eeng2009lec1 - EENG2009 Introduction to Circuits and...

EENG2009 FS 2006 Part 1: Charge, etc.
EENG2009
Introduction to Circuits and
Electronics
Part 1: Charge, Current, Voltage,
Energy, Power, Sources
2.1 Major Topics of the Course
2.2 Circuits Quantities
2.3 Charge
2.4 Current
2.5 Voltage
2.6 Energy and Power
2.7 Practical Examples
2.8 Independent & Dependent Sources
EENG2009 FS 2006 Part 1: Charge, etc.
2.1 Major Topics of the Course:
1. Basics, DC Analysis Techniques
2. Capacitors, Inductors, and Transient
Analysis Techniques
3. AC Circuits, AC Power, Transformers,
and Three-Phase
4. Semiconductors, Diodes, Op
Amplifiers, and Power Supplies

EENG2009 FS 2006 Part 1: Charge, etc.
2.2 Fundamental Circuits Quantities
q (charge: coulombs, C )
i (current: amperes, A )
v (voltage: volts, V )
w (energy: joules, J )
p (power: watts, W )
S
complex power:
volt-amps, VA ,
watts, W
volt-amps-reactive, VAR
f (frequency: hertz, Hz )
EENG2009 FS 2006 Part 1: Charge, etc.
2.3 Charge
When you rub a comb with a woolen cloth, a
negative charge is produced on the comb and a
positive charge is produced on the cloth .
(Benjamin Franklin defined the charge on the
comb as negative .)
The comb acquires its negative charge because
some of the electrons on the cloth are rubbed off
onto the comb .
One coulomb is defined as the amount of charge
on identically charged particles, separated by
one meter in a vacuum, that repel each other
with a force of 10 –7 c 2 N , where c is the velocity
of light (2.997… x 10 8 m/s).
Recall that a single electron charge is –1.602 x
10 –19 C. This is quite small: For example, the
charge needed to plate 1 g of copper is 3,036 C.

EENG2009 FS 2006 Part 1: Charge, etc.
2.4 Current
Charge in motion results in energy transfer. Of
special interest is the case where the motion is
confined to a definite path. In this case, current
“flows.” Current is the movement of
electrical charge—the flow of electrons through
an electronic circuit.
Current (i) is the rate at which charge “flows,” or
is transferred.
1 amp = 1 coulomb/second
It is convenient to think of current as the flowing
motion of positive charge even though we know
that current flow in metallic conductors results
from electron motion (that is, the charge carriers
are negative and move in the opposite direction.)
i = dq/dt amperes
Memorize me!!
EENG2009 FS 2006 Part 1: Charge, etc.
Charge/Current Relationship
++ + + + + + +
The charge and the current flow through the
conductor, passing from one side of the
imaginary surface to the other.
During the time interval from t
a
to t
b
the charge
passing through the imaginary surface is
i(t) i(t)
Memorize me!!
b
a
t
t
idt
q
=
∫

EENG2009 FS 2006 Part 1: Charge, etc.
In defining current, both an arrow and a symbol
are required. It takes two!
Example:
Circuit analysis shows that in the given circuit, i
1
has the value 0.5 A. Give the corresponding
physical interpretation.
1 V
10 Ω
10 Ω
5 Ω
12 V
i
1
= 0.5 A
Solution:
Since i
1
is 0.5 A, this means that 0.5 A, or 0.5
C/s is flowing in the direction of the current-
reference arrow. This current flows through the 5
Ω , to the + side of the battery, through the
battery, exiting from the – side of the battery.