What is the definition of Unit and Dimension ?
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Unit and Dimension
Unit : The chosen standard used for measuring a
physical quantity is called unit,
Unit should be
1. well defined
2, easy to reproduce
3 easy to compare
4• internationally accepted
5. independent of changes in physical
conditions
Units are of two types : 1. Fundamental Unit and
2. Derived Unit
System of Units : Units depend on choice. Each choice Of
units leads to a new system (set) of units. The internationally
accepted systems are 1. CGS system 2. MKS System 3. FPS
System 4. SI Units.
In Sl Units, there are seven fundamental units given in
the following table :
• Physical Quantity
Length
Mass
Time
Electric Current
Luminous intensity
Amount of substance
metre
kilogram
second
ampere
kelvin
candela
mole
m
kg
Cd
mol
Besides these seven fundamental units, two
supplementary units are also defined, viz., radian trad] 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.
Some important derived units
Relation
S. Physical Quantity cgs units SI unit
1. Force
2. work
newton 1 newton 105 dyne
dyne
erg joule 1 joule — erg
Some practical units of length, mass and time
@ength
Masg
Light year — distance travelled by light in one year in
vacuum.
ILY=9.46 x 1015m
1 Astronomical Unit (A.LL) = 1.5 x 1011 m
1 Parsec 3.26 ly – 3.08 x 1016 m
1 Nautical mile or Seamile 6076 ft = 1852 m
1 Micron = 1 gm = 10-6 m
1 Angstron (Å) = 10-10 m
1 Quintal 102 kg
1 Metric ton = 103 kg
1 Atomic Mass Unit (amu) or Dalton = 1,66 x 10-27 kg
1 Slug = 14.59 kg
1 Pound – 0.4537 kg
I Chandrashekhar limit = 1.4 times the mass Of sun
-2.8 1030kg
ISolar day — sec.
i Year=3651/2 h solar days
i Lunar month 27.3 solar days.
time i Tropical year It is the year in which total solar ecli
Leap year It is the year in which the month of
is of 29 days.
Astronomical distances are measured in light year
because speed of light is always the same.
Prefixes used in metric system
*irefik; Symbö@
centi
omicro
pico
femto
atto
zepto
yocto
Dimension
10
10-3
10-4
10-15
10-24
deca
kilo
mega
tera
peta
exa
yotta
da
h
k
c
E
z
102
IOS
106
109
1015
Iou
1021
Dimension Of a physical quantity are the powers
through which different fundamental quantities are raised
to define the quantity.
The expression by which a physical quantity is defined
in terms of fundamental quantities is called dimensional
formula of the quantity.
For example : Dimensional formula of momentum is
Quantity
1. Volume
2, Density
3. Velocity
4. Acceleration
5, Angular Velocib,’f Frequency
6, Momentum, [mpulse
7. Force
8. Work, Energy
9, Power
DimensiOnal
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10. Pressure, Streess, Modulus of Elasticity [ML-IT-21
11. Moment of Inertia
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12. Torque/Moment of Force
13, Angular Momentum, Planck’s Constent
14. Coefficient of Viscosity
15. Surface Tension
16. Universal Gravitational Constant
17. Latent Heat
18. SFific Heat
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