Units & DIMENSIONS - UNITS & RULES

DEFINITIONS FOR S.I. UNITS:
Fundamental quantities
Meter: The distance travelled by light in vacuum in 1 in 299, 792, 458th second is called meter.
Kilogram: kilogram is the mass of a platinum - irridium alloy cylinder proto type kept at Sevres, near Paris.
Second:One second is equal to 1/86,400 of a mean solar day. Mean solar day is time interval between two consecutive mid days in a year. 

Fundamental unit :
    The units used to measure the fundamental quantity is called fundamental unit.
    e.g., Meter for length, kilogram for mass etc...
Derived unit :
    The unit used to measure the derived quantity is called derived unit.
    e.g:- m\(^2\) for area, gm/cm\(^3\)  for density etc…

Radian:  Radian is the angle subtended at the  centre of a circle by an arc whose 
length is equal to the radius.
    2 radian = 360\(^0\) :  1 radian =180\(^0\)
Steradian : The solid angle subtended at the centre of the sphere of radius 1 meter by  its surface of area 1 square meter. Solid angle= normal area/r\(^2\). 
Total solid anglethat can be formed at any point in space or at the centre of a sphere is 4 steradian.
RULES FOR WRITING UNITS:
1.Symbols for a unit named after a scientist should have a capital letter. 
 Ex: N for newton, W for watt, A for ampere.
2. Full names of the units,even when they are named after a scientist should not  be written with a capital letter. 
 Ex: newton, watt, ampere, meter.    
3.Units should be written either in full or in agreed symbols only.Units do not take plural form.
Ex:10kg but not 10 kgs, 20W but not 20 Wsand 2A but not 2As
4.No full stop or punctuation mark should be used within or at the end of symbols for units.
Ex:10W but not 10W.