electric force or Coulomb force Force between two electric charges. The magnitude of the force F is proportional to the product of the two charges, q 1 and q 2 , divided by the square of the distance r between them, or F = kq^q-Jr 1 , where k is a constant that depends on the measurement system being used. The electric force can be one of repulsion, such as the force between two objects having like charges, or it can be attractive, such as the force between two objects having opposite charges.
electric furnace Chamber heated with electricity to very high tem¬ peratures, for melting and alloying metals and refractories. Modern elec¬ tric furnaces generally are either arc furnaces or induction furnaces. Arc furnaces produce roughly two-fifths of the steel made in the U.S. In the induction furnace, a coil carrying alternating electric current surrounds the container or chamber of metal; circulating eddy currents induced in the metal produce extremely high temperatures.
electric potential Amount of work needed to move a unit electric charge from a reference point to a specific point against an electric field. The potential energy of a positive charge increases when it moves against an electric field, and decreases when it moves with the field. Electric potential can be thought of as potential energy per unit charge. The work done in moving a unit charge from one point to another, as in an electric circuit, is equal to the difference in potential energies at each point. Elec¬ tric potential is expressed in units of joules per coulomb, or volts.
electric ray Any of the aquatic rays (families Torpedinidae, Narkidae, and Temeridae) that produce an electrical shock. They are found world¬ wide in warm and temperate seas, mostly in shallow water but some (genus Benthobatis) at depths greater than 3,000 ft (900 m). Slow-moving bottom-dwellers, they feed on fishes and invertebrates. They range in length from less than 1 ft (30 cm) to about 6 ft (1.8 m) and have a short, stout tail. They are soft and smooth-skinned, with a circular or nearly cir¬ cular body disk formed by the head and pectoral fins. They are harmless unless touched or stepped on. The electric organs, composed of modified muscle tissue, are in the disk near the head. The shock from these organs, which may reach 220 volts and is strong enough to fell a human adult, is used for defense, sensory location, and capturing prey.
Electric ray (Narcine brasiliensis)
DOUGLAS FAULKNER
electric shock Physical effect of an electric current that enters the body, ranging from a minor static-electricity discharge to a power-line accident or lightning strike but most often resulting from house current. The effects
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608 I electrical engineering ► electromagnetic field
depend on the current (not the voltage), and the worst damage occurs along its path from the entry to the exit point. Causes of immediate death are ventricular fibrillation and paralysis of the brain’s breathing centre or of the heart. Cardiopulmonary resuscitation is the best first aid. Though most survivors recover completely, aftereffects may include cataract, angina pectoris, or nervous-system disorders.
electrical engineering Branch of engineering concerned with the practical applications of electricity in all its forms, including those of elec¬ tronics. Electrical engineering deals with electric light and power systems and apparatuses; electronics engineering deals with wire and radio com¬ munication, the stored-program electronic computer, radar, and automatic control systems. The first practical application of electricity was the tele¬ graph, in 1837. Electrical engineering emerged as a discipline in 1864 when James Clerk Maxwell summarized the basic laws of electricity in mathematical form and predicted that radiation of electromagnetic energy would occur in a form that later became known as radio waves. The need for electrical engineers was not felt until the invention of the telephone (1876) and the incandescent lamp (1878).
electrical impedance \im-'pe-d 3 ns\ Opposition that a circuit presents to electric current. It includes both resistance and reactance. Resistance arises from collisions of the current-carrying charged particles with the internal structure of the conductor. Reactance is an additional opposition to the movement of electric charge that arises from the changing electric and magnetic fields in circuits carrying alternating current. Impedance in circuits carrying steady direct currents is simply resistance. The magni¬ tude of the impedance Z of a circuit is equal to the maximum value of the potential difference, or voltage V, across the circuit, divided by the maximum value of the current I through the circuit, or simply Z = VII. The unit of impedance is the ohm.
electricity Phenomenon associated with stationary or moving electric charges. The word comes from the Greek elektron (“amber”); the Greeks discovered that amber rubbed with fur attracted light objects such as feathers. Such effects due to stationary charges, or static electricity, were the first electrical phenomena to be studied. Not until the early 19th cen¬ tury were static electricity and electric current shown to be aspects of the same phenomenon. The discovery of the electron, which carries a charge designated as negative, showed that the various manifestations of elec¬ tricity are the result of the accumulation or motion of numbers of elec¬ trons. The invention of the incandescent lightbulb (1879) and the construction of the first central power station (1881) by Thomas Alva Edi¬ son led to the rapid introduction of electric power into factories and homes. See also James Clerk AAaxwell.
electrification, rural See rural electrification
electrocardiography \i-,lek-tro- l kard-e-'ag-r3-fe\ Method of tracing the electric current of a heartbeat to provide information on the heart. Electrocardiograms (ECGs) are made by applying electrodes, usually to the arms, legs, and chest wall, attached to an electrocardiograph, which records the tiny heart current. Upward and downward movements on the tracing reflect contractions of the atria and ventricles. Deviations from a standard point to a possible heart disorder and its site, as well as to pos¬ sible high blood pressure and other diseases.
electrochemistry Branch of chemistry concerned with the relation between electricity and chemical change. Many spontaneous chemical reac¬ tions liberate electrical energy, and some of these reactions are used in batteries and fuel cells to produce electric power. Conversely, electric cur¬ rent can bring about many reactions that do not occur spontaneously. In the process called electrolysis, electrical energy is converted directly into chemical energy, which is stored in the products of the reaction. This pro¬ cess is applied in refining metals, in electroplating, and in producing hydrogen and oxygen from water. Passage of electricity through a gas generally causes chemical changes, a subject that forms a separate branch of electrochemistry. See also oxidation-reduction.
electroconvulsive therapy formerly shock therapy Method of treating psychiatric disorders by inducing shock through electric current. Electroconvulsive, or electroshock, therapy involves passing an electric current through the patient’s head between two electrodes placed over the temples and thus causing a convulsive seizure; it was used to treat bipolar disorder and other types of depression. Shock was previously induced by administering increasingly large doses of insulin until the patient was thrown into a brief coma; the so-called insulin-shock therapy was used
for the treatment of schizophrenia. Both forms of shock therapy were devel¬ oped in the 1930s. Their use declined after the introduction of tranquil- izing drugs and antidepressants.
electrocution Method of execution in which the condemned person is subjected to a heavy charge of electric current. The prisoner is shackled into a wired chair, and electrodes are fastened to the head and one leg so that the current will flow through the body. One electrical shock may not be enough to kill the person; if a doctor does not confirm the death, sev¬ eral shocks may be applied. The electric chair was first used in 1890. Electrocution also refers to death by other causes of electrical shock (e.g., accidental contact with high-voltage wiring).