Devoir de Philosophie

Electricity I INTRODUCTION Electricity, one of the basic forms of energy.

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Electricity I INTRODUCTION Electricity, one of the basic forms of energy. Electricity is associated with electric charge, a property of certain elementary particles such as electrons and protons, two of the basic particles that make up the atoms of all ordinary matter. Electric charges can be stationary, as in static electricity, or moving, as in an electric current. Electrical activity takes place constantly everywhere in the universe. Electrical forces hold molecules together. The nervous systems of animals work by means of weak electric signals transmitted between neurons (nerve cells). Electricity is generated, transmitted, and converted into heat, light, motion, and other forms of energy through natural processes, as well as by devices built by people. Electricity is an extremely versatile form of energy. It can be generated in many ways and from many different sources. It can be sent almost instantaneously over long distances. Electricity can also be converted efficiently into other forms of energy, and it can be stored. Because of this versatility, electricity plays a part in nearly every aspect of modern technology. Electricity provides light, heat, and mechanical power. It makes telephones, computers, televisions, and countless other necessities and luxuries possible. II ELECTRIC CHARGE Electricity consists of charges carried by electrons, protons, and other particles. Electric charge comes in two forms: positive and negative. Electrons and protons both carry exactly the same amount of electric charge, but the positive charge of the proton is exactly opposite the negative charge of the electron. If an object has more protons than electrons, it is said to be positively charged; if it has more electrons than protons, it is said to be negatively charged. If an object contains as many protons as electrons, the charges will cancel each other and the object is said to be uncharged, or electrically neutral. Electricity occurs in two forms: static electricity and electric current. Static electricity consists of electric charges that stay in one place. An electric current is a flow of electric charges between objects or locations. III STATIC ELECTRICITY Static electricity can be produced by rubbing together two objects made of different materials. Electrons move from the surface of one object to the surface of the other if the second material holds onto its electrons more strongly than the first does. The object that gains electrons becomes negatively charged, since it now has more electrons than protons. The object that gives up electrons becomes positively charged. For example, if a nylon comb is run through clean, dry hair, some of the electrons on the hair are transferred to the comb. The comb becomes negatively charged and the hair becomes positively charged. The following materials are named in decreasing order of their ability to hold electrons: rubber, silk, glass, flannel, and fur (or hair). If any two of these materials are rubbed together, the material earlier in the list becomes negative, and the material later in the list becomes positive. The materials should be clean and dry. A Charging by Contact Objects become electrically charged in either of two ways: by contact or by induction. A charged object transfers electric charge to an object with lesser charge if the two touch. When this happens, a charge flows from the first to the second object for a brief time. Charges in motion form an electric current. When charge flows between objects in contact, the amount of charge that an object receives depends on its ability to store charge. The ability to store charge is called capacitance and is measured in units called farads. Charging by contact can be demonstrated by touching an uncharged electroscope with a charged comb. An electroscope is a device that contains two strips of metal foil, called leaves, that hang from one end of a metal rod. A metal ball is at the other end of the rod. When the charged comb touches the ball, some of the charges on the comb flow to the leaves, which separate because they now hold like charges and repel each other. If the comb is removed, the leaves remain apart because they retain their charges. The electroscope has thus been charged by contact with the comb. This flow of charge between objects with different amounts of charge will occur whenever possible. However, it requires a pathway for the electric charge to move along. Some materials, called conductors, allow an electric current to flow through them easily. Other materials, called insulators, strongly resist the passage of an electric current. Under normal conditions, air is an insulator. However, if an object gains a large enough charge of static electricity, part of the charge may jump, or discharge, through the air to another object without touching it directly. When the charge is large enough, the air becomes a conductor. Lightning is an example of a discharge. B Coulomb's Law Objects with opposite charges attract each other, and objects with similar charges repel each other. Coulomb's law, formulated by French physicist Charles Augustin de Coulomb during the late 18th century, quantifies the strength of the attraction or repulsion. This law states that the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. The greater the charges on the objects, the larger the force between them; the greater the distance between the objects, the lesser the force between them. The unit of electric charge, also named after Coulomb, is equal to the combined charges of 6.24 × 1018 protons (or electrons). If two charged objects in contact have the same capacitance, they divide the charge evenly. Suppose, for example, that one object has a charge of +4 coulombs and the other a charge of +8 coulombs. When they touch, charge will flow from the 8-coulomb object to the 4-coulomb object until each has a charge of +6 coulombs. If each object originally had a charge of +6 coulombs, no charge would flow between them. If two objects have different capacitances, they divide the charge in proportion to their capacitances. If an object with a capacitance of 10 farads touches an object with a capacitance of 5 farads, the 10-farad object will end up with twice the amount of charge of the 5-farad object. Suppose that the objects are oppositely charged and that one has a charge of +20 coulombs and the other a charge of -8 coulombs. Their total charge is therefore +12 coulombs. After they touch, the 10-farad object will have a charge of +8 coulombs and the 5-farad object will have +4 coulombs. C Charging by Induction A charged object may induce a charge in a nearby