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.