CHAPTER 1
Variables &
Circuit Elements
School of Computer and Communication
Engineering,
UniMAP
Prepared By:
Shahadah binti Ahmad
EKT 101 [Electric Circuit I]: V2010/11
1
Lecturer : Shahadah Ahmad
shahadah@unimap.edu.my
Office: KKF 8B
Textbook:
(1) Fundamentals of Electric Circuits – Alexander sadiku (4th
edition)
Reference book:
(2) Electric Circuits Fundamentals – FLOYD (7Th edition)
(3) Basic Engineering Circuit Analysis – J.David Irwin &
R.Mark Nelms( 9th edition)
2
Syllabus - EKT 101
Chap 1 : Circuit Elements and Variables
Overview of circuit analysis, SI unit, voltage and currents, power, energy,
elements on the circuit (passive and active) voltage and current source,
Ohm’s Law, Kirchhoff’s Law, circuit model, circuit with dependent source.
Chap 2 : Resistive Circuit
Series / Parallel circuit, voltage divider circuit, current divider circuit,
voltage and current measurement, Wheatstone Bridge, equivalent circuit
for delta-wye (Pi-Tee).
Chap 3 : Circuit Analysis Methods
Introduction to the Node-Voltage Method, the Node-Voltage Method with
dependent sources and special cases, introduction to Mesh-Current
Method, Mesh-Current Method with dependent sources and special cases,
source transformations, Thevenin and Norton equivalent circuit, maximum
power transfer and superposition.
Chap 4 : Inductance and Capacitance
Inductor, relationship between voltage, current, power and energy,
capacitor, relationship between voltage, current, power and energy, seriesparallel combinations for inductance and capacitance.
3
Syllabus - EKT 101
Chap 5 : First-Order and Second-Order Response of RL and RC
Circuit
Natural response of RL and RC Circuit, Step Response of RL and RC
Circuit, general solutions for natural and step response, sequential
switching, introduction to the natural and step response of RLC circuit,
natural response of series and parallel RLC circuit, Step response of
series and parallel RLC circuit.
Chap 6: Sinusoidal Steady-State Analysis
The sinusoidal source, the sinusoidal response, the phasor and phasor
diagram, the passive circuit elements in the frequency domain,
impedances and reactance, Kirchhoff’s Laws in frequency domain,
techniques of circuit analysis in frequency domain
Chap 7 : Sinusoidal Steady-State Power Calculation
Instantaneous power, average (active) and reactive power, the rms value
power calculation, complex and power triangle , the maximum power
transfer
Chap 8 : Three Phase System Circuit
Single and Three Phase System (Y and Δ circuit), balanced three phase
voltage sources, Y – Y circuit analysis, Y - Δ circuit analysis, power
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calculation in three phase balanced circuit, average power measurement
in three phase circuit.
Basic Electric Circuit Concepts
 Review SI
 Know the definition of basic electrical
quantities : voltage, current, and power
 Know the symbols for and definition of
independent and dependent sources
 Be able to calculate the power absorbed by
a circuit element using the passive sign
convention
5
Circuit Elements & Variables




Overview of circuit analysis
SI unit
Voltage, currents, power, energy,
elements on the circuit (passive and active)
voltage and current source
 Ohm’s Law and Kirchhoff’s Law
 circuit model
 circuit with dependent source.
6
SI Unit
SI: International System of Unit is used by all
the major engineering societies and most
engineers throughout the world.
Quantity
Base unit
Symbol
Length
Meter
m
Mass
Kilogram
kg
Time
second
s
Electric current
Ampere
A
Thermodynamic
temperature
Kelvin
K
Luminous
intensity
candela
cd
7
 Standardized prefixes to signify powers of 10
Power
Prefix
Symbol
1012
Tera
T
109
Giga
G
106
Mega
M
103
Kilo
k
10-3
Mili
m
10-6
Micro
µ
10-9
Nano
n
10-12
Pico
p
10-15
Femto
f
10-18
Atto
a
100
8
Circuit Elements & Variables




Overview of circuit analysis
SI unit
Voltage, currents, power, energy,
elements on the circuit (passive and active)
voltage and current source
 Ohm’s Law and Kirchhoff’s Law
 circuit model
 circuit with dependent source.
9
Electric Units





