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Electrical Circuits
Electrical Circuits
 Nearly all branches of electrical engineering are
fundamentally based on circuit theory.
 The only subject in electrical engineering that is
more fundamental than circuit theory is
electromagnetic field theory, which deals with the
physics of electromagnetic fields and waves.
Electrical Circuits
SI base units
 International System of Units
 (SI from the French Système international d'unités)
Base quantity
length
mass
time
electric current
thermodynamic temperature
amount of substance
luminous intensity
Name
Symbol
SI base unit
meter
m
kilogram
kg
second
s
ampere
A
kelvin
K
mole
mol
candela
cd
SI derived unit
plane angle
solid angle
frequency
force
pressure, stress
energy, work, quantity of heat
power, radiant flux
electric charge, quantity of electricity
electric potential difference,
electromotive force
capacitance
electric resistance
electric conductance
magnetic flux
magnetic flux density
inductance
Celsius temperature
luminous flux
illuminance
activity (of a radionuclide)
absorbed dose, specific energy (imparted), kerma
dose equivalent (d)
catalytic activity
radian (a)
steradian (a)
hertz
newton
pascal
joule
watt
coulomb
rad
sr (c)
Hz
N
Pa
J
W
C
N/m2
N·m
J/s
-
m·m-1 = 1 (b)
m2·m-2 = 1 (b)
s-1
m·kg·s-2
m-1·kg·s-2
m2·kg·s-2
m2·kg·s-3
s·A
volt
farad
ohm
siemens
weber
tesla
henry
degree Celsius
lumen
lux
becquerel
gray
sievert
katal
V
F
Omega
S
Wb
T
H
°C
lm
lx
Bq
Gy
Sv
kat
W/A
C/V
V/A
A/V
V·s
Wb/m2
Wb/A
cd·sr (c)
lm/m2
J/kg
J/kg
m2·kg·s-3·A-1
m-2·kg-1·s4·A2
m2·kg·s-3·A-2
m-2·kg-1·s3·A2
m2·kg·s-2·A-1
kg·s-2·A-1
m2·kg·s-2·A-2
K
m2·m-2·cd = cd
m2·m-4·cd = m-2·cd
s-1
m2·s-2
m2·s-2
s-1·mol
Prefix
pico
nano
micro
mili
centi
deci
deca
hecto
kilo
mega
giga
tera
10
9
10
69
10
36
10
23
10
12
10
1
10
21
10
32
10
63
10
96
10
9
SI prefixes
Symbol
P
n
µ
m
c
d
da
h
k
M
G
T
Value
10 12
10 9
10 6
10 3
10 2
10 1
101
10 2
10 3
10 6
10 9
1012
Electrical Circuits
 Electrical circuits can be very simple. such as the
circuit in a flashlight containing two batteries, a light
bulb and a switch.
Electrical Circuits
 The “conductors” that interconnect these components are
usually wires or metal pathways integrated on a printed
circuit hoard.
Electrical Circuits
 Most electrical circuits. however, are much more
complex than a flashlight. A standard television
contains, among other things: power supplies,
amplifiers speakers, and a cathode ray tube.
 The microprocessor in a computer may contain the
equivalent of millions of transistors interconnected
in a single chip that is smaller than a fingernail
Motherboard and CPU
Pentium 4
Electrical Circuits Examples
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PJMM%3A%3BSTfT9SCPeg7BQM%3BvQf0veq4azPJMM%3A
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Electrical Circuits
 The gravitational force is an attractive force that
tends to move objects toward one another, the most
common example being the earth’s gravitational
force that attracts objects toward the center of the
earth.
 Gravitational forces govern the motions of planets.
stars, galaxies. and other celestial objects in the
universe, and yet it is the weakest of all the natural
forces. A type of force that is much stronger than
gravity is electrical in nature.
Electric Charge
 An electrical force is established