neutral object without touching it. For example, if a positively charged object is brought near a neutral object, the electrons in the neutral object are attracted to the positive object. Some of these electrons flow to the side of the neutral object that is nearest to the positive object. This side of the neutral object accumulates electrons and becomes negatively charged. Because electrons leave the far side of the neutral object while its protons remain stationary, that side becomes positively charged. Since the negatively charged side of the neutral object is closest to the positive object, the attraction between this side and the positive object is greater than the repulsion between the positively charged side and the positive object. The net effect is an attraction between the objects. Similarly, when a negatively charged object is brought near a neutral object, the negative object induces a positive charge on the near side of the neutral object and a negative charge on the far side. As before, the net effect is an attraction between the objects. The induced charges described above are not permanent. As soon as the charged object is taken away, the electrons on the other object redistribute themselves evenly over it, so that it again becomes neutral. An object can also be charged permanently by induction. If a negatively charged object, A, is brought near a neutral object, B, the electrons on B are repelled as far as possible from A and flow to the other side of B. If that side of B is then connected to the ground by a good conductor, such as a metal wire, the electrons flow out through the wire into the ground. The ground can receive almost any amount of charge because Earth, being neutral, has an enormous capacitance. Object B is said to be grounded by the wire connecting it to Earth. If this wire is then removed, B has a positive charge, since it has lost electrons to Earth. Thus B has been permanently charged by induction. Even if A is subsequently removed, B still remains positive because the wire has been disconnected and B cannot regain electrons from Earth to neutralize its positive charge. IV ELECTRIC CURRENT An electric current is a movement of charge. When two objects with different charges touch and redistribute their charges, an electric current flows from one object to the other until the charge is distributed according to the capacitances of the objects. If two objects are connected by a material that lets charge flow easily, such as a copper wire, then an electric current flows from one object to the other through the wire. Electric current can be demonstrated by connecting a small light bulb to an electric battery by two copper wires. When the connections are properly made, current flows through the wires and the bulb, causing the bulb to glow. Current that flows in one direction only, such as the current in a battery-powered flashlight, is called direct current. Current that flows back and forth, reversing direction again and again, is called alternating current. Direct current, which is used in most battery-powered devices, is easier to understand than alternating current. Most of the following discussion focuses on direct current. Alternating current, which is used in most devices that are "plugged in" to electrical outlets in buildings, will be discussed in the Alternating Current section of this article. Other properties that are used to quantify and compare electric currents are the voltage (also called electromotive force) driving the current and the resistance of the conductor to the passage of the current. The amount of current, voltage, and resistance in any circuit are all related through an equation called Ohm's law. A Conductors and Insulators Conductors are materials that allow an electric current to flow through them easily. Most metals are good conductors. Substances that do not allow electric current to flow through them are called insulators, nonconductors, or dielectrics. Rubber, glass, and air are common insulators. Electricians wear rubber gloves so that electric current will not pass from electrical equipment to their bodies. However, if an object contains a sufficient amount of charge, the charge can arc, or jump, through an insulator to another object. For example, if you shuffle across a wool rug and then hold your finger very close to, but not in contact with, a metal doorknob or radiator, current will arc through the air from your finger to the doorknob or radiator, even though air is an insulator. In the dark, the passage of the current through the air is visible as a tiny spark. B Measuring Electric Current Electric current is measured in units called amperes (amp). If 1 coulomb of charge flows past each point of a wire every second, the wire is carrying a current of 1 amp. If 2 coulombs flow past each point in a second, the current is 2 amp. See also Electric Meters. C Voltage When the two terminals of a battery are connected by a conductor, an electric current flows through the conductor. One terminal continuously sends electrons into the conductor, while the other continuously receives electrons from it. The current flow is caused by the voltage, or potential difference, between the terminals. The more willing the terminals are to give up and receive electrons, the higher the voltage. Voltage is measured in units called volts. Another name for a voltage produced by a source of electric current is electromotive force. D Resistance A conductor allows an electric current to flow through it, but it does not permit the current to flow with perfect freedom. Collisions between the electrons and the atoms of the conductor interfere with the flow of electrons. This phenomenon is known as resistance. Resistance is measured in units called ohms. The symbol for ohms is the Greek letter omega, ?. A good conductor is one that has low resistance. A good insulator has a very high resistance. At commonly encountered temperatures, silver is the best conductor and copper is the second best. Electric wires are usually made of copper, which is less expensive than silver. The resistance of a piece of wire depends on its length, and its cross-sectional area, or thickness. The longer the wire is, the greater its resistance. If one wire is twice as long as a wire of identical diameter and material, the longer wire offers twice as much resistance as the shorter one. A thicker wire, however, has less resistance, because a thick wire offers more room for an electric current to pass through than a thin wire does. A wire whose cross-sectional area is twice that of another wire of equal length and similar material has only half the resistance of the thinner wire. Scientists describe this relationship between resistance, length, and area by saying that resistance