Electricity

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.