Introduction About Network theorem

1. Basic Concepts

2.  Electric circuit theory and electromagnetic theory are
the two fundamental theories upon which all
branches of electrical engineering are built.
 In electrical engineering, we are often interested in
communicating or transferring energy from one
point to another. To do this requires an
interconnection of electrical devices

3.  Interconnection is referred to as an electric circuit,
and each component of the circuit is known as an
element.
An electric circuit is an interconnection of electrical
elements

4. SYSTEMS OF UNITS
 As electrical engineers, we deal with measurable
quantities. Our measurement, however, must be
communicated in a standard language that virtually
all professionals can understand, irrespective of the
country where the measurement is conducted
 Such an international measurement language is the
International System of Units (SI)

5. SYSTEMS OF UNITS
 there are six principal units from which the units of
all other physical quantities can be derived.

6. The SI prefixes

7. CHARGE AND CURRENT
 The most basic quantity in an electric circuit is the
electric charge. We all experience the effect of
electric
 charge when we try to remove our wool sweater and
have it stick to our body or walk across a carpet and
receive a shock.
Charge is an electrical property of the atomic
particles of which matter consists,
measured in coulombs (C).

8.  The following points should be noted about electric
charge:
 The coulomb is a large unit for charges. In 1 C of
charge, there are 1/(1.602 × 10−19) = 6.24 × 1018
electrons. Thus realistic or laboratory values of
charges are on the order of pC, nC, or μC.1
 The law of conservation of charge states that charge
can neither be created nor destroyed, only
transferred.

9. Electric current is the time rate of change of charge,
measured in amperes (A).

10.  Mathematically, the relationship between current i,
charge q, and time t is
 where current is measured in amperes (A), and
1 ampere = 1 coulomb/second

11.  The charge transferred between time t0 and t is
obtained by integrating both sides
A direct current (dc) is a current that remains constant
with time.

12. An alternating current (ac) is a current that varies
sinusoidal with time.
Conventional current flow

13. VOLTAGE

14. OHM’S LAW
 Materials in general have a characteristic behavior of
resisting the flow of electric charge. This physical
property, or ability to resist current, is known as
resistance and is represented by the symbol R.

15.  where ρ is known as the resistivity of the material in
ohm-meters.
 The uniform cross-sectional area A depends on A
and l its length

16.  Ohm’s law states that the voltage v across a resistor
is directly proportional to the current i flowing
through the resistor.
 v ∝ i
 Ohm defined the constant of proportionality for a
resistor to be the resistance, R. (The resistance is a
material property which can change if the internal or
external conditions of the element are altered, e.g., if
there are changes in the temperature.) Thus, v = iR

17.  The resistance R of an element denotes its ability to
resist the flow of electric current; it is measured in
ohms (Ω).
 A short circuit is a circuit element with
resistance approaching zero.

18.  An open circuit is a circuit element with
resistance approaching infinity

19.  Fixed Resistance
 Variable Resistance

20.  A resistor that obeys Ohm’s law is known as a linear
resistor. It has a constant resistance and thus its
current-voltage characteristic is as illustrated

21.  A useful quantity in circuit analysis is the reciprocal
of resistance R, known as conductance and denoted
by G

22.  The conductance is a measure of how well an
element will conduct electric current.
 The unit of conductance is the mho (ohm spelled
backward) or reciprocal ohm, with symbol , the
inverted omega.
 Although engineers often use the mhos, or siemens
(S), the SI unit of conductance
1 S = 1 Ʊ = 1 A/V

23.  Conductance is the ability of an element to conduct
electric current; it is measured in mhos ( Ʊ) or
siemens (S).
 The power dissipated by a resistor can be expressed
in terms of R.

24. Points to be Remember
 The power dissipated in a resistor is a nonlinear function
of either current or voltage.
 Since R and G are positive quantities, the power
dissipated in a resistor is always positive. Thus, a resistor
always absorbs power from the circuit. This confirms the
idea that a resistor is a passive element, incapable of
generating energy.