I.

Introduction

Electricity, all the phenomena that result from the interaction of electrical charges. Electric and magnetic effects are caused by the relative positions and movements of charged particles of matter. When a charge is stationary (static), it produces electrostatic forces on charged objects, and when it is in motion it produces additional magnetic effects. So far as electrical effects are concerned, objects can be electrically neutral, positively charged, or negatively charged. Positively charged particles, such as the protons that are found in the nucleus of atoms, repel one another. Negatively charged particles, such as the electrons that are found in the outer parts of atoms, also repel one another (see Atom). Negative and positive particles, however, attract each other. This behaviour may be summed up as: like charges repel, and unlike charges attract.

II.

Electrostatics

The electric charge on a body is measured in coulombs (see Electrical Units; International System of Units). The force between particles bearing charges q₁ and q₂ can be calculated by Coulomb’s law, This equation states that the force is proportional to the product of the charges, divided by the square of the distance that separates them. The charges exert equal forces on one another. This is an instance of the law that every force produces an equal and opposite reaction. (see Mechanics: Newton’s Three Laws of Motion.) The term p is the Greek letter pi, standing for the number 3.1415..., which crops up repeatedly in geometry. The term e is the Greek letter epsilon, standing for a quantity called the absolute permittivity, which depends on the medium surrounding the charges. This law is named after the French physicist Charles Augustin de Coulomb, who developed the equation.

Every electrically charged particle is surrounded by a field of force. This field may be represented by lines of force showing the direction of the electrical forces that would be experienced by an imaginary positive test charge within the field. To move a charged particle from one point in the field to another requires that work be done or, equivalently, that energy be transferred to the particle. The amount of energy needed for a particle bearing a unit charge is known as the potential difference between these two points. The potential difference is usually measured in volts (symbol V). The Earth, a large conductor that may be assumed to be substantially uniform electrically, is commonly used as the zero reference level for potential energy. Thus the potential of a positively charged body is said to be a certain number of volts above the potential of the Earth, and the potential of a negatively charged body is said to be a certain number of volts below it.

A.

Electrical Properties of Solids

The first artificial electrical phenomenon to be observed was the property displayed by certain resinous substances such as amber, which become negatively charged when rubbed with a piece of fur or woollen cloth and then attract small objects. Such a body has an excess of electrons. A glass rod rubbed with silk has a similar power; however, the glass has a positive charge, owing to a deficiency of electrons. The charged amber and glass even attract uncharged bodies (see Electric Charges below).

Protons lie at the heart of the atom and are effectively fixed in position in solids. When charge moves in a solid, it is carried by the negatively charged electrons. Electrons are easily liberated in some materials, which are known as conductors. Metals, particularly copper and silver, are good conductors. see Conductor, Electrical.

Materials in which the electrons are tightly bound to the atoms are known as insulators, non-conductors, or dielectrics. Glass, rubber, and dry wood are examples of these materials.

A third kind of material is called a semiconductor, because it generally has a higher resistance to the flow of current than a conductor such as copper, but a lower resistance than an insulator such as glass. In one kind of semiconductor, most of the current is carried by electrons, and the semiconductor is called n-type. In an n-type semiconductor, a relatively small number of electrons can be freed from their atoms in such a manner as to leave a “hole” where each electron had been. The hole, representing the absence of a negative electron, is a positively charged ion (incomplete atom). An electric field will cause the negative electrons to flow through the material while the positive holes remain fixed. In a second type of semiconductor, the holes move, while electrons hardly move at all. When most of the current is carried by the positive holes, the semiconductor is said to be p-type.

If a material were a perfect conductor, a charge would pass through it without resistance, while a perfect insulator would allow no charge to be forced through it. No substance of either type is known to exist at room temperature. The best conductors at room temperature offer a low (but non-zero) resistance to the flow of current. The best insulators offer a high (but not infinite) resistance at room temperature. Most metals, however, lose all their resistance at temperatures near absolute zero; this phenomenon is called superconductivity.

B.

Electric Charges

One quantitative tool used to demonstrate the presence of electric charges is the electroscope. This device also indicates whether the charge is negative or positive and detects the presence of radiation. The device, in the form first used by the British physicist and chemist Michael Faraday, is shown in Figure 1. The electroscope consists of two leaves of thin metal foil (a,a_{_}) suspended from a metal support (b) inside a glass or other non-conducting container (c). A knob (d) collects the electric charges, either positive or negative, and these are conducted along the metal support and travel to both leaves. The like charges repel one another and the leaves fly apart, the distance between them depending roughly on the quantity of charge.