is proportional to length and inversely proportional to cross-sectional area. Usually, the higher the temperature of a wire, the greater its resistance. The resistance of some materials drops to zero at very low temperatures. This phenomenon is known as superconductivity. E Ohm's Law The relationship between current, voltage, and resistance is given by Ohm's law. This law states that the amount of current passing through a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor. Ohm's law can be expressed as an equation, V = IR, where V is the difference in volts between two locations (called the potential difference), I is the amount of current in amperes that is flowing between these two points, and R is the resistance in ohms of the conductor between the two locations of interest. V = IR can also be written R = V/I and I = V/R. If any two of the quantities are known, the third can be calculated. For example, if a potential difference of 110 volts sends a 10-amp current through a conductor, then the resistance of the conductor is R = V/I = 110/10 = 11 ohms. If V = 110 and R = 11, then I = V/R = 110/11 = 10 amp. Under normal conditions, resistance is constant in conductors made of metal. If the voltage is raised to 220 in the example above, then R is still 11. The current I will be doubled, however, since I = V/R = 220/11 = 20 amp. F Heat and Power A conductor's resistance to electric current produces heat. The greater the current passing through the conductor, the greater the heat. Also, the greater the resistance, the greater the heat. A current of I amp passing through a resistance of R ohms for t seconds generates an amount of heat equal to I2Rt joules (a joule is a unit of energy equal to 0.239 calorie). Energy is required to drive an electric current through a resistance. This energy is supplied by the source of the current, such as a battery or an electric generator. The rate at which energy is supplied to a device is called power, and it is often measured in units called watts. The power P supplied by a current of I amp passing through a resistance of R ohms is given by P = I 2R. V HOW ELECTRIC CURRENT IS CONDUCTED All electric currents consist of charges in motion. However, electric current is conducted differently in solids, gases, and liquids. When an electric current flows in a solid conductor, the flow is in one direction only, because the current is carried entirely by electrons. In liquids and gases, however, a two-directional flow is made possible by the process of ionization (see Electrochemistry). A Conduction in Solids The conduction of electric currents in solid substances is made possible by the presence of free electrons (electrons that are free to move about). Most of the electrons in a bar of copper, for example, are tightly bound to individual copper atoms. However, some are free to move from atom to atom, enabling current to flow. Ordinarily the motion of the free electrons is random; that is, as many of them are moving in one direction as in another. However, if a voltage is applied to the two ends of a copper bar by means of a battery, the free electrons tend to drift toward one end. This end is said to be at a higher potential and is called the positive end. The other end is said to be at a lower potential and is called the negative end. The function of a battery or other source of electric current is to maintain potential difference. A battery does this by supplying electrons to the negative end of the bar to replace those that drift to the positive end and also by absorbing electrons at the positive end. Insulators cannot conduct electric currents because all their electrons are tightly bound to their atoms. A perfect insulator would allow no charge to be forced through it, but no such substance is known at room temperature. The best insulators offer high but not infinite resistance at room temperature. Some substances that ordinarily have no free electrons, such as silicon and germanium, can conduct electric currents when small amounts of certain impurities are added to them. Such substances are called semiconductors. Semiconductors generally have a higher resistance to the flow of current than does a conductor, such as copper, but a lower resistance than an insulator, such as glass. B Conduction in Gases Gases normally contain few free electrons and are generally insulators. When a strong potential difference is applied between two points inside a container filled with a gas, the few free electrons are accelerated by the potential difference and collide with the atoms of the gas, knocking free more electrons. The gas atoms become positively charged ions and the gas is said to be ionized. The electrons move toward the high-potential (more positive) point, while the ions move toward the lowpotential (more negative) point. An electric current in a gas is composed of these opposite flows of charges. C Conduction in Liquid Solutions Many substances become ionized when they dissolve in water or in some other liquid. An example is ordinary table salt, sodium chloride (NaCl). When sodium chloride dissolves in water, it separates into positive sodium ions, Na+ , and negative chlorine ions, Cl-. If two points in the solution are at different potentials, the negative ions drift toward the positive point, while the positive ions drift toward the negative point. As in gases, the electric current is composed of these flows of opposite charges. Thus, while water that is absolutely pure is an insulator, water that contains even a slight impurity of an ionized substance is a conductor. Since the positive and negative ions of a dissolved substance migrate to different points when an electric current flows, the substance is gradually separated into two parts. This separation is called electrolysis. VI SOURCES OF ELECTRIC CURRENT There are several different devices that can supply the voltage necessary to generate an electric current. The two most common sources are generators and electrolytic cells. A Generators Generators use mechanical energy, such as water pouring through a dam or the motion of a turbine driven by steam, to produce electricity. The electric outlets on the walls of homes and other buildings, from which electricity to operate lights and appliances is drawn, are connected to giant generators located in electric power stations. Each outlet contains two terminals. The voltage between the terminals drives an electric current through the appliance that is plugged into the outlet. See Electric Power Systems. B Electrolytic Cells Electrolytic cells use chemical energy to produce electricity. Chemical reactions within an electrolytic cell produce a potential difference between the cell's terminals. An electric battery consists of a cell or group of cells connected