03-05-2014, 02:58 PM
Introduction of Electric Circuit
Electric Circuit.pdf (Size: 654.14 KB / Downloads: 29)
Introduction
The interconnection of various electric elements in a prescribed manner
comprises as an electric circuit in order to perform a desired function. The electric
elements include controlled and uncontrolled source of energy, resistors, capacitors,
inductors, etc. Analysis of electric circuits refers to computations required to determine
the unknown quantities such as voltage, current and power associated with one or more
elements in the circuit. To contribute to the solution of engineering problems one must
acquire the basic knowledge of electric circuit analysis and laws. Many other systems,
like mechanical, hydraulic, thermal, magnetic and power system are easy to analyze and
model by a circuit. To learn how to analyze the models of these systems, first one needs
to learn the techniques of circuit analysis. We shall discuss briefly some of the basic
circuit elements and the laws that will help us to develop the background of subject.
Linear Circuit:
Roughly speaking, a linear circuit is one whose parameters do not
change with voltage or current. More specifically, a linear system is one that satisfies (i)
homogeneity property [response of α u (t ) equals α times the response of u (t ) , S (α u (t ))
= α S (u (t )) for all α ; and u (t ) ] (ii) additive property [that is the response of system due
to an input ( α1 u1 (t ) + α 2 u2 (t ) ) equals the sum of the response of input α1 u1 (t ) and the
response of input α 2 u2 (t ) , S (α1 u1 (t ) + α 2 u2 (t)) = α1 S (u1 (t )) + α 2 S (u2 (t )) .] When an
input u1 (t ) or u2 (t ) is applied to a system “ S ”, the corresponding output response of the
system is observed as S (u1 (t )) = y1 (t ) or S (u2 (t )) = y2 (t ) respectively. Fig. 3.1 explains
the meaning of homogeneity and additive properties of a system.
Kirchhoff’s Laws
Kirchhoff’s laws are basic analytical tools in order to obtain the solutions of currents and
voltages for any electric circuit; whether it is supplied from a direct-current system or an
alternating current system. But with complex circuits the equations connecting the
currents and voltages may become so numerous that much tedious algebraic work is
involve in their solutions.
Elements that generally encounter in an electric circuit can be interconnected in
various possible ways. Before discussing the basic analytical tools that determine the
currents and voltages at different parts of the circuit, some basic definition of the
following terms are considered.
Ideal and Practical Current Sources
Another two-terminal element of common use in circuit modeling is `current
source` as depicted in fig.3.17. An ideal current source, which is represented by a
model in fig. 3.17(a), is a device that delivers a constant current to any load
resistance connected across it, no matter what the terminal voltage is developed
across the load (i.e., independent of the voltage across its terminals across the
terminals).
Understanding Delivering and Absorbing Power by the
Source.
It is essential to differentiate between the absorption of power (or dissipating power) and
the generating (or delivering) power. The power absorbed or dissipated by any circuit
element when flows in a load element from higher potential point (i.e +ve terminal)
toward the lower terminal point (i.e., -ve terminal). This situation is observed when
charging a battery or source because the source is absorbing power. On the other hand,
when current flows in a source from the lower potential point (i.e., -ve terminal) toward
the higher potential point (i.e., +ve terminal), we call that source is generating power or
delivering power to the other elements in the electric circuit. In this case, one can note
that the battery is acting as a “source” whereas the other element is acting as a “sink”.
Fig.3.22 shows mode of current entering in a electric element and it behaves either as
source (delivering power) or as a sink (absorbing or dissipating power).