Lecture 1
TE 262
Learning is not attained by chance; it must be sought for with ardour
and attended to with diligence.
- Abigail Adams
(in letter to John Quincy Adams, 1780)
The Electromagnetic Model
Overview
 Electromagnetic theory is a discipline concerned with the study
of charges at rest and in motion.
 Existence of electric charges was discovered by Greek
astronomer and philosopher Thales of Miletus
 He notices that an amber rod, after being rubbed with silk or
wool attracted small bits of paper
 The word electron was derived from the Greek work for amber
which is elektron, from which electronics, electricity, … etc
stems from.
Overview
 An electric field has two sources: a positive charge and a
negative charge.
 Moving electric charges produce current which gives rise to
magnetic field.
 A field is a spatial distribution of a quantity which may or may
not be a function of time.
 Therefore a time-varying electric field is accompanied by a
magnetic field, and vice versa.
 A time-varying electric field coupled with a time-varying
magnetic field results in an electromagnetic field.
Overview
 Why is the concept of electromagnetics essential ?
 The concept of fields and waves are essential in the
explanation of actions at a distance.
 For example, how does an object fall toward the earth surface if
there are no elastic strings connecting a free-falling object and
the earth ??
 This can be explained by the existence of a gravitational field
 Similarly, the possibilities of satellite communication can be
explained only by postulating the existence of electric and
magnetic fields.
Overview
 A mobile phone on transmit sends a
message-carrying current at a certain
frequency from the source to the antenna
ends
 From circuit-theory point of view, the
end of the antenna is connected to an
open space (open circuit). Hence no
current will flow
How is mobile communication possible
then ??
 Electromagnetic concepts must be used
to explain this phenomenon, since
circuit-theory is limited in this regard
Overview
Three essential steps are involved in building a theory or model
Step 1: Define some basic quantities relevant to the subject of study
Step 2: Specify the rules of operation (mathematics) of these quantities
Step 3: Postulate some fundamental relations or Laws
For instance in Circuit theory (Circuit model):
Step 1 involves defining basic quantities like resistors, capacitors,
inductors, voltages and currents
Step 2 involves defining algebraic rules, differential equations, and
Laplace transformations.
Step 3 involves creating laws like Kirchhoff's voltage and current laws
Overview
Similarly, in electromagnetic model:
Step 1 will be defining the basic quantities of electromagnetics
Step 2 will be defining the rules of operation. This includes vector
algebra and vector calculus
Step 3 will be presenting the fundamental postulates/laws of
electromagnetics
Overview
 Electromagnetic principles are fundamental to the study of
Electrical Engineering and Physics.
 Electromagnetic theory is also indispensable to the
understanding, analysis and design of various electrical,
electromechanical and electronic systems.
 Some of the branches of study where electromagnetic principles
find application are:
Overview
 RF communication
 Microwave Engineering
 Antennas
 Electrical Machines
 Satellite Communication
 Atomic and nuclear research
 Radar Technology
 Remote sensing
 EMI EMC
 Quantum Electronics
 VLSI
Overview
 Electromagnetic theory is a prerequisite for a wide spectrum of
studies in the field of Electrical Sciences and Physics.
 Electromagnetic theory can be thought of as generalization of
circuit theory.
 There are certain situations that can be handled exclusively in
terms of field theory.
 In electromagnetic theory, the quantities involved can be
categorized as source quantities and field quantities.
Overview
 Source of electromagnetic field is electric charges: either at rest
or in motion.
 However an electromagnetic field may cause a redistribution of
charges that in turn change the field and hence the separation of
cause and effect is not always visible.
 We use the symbol q (sometimes Q) to denote electric charge.
Electric charge is a fundamental property of matter and exists
only in positive or negative integral multiples of the charge of
an electron, e.
 = 1.602 × 10
( )
Overview
 where C is the abbreviation of the unit of charge, coulomb.
 A coulomb is a very large unit for electric charge; it takes
1/1.602 × 10
to make up 1
 The principle of conservation of electric charge, like the principle
of conservation of energy, is a fundamental postulate or law of
physics.
 It states that electric charge is conserved; that is , it can neither
be created nor be destroyed.
Overview
Source quantities:
∆
Volume charge density,
= lim
/
∆
∆ →
∆ is the amount of charge in a very small volume ∆!
∆
#
Surface charge density,
=
lim
/
"
∆"→ ∆"
Line charge density,
∆
∆$→ ∆$
$ = lim
 Charge densities vary from point to point, hence
point functions of space coordinates
,
/
" and
$ are
&
 Current is the rate of change of charge with respect to time, % =
&'
and the unit is /( or ).
Overview
Current Density
 Current must flow through a finite area, and hence it is not a
point function
 Current density measures the amount of current flowing
through a unit area normal to the direction of current flow. It
is represented by *.
 It is a vector whose magnitude is the current per unit area
A/ # and whose direction is the direction of current flow
Overview
Field Quantities: Four fundamental vector field quantities
 Electric field intensity, , – needed in discussing electrostatics (effects
of stationary electric charges) in free space. It is defined as the force
experienced by a unit positive charge placed at a point.
 Electric flux density, - – useful in the study of electric field in a
material. It is a measure of the strength of an electric field generated
by a free electric charge, corresponding to the number of electric
lines of force passing through a given area
 Magnetic flux density, . – defined as magnetic force acting on a
charge moving with a given velocity.
 Magnetic field intensity, / – measures how strong or weak any
magnetic field is.
Overview
Field Quantities
 All four quantities are point functions; they are defined at
every point in space, and in general, are functions of space
coordinates.
 Material properties determine the relations between 0 and 1
and between 2 and 3.
Overview
TABLE 1: FUNDAMENTAL ELECTROMAGNETIC
FIELD QUANTITIES
Symbol and
Units For Field
Quantities
Electric
Magnetic
Field Quantity
Symbol
Unit
Electric filed intensity
E
V/m
Electric flux density
D
C/m2
Magnetic flux density
B
T
Magnetic field
intensity
H
A/m
Overview
SI Units
 In electromagnetic, four
main SI units are used
Overview
Universal Constants: Three universal constants
 Speed of an EM wave, 4
4=
56 76
 Permittivity of free space, 8
 Permeability of free space, 9
TABLE 2: UNIVERSAL CONSTANTS
Constants
Symbols
Value
Unit
Velocity of light in free space
4
3 × 10;
m/s
Permittivity of free space
8<
1
× 10
36=
F/m
Permeability of free space
9<
4= × 10 ?
H/m
Overview
Relation between fundamental quantities
 @ = AB 0 in free space
 C = DB 3 in free space
Overview
 Difference between Circuit Theory and Field Theory
Circuit Theory
Deals with voltage and current
Field Theory
E and % are scalars
Deals with electric field and magnetic field
G are vectors
,F and /
Radiation effects are neglected
Radiation effects are considered
Transmitter and receiver circuits can be
analysed. However, medium like free space
cannot be analysed
All media can be analysed
THANK YOU