Three methods may be used to charge an object electrically: (1) by contact with another object of a different material (for example, touching amber to fur), followed by separation; (2) by contact with another charged body; and (3) by induction.

Electrical induction is shown in Figure 2. A negatively charged body, A, is placed between a neutral conductor, B, and a neutral non-conductor, C. The free electrons in the conductor are repelled to the side of the conductor away from A, leaving a net positive charge at the nearer side. The entire body B is attracted towards A, because the attraction of the unlike charges that are close together is greater than the repulsion of the like charges that are farther apart. As stated above, the forces between electrical charges vary inversely according to the square of the distance between the charges. In the non-conductor, C, the electrons are not free to move, but the atoms or molecules of the non-conductor are stretched and reoriented so that their constituent electrons are as far as possible from A; the non-conductor is therefore also attracted to A, but to a lesser extent than the conductor.

The movement of electrons in the conductor B of Figure 2 and the reconfiguration of the atoms of the non-conductor C give these bodies positive charges on the sides nearest A and negative charges on the sides away from A. Charges produced in this manner are called induced charges and the process of producing them is called induction.

III.

Electrical Measurements

The flow of charge in a wire is called current. It is expressed in terms of the number of coulombs per second going past a given point on a wire. One coulomb/sec equals 1 ampere (symbol A), a unit of electric current named after the French physicist André Marie Ampère. See Current Electricity below.

When 1 coulomb of charge travels across a potential difference of 1 volt, the work done equals 1 joule, a unit named after the English physicist James Prescott Joule. This definition facilitates transitions from mechanical to electrical quantities.

A widely used unit of energy in atomic physics is the electronvolt (eV). This is the amount of energy gained by an electron that is accelerated by a potential difference of 1 volt. This is a small unit and is frequently multiplied by 1 million or 1 billion, the result being abbreviated to 1 MeV or 1 GeV, respectively.

IV.

Current Electricity

If two equally and oppositely charged bodies are connected by a metallic conductor such as a wire, the charges neutralize each other. This neutralization is accomplished by means of a flow of electrons through the conductor from the negatively charged body to the positively charged one. (Electric current is often conventionally assumed to flow in the opposite direction—that is, from positive to negative; nevertheless, a current in a wire consists only of moving negatively charged electrons.) In any continuous system of conductors, electrons will flow from the point of lowest potential to the point of highest potential. A system of this kind is called an electric circuit. The current flowing in a circuit is described as direct current (DC) if it flows continuously in one direction, and as alternating current (AC) if it flows alternately in each direction.

Three interdependent quantities characterize direct current. The first is the potential difference in the circuit, which is sometimes called the electromotive force (emf) or voltage. The second is the rate of current flow. This quantity is usually given in terms of the ampere, which corresponds to a flow of about 6.24 × 10¹⁸ electrons per second past any point of the circuit. The third quantity is the resistance of the circuit. Under ordinary conditions all substances, conductors as well as non-conductors, offer some opposition to the flow of an electric current, and this resistance necessarily limits the current. The unit used for expressing the quantity of resistance is the ohm, which is defined as the amount of resistance that will limit the flow of current to 1 ampere in a circuit with a potential difference of 1 volt. The symbol for the ohm is the Greek letter Ω, omega. The relationship may be stated in the form of the algebraic equation E = I × R, in which E is the electromotive force in volts, I is the current in amperes, and R is the resistance in ohms. From this equation any of the three quantities for a given circuit can be calculated if the other two quantities are known. Another formulation is I = E/R. see Electric Circuit; Electric Meters.

Ohm’s law is the generalization that for many materials over a wide range of circumstances, R is constant. It is named after the German physicist Georg Simon Ohm, who discovered the law in 1827.

When an electric current flows through a wire, two important effects can be observed: the temperature of the wire is raised, and a magnet or a compass needle placed near the wire will be deflected, tending to point in a direction perpendicular to the wire. As the current flows, the electrons making up the current collide with the atoms of the conductor and give up energy, which appears in the form of heat. The amount of energy expended in an electric circuit is expressed in terms of the joule. Power is expressed in terms of the watt, which is equal to 1 J/sec. The power expended in a given circuit can be calculated from the equation P = E × I or P = I ² × R. Power may also be expended in doing mechanical work, in producing electromagnetic radiation such as light or radio waves, and in chemical decomposition.

V.

