# electric circuit

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## 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, electricfuse, electric,
safety device used to protect an electric circuit against an excessive current. A fuse consists essentially of a strip of low-melting alloy enclosed in a suitable housing. It is connected in series with the circuit it is to protect.
; circuit breakercircuit breaker,
electric device that, like a fuse, interrupts an electric current in a circuit when the current becomes too high. The advantage of a circuit breaker is that it can be reset after it has been tripped; a fuse must be replaced after it has been used once.
; short circuitshort circuit,
abnormal connection of low resistance between two points of a circuit that usually causes a high, potentially damaging current to flow. To protect against damage, devices such as a fuse or a circuit breaker are used.
). 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 resistanceresistance,
property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. Optimum resistance is provided by a conductor that is long, small in cross section, and of a material that conducts
(impedanceimpedance,
in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals.
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 circuitdigital circuit,
electronic circuit that can take on only a finite number of states. That is contrasted with analog circuits, whose voltages or other quantities vary in a continuous manner. Binary (two-state) digital circuits are the most common.
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 resistorresistor,
two-terminal electric circuit component that offers opposition to an electric current. Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistance).
, and an inductorinductor,
electric device consisting of one or more turns of wire and typically having two terminals. An inductor is usually connected into a circuit in order to raise the inductance to a desired value.
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 computercomputer,
device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical
).

## circuit, electric:

see electric circuitelectric 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
.

## 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

## 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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