Basic Definitions & Basic Laws
Dr. Mohamed Refky Amin
Electronics and Electrical Communications Engineering Department (EECE)
Cairo University
http://scholar.cu.edu.eg/refky/
Outline of this Lecture
• Course Contents, References, Course Plan
• Basic Definitions
• Topology of Electric Circuit
• Basic Laws
– Ohm’s Law
– Kirchhoff's Voltage Law
– Kirchhoff's Current Law
• Power and Energy
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References
• Textbook:
– “Fundamentals of Electric Circuits”, Alexander and Sadiku, 5th
edition
• References:
– J. W. Nilsson and S. Riedel, Electric Circuits. 8th Edition, Prentice Hall,
2008.
– Syed A. Nasar, “Electric Circuits,” Schaum’s Solved Problems Series,
Mcgraw-Hill, New York, 1988.
– Cunningham Stuller, “Circuit Analysis,” John Wiley & Sons Inc., New
York, 1995.
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Course Plan
• Instructor:
• TA:
• Grading:
– 40% Activity
• Quizzers
• Assignments
• Attendance
– 20 % Midterm
– 40% Final Exam
• Office Hours: TBA
Prof. Mohamed Fathy
Dr. Mohamed Refky
Dr. Mohamed Refky
TBA
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Course Contents
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Week Topic
1 Basic Definitions & Basic Laws
2
Methods of solution of DC circuits3
4
8Network Theorems
6
Midterm
7 Introduction to Capacitors and Inductors
8Sinusoids and phasors
9
10Sinusoidal steady state analysis
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12 Three Phase Circuits
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Introduction
Dr. Mohamed Refky
For an electrical engineering
student, basic electric circuit
theory is an essential to
understand several other courses.
Many branches of electrical
engineering, such as power,
electric machines, electronics,
and instrumentation, are based on
electric circuit theory.
Importance of this Course
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Outline of this Lecture
• Course Contents, References, Course Plan
• Basic Definitions
• Topology of Electric Circuit
• Basic Laws
– Ohm’s Law
– Kirchhoff's Voltage Law
– Kirchhoff's Current Law
• Power and Energy
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Basic Definitions
Dr. Mohamed Refky
In general, electrical engineering is interested in transferring
electrical energy from one point to another.
Introduction
Transfer of electrical energy requires an
interconnection of electrical component.
The interconnection is referred to as an
electric circuit, and each component of
the circuit is known as a circuit element.
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Basic Definitions
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Generally, any electrical circuits has three main components:
▪ Source of power (Battery)
▪ Load (Lamp)
▪ Connecting wires
▪ Switch
Elements of Electrical Circuits
Electrical circuits are modeled using a
simple schematic to represent the circuit
components.
The circuit load such as lamp, speaker, …
is usually modeled as a resistor.
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Basic Definitions
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Elements of Electrical Circuits
Electrical Circuit
Active Elements Passive Elements
Voltage
Source
Current
Source
Resistor Capacitor Coil
Active elements are the components that are capable of
generating energy. On the other hand, passive elements are the
component that are not capable of generating energy
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Basic Definitions
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Elements of Electrical Circuits
Electrical Circuit
Active Elements Passive Elements
Voltage
Source
Current
Source
Resistor Capacitor Coil
Independent
Source
Dependent
Source
An independent source is the source
whose value is independent of any
signal (voltage/current) in the
circuit.
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Basic Definitions
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Elements of Electrical Circuits
Electrical Circuit
Active Elements Passive Elements
Voltage
Source
Current
Source
Resistor Capacitor Coil
Independent
Source
Dependent
Source
A dependent source is the source
whose value depends on another
signal (voltage/current) in the
circuit.
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Basic Definitions
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Elements of Electrical Circuits
Electrical Circuit
Active Elements Passive Elements
Voltage
Source
Current
Source
Resistor Capacitor Coil
Independent
Source
Dependent
Source
Voltage dependent Source
Current dependent Source
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Basic Definitions
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Elements of Electrical Circuits
Electrical Circuit
DC Circuit AC Circuit
DC Active
elements
Passive
element
AC Active
elements
Passive
element
In Direct Current (DC) circuits, currents and voltages are
constant with time. While in Alternating Current (AC) circuits,
current and voltage are time varying.
