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electricityAll phenomena caused by electric charge. There are two types of electricity: static and current. Electric charge is caused by an excess or deficit of electrons in a substance, and an electric current is the movement of charge through a material. Materials having equal numbers of positive and negative charges are termed neutral, as the charges balance out. Substances may be electrical conductors, such as metals, which allow the passage of electricity through them readily, or insulators, such as rubber, which are extremely poor conductors. Substances with relatively poor conductivities that increase with a rise in temperature or when light falls on the material are known as semiconductors. Electric currents also flow through the nerves of organisms. For example, the optic nerve in humans carries electric signals from the eye to the brain. Electricity cannot be seen, but the effects it produces can be clearly seen; for example, a flash of lightning, or the small sparks produced by rubbing a nylon garment. Electricity is essential to modern society. Electrical devices are used in the home, office, and industry, powered by mains electricity. The energy provided by the mains supply is measured in kilowatt hours. Electrical energy is used to power devices used for communications such as fax machines, telephones, computers, and satellites. Electrical properties of solids The first artificial electrical phenomenon to be observed was that some naturally occurring materials such as amber, when rubbed with a piece of cloth, would then attract small objects such as dust and pieces of paper. Rubbing the object caused it to become electrically charged so that it had an excess or deficit of electrons. When the amber is rubbed with a piece of cloth electrons are transferred from the cloth to the amber so that the amber has an excess of electrons and is negatively charged, and the cloth has a deficit of electrons and is positively charged. This accumulation of charge is called static electricity. This charge on the object exerts an electric field, in the space around the object, that can attract or repel other objects. It was discovered that there are only two types of charge, positive and negative, and that they neutralize each other. Objects with a like charge always repel each other while objects with an opposite charge attract each other. Neutral objects (such as pieces of paper) can be attracted to charged bodies by electrical induction. For example, the charge on a negatively-charged body causes a separation of charge across the neutral body by induction. The negative charges tend to move away from the side near the negatively-charged body, leaving that body with a deficit of negative charges (that is, positively charged). The positive side of the neutral body is then weakly attracted to the charged body. The electroscope is a device used to demonstrate the presence of electric charges and to measure its size and whether it is positive or negative. The electroscope was invented by Michael Faraday. |
Current, charge, and energy An electric current in a material is the passage of charge through it. In metals and other conducting materials, the charge is carried by free electrons that are not bound tightly to the atoms and are thus able to move through the material. For charge to flow in a circuit there must be a potential difference (PD) applied across the circuit. This is often supplied by a battery, which has a positive terminal and a negative terminal. Under the influence of the potential difference, the electrons are repelled from the negative terminal side of the circuit and attracted to the positive terminal of the battery. A steady flow of electrons around the circuit is produced. Current flowing through a circuit can be measured using an ammeter and is measured in amperes (or amps). A coulomb (C) is the unit of charge, defined as the charge passing a point in a wire each second when the current is exactly 1 amp. The unit of charge is named after Charles Augustin de Coulomb. Direct current (DC) flows continuously in one direction; alternating current (AC) flows alternately in each direction. In a circuit the battery provides energy to make charge flow through the circuit. The amount of energy supplied to each unit of charge is called the electromotive force (emf). The unit of emf is the volt (V). A battery has an emf of 1 volt when it supplies 1 joule of energy to each coulomb of charge flowing through it. The energy carried by flowing charges can be used to do work, for example to light a bulb, to cause current to flow through a resistor, to emit radiation, or to produce heat. When the energy carried by a current is made to do work in this way, a potential difference can be measured across the circuit component concerned by a voltmeter or a cathode-ray oscilloscope. The potential difference is also measured in volts. Power, measured in watts, is the product of current and voltage. Potential difference and current measure are related to each other. This relationship was discovered by Georg Ohm, and is expressed by Ohm's law: the current through a wire is proportional to the potential difference across its ends. The potential difference divided by the current is a constant for a given piece of wire. This constant for a given material is called the resistance. |
Conduction in liquids and gases In liquids, current can flow by the movement of charged ions through a solution or molten salt (the electrolyte), resulting in the migration of ions to the electrodes: positive ions (cations) to the negative electrode (cathode) and negative ions (anions) to the positive electrode (anode). This process is called electrolysis and represents bidirectional flow of charge as opposite charges move to oppositely-charged electrodes. In metals, charges are only carried by free electrons and therefore move in only one direction. |
