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SI Units

  • "Systeme International", or SI Units, is a standardised system of measurement based on internationally agreed definitions.
  • It has 7 Base Units, from which all other units are derived:
    • Metre (m) - Length
    • Kilogram (Kg) - Mass
    • Second (s) - Time
    • Kelvin (°K) - temperature
    • Ampere (A) - Current
    • Candela (Cd) - Luminous Intensity
    • Mole (mol) - Amount of a substance
  • All the base units have standard definition, for example, the metre is the distance travelled by light in a vacuum in seconds.
  • Other units are derived from the base units. For example:
    • Velocity (ms-1)
    • Acceleration (ms-2)
    • Density (Kgm-2)
  • Some of the derived units also have their own special unit names. For example:
    • Coulomb (C) - (As) Charge
    • Pascal (Pa) - (Kgm-1s-2) Pressure
    • Ohms (Ω) - (Kgm2s-3A-2) Resistence

Prefixes

  • In order to make working with large or small numbers more convenient, a system of prefixes is used, where the unit is multiplied by a certain power of ten:
    • Yotta (Y) - 1024
    • Zetta (Z) - 1021
    • Exa (E) - 1018
    • Peta (P) - 1015
    • Tera (T) - 1012
    • Giga (G) - 109
    • Mega (M) - 106
    • Kilo (k) - 103
    • Milli (m) - 10-3
    • Micro (μ) - 10-6
    • Nano (n) - 10-9
    • Pico (p) - 10-12
    • Femto (f) - 10-15
    • Atto (a) - 10-18
    • Zepto (z) - 10-21
    • Yocto (y) - 10-24
  • In addition, these prefixes are also used:
    • Hecto (h) - 102
    • Deca (da) - 101
    • Deci (d) - 10--1
    • Centi (c) - 10-2
  • For example, 0.005 A can be written as 5 mA, and 6900000000000000000000000 m can be written as 6.9 Ym.