together. C Other Sources There are many sources of electric current other than generators and electrolytic cells. Fuel cells, for example, produce electricity through chemical reactions. Unlike electrolytic cells, however, fuel cells do not store chemicals and therefore must be constantly refilled. Certain sources of electric current operate on the principle that some metals hold onto their electrons more strongly than other metals do. Platinum, for example, holds its electrons less strongly than aluminum does. If a strip of platinum and a strip of aluminum are pressed together under the proper conditions, some electrons will flow from the platinum to the aluminum. As the aluminum gains electrons and becomes negative, the platinum loses electrons and becomes positive. The strength with which a metal holds its electrons varies with temperature. If two strips of different metals are joined and the joint heated, electrons will pass from one strip to the other. Electricity produced directly by heating is called thermoelectricity. Some substances emit electrons when they are struck by light. Electricity produced in this way is called photoelectricity. When pressure is applied to certain crystals, a potential difference develops across them. Electricity thus produced is called piezoelectricity. Some microphones work on this principle. VII ELECTRIC CIRCUITS An electric circuit is an arrangement of electric current sources and conducting paths through which a current can continuously flow. In a simple circuit consisting of a small light bulb, a battery, and two pieces of wire, the electric current flows from the negative terminal of the battery, through one piece of connecting wire, through the bulb filament (also a type of wire), through the other piece of connecting wire, and back to the positive terminal of the battery. When the electric current flows through the filament, the filament heats up and the bulb lights. A switch can be placed in one of the connecting wires. A flashlight is an example of such a circuit. When the switch is open, the connection is broken, electric current cannot flow through the circuit, and the bulb does not light. When the switch is closed, current flows and the bulb lights. The bulb filament may burn out if too much electric current flows through it. To prevent this from happening, a fuse (circuit breaker) may be placed in the circuit. When too much current flows through the fuse, a wire in the fuse heats up and melts, thereby breaking the circuit and stopping the flow of current. The wire in the fuse is designed to melt before the filament would melt. The part of an electric circuit other than the source of electric current is called the load. The load includes all appliances placed in the circuit, such as lights, radios, fans, buzzers, and toasters. It also includes the connecting wires, as well as switches, fuses, and other devices. The load forms a continuous conducting path between the terminals of the current source. There are two basic ways in which the parts of a circuit are arranged. One arrangement is called a series circuit, and the other is called a parallel circuit. A Series Circuits If various objects are arranged to form a single conducting path between the terminals of a source of electric current, the objects are said to be connected in series. The electron current first passes from the negative terminal of the source into the first object, then flows through the other objects one after another, and finally returns to the positive terminal of the source. The current is the same throughout the circuit. In the example of the light bulb, the wires, bulb, switch, and fuse are connected in series. When objects are connected in series, the electric current flows through them against the resistance of the first object, then against the resistance of the next object, and so on. Therefore the total resistance to the current is equal to the sum of the individual resistances. If three objects with resistances R1, R2, and R3 are connected in series, their total resistance is R1 + R2 + R3. For example, if a motor with a resistance of 48 ohms is connected to the terminals of a current source by two wires, each with a resistance of 1 ohm, the total resistance of the motor and wires is 48 + 1 + 1 = 50 ohms. If the voltage is 100 volts, a current of 100/50 = 2 amp will flow through the circuit. Voltage can be thought of as being used up by the objects in a circuit. The voltage that each object uses up is called the voltage drop across that object. Voltage drop can be calculated from the equation V = IR, where V is the voltage drop across the object, I is the amount of current, and R is the resistance of the object. In the example of the motor, the voltage drop in each wire is V = IR = 2 × 1 = 2 volts, and the voltage drop in the motor is 2 × 48 = 96 volts. Adding up the voltage drops (2 + 2 + 96) gives a total drop of 100 volts. In a series circuit the sum of the voltage drops across the objects always equals the total voltage supplied by the source. B Parallel Circuits If various objects are connected to form separate paths between the terminals of a source of electric current, they are said to be connected in parallel. Each separate path is called a branch of the circuit. Current from the source splits up and enters the various branches. After flowing through the separate branches, the current merges again before reentering the current source. The total resistance of objects connected in parallel is less than that of any of the individual resistances. This is because a parallel circuit offers more than one branch (path) for the electric current, whereas a series circuit has only one path for all the current. The electric current through a parallel circuit is distributed among the branches according to the resistances of the branches. If each branch has the same resistance, then the current in each will be equal. If the branches have different resistances, the current in each branch can be determined from the equation I = V/R, where I is the amount of current in the branch, V is the voltage, and R is the resistance of the branch. The total resistance of a parallel circuit can be calculated from the equation where R is the total resistance and R1, R2, ... are the resistances of the branches. For example, if a parallel circuit consists of three branches with resistances of 10, 15, and 30 ohms, then Therefore, R = 5 ohms. In this circuit, a voltage of 150 volts would produce an electric current of I = V/R = 150/5 = 30 amp. The greater the resistance of a given branch, the smaller the portion of the electric current flowing through that branch. If a parallel circuit of three branches, with resistances of 10, 15, and 30 ohms, is connected to a 150-volt source, the