Fundamentals of Electric Circuit - Chapter 1: Basic concepts

Fundamentals of Electric Circuits – Viet Son Nguyen - 2011
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FUNDAMENTALS OF ELECTRIC CIRCUITS
Part 1: DC CIRCUITS
Chapter 1: Basic concepts
I. Introduction.
II. Systems of units.
III. Charge and current.
IV. Voltage.
V. Power and Energy.
VI. Circuit elements.
Fundamentals of Electric Circuits – Viet Son Nguyen - 2011
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Chapter 1: Basic concepts
I. Introduction.
 Electric circuit theory and electromagnetic theory are the two fundamental
theories upon which all branches of electrical engineering are built, such as:
 Power
 Electric machines
 Control
 Electronics
 Communications
 Instrumentation
 etc.
 The basic electric engineering theory course is:
 The most important course for an electrical engineering student.
 Excellent starting point for a beginning student in electrical engineering
education.
 Valuable to students specializing in other branches of the physical sciences.
 An electric circuit is an interconnection of electrical elements.
Ex1: A simple electric circuit consists of 4 basic components:
 A battery.
 A lamp.
 A contact.
 Connecting wires.
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Chapter 1: Basic concepts
I. Introduction.
 Electric circuit are used in many electrical systems to accomplish different task.
 Objective in this course:
 Not the study of various uses and applications of circuits.
 The analysis of the circuits:
 How does it respond to a give input ?
 How do the interconnected elements and devices in the circuit interact?
 Study commence by defining some basic concepts:
 Charge
 Current
 Voltage
 Circuit element.
 Power
 Energy
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Chapter 1: Basic concepts
II. Systems of units.
 An international measurement language is the International System of Unit (SI), 
adopt by the General Conference on Weight and Measures in 1960.
 In SI, there are six principal units:
Quantity Basic unit Symbol
Length Meter m
Mass Kilogram Kg
Time Second kg
Electric current Ampere A
Thermodynamic temperature Kelvin K
Luminous intensity Candela cd
 From which the units of all
other physical quantities can
be derived.
 Use prefixes base on the
power of 10 to relate larger
and smaller unit to the basic
unit.
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Chapter 1: Basic concepts
III. Charge and current.
 The concept of electric charge is the underlying principle for explainning all
electrical phenomena.
 Charge is an electrical property of the atomic particles of which matter consists,
measured in coulombs (C)
 In atom physic, each atom consists of electrons, protons and neutrons:
 Electron charge e: -1.602 x 10-19 C
 Proton charge: 1.602 x 10-19 C
 Notes:
 1C = 6.24 x 1018 electrons pC, nC, μC
 Only charges occuring in nature are integral multiples of the electronic
charge e = -1.602 x 10-19 C.
 Law of conservation of charge: Charge can neither be created nor
destroyed, only transferred.
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Chapter 1: Basic concepts
III. Charge and current.
 Electric charge is mobile:
 Can be transferred from one place to another
 Can be converted to another form of energy.
 When a conducting wire is connected to a battery:
 Positive charges move in one directions
 Negative charges move in the opposite direction
creat an 
electric current
current
battery
 It it conventional to take the current flow as the movement of positive charge,
opposite to the flow of negative charges (the current In metallic conductors is
due to negatively charged electrons).
 Electric current is the time rate of change of charge, measured in amperes (A)
dq C
i A
dt s
[ ]
; [ ]
[ ]
 
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Chapter 1: Basic concepts
III. Charge and current.
 There are two common types of current:
 A direct current (dc) is a current that remains constant with time (I).
 An alternating current (ac) is a current that varies sinusoidally with time (i).
i
tt
0
I
2A 2A
a current of 2A may be represented 
positively or negatively
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Chapter 1: Basic concepts
IV. Voltage.
 In order to move the electron in a conductor in a particular direction requires
some work or energy transfer (external electromotive force, emf, as known as
voltage or potential difference).
 Voltage (or potential difference) is the energy required to move a unit charge
through an element, measured in volts (V).
ab
dw J Nm
v V
dq C C
[ ] [ ]
; [ ]
[ ] [ ]
  
b
a +
-
vab
 The vab ca be interpreted in two ways:
 Point a is at a potential of vab volts higher than point b.
 The potential at point a with respect to point b is vab.
ab ba
v v  
There are two common types of voltage:
 dc voltage: produced by a battery, represented by V
 ac voltage: produced by an electric generator, represented by v
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Chapter 1: Basic concepts
V. Power and energy.
 Although current and voltage are the two basic variables in an electric circuit, 
they are not sufficient by themselves.
 In fact:
 Need to know how much power an electric device can handle ?
 The EVN’s bills are paid for the electric energy consumed over a certain 
period of time.
 power and energy calculations are important in circuit analysis.
 Power is the time rate of expending or absorbing energy, measured in watts (W)
  
dw dw dq
p vi
dt dq dt
.
p: the instantaneous power
b a
+- v
i
b a
+- v
i
p = -vi
power is being supplied 
by the element
p = vi
power is being delivered 
to the element
Passive sign convension is satisfied
when the current enters through the
positive terminal of an element, and p
= vi. If the current enters through the
negative terminal, p = -vi
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Chapter 1: Basic concepts
V. Power and energy.
 Law of conservation of energy:
 Energy is the capacity to do work, measured in Joules (J)
 The electric power utility companies measure energy in Watt-hours (Wh)
p 0
1 Wh = 3,600 J
t t
t t
pdt vidt
0 0
   
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Chapter 1: Basic concepts
VI. Circuit elements.
 An electric circuit is simply an interconnection of the elements.
 Circuit analysis is the process of determining voltages across (or the currents
through) the element of the circuit.
 An element is the basic building block of a circuit.
 Types of circuit elements:
 Passive elements: Not be able to generate energy.
Ex. : Resistors, capacitors, inductors, 
 Active elements: To be able to generate energy.
Ex. : Generators, batteries, operational amplifiers, voltage sources, current
sources 
 Two kinds of sources:
 Independent sources.
 Dependent sources.
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Chapter 1: Basic concepts
VI. Circuit elements.
 An ideal independent source is an active element that provides a specified
voltage or current that is completely independent of other circuit variables.
 An ideal independent voltage source delivers to the circuit whatever current
is necessary to maintain its terminal voltage.
 An ideal independent current source is an active
element that provides a specified current completely
independent of the voltage across the source (the
arrow indicates the direction of current i)
DC or time varying independent 
voltage source
DC independent voltage 
source
Independent current 
source
v
v
i
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Chapter 1: Basic concepts
VI. Circuit elements.
 An ideal dependent (or controlled) source is an active element in which the
source quantity is controlled by another voltage or current.
 A voltage-controlled voltage source (VCVS)
 A current-controlled voltage source (CCVS)
 A voltage-controlled current source (VCCS)
 A current-controlled current source (CCCS)
 There are 4 possible types of dependent sources:
Dependent current 
source
Dependent voltage 
source
 Dependent sources are useful in modeling elements:
 Transistors.
 Operational amplifier.
 Integrated circuits.
 
i
+
-
v