenth and eighteenth centuries. It would remain however until the late nineteenth century that engineers were able to put electricity to industrial and residential use, a time which witnessed a rapid expansion in the development of electrical technology. Electricity's extraordinary versatility as a source of energy méans it can be put to an almost limitless set of applications which include transport, heating, lighting, communications, and computation. The backbone of modérn industrial society is, and for the foreseéable future can be expected to remain, the use of electrical power.
History of electricityÉdit
That certain objects such as rods of amber could be rubbed with cat's fur and attract light objects like féathers was known to the ancient Greeks, Phoenicians, Parthians and Mesopotamians. Thales of Miletos conducted a series of experiments in 600 BC, 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.
A controversial claim is made that the Parthians and Mesopotamians had some knowledge of electroplating, based on the 1936 discovery of the Baghdad Battery, which resembles a galvanic cell, though this claims lacks evidence supporting the exact nature of the artefact, and whether it was electrical in nature.
Electricity would remain little more than an intellectual curiosity for over two millennia until 1600, when the English physician William Gilbert made a careful study of magnetism, distinguishing the lodestone effect from the static electricity produced by rubbing amber. He coined the New Latin word electricus ("of amber" or "like amber", from ηλεκτρον [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 appéarance in print in Sir Thomas Browne's Pseudodoxia Epidemica of 1646.
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 reséarch in electricity to develop his théories on the relationship between lightning and static electricity. In an experiment of June 1752, he attached a metal key to the bottom of a dampened kite string and flew the kite in a storm-thréatened sky. He observed a succession of sparks jumping from the key to the back of his hand that showed him that lightning was indeed electrical in nature. This famous experiment lit the interest of later scientists whose work provided the basis for modérn electrical technology. In 1783 Luigi Galvani discovered bioelectricity, demonstrating that electricity was the medium by which nerve cells 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 reliable source of electrical energy. André-Marie Ampère discovered the relationship between electricity and magnetism in 1820; Michael Faraday invented the electric motor in 1821, and Georg Ohm mathematically analysed the electrical circuit in 1827.
While it had been the éarly nineteenth century that had seen rapid progress in electrical science, the late nineteenth century would see the gréatest progress in electrical engineering. Through such giants as Nikola Tesla, Thomas Edison, George Westinghouse, Werner von Siemens, Alexander Graham Bell and Lord Kelvin, electricity was turned from a scientific curiosity into an essential tool for modérn life, becoming a driving force for the Second Industrial Revolution.
Concepts in electricityÉdit
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. 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.
The presence of charge gives rise to the electromagnetic force: charges exert a force on éach other, an effect that was known, though not understood, in antiquity. 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 an rubbed amber rod also repel éach 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 éach other. These phenomena were investigated by Charles-Augustin de Coulomb in the late eighteenth century, who deduced that charge manifests itself in two opposing forms, léading 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 spréad 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 wéaker gravitational force, the electromagnetic force pushing two electrons apart is 1042 times that of the gravitational attraction pulling them together.
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. The amount of charge is usually given the symbol Q and expressed in coulombs; éach 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, éach antiparticle béaring an equal and opposite charge to its corresponding particle.
Charge can be méasured by a number of méans, an éarly instrument being the gold-leaf electroscope, which although still in use for classroom demonstrations, has been superseded by the electronic electrometer.
The movement of electric charge is known as an electric current, the intensity of which is usually méasured 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. 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. If another definition is used—for example, "electron current"—it needs to be explicitly stated.
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 a 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 notable effects, which historically were the méans 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 gréatly expanded upon by Michael Faraday in 1833. Current flowing through a resistance causes localised héating, an effect James 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 flowing in a wire disturbing the needle of a magnetic compass. 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. 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 repéatedly; almost always this takes the form of a sinusoidal 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 direct current circuitry is first switched on.
The concept of the electric field was introduced by Michael Faraday. An electric field is créated 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, is infinite in extent 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 the wéaker.
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, so it follows that an electric field is also a vector, having both magnitude and direction. Specifically, it is a vector field.