Charge
Current
Voltage
Resistance
Power
»»» Coulomb (C)
»»» Ampere (A)
»»» Volt (V)
»»» Ohm ()
»»» Watt (W)
10
Electric charge is a property possessed by
both electrons and protons.
Quantity is
CHARGE (Q)
Base Unit is
COULOMB (C)
Examples of correct usage:
Charge = 15 Coulombs
Q = 15 C
11
Current
Current is the movement
of charge in a specified
direction.
12
Electric Current Terminology
Quantity is
CURRENT (I)
Base Unit is
AMPERE (A)
An ampere equals a coulomb per second.
Examples of correct usage:
Current = 12 Amperes
I = 12 A
13
Electric Current Relationships
Charge
Current =
Time
Q
I= t
Examples:
Q
14 C = 1.4 A
I = t = 10 s
Q
14 C = 10 s
t = I = 1.4 A
14
Types of Current:
i
DC
AC
ex batteries –
used in
automobiles
or flashlight
t
Alternating current
(arus ulangalik)
Direct current (arus terus)
i
t
Damped alternating current
(arus ulangalik teredam)
Exponential current15
Voltage
Definition of Voltage
Voltage is the electric pressure
or force that causes current.
It is a potential energy
difference between two points.
It is also known as an
electromotive force (emf) or
potential.
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Voltage Terminology
Quantity is
VOLTAGE (V)
Base Unit is VOLT (V)
A volt equals a joule per coulomb.
Examples of correct usage:
Voltage = 32 Volts
V = 32 V
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Voltage Relationships
Energy
Voltage =
Charge
Examples:
W
V= Q =
W
V= Q
56 J = 28 V
2C
84 J = 4 C
W
Q = V = 21 V
18
Resistance
Definition of Resistance
Resistance is the opposition
a material offers to current.
Resistance is determined by:
 Type of material (resistivity)
 Temperature of material
 Cross-sectional area
 Length of material
19
Some Factors That Determine Resistance
For a specific material and temperature,
this block has given amount of resistance.
Doubling the length of the block, doubles the resistance.
20
Doubling the cross-sectional area, halves the resistance.
Resistance Terminology
Quantity is
RESISTANCE (R)
Base Unit is
OHM ()
An ohm equals a volt per ampere.
Examples of correct usage:
Resistance = 47 ohms
R = 47 
21
Resistance Relationships
Resistivity x length
KL
Resistance =
R= A
area
Example:
1.4 x10-6  cm x 2 x104 cm
KL
=
R=
A
0.28 cm2
= 0.1 
22
ENERGY
Work (W)
consists of a force moving
through a distance.
Energy (W)
is the capacity to do
work.
The
joule (J)
is the base unit for both
energy and work.
The amount of
work done
equals the
amount of
energy used
(converted).
Fifty joules of
energy are
required to do
fifty joules of
work.
23
POWER
Definition of Power
Power is the rate of using
energy or doing work.
“Using energy” means that energy
is being converted to a different
form.
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Power Terminology
Quantity is
POWER (P)
Base Unit is WATT (W)
A watt equals a joule per second.
Examples of correct usage:
Power = 120 Watts
P = 120 W
25
Power Relationships
Energy
Power =
Time
W
P= t
Examples:
158 J = 7.9 W
W
P = t = 20 s
W = Pt = 75 W x 25 s = 1875 J
26
Circuit Elements & Variables




Overview of circuit analysis
SI unit
voltage and currents, power, energy,
elements on the circuit (passive and active)
voltage and current source
 Ohm’s Law and Kirchhoff’s Law
 circuit model
 circuit with dependent source.
27
Active and Passive Elements
Circuit
Elements
Active elements
•capable of generating
electric energy
•Example : voltage and
current sources
Passive elements
•incapable of generating
electric energy
•Example : resistor,
inductor, capacitor,
diode and etc
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Independent Source
Current
Voltage
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Dependent Source
Vs  ix
Voltage
is  Vx
Current
30
Ideal Voltage Source Connected in
Series
31
Ideal Current Source Connected
in Parallel
32
Symbol of Circuit Elements
 Resistor
R
UNIT: Ohm (Ω)
33
Resistor Color Code
34
Resistor Color Code
Yellow
Violet
Red
Silver
4700 ±10 %
35
Resistor Color Code
Green = 5
white= 9
Orange = 3
Gold = ± 5 %
59 x 103 ± 5 % = 59,000 ± 5 % = 59 K ± 5 %
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Resistor Color Code
4 6 4 0 0 0 ± 2% = 464 k ± 2%
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Conductance
 Conductance is a measure of the ability of an
element to conduct electric current
 Inverse of resistance
 The units is Siemens (S) or mhos
1
i
G

