An electric circuit (or network) is an interconnection of physical electrical devices such as an energy source (or sources), an energy convertor or convertors (load or loads), and conductors that connect them.

An energy source (or source), such as a primary or secondary cell, a generator, and the like, is a device that converts chemical, mechanical, thermal or some other form of energy into electrical energy.

An energy convertor, also called the load, (such as lamp, heating appliance, or an electric motor) converts electrical energy into light, heat, mechanical work and so on.

Events in an electrical circuit may be defined in terms of emf (or voltage) and current.

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When electrical energy is generated, transmitted and converted under conditions such that the currents and voltages involved remain constant with time, the electric circuit is identified as direct current (dc) circuit. If the currents and voltages do change with time, the circuit is defined as alternating current (ac) circuit.

A graphic representation of an electric circuit is called a circuit diagram (Fig. 2.1). Such a diagram consists of an interconnected symbol called circuit elements or circuit parameters. Two elements are necessary to represent processes in a dc circuit. These are source of emf ES and of internal (or source) resistance RS and the load resistance (which includes the resistance of the conductors) R.

In any electric circuit the energy convertor (or load) and the conductors connecting it to the source make up the external circuit in which current flows from the + ve side to the – ve side of the source whereas inside the source, current flows in the opposite direction, i.e., from the – ve side to the + ve side. The source emf is directed from the terminal at a lower potential to that at a higher one. In diagrams this is shown by arrows.

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The source emf (or open-circuit voltage) is the voltage that appears across the source when no load is connected across it.

When a load is connected to the source terminals and the circuit is closed, an electric current starts flowing through the circuit. Now voltage across source terminals (called the terminal voltage) is not equal to source emf. It is due to voltage drop inside the source, i.e., across the source resistance.

Voltage drop inside the source = I RS.

The relationship between the current through a resistance and the voltage across the same resistance is called its volt-ampere (or voltage-current) characteristic. When represented graphically, voltages are laid off as abscissae and currents as ordinates.

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There are two types of volt-ampere characteristics-straight line and non-linear (curve), as shown in Figs. 2.2 (a) and 2.2 (b) respectively.

Resistive elements for which the volt-ampere characteristic is a straight line [Fig. 2.2 (a)] are called linear, and the electric circuits containing only linear resistances are called linear circuits.

Resistive elements for which the volt-ampere characteristic is other than a straight line are termed non­linear, and so the electric circuits containing them are called non-linear circuits. Examples of non-linear elements are tungsten lamps, vacuum tubes and transistors, etc.

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An electric circuit, whose characteristics or properties are same in either direction (e.g., a distribution or transmission line), is called the bilateral circuit. The distribution or transmission line can be made to perform its function equally well in either direction.

An electric circuit, whose characteristics or properties change with the direction of its operation (e.g., a diode rectifier), is called the unilateral circuit. A diode rectifier cannot perform rectification in both directions.

A network is said to be passive if it contains no source of emf in it. The equivalent resistance between any two terminals of a passive network is the ratio of potential difference across the two terminals to the current flowing into (or out of) the network. When a network contains one or more sources of emf and/ or current, it is said to be active.

In case, a branch is removed from an electric network, the remainder of the network is left with a pair of terminals. The part of the network, which is considered with respect to the removed branch or terminal pair or port, is termed as one-port network.

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When two branches are removed so that the network is left with four terminals or two pairs of terminals, the remainder network is called the two-port network. Usually one port accepts a source and the other port is coupled to a load, so that there is an input port and an output port in any two-port system.