Electromagnetism Magnetic fields are produced either by current-carrying conductors or by permanent magnets. In current-carrying wires, the magnetic field lines are concentric circles around the wire. Their direction depends on the direction of the current, and their strength on the size of the current. If a conducting wire is moved within a magnetic field, the magnetic field acts on the free electrons within the conductor, displacing them and causing a current to flow. The force acting on the electrons and causing them to move is greatest when the wire is perpendicular to the magnetic field lines. The direction of the current is given by the left-hand rule. The generation of a current by the relative movement of a conductor in a magnetic field is called electromagnetic induction. This is the basis of how a dynamo works. |
Generation of electricity Electricity is the most useful and most convenient form of energy, readily convertible into heat and light and used to power machines. Electricity can be generated in one place and distributed anywhere because it readily flows through wires. It is generated at power stations where a suitable energy source is harnessed to drive turbines that spin electricity generators. Current energy sources are coal, oil, water power (hydroelectricity), natural gas, and nuclear energy. Research is under way to increase the contribution of wind, tidal, solar, and geothermal power. Nuclear fuel has proved a more expensive source of electricity than initially anticipated and worldwide concern over radioactivity may limit its future development. Electricity is generated at power stations at a voltage of about 25,000 volts, which is not a suitable voltage for long-distance transmission. For minimal power loss, transmission must take place at very high voltage (400,000 volts or more). The generated voltage is therefore increased (‘stepped up’) by a transformer. The resulting high-voltage electricity is then fed into the main arteries of the grid system, an interconnected network of power stations and distribution centres covering a large area. After transmission to a local substation, the line voltage is reduced by a step-down transformer and distributed to consumers. Among specialized power units that convert energy directly to electrical energy without the intervention of any moving mechanisms, the most promising are thermionic converters. These use conventional fuels such as propane gas, as in portable military power packs, or, if refuelling is to be avoided, radioactive fuels, as in uncrewed navigational aids and spacecraft. UK electricity generation was split into four companies in 1990 in preparation for privatization. The nuclear power stations remain in the hands of the state through Nuclear Electric (accounting for 20% of electricity generated); National Power (50%) and PowerGen (30%) generate electricity from fossil-fuel and renewable sources. Transmission lines and substations are owned by the National Grid, which was privatized in 1996. Electricity generated on a commercial scale was available from the early 1880s and used for electric motors driving all kinds of machinery, and for lighting, first by carbon arc, but later by incandescent filaments (first of carbon and then of tungsten), enclosed in glass bulbs partially filled with noble gas under vacuum. Light is also produced by passing electricity through a gas or metal vapour or a fluorescent lamp. Other practical applications include telephone, radio, television, X-ray machines, and many other applications in electronics. An important consideration in the design of electrical equipment is electrical safety. This includes measures to minimize the risk of electric shock or fire caused by electrical faults. Safety measures include the fitting of earth wires, and fuses or circuit breakers, and the insulation of wires. |
History The fact that amber has the power, after being rubbed, of attracting light objects, such as bits of straw and feathers, is said to have been known to Thales of Miletus and to the Roman naturalist Pliny. William Gilbert, Queen Elizabeth I's physician, found that many substances possessed this power, and he called it ‘electric’ after the Greek word meaning ‘amber’. In the early 1700s, it was recognized that both positive and negative charges exist and that objects of like charge attract each other whereas those of opposite charge repel. The charge on glass rubbed with silk came to be known as positive electricity, and the charge on amber rubbed with wool as negative electricity. These two charges were found to cancel each other when brought together. In 1800 Alessandro Volta found that a series of cells containing brine, in which were dipped plates of zinc and copper, gave an electric current, which later in the same year was shown to evolve hydrogen and oxygen when passed through water (see electrolysis). Humphry Davy, in 1807, decomposed soda and potash (both thought to be elements) and isolated the metals sodium and potassium, a discovery that led the way to electroplating. Other properties of electric currents discovered were the heating effect, now used in lighting and central heating, and the deflection of a magnetic needle, described by Hans Oersted in 1820 and elaborated by André Ampère in 1825. This work made possible the electric telegraph. For Michael Faraday, the fact that an electric current passing through a wire caused a magnet to move suggested that moving a wire or coil of wire rapidly between the poles of a magnet would induce an electric current. He demonstrated this in 1831, producing the first dynamo, which became the basis of electrical engineering. The characteristics of currents were formalized about 1827 by Georg Ohm, who showed that the current passing along a wire was equal to the electromotive force across the wire multiplied by a constant, which was the conductivity of the wire. The unit of resistance (ohm) is named after Ohm, the unit of emf (volt) is named after Volta, and the unit of current (amp) after Ampère. The work of the late 1800s indicated the wide interconnections of electricity (with magnetism, heat, and light), and about 1855 James Clerk Maxwell formulated a single electromagnetic theory. The universal importance of electricity was decisively proved by the discovery that the atom, until then thought to be the ultimate particle of matter, is composed of a positively-charged central core, the nucleus, about which negatively-charged electrons rotate in various orbits. |
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