Here's a quick introduction to some of the important concepts which will be introduced in this tutorialTarget Audience: High School Students, College Freshmen and Sophomores, Class 11/12 Students in India preparing for ISC/CBSE and Entrance Examinations like the IITJEE Main or Advanced/AIEEE, and anyone else who needs this Tutorial as a reference!
Contents:
1. Classification of Networks
2. Voltage and current sources
3. Network theorems: Superposition theorem, Thevenin’s theorem
1) Classification of a network is done based on the type of elements being used in that network. The operational characteristics of a network depend on the behaviour of its elements. Before going into the contents one should be aware of the following terms used to specify the network, they are described below.
Active and Passive Elements
Elements which supply the energy to the circuit are known as active elements. A network which contains active elements is known as active networks. Ex: batteries, generators, transistors etc. Elements which absorb the energy are known as passive elements. A network containing only passive elements is known as passive networks. Ex: resistors, inductors and capacitors.
Unilateral/Bilateral Elements
An element whose operational behaviour is dependent on the direction of flow of current through is known as unilateral elements. Elements like semiconductor diode, which allow the current to pass through them only in one direction.
An element whose behaviour is same irrespective of the direction of flow of current through it is known as bilateral element. Passive elements that allow the current to pass through them in both directions are known as bilateral elements.
Lumped and Distributed Networks
Networks consisting of elements which can be physically separated are known as lumped networks. Most of the networks we deal with, are lumped in nature and consists of R, L,C and sources. Networks, like transmission lines, having inseparable elements are known as distributed networks.
Linear and NonLinear Elements
A linear element is one which has linear output/input relation and always follows superposition and homogeneity principles. Ohm’s can be applied to such networks.
The element that which does not follow these is known as a nonlinear element. Ohm’s law cannot be applied to such networks.
2) Voltage and current sources:
There are two principal types of source, voltage source and current source. Sources can be either independent or dependent upon some other source quantities.
Independent sources: An independent voltage source (AC or DC) maintains a voltage which is not affected by any other quantity. Similarly an independent current source maintains a current which is unaffected by any other quantity. The usual representation of independent voltage and current source is shown in the tutorial document.
Dependent sources: Some voltage (current) sources have their voltage (current) values varying with some other variables. They are called dependent voltage (current) sources or controlled voltage (current) sources, and their usual representation of dependent sources is as shown in the tutorial document.
VCVS: voltage controlled voltage source: voltage source depends on a voltage value of V1. VCCS: voltage controlled current source: current source depends on a voltage value of V1. CCVS: current controlled voltage source: voltage source depends on a current value of I1. CCCS: current controlled current source: current source depends on a current value of I1.
Superposition Theorem: It states that in an active, linear, bilateral network consisting of active and passive elements with more than one source, the overall response (voltage or current) is equal to the sum of the responses due to each of the sources acting independently.
Statement:
In any linear network having number of voltage or current sources and resistances, the voltage across or the current through any branch is given by algebraic sum of all the individual voltages or currents caused by each independent source acting alone, with
all other independent voltage sources replaced by short circuits and all other independent current sources replaced by open circuit.
Number of networks to be analyzed = Number of independent sources
Note: the total power delivered to a resistance element must be determined using the total current through or the total voltage across the element and cannot be determined by a simple sum of the power levels established by each source.
Thevenin’s Theorem (or Helmholtz’s Theorem)
Statement: Any twoterminal, linear bilateral network can be replaced by an equivalent network having a single voltage source called Thevenin’s voltage (V Th) and a single series resistance called Thevenin’s resistance (RTh). The Thevenin’s equivalent circuit provides equivalence at the terminals only—the internal construction and characteristics of the original network and the Thevenin’s equivalent are usually quite different.
For finding the thevenin’s resistance, the equivalent resistance across the terminals have to be found with all the independent sources being zero.
Here are kind of the problems which have been solved in the tutorial document :
 Verifying the superposition theorem for given networks.
 Gain an understanding of how to apply Kirchoff laws for voltage and current, KVL and KCL, and solve mesh equations using them
 Reduce circuits to simpler forms using the theorems above
 Finding current in a link using the superposition theorem.
 Finding the current in a resistor in a given circuit using the Thevenin Theorem.
 Using superposition theorem to find the current through a link that is to be connected between two given terminals ab, assuming the link resistance to be zero.
 In a given network, finding the current through a given resistor utilising Thevenin’s theorem.
 Finding the current through a given resistance, given a network of voltage, current sources and resistors.
Complete Tutorial with Solved Problems :
Related Tutorials ( Introduction to Electrical Circuits  DC ) :
Circuit Theory 1a  Introduction to Electrical Engineering, DC Circuits, Resistance and Capacitance, Kirchoff Law

Resistors, Capacitors, problems related to these. 
Circuit Theory 1b  More solved problems related to DC Circuits with Resistance and Capacitance

Capacitors, computing capacitance, RC Circuits, time constant of decay, computing voltage and electrostatic energy across a capacitance 
Circuit Theory 2a  Introducing Inductors

Inductors, inductance, computing selfinductance, fluxlinkages, computing energy stored as a magnetic field in a coil, mutual inductance, dot convention, introduction to RL Circuits and decay of an inductor. 
Circuit Theory 2b  Problems related to RL, LC, RLC circuits

Introducing the concept of oscillations. Solving problems related to RL, LC and RLC circuits using calculus based techniques. 
Circuit Theory 3a  Electrical Networks and Network Theorems

Different kind of network elements: Active and passive, linear and nonlinear, lumped and distributed. Voltage and current sources. Superposition theorem, Thevenin (or Helmholtz) theorem and problems based on these. 
Circuit Theory 3b  More network theorems, solved problems

More solved problems and examples related to electrical networks. Star and Delta network transformations, maximum power transfer theorem, Compensation theorem and Tellegen's Theorem and examples related to these. 
