1 describes some the basic units
that shall using our electrical studies.
Definition
SI units are based upon small
number fundamental units from
which all other units may derived.
Units
Very early units measurement were based the things easily available –
the length stride, the distance from the nose the outstretched hand,
the weight stone and the time-lapse one day.6 106
J kWh
Force The effect body Newton N
Frequency Number cycles Hertz mains frequency Hz
Length Distance Metre m
Mass Amount material kilogram One metric tonne 1000kg
Magnetic flux Magnetic energy Weber Wb
Magnetic flux
density B
Number lines of
magnetic flux
Tesla T
Potential or
pressure
Voltage Volt V
Period Time taken to
complete one cycle
Second The 50Hz mains supply has a
period 20ms
Power Rate doing work Watt W
Resistance Opposition to
current flow
Ohm Ω
Resistivity Resistance a
sample piece of
material
Ohm metre Resistivity copper 17.
SI units are based upon small number fundamental units from which
all other units may derived.
Like all metric systems, units have the advantage that prefixes represent-
ing various multiples sub-multiples may used increase decrease
the size the unit various powers 10. Table 4. Different branches of
science and engineering were working isolation, using their own units,
and the result was overwhelming variety units.
In all branches science and engineering there need for practical
system units which everyone can use.5
109
Ωm
Temperature Hotness coldness Kelvin 0°C 273K. 1960, the General Conference
of Weights and Measures agreed international system called the
Système International d’Unités (abbreviated units). Over the years, new
units were introduced and old ones were modified.Basic Electrical Installation Work
80
Table 4.2. Some the more common
prefixes and their symbols are shown Table 4. change is
the same 1°C
Time Time Second 60s min
60min h
Weight Force exerted a
mass
Kilogram 1000kg tonne
Note: more detailed description can found this chapter.
.1 Basic Units
Quantity Measure Basic unit Symbol Notes
Area Length length Metre squared m2
Current Electric current Ampere A
Energy Ability work Joule Joule very small unit 3
