Transcript EE302 Lesson 1: Introduction

Lesson 1: Introduction,
Voltage, Current and
Resistance
Learning Objectives

Apply SI units and engineering notation for standard
electrical quantities.

Apply unit conversion factors when solving engineering
problems.

Describe the concepts of voltage potential and current.
S.I.
Example Problem 1
Given a speed of 60 miles per hour (mph),
a. convert it to kilometers per hour.
b. convert it to meters per second.
Some common unit conversions
are found in Appendix A.
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)
Power
10-3
10-6
10-9
10-12
Prefix
milli (m)
micro ()
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
not common engineering practice
0.15 msec
common engineering practice
150 sec
Example Problem 2
Express the following using engineering notation:
a. 10  104 volts
b. 0.1  10-3 watts
c. 250  10-7 seconds
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 pont.
INTRODUCING VOLTAGE AND 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.
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.
Charge

1
19
Qe 

1.602

10
C
18
6.24  10
VOLTAGE

If we separate the 29th
electron in 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 Fig.
Defining the positive ion.
VOLTAGE
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.

What is Voltage?



Work is required to separate positive and
negative charges.
These separated charges have potential energy.
The voltage (or potential difference) between
two points is defined as one volt if
it requires one joule of energy to
move one coulomb of charge from
one point to another.
W
V
Q
[volts, V]
VOLTAGE

W
V
Q
[volts, V]
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 was defined as follows:
 One
coulomb 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.
Example Problem 3
If 600J of energy are required to move 9.36x1019
electrons from one point to another, what is the
potential difference between the two points?
W
V
Q
[volts, V]
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 the two points, or the potential
difference between a point and ground is 12
V, and so on.
VOLTAGE SOURCES

In general, dc voltage sources can be
divided into three basic types:
 Batteries
(chemical action or solar energy)
 Generators (electromechanical)
 Power supplies (rectification—a conversion
process to be described in your electronics
courses).
What is Current?


The rate of flow of charge is known as electric
current.
The measure of current, an ampere is defined
as a rate of flow of one coulomb of charge per
second.
Q
I
t
[amperes, A]
Example Problem 4
If 840 coulombs of charge pass through the
imaginary plane of below during a time interval of
2 minutes, what is the current?
Q
I
t
[amperes, A]
Current direction



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.
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.
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.
CURRENT

In summary, therefore, 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.
QUESTIONS