Electrical Engineering
Basic Laws
Noman Khan
(JE/Lecturer)
http://faculty.pieas.edu.pk/nomankhan/
PIEAS
Department of Electrical Engineering
Ref2: Anant Agarwaland Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007.
MIT OpenCourseWare(http://ocw.mit.edu/), Massachusetts Institute of Technology
Course Learning Outcomes (EE-151T)
Introduction
An electric circuit is an interconnection of electrical elements.
A simple electric circuit
Electric circuit of
a radio receiver
Motivation
Electrolyzers
Super Capacitors
Units
Six basic SI units and one** derived unit relevant to this course.
Quantity
Basic Unit Symbol
Length
meter
m
Mass
kilogram
kg
Time
second
s
Electric current**
ampere
A
Thermodynamic Temperature kelvin
K
Luminous intensity
candela
cd
Charge**
coulomb
C
Prefixes
Prefix
Most
frequently
used
SI Prefixes
micro
pico
Circuit Diagrams
• Pictorial Diagram
• What is a Circuit Diagram?
Circuit Diagrams
Symbols of Components used in Electrical Circuits
Basic Definitions
•
•
•
•
•
•
Conductors
Insulators
Semiconductors
Charge and Current
Voltage
Power and Energy
Charge and Current
•
Charge is an electrical property of the atomic particles of
matter, measured in coulombs (C).
▪ 1 C of charge requires 6.24 x 1018 electrons.
▪ 1 electron charge e = −1.602 x 10−19 C .
▪ Law of Conservation of Charge: Charge can only
be transferred. Cannot be created/destroyed.
Charge Direction
Charge and Current
•
•
Electric current is the time rate of change of charge, in
other words, the flow of charge, measured in amperes (A).
Current is defined by:
dq
i =
dt
where i = current in amperes (A),
q = charge in coulombs (C),
t = time in seconds (s).
1 A = 1 C/s
•
Charge transferred between time t0 and t :
Q =

t
t0
i dt
Charge and Current
•
Direct Current (dc) is the current that
remains constant with time.
•
•
Alternating Current (ac) is the current
that varries sinusoidally with time.
dc current is represented by I and ac current is represented by i.
•
Conventional current flow: Both
methods represents the same
current. (a) positive current flow, (b)
negative current flow.
Charge and Current : Problems
Ex. 1 :
Ex. 2:
Charge and Current : Problems
Ex. 3:
Voltage
•
Voltage (potential difference/electromotive force) is the energy
required to move a unit charge through an element, measured in
volts (V).
•
The voltage,
between two points a and b:
•
υ ab
dω
=
dq
1 volt = 1 joule/coulomb
υ = voltage in volts (V),
ω = energy in joules (J),
q = charge in coulombs (C).
Voltage
•
•
Voltage pushes charge in one direction.
We use polarity (+ and −) on batteries to indicate which
direction the charge is being pushed.
Two equivalent representation of the
same voltage: (a) point a is 9 V above
point b, (b) point b is −9 V above point a.
•
dc voltage is represented by V and ac voltage is represented by υ.
Power and Energy
•
Power is the rate of expending and absorbing energy,
measured in watts(W).
p = power in watts (W = J/s),
dω
p =
=i
dt
ω = energy in joules (J),
t = time in seconds (s).
υ = voltage in volts (V).
i = current in amperes (A).
p=±υi
• Circuit elements that absorb power has positive value of p.
• Circuit elements that supply (produce) power has negative value of p.
(a) Absorbing power (b) supplying power.
Power and Energy
•
Energy is the capacity to do work, measured in joules(J).
p = power in watts (W),
=
•

t
t
0
p d t = t i d t
t
0
ω = energy in joules (J),
t = time in seconds (s).
υ = voltage in volts (V).
i = current in amperes (A).
= t pdt =p(t − t0 )
t
If current and voltage are constant (dc). The power is:
0
• Law of conservation of energy. Total power in a circuit at any instant is must be
zero.
p=0
+Power absorbed = −Power supplied
Power and Energy : Problems
Ex. 4:
Passive Sign Convention
•
Passive sign Convention (PSC) is satisfied when current
enters through the positive terminal of an element.
•
This is also true for power
▪ If PSC is satisfied: p= υi
▪ If PSC is not satisfied: p=−υi
Passive Sign Convention
Example:
Circuit Elements
•
Ideal Independent Source: provides a specified voltage or
current that is completely independent of other circuit variables
•
Ideal Independent Voltage source:
(a) Independent voltage source
(constant / time varying)
(b) Independent voltage source (battery).
•
Ideal Independent Current source:
Circuit Elements
•
Ideal dependent sources: controlled by other voltage or current.
(a) dependent voltage source
(b) dependent current source