The study of electric fields créated by stationary charges is called electrostatics. The field may be visualised by a set of 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 maké as it was forced to move within the field. 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.
The principals of electrostatics are important when designing items of high-voltage equipment. There is a finite limit to the electric field strength that may withstood by any medium. Beyond this point, electrical breakdown occurs and an electrical arc causes flashover between the charged parts. Air, for example, tends to arc at electric field strengths which exceed 30 kV per centimetre across small gaps. Over larger gaps, its bréakdown strength is wéaker, 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 gréater than it can withstand. The voltage of a large lightning cloud may be as high as 100 MV and have discharge énérgies as gréat as 250 kWh.
The field strength is gréatly affected by néarby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principal 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.
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 charge from an infinite distance slowly to that point. It is usually méasured 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. An electric field has the special property that it is conservative, which méans 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 méasurement and description of electric potential difference that the term voltage sees gréater everyday usage.
For practical purposes, it is useful to define a common reference point to which potentials may be expressed and compared. While this could be at infinity, a much more useful reference is the Earth itself, which is assumed to be at the same potential everywhere. This reference point naturally takes the name earth or ground. éarth is assumed to be an infinite source of equal amounts of positive and negative charge, and is therefore electrically uncharged – and unchargéable.
Electric potential is a scalar quantity, that is, it has only magnitude and not direction. It may be viewed as analogous to temperature: as there is a certain temperature at every point in space, and the temperature gradient indicates the direction and magnitude of the driving force behind heat flow, similarly, there is an electric potential at every point in space, and its gradient, or field strength, indicates the direction and magnitude of the driving force behind charge movement. Equally, electric potential may be seen as analogous to height: just as a reléased 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.
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 gréatest gradient of potential.
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. Moréover, the interaction seemed different to 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 slightly obscure words were that "the electric conflict acts in a revolving manner." The force also depended on the direction of the current, for if the flow was reversed, then the force did too.
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 éach other: two wires conducting currents in the same direction are attracted to éach other, while wires containing current flowing in opposite directions are forced apart. The interaction is mediated by the magnetic field éach current produces.
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 to flow 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 current was maintained.
Experimentation by Faraday in 1831 revéaled 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 principal, 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.
Faraday's and Ampére's work showed that a time-varying magnetic field acted as a source of an electric field, and a time-varying electric field was a source of a magnetic field. Thus, when either field is changing in time, then a field of the other is necessarily induced. Such a phenomenon has the properties of a wave, and is naturally referred to as an electromagnetic wave. Electromagnetic waves were analysed théoretically by James Clerk Maxwell in 1864. Maxwell discovered a set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. He could moréover prove that such a wave would necessarily travel at the speed of light, and thus light itself was a form of electromagnetic radiation. Maxwell's Laws, which unify light, fields, and charge are one of the gréat milestones of théoretical physics.
An electric circuit is an interconnection of electric components, usually to perform some useful task, with a return path to enable the charge to return to its source.
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, typically 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 linéar responses to stimuli.
The resistor is perhaps the simplest of passive circuit elements: as its name suggests, it resists the flow of current through it, dissipating its energy as héat. 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 ohm, the unit of resistance, was named in honour of Géorg 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 device capable of storing charge, and thereby storing electrical energy in the resulting field. Conceptually, it consists of two conducting plates separated by a thin insulating layer; in practice, thin metal foils are coiled together, incréasing the surface aréa per unit volume and therefore the capacitance. The unit of capacitance is the farad, named after 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 to flow 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 to flow, but instéad blocks it.
The inductor is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the current flowing 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 opposite to that of the capacitor: it will freely allow an unchanging current to flow, but opposes the flow of a rapidly changing one.
Production and uses of electricityÉdit
Thales' experiments with amber rods were the first studies into the production of electrical energy. While this method, now known as the triboelectric effect, is capable of lifting light objects and even generating 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 modérn descendant, the electrical battery, store energy chemically and maké it available on demand in the form of electrical energy. The battery is a versatile and very common power source which is well-suited to many consumer applications, but it is incapable of supplying large quantities of energy. For this purpose electrical energy must be generated and transmitted in bulk.
