Measurement of any physical quantity involves comparison with a certain basic arbitrarily chosen, internationally accepted reference standard known as unit. A Unit should be - well defined, easy to reproduce, easy to compare, internationally accepted and independent of changes in physical conditions.
Units are of two types :
1. Fundamental Unit
2. Derived Unit
The physical quantities which do not depend upon other quantity are known as fundamental quantities and their units are known as fundamental units.
There are seven fundamental S.I. units given below :
Physical quantity | Name of the unit | Symbol |
Length | meter | m |
Mass | kilogram | kg |
Time | second | S |
Electric current | Ampere | A |
Temperature | Kelvin | K |
Amount of substance | Mole | Mol |
Luminous intensity | Candela | Cd |
Besides these seven fundamental units, two supplementary units are also defined, viz., radian (rad) for plane angle and steradian (sr) for solid angle.
All the units which are defined/expressed in terms of fundamental units are called derived units.
Length | Mass | Time |
light year = 9.46 x 10^{15} m (distance that light travels with velocity of 3 x 10^{8} m/s in 1 year) | 1 tonne or 1 metric ton = 10^{3} kg | Solar day : It is the time taken by earth to complete one rotation about its axis w.r.t. then sun. 1 year= 365.25 solar days. |
1 astronomical unit = 1 AU = 1.496 x 10 ^{11} m. (average distance of the sun from the earth) | 1 quintal = 10^{2} kg | Lunar month : It is the time taken by moon to complete one revolution around the earth in its orbit. 1 lunar month = 27.3 days |
1 parsec = 3.08 x 10^{16} m (parsec is the distance at which average radius of earthâ€™s orbit subtends an angle of 1 arc second) | 1 pound = 0.4536 kg | Solar year : It is the time taken by the earth to complete one revolution around the sun in its orbit. |
1 micron = 10^{-6} m | 1 atomic mass unit = 1 amu or 1 u = 1.66 x 10^{27} kg | Tropical year : It is that year in which solar eclipse occurs. |
1 angstrom = 10^{-10} m | Leap year : It is that year in which solar eclipse occurs. | |
1 fermit = 1 f = 10^{-15} m | 1 solar day = 86400 s |
Prefix | Symbol | Multiplier |
deci | d | 10^{-1} |
centi | c | 10^{-2} |
mili | 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} |
deca | da | 10^{1} |
hecto | 10^{2} | |
kilo | k | 10^{3} |
mega | M | 10^{6} |
giga | G | 10^{9} |
tera | T | 10^{12} |
peta | P | 10^{15} |
exa | E | 10^{18} |
zatta | Z | 10^{21} |
yotta | Y | 10^{24} |