Network theory is the study of solving the problems of electric circuits or electric networks. In this introductory chapter, let us first discuss the basic terminology of electric circuits and the types of network elements.
Basic Terminology
In Network Theory, we will frequently come across the following terms −
 Electric Circuit
 Electric Network
 Current
 Voltage
 Power
So, it is imperative that we gather some basic knowledge on these terms before proceeding further. Let’s start with Electric Circuit.
Electric Circuit
An electric circuit contains a closed path for providing a flow of electrons from a voltage source or current source. The elements present in an electric circuit will be in series connection, parallel connection, or in any combination of series and parallel connections.
Electric Network
An electric network need not contain a closed path for providing a flow of electrons from a voltage source or current source. Hence, we can conclude that “all electric circuits are electric networks” but the converse need not be true.
Current
The current “I” flowing through a conductor is nothing but the time rate of flow of charge. Mathematically, it can be written as
Where,

Q is the charge and its unit is Coloumb.

t is the time and its unit is second.
As an analogy, electric current can be thought of as the flow of water through a pipe. Current is measured in terms of Ampere.
In general, Electron current flows from negative terminal of source to positive terminal, whereas, Conventional current flows from positive terminal of source to negative terminal.
Electron current is obtained due to the movement of free electrons, whereas, Conventional current is obtained due to the movement of free positive charges. Both of these are called as electric current.
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Voltage
The voltage “V” is nothing but an electromotive force that causes the charge (electrons) to flow. Mathematically, it can be written as
Where,

W is the potential energy and its unit is Joule.

Q is the charge and its unit is Coloumb.
As an analogy, Voltage can be thought of as the pressure of water that causes the water to flow through a pipe. It is measured in terms of Volt.
Types of Network Elements
We can classify the Network elements into various types based on some parameters. Following are the types of Network elements −

Active Elements and Passive Elements

Linear Elements and Nonlinear Elements

Bilateral Elements and Unilateral Elements
Active Elements and Passive Elements
We can classify the Network elements into either active or passive based on the ability of delivering power.

Active Elements deliver power to other elements, which are present in an electric circuit. Sometimes, they may absorb the power like passive elements. That means active elements have the capability of both delivering and absorbing power. Examples: Voltage sources and current sources.

Passive Elements can’t deliver power (energy) to other elements, however they can absorb power. That means these elements either dissipate power in the form of heat or store energy in the form of either magnetic field or electric field. Examples: Resistors, Inductors, and capacitors.
Linear Elements and NonLinear Elements
We can classify the network elements as linear or nonlinear based on their characteristic to obey the property of linearity.

Linear Elements are the elements that show a linear relationship between voltage and current. Examples: Resistors, Inductors, and capacitors.

NonLinear Elements are those that do not show a linear relation between voltage and current. Examples: Voltage sources and current sources.
Bilateral Elements and Unilateral Elements
Network elements can also be classified as either bilateral or unilateral based on the direction of current flows through the network elements.
Bilateral Elements are the elements that allow the current in both directions and offer the same impedance in either direction of current flow. Examples: Resistors, Inductors and capacitors.
The concept of Bilateral elements is illustrated in the following figures.
In the above figure, the current (I) is flowing from terminals A to B through a passive element having impedance of Z Ω. It is the ratio of voltage (V) across that element between terminals A & B and current (I).
In the above figure, the current (I) is flowing from terminals B to A through a passive element having impedance of Z Ω. That means the current (–I) is flowing from terminals A to B. In this case too, we will get the same impedance value, since both the current and voltage having negative signs with respect to terminals A & B.
Unilateral Elements are those that allow the current in only one direction. Hence, they offer different impedances in both directions.
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Network Theory – Active Elements
Active Elements are the network elements that deliver power to other elements present in an electric circuit. So, active elements are also called as sources of voltage or current type. We can classify these sources into the following two categories −
 Independent Sources
 Dependent Sources
Independent Sources
As the name suggests, independent sources produce fixed values of voltage or current and these are not dependent on any other parameter. Independent sources can be further divided into the following two categories −
 Independent Voltage Sources
 Independent Current Sources
Independent Voltage Sources
An independent voltage source produces a constant voltage across its two terminals. This voltage is independent of the amount of current that is flowing through the two terminals of voltage source.
Independent ideal voltage source and its VI characteristics are shown in the following figure.
The VI characteristics of an independent ideal voltage source is a constant line, which is always equal to the source voltage (VS) irrespective of the current value (I). So, the internal resistance of an independent ideal voltage source is zero Ohms.
Hence, the independent ideal voltage sources do not exist practically, because there will be some internal resistance.
Independent practical voltage source and its VI characteristics are shown in the following figure.
There is a deviation in the VI characteristics of an independent practical voltage source from the VI characteristics of an independent ideal voltage source. This is due to the voltage drop across the internal resistance (R_{S}) of an independent practical voltage source.