Notes on Units and Measurements

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Units and Measurements


Units

  • The chosen standard used for measuring a physical quantity.
  • Unit should be:
    • well define
    • easy to reproduce
    • easy to compare
    • internationally accepted
    • independent of changes in physical conditions
  • Two different types:
    • Fundamental unit
    • Derived unit
  • internationally accepted system of units include:
        • CGS system
        • FPS System
        • MKS system
        • SI Units
  • In SI Units, there are seven fundamental units that are given below:
Physical Quantity SI unit Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd
  • Besides these seven fundamental units, two supplementary units are: radian (rad) for plane angle and steradian (sr) for solid angle.
  • Derived units: Defined/expressed in terms of fundamental units
Derived quantity Name Symbol
area square meter m2
volume cubic meter m3
speed, velocity meter per second m/s
acceleration meter per second squared m/s2
wave number reciprocal meter m-1
mass density kilogram per cubic meter kg/m3
specific volume cubic meter per kilogram m3/kg
current density ampere per square meter A/m2
magnetic field strength ampere per meter A/m
amount-of-substance concentration mole per cubic meter mol/m3
luminance candela per square meter cd/m2

Prefixes used in the metric system:

Purpose Prefix Name Prefix Symbol Value
larger quantities
or whole units
yotta Y 1024 Septillion
zetta Z 1021 Sextillion
exa E 1018 Quintillion
peta P 1015 Quadrillion
tera T 1012 Trillion
giga G 109 Billion
mega M 106 Million
kilo k 103 Thousand
hecto h 102 Hundred
deka da 101 Ten
100 One
smaller quantities
or sub units
deci d 10-1 Tenth
centi c 10-2 Hundredth
milli m 10-3 Thousandth
micro μ 10-6 Millionth
nano n 10-9 Billionth
pico p 10-12 Trillionth
femto f 10-15 Quadrillionth
atto a 10-18 Quintillionth
zepto z 10-21 Sextillionth
yocto y 10-24 Septillionth
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