Technology is rapidly changing the way we do things; we now have computers
in our homes, electronic control systems in our cars, cellular phones that
can be used just about anywhere, robots that assemble products on production
lines, and so on.
A first step to understanding these technologies is electric circuit theory.
Circuit theory provides you with the knowledge of basic principles that you
need to understand the behavior of electric and electronic devices, circuits,
and systems. In this book, we develop and explore its basic ideas.
After studying this chapter, you will be
• describe the SI system of measurement,
• convert between various sets of units,
• use power of ten notation to simplify
handling of large and small numbers,
• express electrical units using standard
prefix notation such as mA, kV, mW, etc.,
• use a sensible number of significant digits
• describe what block diagrams are and
why they are used,
• convert a simple pictorial circuit to its
• describe generally how computers fit in
the electrical circuit analysis picture.
An electrical circuit is a system of interconnected components such as resistors,
capacitors, inductors, voltage sources, and so on. The electrical behavior
of these components is described by a few basic experimental laws. These
laws and the principles, concepts, mathematical relationships, and methods of
analysis that have evolved from them are known as circuit theory.
Much of circuit theory deals with problem solving and numerical analysis.
When you analyze a problem or design a circuit, for example, you are typically
required to compute values for voltage, current, and power. In addition to a
numerical value, your answer must include a unit. The system of units used for
this purpose is the SI system (Systéme International). The SI system is a unified
system of metric measurement; it encompasses not only the familiar MKS
(meters, kilograms, seconds) units for length, mass, and time, but also units for
electrical and magnetic quantities as well.
Quite frequently, however, the SI units yield numbers that are either too
large or too small for convenient use. To handle these, engineering notation and
a set of standard prefixes have been developed. Their use in representation and
computation is described and illustrated. The question of significant digits is also
Since circuit theory is somewhat abstract, diagrams are used to help present
ideas. We look at several types—schematic, pictorial, and block diagrams—and
show how to use them to represent circuits and systems.
We conclude the chapter with a brief look at computer usage in circuit analysis
and design. Several popular application packages and programming languages
are described. Special emphasis is placed on OrCAD PSpice and Electronics
Workbench, the two principal software packages used throughout this book.
Hints on Problem Solving
DURING THEANALYSIS of electric circuits, you will find yourself solving quite a few
problems.Anorganized approach helps. Listed beloware some useful guidelines:
1. Make a sketch (e.g., a circuit diagram), mark on it what you know, then identify
what it is that you are trying to determine. Watch for “implied data” such
as the phrase “the capacitor is initially uncharged”. (As you will find out
later, this means that the initial voltage on the capacitor is zero.) Be sure to
convert all implied data to explicit data.
2. Think through the problem to identify the principles involved, then look for
relationships that tie together the unknown and known quantities.
3. Substitute the known information into the selected equation(s) and solve for
the unknown. (For complex problems, the solution may require a series of
steps involving several concepts. If you cannot identify the complete set of
steps before you start, start anyway.As each piece of the solution emerges, you
are one step closer to the answer. You may make false starts. However, even
experienced people do not get it right on the first try every time. Note also that
there is seldom one “right” way to solve a problem. You may therefore come
up with an entirely different correct solution method than the authors do.)
4. Check the answer to see that it is sensible—that is, is it in the “right ballpark”?
Does it have the correct sign? Do the units match?