branch with a resistance of 10 ohms would receive a current of V/R = 150/10 = 15 amp. Similarly, the 15-ohm branch receives 10 amp, and the 30-ohm branch receives 5 amp. These branch currents add up to a total current of 30 amp, which is the value obtained by dividing the voltage by the total resistance. C Series-Parallel Circuits Many circuits combine series and parallel arrangements. One branch of a parallel circuit, for example, may have within it several objects in a series. The resistances of these objects must be combined according to the rules for a series circuit. On the other hand, a series circuit may at one point divide into two or more branches and then rejoin. The branches are parallel and must be treated by the rules for parallel circuits. Complicated series-parallel circuits may be analyzed by means of two rules called Kirchhoff's laws. These rules make it possible to find the amount of electric current flowing through each part of any circuit, as well as the voltage across it. The first of Kirchhoff's laws states that at any junction in a circuit through which a steady current is flowing, the sum of the currents flowing to the junction is equal to the sum of the currents flowing away from that point. The second law states that, starting at any point in a circuit and following any closed path back to the starting point, the net sum of the voltage encountered will be equal to the net sum of the products of the resistances encountered and the currents flowing through them. In other words, Ohm's law applies not only to a circuit as a whole, but also to any given section of a circuit. D Series and Parallel Sources Sources of electric current can also be connected in various ways. Sources can be arranged in series by connecting a terminal of one source to the opposite terminal of the next source. For example, if the positive terminal of battery A is connected to the negative terminal of battery B, and the positive terminal of battery B to the negative terminal of battery C, then batteries A, B, and C are in series. The load is then placed between the positive terminal of battery C and the negative terminal of battery A. When sources of electric current are connected in series, their total voltage is equal to the sum of their individual voltages. For example, three 1.5-volt batteries connected in series furnish a total of 4.5 volts. If the load is 9 ohms, the batteries send a current of 4.5/9 = 0.5 amp through the load. Current sources may be arranged in parallel by connecting all the positive terminals together and all the negative terminals together. The load is then placed between the group of positive terminals and the group of negative terminals. Arranging sources in parallel does not increase the voltage. If three 1.5-volt batteries are connected in parallel, the total voltage is still 1.5 volts. Batteries should not be connected in parallel unless they have approximately the same voltage. If a high voltage battery is connected in parallel with a low voltage battery, the high voltage battery will force an electric current through the low voltage battery and damage it. VIII ELECTRIC FIELDS A single electric charge can attract or repel, and it will demonstrate this ability as soon as another charge is brought near it. The ability to attract or repel can be thought of as being stored in the region around the charge. This region is called the electric field of force of the charge. All charged objects have electric fields around them. A Lines of Force An electric field can be visualized as consisting of imaginary lines called lines of force. Each line corresponds to the path that a positive charge would take if placed in the field on that line. The lines in the field around a positively charged object radiate in all directions away from the object, since the object repels positive charges. Conversely, the lines in the field around a negatively charged object are directed toward the object. If a positive and a negative object are placed near each other, their lines of force connect. If two objects with similar charges are placed near each other, the lines do not connect. Lines of force never cross each other. Lines of force are only imaginary. Nevertheless, the idea of lines of force helps in visualizing an electric field. B Field Direction When a charge is placed at any given point in an electric field, it is acted on by a force that tends to push it in a certain direction. This direction is called the direction of the field at that point. The field direction can be represented graphically by the lines of force near an electric charge. C Field Strength The strength, or intensity, of a field at any point is defined as the force exerted on a charge of 1 coulomb placed at that point. For example, if a point charge of 1 coulomb is subjected to a force of 10 newtons, the electric field is 10 newtons per coulomb at that point. An object with a charge of 5 coulombs would be subjected to a force of 50 newtons at the same point. Field strength is represented graphically by the closeness (density) of the lines of force. Where the lines are close together, the field is strong. Where they are far apart, the field is weak. Near a charge, the field is strong and the lines are close together. At greater distances from the charge, the field weakens and the lines are not as close together. The field strength values that the lines represent are relative, since a field can be drawn with as many lines as desired. IX ELECTRICITY AND MAGNETISM Many similarities exist between electric and magnetic phenomena. A magnet has two opposite poles, referred to as north and south. Opposite magnetic poles attract each other, and similar magnetic poles repel each other, exactly as happens with electric charges. The force with which magnetic poles attract or repel each other depends on the strength of the poles and the distance between them. This relationship is similar to the Coulomb's inverse square law for electric charges. See also Magnetism. The similarities between electric and magnetic phenomena indicate that electricity and magnetism are related. Electricity produces magnetic effects and magnetism produces electric effects. The relationship between electricity and magnetism is called electromagnetism. See also Quantum Electrodynamics. A Magnetic Effects of Electricity It has been noted that an electric field exists around any electric charge. If electric charges are moving, they constitute an electric current. The magnetic effect of electricity is demonstrated by the fact that a magnetic field exists around any electric current. The field can be detected when a magnet is brought close to the currentcarrying conductor. The magnetic field around an electric current can be thought of as lines of