Electric Circuit

electric circuit, unbroken path along which an electric current exists or is intended or able to flow. A simple circuit might consist of an electric cell (the power source), two conducting wires (one end of each being attached to each terminal of the cell), and a small lamp (the load) to which the free ends of the wires leading from the cell are attached. When the connections are made properly, current flows, the circuit is said to be "closed," and the lamp will light. The current flows from the cell along one wire to the lamp, through the lamp, and along the other wire back to the cell. When the wires are disconnected, the circuit is said to be "open" or "broken." In practice, circuits are opened by such devices as switches, fuses, and circuit breakers (see fuse, electric; circuit breaker; short circuit). Two general circuit classifications are series and parallel. The elements of a series circuit are connected end to end; the same current flows through its parts one after another. The elements of a parallel circuit are connected so that each component has the same voltage across its terminals; the current flow is divided among its parts. When two circuit elements are connected in series, their effective resistance (impedance if the circuit is being fed alternating current) is equal to the sum of the separate resistances; the current is the same in each component throughout the circuit. When circuit elements are connected in parallel, the total resistance is less than that of the element having the least resistance, and the total current is equal to the sum of the currents in the individual branches. A battery-powered circuit is an example of a direct-current circuit; the voltages and currents are constant in magnitude and do not vary with time. In alternating-current circuits, the voltage and current periodically reverse direction with time. A standard electrical outlet supplies alternating current. Lighting circuits and electrical machinery use alternating current circuits. Many other devices, including computers, stereo systems, and television sets, must first convert the alternating current to direct current. That is done by a special internal circuit usually called a power supply. A digital circuit is a special kind of electronic circuit used in computers and many other devices. Magnetic circuits are analogous to electric circuits, where magnetic materials are regarded as conductors of magnetic flux. Magnetic circuits can be part of an electric circuit; a transformer is an example. Equivalent circuits are used in circuit analysis as a modeling tool; a simple circuit made up of a resistor, and an inductor might be used to electrically represent a loudspeaker. Electrical circuits can also be used in other fields of studies. In the study of heat flow, for example, a resistor is used to represent thermal insulation. Operating electric circuits can be used for general problem solving (as in an analog computer).

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circuit

 or electric circuit

Path that transmits electric current. A circuit includes a battery or a generator that gives energy to the charged particles; devices that use current, such as lamps, motors, or electronic computers; and connecting wires or transmission lines. Circuits can be classified according to the type of current they carry (see alternating current, direct current) or according to whether the current remains whole (series) or divides to flow through several branches simultaneously (parallel). Two basic laws that describe the performance of electric circuits are Ohm's law and Kirchhoff's circuit rules. See also tuned circuit.

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electric circuit [i¦lek·trik ′sər·kət]

(electricity)

Also known as circuit.

A path or group of interconnected paths capable of carrying electric currents.

An arrangement of one or more complete, closed paths for electron flow.

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Circuit, Electric 

an aggregate of sources and receivers of electric energy and their interconnecting wires. In addition, an electric circuit may include circuit breakers, switches, fuses, and other protective and switching gear, as well as measuring and monitoring instruments. Occurring within a circuit is the transmission, distribution, and conversion of electric (electromagnetic) or other form of energy linked with the presence in the circuit of an electric current, a potential difference, an electromotive force (emf), and so on. In the sources, some form of energy is converted into electric energy, while in the receivers, electric energy is converted into heat, mechanical energy, or some other form of energy.

The operation of an electric circuit is characterized by the values of the currents and voltages in all its parts, and the relations between these quantities are described by Kirchhoff’s laws (seeKIRCHHOFF’S LAWS). The principal components of an electric circuit are resistors, in which electric energy is converted into heat; inductance coils, which store energy in the magnetic fields of the currents flowing through their windings; and capacitors, which store energy in the electric fields of charges on their plates (seeRESISTOR; INDUCTANCE COIL; and CAPACITOR).

An electric circuit with lumped parameters is a circuit in which each of its components can be referred to a single point of the circuit; the processes in such circuits are described by ordinary differential equations. Circuits with distributed parameters are circuits in which it is necessary to take the geometrical dimensions of its components into account; such circuits are described by partial differential equations.

A linear electric circuit is a circuit consisting of components for which the relations between current and voltage, current and flux-linkage, and charge and voltage are linear. Otherwise a circuit is nonlinear. For linear circuits, Kirchhoff’s laws are written in the form of a system of linear equations, which, when solved, determine the operating conditions of the circuit. For such circuits, the principle of superposition is valid. Nonlinear circuits are computed by graphical or numerical methods using approximations and interpolations for the functions.

A distinction is made between direct-current and alternating-current circuits. The most common among the latter are circuits with harmonic currents. In such circuits, the emf’s and currents are sinusoidal time functions of a single frequency, and their operating conditions are computed by a symbolic method. Three-phase circuits have become very common.

Electric circuits can be represented by a combination of two-terminal networks (sources and receivers of electric energy), four-terminal networks (communication lines, amplifiers, transformers), or multiterminal networks (computer adders and memories).

The concept of electric circuit is used in electrical engineering, radio engineering, automation, bionics, and other branches of science.

REFERENCE

Osnovy teorii tsepei, 4th ed. Moscow, 1975.

P. V. ERMURATSKII