between two charged particles.
The force between the particles
is attractive if the charges are
unlike (i.e.. if one charge is
positive and the other is
negative).
The force is repulsive if the
charges are alike, that is. if both
charges are either positive or
negative.
This force is referred to as an
electrostatic force because the
charges are static or stationary.
The branch of electrical studies
that deals with static charges is
called electrostatics.
q  1.602 10
19
C
Coulomb's Law
 Like charges repel, unlike charges attract. The electric force acting
on a point charge q1 as a result of the presence of a second point
charge q2 is given by Coulomb's Law:
 0 is the permitivit y of space
Electron Mass
e
Q
n
m
M
1.60E-19
1
6.24E+18
9.11E-31
5.68626E-12
C
C
Number of Electrons in 1C
kg
kg
Mass of 1C of electrons
Electric Current
 En electric circuit theory
current is generally
considered to he the
movement of positive charges
 This convention is based on
the work of Benjamin
Franklin (1706—1790), who
conjectured that electricity
flowed from positive to
negative.
 Today we know that electric
current in wires and other
conductors is due to the drift
of free electrons (negatively
charged particles) in the
atoms of the conductor.
Coulomb's Law
Coulomb's Law
Electrical Circuits
 The Ideal Basic Circuit Element
 Has Only two terminal
 It is described mathematically in terms of current and/or
voltage
 It cannot be subdivided into other elements
 An electrical circuit may be defined as two or more
Basic Circuit Elements interconnected by
conductors.
Resistance
 Electrical resistance may be defined as an impedance
to current flow through a circuit element.
 All circuit elements, including even the conductors
(wires) that connect them impede the flow of current
to some extent.
The resistance element
i  v/R
I V / R
Different Types of Current
 Direct Current (DC)
 Alternating Current (AC)
 Others
 Electric current is
measured by means of an
instrument called an
ammeter. There are
basically two types of
ammeters:


analog and
digital.
Electric Potential Energy and Voltage
 Potential energy can be defined as the capacity for doing work which
arises from position or configuration.
 Voltage is electric potential energy per unit charge, measured in joules
per coulomb ( = volts). It is often referred to as "electric potential",
which then must be distinguished from electric potential energy by
noting that the "potential" is a "per-unit-charge" quantity.
Voltage Difference
 The word difference denotes that voltage is always
taken between two points.
 To speak of voltage “at a point is meaningless, unless
a second point (reference point) is implied.
 A voltage difference exists across the positive and
negative terminals of a battery.
Voltage
Notation
 Time varying quantities - lower case
e.g. v(t), i(t)
 sometimes assume time: v(t) = v
 Time invariant quantities upper case
e.g. V, R,
 Remember to include units of measure
e.g. 15 V, 7A
Electric Power
w
q
w q w
v
;i
; v i 


; hence,
q
t
q t t
p  v  i ( J / s)
P  VI ( J / s)
Resistors Combinations
 Resistance is measured by
means of an instrument called
an ohmmeter. Like ammeters
and voltmeters that measure
current and voltage, there are
basically two types of
ohmmeters: analog and
digital.


An analog ohmmeter provides
a resistance reading by means
of a needle or pointer that
moves across a calibrated
scale.
Digital ohmmeters provide a
resistance reading by
displaying numbers in a
window.
Example
 Find the total resistance
for the resistor circuit
shown in the Figure
 Suppose that we are
designing a powersupply circuit. Our
circuit design calls for a
resistor that carries a
direct current of 800 mA
and has a voltage drop of
24 V.


What is the resistance of
the resistor?
What power rating must
the resistor have?
Electrical Circuits
 In electrical circuits, there are numerous types of
electrical components such as resistors. capacitors.
inductors, diodes, transistors, transformers,
batteries, lamps. fuses, switches, and motors.
Common circuit elements and their schematic
symbols
Independent Current and Voltage Sources
Nodes and Branches
 A node is defined as a
point of connection of
two or more circuit
elements.
 An essential node is
defined as a point of
connection of three or
more circuit elements.
 Branch, an open path in
a circuit including one or
more circuit elements
and no essential nodes
Kirchhoff's Circuit Laws
Example: Kirchhoff's Voltage Law
 The DC circuit shown in
the Figure consists of a
10-V independent
voltage source connected
to two resistors in series.
Find:
1. The current.
2. The voltage across each
resistor, and
3. The power dissipated by
each resistor.
Example: Kirchhoff's Current Law
 The DC circuit shown in
the Figure consists of a
200-mA independent
current source connected
to two resistors in
parallel. Find:
1. the voltage across the
resistors and
2. the current in each
resistor.
Short and Open Circuits
 Short Circuit.
 Basic Circuit element whose voltage is always 0. (Resistance
=0)
 Symbol
 Open Circuit
 Basic Circuit element whose current is always 0. (Resistance =
infinity)
 Symbol
Practice
3. Find the total resistance
for the resistor circuit
shown in the Figure.
4. For the DC circuit
shown in the Figure,
find the voltage across
each resistor and the
current in each resistor.
Practice
For the DC circuit shown
in the Figure, find the
voltage across each
resistor and the current in
each resistor. Find the
power dissipations in the
2 Ω, 5 Ω and 22 Ω
resistors.
6. For the DC circuit shown
in the Figure, find the
voltage across each
resistor and the current in
each resistor.
5.