Unit: – The chosen standard used for measuring a physical quantity is called unit.
Unit should be:
(i) Well defined
(ii) easy to reproduce
(iii) Easy to compare
(iv) internationally accepted
(v) Independent of changes in physical conditions
Units are two types
(I) Fundamental Unit
(II) Derived Unit
Systems of Units depend on choice Each Choice of units leads to a new system (set) of units. The internationally accepted systems are (i) CGS system (ii) MKS System (iii) FPS System (iv) SI Units.
In SI Units, there are seven fundamental units given in the following table:
Physical Quantity SI Unit Symbol Physical Quantity SI Unit Symbol
Length metre m Temperature Kelvin K
Mass Kilogram Kg Luminous intensity candela Cd
Time second s Amount of substance mole mol
Electric Current ampere A
Besides these seven fundamental units, two supplementary units are also defined viz radian ( rad ) for plane angle and steradian (sr) for the solid angle. All the units which are defined/expressed in terms of fundamental units are called derived units.
Some important derived units.
S.No. Physical Quantity CGS units SI unit Relation
1. Force dyne Newton 1 Newton=10^5 dyne
2. Work erg joule 1 joule= 10 ^7 erg
Some practical units of length mass and time.
Light Mass Time
Light year = distance 1 quintol = 10^2 kg 1 solar day = 86400 sec.
Travelled by light in one 1 metric ton = 10^3 kg 1 year=365 ½ solar days.
Year in vaccum. 1 atomic mass unit (amu) 1 lunar month
1 astronomical unit(A.U) or Dalton = 1.66 X 10^-27 kg = 27.3 solar days.
=1.5 x10^11 m 1 slug = 14.59 kg Tropical year = it is the
1 parsec = 3.26 1 pound = 0.4537 kg year in which total solar
=3.08×10^16 m eclipse occurs.
1 nautical mile or seamile 1Chandrashekhar limit = Leap year = it is the year
=6020 ft. 1.4 times the mass of sun= in which the month of
1 micron = 1 um=10^-6 m 2.8 x 10 kg February is of 29 days.
1 Angstron (A) =10^-15m