A unit is a particular
physical quantity, defined and adopted by convention, with which
other particular quantities of the same kind are compared to
express their value. A physical quantity is a quantity
that can be used in the mathematical equations of science and
engineering. The value of a physical quantity is the
quantitative expression of a particular physical quantity as the
product of a number and a unit, the number being its numerical
value. Thus, the numerical value of a particular physical
quantity depends on the unit in which it is expressed. When
making measurements, it is customary to record both the quantity
(how much) and the unit (of what). Science and technology depend
largely on the unit of measurement. For example, the
value of the height h of building is h = 120 m.
Here h is the physical quantity, its value expressed in
the unit “meter,” unit symbol m, is 120 m, and its numerical
value when expressed in meters is 120.
The SI System of Measurement
A
system of units is a class of units defined by composition
from a base set of units, such that every instance of the class
is “standard” unit for a physical dimension and every physical
dimension has an associated unit.
A measurement of any physical quantity must be
expressed as a number followed by a unit. A unit is a
standard by which a dimension can be expressed numerically. The
units for the fundamental dimensions are called the
fundamental or base units.
While carrying out engineering calculations,
there are several systems of base units that are available.
However, they may be broken into two main groups. First, the
International System of Units (also called SI, from the
French "Système International des Unités")
introduced by Griorgi in 1901, including the
meterkilogramsecondampere (MKSA) subsystem representing
the four fundamental dimensions length, mass,
time, and electric current, respectively. Second is
the centimetergramsecond (CGS) system. The units for
other dimensions are called secondary or derived
units and are based on the above fundamental units.
The International System of Units
has seven base units, several derived units with special names,
and many derived units with compound names.
The seven base units are
the building blocks from which the derived units are
constructed. Each base unit is defined by a very precise
measurement standard that gives the exact value of the unit. The
base units are not related to one other, no do they depend on
each other for their definition. The complete SI system involves
units and other recommendations, one of which is that multiple
and submultiples of the MKSA units be set in steps of 10^{3
}or 10^{3}. The base units SI units and
abbreviations are listed in Table 1
The SI has tremendous advantage over previous
systems because it uses a unique unit name for each physical
quantity and it assigns a unique symbol for each name. This book
employs the SI units that are commonly adhered to by virtually
allengineering professional societies.
The SI is the standard system used in today’s
scientific literature.
Table
1
The Seven Fundamental SI Units
Quantity 
Unit 
Abbreviation 
Length
Mass
Time
Electric current
Temperature
Luminous intensity
Matter 
meter
kilogram
second
ampere
kelvin
candela
mole 
m
kg
s
A
K
cd
mol 
The SI derived units are
formed from the previously defined SI base units. Table 2 lists
many of the SI derived units used in electric and electronic
circuits.
The SI uses the decimal system to
relate larger and smaller units to the basic units, and employs
prefixes to signify the various powers of 10. A list of prefixes
and their symbols is given in Table 3. These prefixes are very
important in engineering studies and are worth memorizing.
Table 2
SI Derived Units
Quantity 
Symbol 
Unit 
Unit Symbol 
Angle 
q 
radian 
rad 
Capacitance 
C 
farad 
F 
Conductance 
G 
siemens 
S 
Electric charge 
Q 
coulomb 
C 
Electromotive force 
E 
volt 
V 
Energy, work 
W 
joule 
J 
Force 
F 
newton 
N 
Frequency 
f 
hertz 
Hz 
Inductance 
L 
henry 
H 
Power 
P 
watt 
W 
Resistance 
R 
ohm 
W 
Pressure 
p 
pascal 
Pa 
Magnetic Flux

f 
weber 
Wb 
Magnetic Induction

B 
tesla 
T 
Light Flux

L 
lumen 
lm 
Table 3
SI Prefixes
Metric Symbol 
Metric Prefix 
Value 
Power of Ten 
T 
One Trillion 
Tera 
10^{12} 
G 
One Billion 
Giga 
10^{9} 
M 
One Million 
Mega 
10^{6} 
K 
One Thousand 
Kilo 
10^{3} 
m 
One Thousandth 
Milli 
10^{3} 
m 
One Millionth 
Micro 
10^{6} 
n 
One Billionth 
Nano 
10^{9} 
p 
One Trillionth 
Pico 
10^{12} 