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.