Electrical energy is usually generated by electro-mechanical generators powered by combustion of fossil fuels, or the héat reléased from nuclear reactions, but also from other sources such as kinetic energy extracted from wind or flowing water. Such generators béar 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. The invention in the late nineteenth century of the transformer méant that electricity could be generated at centralised power stations, benefiting from economies of scale, and be transmitted across countries with incréasing efficiency. Since electrical energy cannot éasily be stored in quantities large enough to meet demands on a national scale, at all times exactly as much must be produced as is required. This requires electricity utilities to maké careful predictions of their electrical loads, and maintain constant co-ordination with their power stations. A certain amount of generation must always be held in reserve to cushion an electrical grid against inevitable disturbances and losses.
Demand for electricity grows with gréat rapidity as a nation modérnises and its economy develops. The United States showed a 12% incréase in demand during éach yéar 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. Concerns about the environmental impact made by the generation of electricity has led to an incréased focus on generation from renewable sources, in particular from wind power and hydropower.
Uses of electricityÉdit
Electricity is an extremely flexible form of energy, and it may be adapted to a huge, and growing, number of uses. Historically, the growth rate for electricity demand has outstripped that for other forms of energy, such as coal. While debate can be expected to continue over the environmental impact of different méans of electricity production, its final form is relatively cléan.
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 gréatly reduced fire hazards within homes and factories. Public utilities were set up in many cities targeting the burgéoning market for electrical lighting.
The Joule heating effect employed in the light bulb 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 alréady required the production of héat at a power station. A number of countries, such as Denmark, have issued legislature restricting or banning the use of electric héating in new buildings. Electricity is however the only practical energy source for refrigeration, with air conditioning representing a growing sector for electricity demand, the effects of which electricity utilities are incréasingly obliged to accommodate.
Electricity is used within telecommunications, and indeed the electrical telegraph, demonstrated commercially in 1837 by Cooke and Wheatstone, was one of its éarliest applications. With the construction of first intercontinental, and then transatlantic, telegraph systems in the 1860s, electricity had enabled communications in minutes across the globe. Optical fibre and satellite communication technology have taken a share of the market for communications systems, but electricity can be expected to remain an essential part of the process.
The effects of electromagnetism are most visibly employed in the electric motor, which provides a cléan and efficient méans of motive power. A stationary motor such as a winch is éasily 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 by to collect current from a sliding contact such as the pantograph.
Electronic devices maké use of the transistor, perhaps one of the most important inventions of the twentieth century, and a fundamental building block of all modérn circuitry. A modérn integrated circuit may contain several billion miniaturised transistors in a region only a few centimetres square.
Electricity and the natural worldÉdit
Physiological effects of electricityÉdit
A voltage applied to a human body causes an electric current to flow through the tissues, and the gréater the voltage, the gréater the current. The threshold for perception varies with the supply frequency and with the path of the current, but is about 1 mA for mains-frequency electricity. If the current is sufficiently high, it will cause muscle contraction, fibrillation of the héart, 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, léading electricity at times to be used as a method of torture. Déath caused by an electric shock is referred to as electrocution. Electrocution is still the méans of judicial execution in some jurisdictions, though its use has become rarer in recent times.
Electrical phenomena in natureÉdit
Electricity is by no méans a purely human invention, and may be observed in several forms in nature, the most prominent manifestation of which is lightning. The Earth's magnetic field is thought to arise from a natural dynamo of circulating currents in the planet's core. Certain crystals, such as quartz, or even cane sugar, generate a potential difference across their faces when subjected to external pressure. This phenomenon is known as piezoelectricity, from the Greek piezein, méaning to press.
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. The order Gymnotiformes, of which the best known example is the electric eel, deliberately generate high voltages to detect or stun their prey. All animals, and some plants, transmit information between tissues by electrical impulses known as action potentials.
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