R
v
38
 Capacitor
C
UNIT: Farad (F)
L
UNIT: Henry (H)
 Inductor
39
Circuit Elements & Variables




Overview of circuit analysis
SI unit
voltage and currents, power, energy,
elements on the circuit (passive and active)
voltage and current source
 Ohm’s Law and Kirchhoff’s Law
 circuit model
 circuit with dependent source.
40
Short Circuit
 R = 0  no voltage difference exists
 all points on the wire are at the same
potential.
 Current can flow, as determined by the
circuit
41
Open circuit
 R =   no current flows
 Voltage difference can exist, as
determined by the circuit
42
Circuit Nodes and Loops
 A node is a point where two or more circuit
elements are connected.
 A loop is formed by tracing a closed path in
a circuit through selected basic circuit
elements without passing through any
intermediate node more than once
43
Example: Find the Nodes
+
Vs
-
node
44
Example: Find the loops
loop
45
Ohm’s Law
 George Simon Ohm (1787-1854)
formulated the relationships among
voltage, current, and resistance as follows:
 The current in a circuit is directly
proportional to the applied voltage and
inversely proportional to the resistance of the
circuit.
V  IR
46
Kirchhoff Law
 Gustav Robert Kirchhoff (1824–1887)
 Models relationship between:
 circuit element currents (KCL)
 circuit element voltages (KVL)
 Introduce two laws:
 Kirchhoff Current Law (KCL)
 Kirchhoff Voltage Law (KVL)
47
Kirchhoff’s Current Law (KCL)
 Current entering node = current exiting
(What goes in, must come out)
 Convention: +i is exiting, -i is entering
 For any circuit node:
i

0

48
Kirchhoff’s Current Law (KCL)
 No matter how many paths into and out of a
single point all the current leaving that point
must equal the current arriving at that point.
49
Kirchhoff’s Voltage Law (KVL)
 voltage increases = voltage decreases
(What goes up, must come down)
 Convention: hit minus (-) side first, write
negative
 For any circuit loop:
v

0

50
Kirchhoff’s Voltage Law (KVL)
 The voltage drops around any closed loop must
equal the applied voltages
51
Circuit Elements & Variables




Overview of circuit analysis
SI unit
voltage and currents, power, energy,
elements on the circuit (passive and active)
voltage and current source
 Ohm’s Law and Kirchhoff’s Law
 circuit model
 circuit with dependent source.
52
Circuit Model
SERIES
PARALLEL
PARALLEL-SERIES
53
This complete circuit uses the following:
• An energy or power source
• A control device
• A load
• Conductors
• Insulation (not shown)
54
Calculating Current
S1
B1
36 V
SPST
R
1.8 k
V
36
V
= 0.02 A = 20 mA
I=
=
R
1800 
55
Calculating Resistance
A
B1
24 V
0.03 A
R
V
24 V
R= I =
= 800  = 0.8 k 
0.03 A
56
Calculating Voltage
A
B1
0.15 A
R
270 
V = IR = 0.15 A * 270  = 40.5 V
57
Calculating Power
A
V
0.2 A
54 V
270 
P = IV = 0.2 A * 54 V = 10.8 W
P = I2R = 0.2 A * 0.2 A * 270  = 10.8 W
P = V2/R = (54 V * 54 V) / 270  = 10.8 W
58
Circuit Elements & Variables




Overview of circuit analysis
SI unit
voltage and currents, power, energy,
elements on the circuit (passive and active)
voltage and current source
 Ohm’s Law and Kirchhoff’s Law
 circuit model
 circuit with dependent source.
59
Circuit With Dependent Source
60
 Using KVL on the first loop,
500  5 i  20 i0
 Using KCL on the second loop,
i0  i  5 i
i0  6i

Solve the equations,
i  4 A
i0  24 A
61
 Using Ohm law for the resistor,
v 0  i o (20)
 480V
62