Electromagnetism

The movement of a compass needle near a conductor through which a current is flowing indicates the presence of a magnetic field (see Magnetism) around the conductor. When currents flow through two parallel conductors in the same direction, the magnetic fields cause the conductors to attract each other; when the flows are in opposite directions, they repel each other. The magnetic field caused by the current in a single loop or wire is such that the loop will behave like a magnet or compass needle and swing until it is perpendicular to a line running from the north magnetic pole to the south.

The magnetic field about a current-carrying conductor can be visualized as encircling the conductor. The direction of the magnetic lines of force in the field is anticlockwise when observed in the direction in which the electrons are moving. The field is stationary so long as the current is flowing steadily through the conductor.

When a moving conductor cuts the lines of force of a magnetic field, the field acts on the free electrons in the conductor, displacing them and causing a potential difference and a flow of current in the conductor. The same effect occurs whether the magnetic field is stationary and the wire moves, or the field moves and the wire is stationary.

When a current increases in strength, the field increases in strength, and the circular lines of force may be imagined to expand from the conductor. These expanding lines of force cut the conductor itself and induce a current in it in the direction opposite to the original flow. With a conductor such as a straight piece of wire this effect is very slight, but if the wire is wound into a helical coil the effect is much increased, because the fields from the individual turns of the coil cut the neighbouring turns and induce a current in them as well. The result is that such a coil, when connected to a source of potential difference, will impede the flow of current when the potential difference is first applied.

Similarly, when the source of potential difference is removed the magnetic field “collapses”, and again the moving lines of force cut the turns of the coil. The current induced under these circumstances is in the same direction as the original current, and the coil tends to maintain the flow of current. Because of these properties, a coil resists any change in the flow of current and is said to possess electrical inertia, or inductance. This inertia has little importance in DC circuits, because it is not observed when current is flowing steadily, but it has great importance in AC circuits. See Alternating Currents below.

VI.

Conduction in Liquids and Gases

When an electric current flows in a metallic conductor, the flow of particles 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). In a liquid solution, the positive ions move from higher potential to lower; the negative ions move in the opposite direction. Similarly, in gases that have been ionized by radioactivity, by the ultraviolet rays of sunlight, by electromagnetic waves, or by a strong electric field, a two-way drift of ions takes place to produce an electric current through the gas. see Electric Arc; Electric Lighting.

VII.

Sources of Electromotive Force

To produce a flow of current in any electrical circuit, a source of electromotive force or potential difference is necessary. The available sources are: (1) electrostatic machines such as the Van de Graaff generator, which operate on the principle of inducing electric charges by mechanical means ; (2) electromagnetic machines, which generate current by mechanically moving conductors through a magnetic field or a number of fields (see Electric Motors and Generators); (3) batteries, which produce an electromotive force through electrochemical action; (4) devices that produce electromotive force through the action of heat (see Crystal: Other Crystal Properties; Thermoelectricity); (5) devices that produce electromotive force by the photoelectric effect, the action of light; and (6) devices that produce electromotive force by means of physical pressure—the piezoelectric effect.

VIII.

Alternating Currents

When a conductor is moved back and forth in a magnetic field, the flow of current in the conductor will change direction as often as the physical motion of the conductor changes direction. Several electricity-generating devices operate on this principle, and the oscillating current produced is called alternating current. Alternating current has several valuable characteristics, as compared to direct current, and is generally used as a source of electric power, both for industrial installations and in the home. The most important practical characteristic of alternating current 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 passes through a coil of wire, the magnetic field about the coil first expands and then collapses, then expands with its direction reversed, and again collapses. If another conductor, such as a coil of wire, is placed in this field, but not in direct electric connection with the coil, the changes of the field induce an alternating current in the second conductor. If the second conductor is a coil with a larger number of 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 the number of turns in the second coil is smaller, the secondary, or induced, voltage will be smaller than the primary voltage.

The action of a transformer makes possible the economical transmission of current over long distances in electric power systems (see Electricity Supply). 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 ampere or by a potential of 2,000 volts and a current of 100 amperes, because power is equal to the product of voltage and current. However, the power lost in the line through heating 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.

The magnetic field surrounding a coil in an AC circuit is constantly changing, and constantly impedes the flow of current in the circuit because of the phenomenon of inductance mentioned above. The relationship between the voltage impressed on an ideal coil (that is, a coil having no resistance) and the current flowing in it is such that the current is zero when the voltage is at a maximum, and the current is at a maximum when the voltage is zero. Furthermore, the changing magnetic field induces a potential difference in the coil, called a back emf, that is equal in magnitude and opposite in direction to the impressed potential difference. So the net potential difference across an ideal coil is always zero, as it must necessarily be in any circuit element with zero resistance.

