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E E 2315 Circuits I
Lecture 1 - Introduction to
Electric Circuits
Overview of Electrical Engineering
• The electrical engineer combines the
physicist’s models of natural phenomena
with the mathematician’s tools for
manipulating those models to produce
systems that meet practical needs.
• Electrical systems can be divided into five
major classifications.
5 Linked Classifications of Electrical
Systems
• Communications Systems
• Computer Systems
• Control Systems
• Power Systems
• Signal Processing Systems
Communications Systems
• Telephony
– Analog and Digital
– Switched Channels
• Radio
– Broadcast AM, FM, & SW
– Two-Way
• Television
Computer Systems
• Programmable Microcircuits
• High-speed switching of logic circuits
• Used for
– Computation
– Control
Control Systems
• Automated
• Adaptable
• Faster Operation than Manual
• More Reliable than Manual
• Modern high-performance aircraft rely on
automated control systems
Power Systems
• Large networks connected by low
frequency a-c transmission lines
• Small networks in aircraft and spacecraft
• Electromechanical Energy Conversion
• Power Electronics
– High frequency switching converters
– High efficiency, high power density
Signal Processing Systems
• Transform and manipulate signals and the
information they contain
• Image processing
– Data from weather satellites
– MRI scans of the human body
• Noise reduction
• Encryption
Circuit Theory
• Mathematical modeling of physical
phenomena
• Circuit theory is a special case of
electromagnetic field theory
– General EM theory is more complex than
circuit theory
– General EM theory requires more complex
mathematics
Three Basic Assumptions of Circuit
Theory
• Electric effects happen
instantaneously throughout a
system
• Net Charge on every
component in the system is
zero
• No magnetic coupling
between components
Problem Solving
• Identify what is given and what is to be
found.
• Sketch a circuit diagram or other visual
model
• Think of several solution methods and a
way of choosing between them
• Calculate a solution
Problem Solving (continued)
• Use your creativity
– If your efforts are not converging to a
solution, you may want to rethink
your assumptions.
• Test your solution
– Is your answer reasonable?
– Does your answer validate your
assumptions?
International System of Units
–
–
–
–
–
–
–
–
Frequency
Force
Energy or work
Power
Electric charge
Electric potential
Electric resistance
Electric conductance
hertz (Hz) s-1
newton (N)kg·m/s2
joule (J)
N·m
watt (W) J/s
coulomb A·s
volt (V)
W/A
ohm () V/A
siemens (S) A/V
International System of Units
(continued)
– Electric capacitance
– Magnetic flux
– Inductance
farad (F)
weber (Wb)
henry (H)
C/V
V·s
Wb/A
Standardized Prefixes
•
•
•
•
•
•
•
•
atto
femto
pico
nano
micro
milli
centi
deci
a
f
p
n

m
c
d
10-18
10-15
10-12
10-9
10-6
10-3
10-2
10-1
•
•
•
•
•
•
deka
hecto
kilo
mega
giga
tera
da
h
k
M
G
T
10
102
103
106
109
1012
Circuit Analysis: An Overview
• A circuit model is used to connect our
visualization to our analysis of a
physical system
• The elements of our circuit model are
ideal circuit components.
• The behavior of output parameters is
governed by physical/mathematical
laws for the elements of the circuit
model.
Voltage and Current
• Voltage is the energy per unit of charge.
• Current is the rate of flow of charge.
Voltage and Current (continued)
• The relationship between voltage and
current in a circuit element defines that
circuit element.
• Both voltage and current have associated
polarities.
• These polarities determine the direction of
power flow.
Ideal Basic Circuit Element
• Three attributes of an ideal circuit
element:
– There are only two terminals
– Described mathematically in terms of
current and/or voltage
– Cannot be subdivided into smaller
components
Voltage Polarity Definitions
• Positive v
– voltage drop from 1 to 2 or
– voltage rise from 2 to 1
• Negative v
– voltage drop from 2 to 1 or
– voltage rise from 1 to 2
Current Polarity Definitions
• Positive i
– Positive charge flowing from 1 to 2
– Negative charge flowing from 2 to 1
• Negative i
– Positive charge flowing from 2 to 1
– negative charge flowing from 1 to 2
Passive Sign Convention
• Whenever the reference direction for the
current in an element is in the direction of
the reference voltage drop, use a positive
sign in any expression that relates voltage
to current. Otherwise, use a negative
sign.
Power and Energy
• Power associated with a circuit element is
consumed by that circuit element when
the value of power is positive.
• Conversely, power is generated, or
produced by the element if the value
consumed is negative.
Expression of Power