EE301 Lesson 1:
Introduction (Chapter 1),
Voltage & Current
(Chapter 2)
1
Learning Objectives
• Apply SI (International System of Units) units and
engineering notation for standard electrical
quantities.
• Apply unit conversion factors when solving
engineering problems.
• Describe the concepts of voltage potential and
current.
2
S.I.
3
Example Problem 1
Given a speed of 60 miles per hour (mph),
a. convert to kilometers per hour:
60miles 1.609344km
*
 96.561km / hr
hour
1mile
b. convert answer for part (a) to meters per second:
96.561km 1000m 1hr
*
*
 26.823m / s
hr
1km 3600 s
Some common unit conversions
are found in Appendix A.
4
Engineering prefixes
• In the SI system, common multiple powers of 10 are
denoted using engineering prefixes.
Power
1012
109
106
103
Prefix
tera (T)
giga (G)
mega (M)
kilo (k)
5
Power
10-3
10-6
Prefix
milli (m)
micro ()
10-9
10-12
nano (n)
pico (p)
Engineering notation
• It is common practice in engineering to avoid using
exponential notation if a suitable engineering prefix exists.
• For example:
15  10-5 sec
150 s
not common engineering practice
common engineering practice
• General guideline: use closest prefix so that you have at least
one NON-ZERO number to the left of the decimal place
0.15 msec not common engineering practice
common engineering practice
150 sec
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Example Problem 2
Express the following using engineering notation:
a.
10  104 volts
100 * 103 Volts
or
100 kilo-Volts (kV)
b.
0.1  10-3 watts
100 * 10-6 Watts
or
100 microWatts (W)
c.
250  10-7 seconds
25 * 10-6 seconds
or
25 microseconds (s)
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Significant Digits
• Keep all digits in calculator while performing
computations.
• Include at least 3 significant digits in all answers.
• Try to keep at least one digit to the right of the
decimal point.
8
Voltage & Current
•
The term voltage is encountered practically every
day.
•
We are aware that most outlets in our homes are 120
volts.
•
Although current may be a less familiar term, we
know what happens when we place too many
appliances on the same outlet; the circuit breaker
opens due to the excessive current that results.
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Atomic theory
• Electrons have a negative charge (-).
• Electrons orbit the nucleus at distinct orbital radiuses
known as shells.
• The outermost shell is called the valence shell.
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Charge
• Materials accumulate charge by the transfer of
electrons.
• The amount of charge is denoted by the letter Q and
the unit of measurement is called a Coulomb (C).
• 1 Coulomb of charge is the total charge associated
with 6.24 * 1018 electrons.
• Therefore, the charge on 1 electron (Qe) is:
1
19
Qe 

1.602

10
C
18
6.24 10
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Voltage
• If we separate the 29th electron from the
rest of the atomic structure of copper by
a dashed line as shown, we create
regions that have a net positive and
negative charge as shown in the figure to
the right.
• This positive region created by separating
the free electron from the basic atomic
structure is called a positive ion.
• In general, every source of voltage is
established by simply creating a
separation of positive and negative
charges.
Defining the positive ion
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What is Voltage?
• Work is required to separate positive and negative
charges.
• These separated charges have potential energy.
• The voltage (or potential difference) (V) between
two points is defined as one volt if
it requires one joule (J) of energy (W) to
move one coulomb of charge (Q)
from
one point to another.
W
V
[volts, V]
Q
Where:
V = volts
W = joules (J)
Q = coulombs (C)
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Voltage
W
V
Q
[volts, V]
Defining the voltage between
two points.
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Voltage
• Since it would be inconsequential to talk about the
voltage established by the separation of a single
electron, a package of electrons called a coulomb (C)
of charge is defined as follows:
− One coulomb (C) of charge is the total charge associated
with 6.242 x 1018 electrons.
− If a total of 1 joule (J) of energy is used to move the
negative charge of 1 coulomb (C), there is a difference of 1
volt (V) between the two points.
W
V
Q
[volts, V]
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Example Problem 3
If 600J of energy are required to move 9.36x1019
electrons from one point to another, what is the
potential difference (voltage) between the two points?
W
V
Q
Recall:
Q = 1 C = 6.24*1018 electrons
9.36*1019 e
Q
 15C
18
6.24*10 e
600 J
V
15C
V  40V
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Voltage
• Since the potential energy associated with a body is
defined by its position, the term potential is often
applied to define voltage levels.
− For example, the difference in potential is 4 V between two
points of a circuit, or the potential difference between a
point and ground is 12 V, and so on…
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Example Problem 4
Determine the energy expended moving a charge of 50
μC through a potential difference of 6 V.
W  Q *V
W  (50*106 C ) *(6V )
W  300 * 10-6 J  300  J
18
Voltage Sources
• In general, direct current (dc) voltage sources can be
divided into three basic types:
1. Batteries (chemical action or solar energy).
2. Generators (electromechanical).
3. Power supplies (rectification—a conversion process to be
described in your electronics courses).
Symbol For DC Voltage Source
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What is Current?
• The rate of flow of charge is known as electric
current.
• The measure of current, an ampere (I) is defined as a
rate of flow of one coulomb of charge per second.
Where:
I = amperes (A)
Q = coulombs (C)
t = time (s)
Q
I
[amperes, A]
t
20
Example Problem 5
If 840 coulombs of charge pass through the imaginary
plane of below during a time interval of 2 minutes,
what is the current?
840C
Q
 7 Amperes ( A)
I 
t 2minutes *( 60seconds )
1minute
21
Direction Of Current
• Initially it was believed that current was the flow of
positive charges. This is called conventional
current direction.
• The actual flow of charge is by electrons (negative
charge) called electron flow direction.
• We will use conventional current (figure (a) below).
22
Current
Safety Considerations
• It is important to realize that even small levels of
current through the human body can cause serious,
dangerous side effects.
• Experimental results reveal that the human body
begins to react to currents of only a few milliamperes
(mA).
• Although most individuals can withstand currents up
to perhaps 10 mA for very short periods of time
without serious side effects, any current over 10 mA
should be considered dangerous and potentially
lethal.
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DC Terminal Characteristics
• A dc voltage source will provide ideally a fixed terminal
voltage, even though the current demand from the electrical
system may vary.
• Figure shows terminal characteristic of:
a) Ideal voltage source
b) Ideal current source
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Current
• In summary; the applied voltage (or potential
difference) in an electrical/electronics system is the
“pressure” to set the system in motion, and the
current is the reaction to that pressure.
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QUESTIONS?
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