If a capacitor (or condenser), a charge-storage device, is placed in an AC circuit, the current is proportional to its capacitance and to the rate of change of the voltage across the capacitor. Therefore, twice as much current will flow through a 2-farad capacitor as through a 1-farad capacitor. In an ideal capacitor the voltage is exactly out of phase with the current. No current flows when the voltage is at its maximum because then the rate of change of voltage is zero. The current is at its maximum when the voltage is zero, because then the rate of change of voltage is maximal. Current may be regarded as flowing through a capacitor even if there is no direct electrical connection between its plates; the voltage on one plate induces an opposite charge on the other, so, when electrons flow into one plate, an equal number always flow out of the other. From the point of view of the external circuit, it is precisely as if electrons had flowed straight through the capacitor.

It follows from the above effects that if an alternating voltage were applied to an ideal inductance or capacitance, no power would be expended over a complete cycle. In all practical cases, however, AC circuits contain resistance as well as inductance and capacitance, and power is actually expended. The amount of power depends on the relative amounts of the three quantities present in the circuits.

IX.

History

The fact that amber acquires the power to attract light objects when rubbed may have been known to the Greek philosopher Thales of Miletus, who lived about 600 bc. Another Greek philosopher, Theophrastus, in a treatise written about three centuries later, stated that this power is possessed by other substances. The first scientific study of electrical and magnetic phenomena, however, did not appear until ad 1600, when the researches of the English doctor William Gilbert were published. Gilbert was the first to apply the term electric (Greek elektron, “amber”) to the force that such substances exert after rubbing. He also distinguished between magnetic and electric action.

The first machine for producing an electric charge was described in 1672 by the German physicist Otto von Guericke. It consisted of a sulphur sphere turned by a crank on which a charge was induced when the hand was held against it. The French scientist Charles François de Cisternay Du Fay was the first to make clear the two different types of electric charge: positive and negative. The earliest form of condenser, the Leyden jar, was developed in 1745. It consisted of a glass bottle with separate coatings of tinfoil on the inside and outside. If either tinfoil coating was charged from an electrostatic machine, a violent shock could be obtained by touching both foil coatings at the same time.

Benjamin Franklin spent much time in electrical research. His famous kite experiment proved that the atmospheric electricity that causes the phenomena of lightning and thunder is identical with the electrostatic charge on a Leyden jar. Franklin developed a theory that electricity is a single “fluid” existing in all matter, and that its effects can be explained by excesses and shortages of this fluid.

The law that the force between electric charges varies inversely with the square of the distance between the charges was proved experimentally by the British chemist Joseph Priestley about 1766. 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. Coulomb invented a torsion balance to measure accurately the force exerted by electrical charges. With this apparatus he confirmed Priestley’s observations and showed that the force between two charges is also proportional to the product of the individual charges. Faraday, who made many contributions to the study of electricity in the early 19th century, was also responsible for the theory of lines of electrical force.

The Italian physicists Luigi Galvani and Alessandro Volta conducted the first important experiments in electrical currents. Galvani produced muscle contraction in the legs of frogs by applying an electric current to them. In 1800 Volta demonstrated the first electric battery. The fact that a magnetic field exists around an electric current was demonstrated by the Danish scientist Hans Christian Oersted in 1819, and in 1831 Faraday proved that a current flowing in a coil of wire can induce electromagnetically a current in a nearby coil. About 1840 James Prescott Joule and the German scientist Hermann von Helmholtz demonstrated that electric circuits obey the law of conservation of energy and that electricity is a form of energy.

An important contribution to the study of electricity in the 19th century was the work of the British mathematical physicist James Clerk Maxwell, who proposed the idea of electromagnetic radiation and developed the theory that light consists of such radiation. His work paved the way for the German physicist Heinrich Hertz, who produced and detected electromagnetic waves in 1886, and for the Italian engineer Guglielmo Marconi, who in 1896 harnessed these waves to produce the first practical radio signalling system.

The electron theory, which is the basis of modern electrical theory, was first advanced by the Dutch physicist Hendrik Antoon Lorentz in 1892. The charge on the electron was first accurately measured by the American physicist Robert Andrews Millikan in 1909. The widespread use of electricity as a source of power is largely due to the work of such pioneering American engineers and inventors as Thomas Alva Edison, Nikola Tesla, and Charles Proteus Steinmetz. See also Electronics.