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Basic Definitions
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Elements of Electrical Circuits
Electrical Circuit
DC Circuit AC Circuit
DC Active
elements
Passive
element
Resistor Capacitor Coil
AC Active
elements
Passive
element
Resistor
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Basic Definitions
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Elements of Electrical Circuits
Passive
element
independent
Sources
Dependent
Sources
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Basic Definitions
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Basic Quantities
Quantity Symbol Unit
Charge
Time
Work
Current
Voltage
Power
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Basic Definitions
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Basic Definitions
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The conservation of charge law states that
Charge can neither be created nor destroyed, only transferred
Thus the algebraic sum of the electric charges in a system does
not change.
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Basic Definitions
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Basic Definitions
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A battery causes current to flow in
a resistor.
Current flows in a resistor from
high voltage to low voltage.
Important: the direction of the conventional current
is opposite to the direction of the electrons flow.
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Basic Definitions
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Outline of this Lecture
• Course Contents, References, Course Plan
• Basic Definitions
• Topology of Electric Circuit
• Basic Laws
– Ohm’s Law
– Kirchhoff's Voltage Law
– Kirchhoff's Current Law
• Power and Energy
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Topology of Electric Circuit
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An electric circuit consists of
Electric Circuit
Branches Nodes Loops
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Topology of Electric Circuit
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Branch
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Topology of Electric Circuit
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NodeA node is a common terminal between two or more elements.
• Simple node is a common terminal inside a branch 1,4,5,6
• Non-simple node is a common terminal of more than two
branches 2, 3, 0
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Topology of Electric Circuit
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A Loop is a set of branches making a closed path.
• Independent loops 12501, 23052, 34603
• Non-independent loops 12301, 1234601, 2346052
Loop
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Topology of Electric Circuit
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Branch, Nodes and Loops
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Outline of this Lecture
• Course Contents, References, Course Plan
• Basic Definitions
• Topology of Electric Circuit
• Basic Laws
– Ohm’s Law
– Kirchhoff's Voltage Law
– Kirchhoff's Current Law
• Power and Energy
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Basic Laws
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Ohm’s low
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Basic Laws
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Kirchhoff's Current Law (KCL)Kirchhoff's Current Law (KCL) states that
The algebraic sum of the currents entering any node is zero
To do an algebraic sum, algebraic sign corresponding to a
reference direction must be assigned to every current at the node
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Basic Laws
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Kirchhoff's Current Law (KCL)Kirchhoff's Current Law (KCL) states that
The algebraic sum of the currents entering any node is zero
To do an algebraic sum, algebraic sign corresponding to a
reference direction must be assigned to every current at the node
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Basic Laws
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Kirchhoff's Current Law (KCL)Kirchhoff's Current Law (KCL) states that
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Basic Laws
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Kirchhoff's Current Law (KCL)
KCL is usually applied only for
non-simple node.
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Kirchhoff's Voltage Law (KVL)
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Basic Laws
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Kirchhoff's Voltage Law (KVL)
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Basic Laws
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For loop 2:
For loop 3:
Kirchhoff's Voltage Law (KVL)
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Series Connection
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Basic Laws
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Series Connection
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Basic Laws
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Series Connection
The series combination of resistors results in increasing the total
resistance40
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Parallel Connection
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Parallel Connection
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Parallel Connection
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Parallel Connection
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Basic Laws
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Parallel Connection
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Basic Laws
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Voltage DividerWhen resistors are connected in series, the total voltage across
these resistors is divided between them with a ratio that depends
on the values of theses resistors.
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Basic Laws
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Voltage DividerWhen resistors are connected in series, the total voltage across
these resistors is divided between them with a ratio that depends
on the values of theses resistors.
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Basic Laws
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Current DividerWhen resistors are connected in parallel, the total current is
divide between these resistor with a ratio that depends on the
values of theses resistors.
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Basic Laws
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Current DividerWhen resistors are connected in parallel, the total current is
divide between these resistor with a ratio that depends on the
values of theses resistors.
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Outline of this Lecture
• Course Contents, References, Course Plan
• Basic Definitions
• Topology of Electric Circuit
• Basic Laws
– Ohm’s Law
– Kirchhoff's Voltage Law
– Kirchhoff's Current Law
• Power and Energy
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Power and Energy
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Definition
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Power and Energy
Power Calculations
Passive Sign Convention
Power is
absorbed
Power is
absorbed
Power is
delivered
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Power is
delivered
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Power and Energy
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Power Calculations
Passive Sign Convention
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Power and Energy
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For a source, power is either delivered or absorbed
Power Calculations
Passive Sign Convention
Power is delivered
Power is absorbed
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Power Balance
The total delivered power to the circuit equal to the
total power dissipated
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Power and Energy
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Power Balance
The total delivered power to the circuit equal to the
total power dissipated
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