Chapter 8

Units, prefixes and

engineering notation

8.1 Introduction

Of considerable importance in engineering is a knowl-
edge ofunitsof engineeringquantities,theprefixes used
with units, and engineering notation.
We need to know, for example, that

80kV= 80 × 103 V,which means 80000 volts

and 25mA= 25 × 10 −^3 A,

which means 0.025 amperes

and 50nF= 50 × 10 −^9 F,

which means 0.000000050 farads

This is explained in this chapter.

8.2 SI units

The system of units used in engineering and science
is the Système Internationale d’Unités (International
System of Units), usually abbreviated to SI units,and is
based on the metric system. This was introduced in 1960
and has now been adopted by the majority of countries
as the official system of measurement.
The basic seven units used in the SI system are listed in
Table 8.1 with their symbols.
There are, of course, many units other than these seven.
These other units are called derived unitsand are
defined in terms of the standard units listed in the table.
For example, speed is measured in metres per second,
therefore using two of the standard units, i.e. length and
time.

Table 8.1Basic SI units

Quantity Unit Symbol

Length metre m(1m=100cm

=1000mm)

Mass kilogramkg (1kg=1000g)

Time second s

Electric current ampere A

Thermodynamic

temperature

kelvin K(K=◦C+273)

Luminous

intensity

candela cd

Amount of

substance

mole mol

Some derived units are givenspecial names.Forexam-

ple, force=mass×acceleration has units of kilogram

metre per second squared, which uses three of the base

units, i.e. kilograms, metres and seconds. The unit of kg

m/s^2 is given the special name of aNewton.

Table 8.2 contains a list of some quantities and their

units that are common in engineering.

8.3 Common prefixes

SI units may be made larger or smaller by usingprefixes

which denote multiplication or division by a particular

amount.

DOI: 10.1016/B978-1-85617-697-2.00008-9