magnetic force that form closed circular loops around the wire that carries the current. The direction of the magnetic field can be determined by a convenient rule called the right-hand rule. To apply this rule, the thumb of the right hand is pointed in the direction in which the current is flowing and the fingers are curled around the wire. The direction of the fingers then indicates the direction of the lines of magnetic force. (The right-hand rule assumes that current flows from positive to negative.) B Motor Effect As already stated, a magnetic field exists around a wire carrying an electric current, and a magnetic field exists between the two poles of a magnet. If the wire is placed between the poles, the magnetic fields interact to produce a force that tends to push the wire out of the field. This phenomenon, known as the motor effect, is used in electric motors. See also Electric Motors and Generators. C Solenoids If a wire is bent into many continuous loops to form a long spiral coil, then the magnetic lines of force tend to go through the center of the coil from one end to the other rather than around the individual loops of wire. Such a coil, called a solenoid, behaves in the same way as a magnet and is the basis for all electromagnets. The end from which the lines exit is the north pole and the end into which the lines reenter is the south pole. The polarity of the coil can be determined by applying the lefthand coil rule. If the left hand grasps the coil in such a way that the fingers curl around in the direction of the electron current, then the thumb points in the direction of the north pole. D Electric Effects of Magnetism If a wire is moved through a magnetic field in such a way that it cuts the magnetic lines of force, a voltage is created across the wire. An electric current will flow through the wire if the two ends of the wire are connected by a conductor to form a circuit. This current is called an induced current, and the induction of a current in this manner is called electromagnetic induction. It does not matter whether the wire moves or the magnetic field moves, provided that the wire cuts through lines of force. If a magnet is moved near a stationary wire, the lines of magnetic force are cut by the wire and an electric current is induced in the wire. Like any electric current, an induced current generates a magnetic field around it. Lenz's law expresses an important fact concerning this magnetic field: The motion of an induced current is always in such a direction that its magnetic field opposes the magnetic field that is causing the current. X ALTERNATING CURRENT An alternating current is an electric current that changes direction at regular intervals. When a conductor is moved back and forth in a magnetic field, the flow of current in the conductor will reverse direction as often as the physical motion of the conductor reverses direction. Most electric power stations supply electricity in the form of alternating currents. The current flows first in one direction, builds up to a maximum in that direction, and dies down to zero. It then immediately starts flowing in the opposite direction, builds up to a maximum in that direction, and again dies down to zero. Then it immediately starts in the first direction again. This surging back and forth can occur at a very rapid rate. Two consecutive surges, one in each direction, are called a cycle. The number of cycles completed by an electric current in one second is called the frequency of the current. In the United States and Canada, most currents have a frequency of 60 cycles per second. Although direct and alternating currents share some characteristics, some properties of alternating current are somewhat different from those of direct current. Alternating currents also produce phenomena that direct currents do not. Some of the unique traits of alternating current make it ideal for power generation, transmission, and use. A Amperage and Voltage The strength, or amperage, of an alternating current varies continuously between zero and a maximum. Since it is inconvenient to take into account a whole range of amperage values, scientists simply deal with the effective amperage. Like a direct current, an alternating current produces heat as it passes through a conductor. The effective amperage of an alternating current is equal to the amperage of a direct current that produces heat at the same rate. In other words, 1 effective amp of alternating current through a conductor produces heat at the same rate as 1 amp of direct current flowing through the same conductor. Similarly, the voltage of an alternating current is considered in terms of the effective voltage. B Impedance Like direct current, alternating current is hindered by the resistance of the conductor through which it passes. In addition, however, various effects produced by the alternating current itself hinder the alternating current. These effects depend on the frequency of the current and on the design of the circuit, and together they are called reactance. The total hindering effect on an alternating current is called impedance. It is equal to the resistance plus the reactance. The relationship of effective current, effective voltage, and impedance is expressed by V = IZ, where V is the effective voltage in volts, I is the effective current in amperes (amp), and Z is the impedance in ohms. C Advantages of Alternating Current Alternating current has several characteristics that make it more attractive than direct current as a source of electric power, both for industrial installations and in the home. The most important of these characteristics is that the voltage or the current may be changed to almost any value desired by means of a simple electromagnetic device called a transformer. When an alternating current surges back and forth through a coil of wire, the magnetic field about the coil expands and collapses and then expands in a field of opposite polarity and again collapses. In a transformer, a coil of wire is placed in the magnetic field of the first coil, but not in direct electric connection with it. The movement of the magnetic field induces an alternating current in the second coil. If the second coil has more turns than the first, the voltage induced in the second coil will be larger than the voltage in the first, because the field is acting on a greater number of individual conductors. Conversely, if there are fewer turns in the second coil, the secondary, or induced, voltage will be smaller than the primary voltage. The action of a transformer makes possible the economical transmission of electric power over long distances. If 200,000 watts of power is supplied to a power line, it may be equally well supplied by a potential of 200,000 volts and a current of 1 amp or by a potential of 2,000 volts and a current of 100 amp, because power is equal to the product of voltage and current. The power lost in the line through heating, however, is equal to the square of the current times the resistance. Thus, if the resistance of the line is 10 ohms, the loss on the 200,000-volt line will be 10 watts, whereas the loss on the 2,000-volt line will be 100,000 watts, or half the available power. Accordingly, power companies tend to favor high voltage lines for long distance transmission. XI HISTORY Humans have known about the existence of static electricity for thousands of years, but scientists did not make great progress in understanding electricity until the 1700s. A Early Theories The ancient Greeks observed that amber, when rubbed, attracted small, light objects. About 600 BC Greek philosopher Thales of Miletus held that amber had a soul, since it could make other objects move. In a treatise written about three centuries later, another Greek philosopher, Theophrastus, stated that other substances also have this power. For almost 2,000 years after Theophrastus, little progress was made in the study of electricity. In 1600 English physician William Gilbert published a book in which he noted that many substances besides amber could be charged by rubbing. He gave these substances the Latin name electrica, which is derived from the Greek word elektron (which means "amber"). The word electricity was first used by English writer and physician Sir Thomas Browne in 1646. The fact that electricity can flow through a substance was discovered by 17th-century German physicist Otto von Guericke, who observed conduction in a linen thread. Von Guericke also described the first machine for producing an electric charge in 1672. The machine consisted of a sulfur sphere turned by a crank. When a hand was held against the sphere, a charge was induced on the sphere. Conduction was rediscovered independently by Englishman Stephen Gray during the early 1700s. Gray also noted that some substances are good conductors while others are insulators. Also during the early 1700s, Frenchman Charles Dufay observed that electric charges are of two kinds. He found that opposite kinds attract each other while similar kinds repel. Dufay called one kind vitreous and the other kind resinous. American scientist Benjamin Franklin theorized that electricity is a kind of fluid. According to Franklin's theory, when two objects are rubbed together, electric fluid flows from one object to the other. The object that gains electric fluid acquires a vitreous charge, which Franklin called positive charge. The object that loses electric fluid acquires a resinous charge, which Franklin called negative charge. Franklin demonstrated that lightning is a form of electricity. In 1752 he constructed a kite and flew it during a storm. When the string became wet enough to conduct, Franklin, who stood under a shed and held the string by a dry silk cord, put his hand near a metal key attached to the string. A spark jumped. Electric charge gathered by the kite had flowed down the wet string to the key and then jumped across an air gap to flow to the ground through Franklin's body. Franklin also showed that a Leyden jar, a device able to store electric charge, could be charged by touching it to the key when electric current was flowing down the string. Around 1766 British chemist Joseph Priestley proved experimentally that the force between electric charges varies inversely with the square of the distance between the charges. Priestley also demonstrated that an electric charge distributes itself uniformly over the surface of a hollow metal sphere and that no charge and no electric field of force exists within such a sphere. French physicist Charles Augustin de Coulomb reinvented a torsion balance to measure accurately the force exerted by electric charges. With this apparatus he confirmed Priestley's observations and also showed that the force between two charges is proportional to the product of the individual charges. In 1791 Italian biologist Luigi Galvani published the results of experiments that he had performed on the muscles of dead frogs. Galvani had found earlier that the muscles in a frog's leg would contract if he applied an electric current to them. B 19th and 20th Centuries In 1800 another Italian scientist, Alessandro Volta, announced that he had created the voltaic pile, a form of electric battery. The voltaic pile made the study of electric current much easier by providing a reliable, steady source of current. Danish physicist Hans Christian Oersted demonstrated that electric currents are surrounded by magnetic fields in 1819. Shortly afterward, André Marie Ampère discovered the relationship known as Ampere's law, which gives the direction of the magnetic field. Ampère also demonstrated the magnetic properties of solenoids. Georg Simon Ohm, a German high school teacher, investigated the conducting abilities of various metals. In 1827 Ohm published his results, including the relationship now known as Ohm's law. In 1830 American physicist Joseph Henry discovered that a moving magnetic field induces an electric current. The same effect was discovered a year later by English scientist Michael Faraday. Faraday introduced the concept of lines of force, a concept that proved extremely useful in the study of electricity. About 1840 British physicist James Prescott Joule and German scientist Hermann Ludwig Ferdinand von Helmholtz demonstrated that electricity is a form of energy and that electric circuits obey the law of the conservation of energy. Also during the 19th century, British physicist James Clerk Maxwell investigated the properties of electromagnetic waves and light and developed the theory that the two are identical. Maxwell summed up almost all the laws of electricity and magnetism in four mathematical equations. His work paved the way for German physicist Heinrich Rudolf Hertz, who produced and detected electric waves in the atmosphere in 1886, and for Italian engineer Guglielmo Marconi, who harnessed these waves in 1895 to produce the first practical radio signaling system. The electron theory, which is the basis of modern electrical theory, was first advanced by Dutch physicist Hendrik Antoon Lorentz in 1892. American physicist Robert Andrews Millikan accurately measured the charge on the electron in 1909. The widespread use of electricity as a source of power is largely due to the work of pioneering American engineers and inventors such as Thomas Alva Edison, Nikola Tesla, and Charles Proteus Steinmetz during the late 19th and early 20th centuries. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« electrons in the neutral object are attracted to the positive object.

Some of these electrons flow to the side of the neutral object that is nearest to the positive object.This side of the neutral object accumulates electrons and becomes negatively charged.

Because electrons leave the far side of the neutral object while its protonsremain stationary, that side becomes positively charged. Since the negatively charged side of the neutral object is closest to the positive object, the attraction between this side and the positive object is greater than therepulsion between the positively charged side and the positive object.

The net effect is an attraction between the objects.

Similarly, when a negatively charged object isbrought near a neutral object, the negative object induces a positive charge on the near side of the neutral object and a negative charge on the far side.

As before, thenet effect is an attraction between the objects. The induced charges described above are not permanent.

As soon as the charged object is taken away, the electrons on the other object redistribute themselves evenlyover it, so that it again becomes neutral. An object can also be charged permanently by induction.

If a negatively charged object, A, is brought near a neutral object, B, the electrons on B are repelled as far aspossible from A and flow to the other side of B.

If that side of B is then connected to the ground by a good conductor, such as a metal wire, the electrons flow outthrough the wire into the ground.

The ground can receive almost any amount of charge because Earth, being neutral, has an enormous capacitance.

Object B is said tobe grounded by the wire connecting it to Earth. If this wire is then removed, B has a positive charge, since it has lost electrons to Earth.

Thus B has been permanently charged by induction.

Even if A is subsequentlyremoved, B still remains positive because the wire has been disconnected and B cannot regain electrons from Earth to neutralize its positive charge. IV ELECTRIC CURRENT An electric current is a movement of charge.

When two objects with different charges touch and redistribute their charges, an electric current flows from one object tothe other until the charge is distributed according to the capacitances of the objects.

If two objects are connected by a material that lets charge flow easily, such as acopper wire, then an electric current flows from one object to the other through the wire.

Electric current can be demonstrated by connecting a small light bulb to anelectric battery by two copper wires.

When the connections are properly made, current flows through the wires and the bulb, causing the bulb to glow. Current that flows in one direction only, such as the current in a battery-powered flashlight, is called direct current.

Current that flows back and forth, reversingdirection again and again, is called alternating current.

Direct current, which is used in most battery-powered devices, is easier to understand than alternating current.Most of the following discussion focuses on direct current.

Alternating current, which is used in most devices that are “plugged in” to electrical outlets in buildings, will bediscussed in the Alternating Current section of this article. Other properties that are used to quantify and compare electric currents are the voltage (also called electromotive force) driving the current and the resistance of theconductor to the passage of the current.

The amount of current, voltage, and resistance in any circuit are all related through an equation called Ohm’s law. A Conductors and Insulators Conductors are materials that allow an electric current to flow through them easily.

Most metals are good conductors. Substances that do not allow electric current to flow through them are called insulators, nonconductors, or dielectrics.

Rubber, glass, and air are common insulators.Electricians wear rubber gloves so that electric current will not pass from electrical equipment to their bodies.

However, if an object contains a sufficient amount ofcharge, the charge can arc, or jump, through an insulator to another object.

For example, if you shuffle across a wool rug and then hold your finger very close to, butnot in contact with, a metal doorknob or radiator, current will arc through the air from your finger to the doorknob or radiator, even though air is an insulator.

In thedark, the passage of the current through the air is visible as a tiny spark. B Measuring Electric Current Electric current is measured in units called amperes (amp).

If 1 coulomb of charge flows past each point of a wire every second, the wire is carrying a current of 1 amp.If 2 coulombs flow past each point in a second, the current is 2 amp.

See also Electric Meters. C Voltage When the two terminals of a battery are connected by a conductor, an electric current flows through the conductor.

One terminal continuously sends electrons into theconductor, while the other continuously receives electrons from it.

The current flow is caused by the voltage, or potential difference, between the terminals.

The morewilling the terminals are to give up and receive electrons, the higher the voltage.

Voltage is measured in units called volts.

Another name for a voltage produced by asource of electric current is electromotive force. D Resistance A conductor allows an electric current to flow through it, but it does not permit the current to flow with perfect freedom.

Collisions between the electrons and the atomsof the conductor interfere with the flow of electrons.

This phenomenon is known as resistance.

Resistance is measured in units called ohms.

The symbol for ohms is theGreek letter omega, Ω. A good conductor is one that has low resistance.

A good insulator has a very high resistance.

At commonly encountered temperatures, silver is the best conductor andcopper is the second best.

Electric wires are usually made of copper, which is less expensive than silver. The resistance of a piece of wire depends on its length, and its cross-sectional area, or thickness.

The longer the wire is, the greater its resistance.

If one wire is twice aslong as a wire of identical diameter and material, the longer wire offers twice as much resistance as the shorter one.

A thicker wire, however, has less resistance,because a thick wire offers more room for an electric current to pass through than a thin wire does.

A wire whose cross-sectional area is twice that of another wire ofequal length and similar material has only half the resistance of the thinner wire.

Scientists describe this relationship between resistance, length, and area by sayingthat resistance is proportional to length and inversely proportional to cross-sectional area. Usually, the higher the temperature of a wire, the greater its resistance.

The resistance of some materials drops to zero at very low temperatures.

This phenomenon isknown as superconductivity. E Ohm’s Law. »

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