Unit :- The chosen standard used for measuring a physical quantities is called unit.
Unit should be:-
(i) Well defined (ii) Easy to reproduce
(iii) Easy to compare (iv) Internationally accepted
(v)Independent of change in physical conditions
Units are of two types:- (i) Fundamental Unit (ii) Derived Unit
System of Units:- Units depend on choice.each choice of units leads to a new system(set) of units. The Internationally accepted unit systems are:-
(i) CGS System (ii) MKS System (iii) FPS System (iv) SI Units
In SI units there are seven fundamental units as given in the table below:-
Besides these two 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 the terms of fundamental units are called derived units.
Some important derived units are:-
Some practical units of length mass and time:-
Prefixes used in metric system:
Unit should be:-
(i) Well defined (ii) Easy to reproduce
(iii) Easy to compare (iv) Internationally accepted
(v)Independent of change in physical conditions
Units are of two types:- (i) Fundamental Unit (ii) Derived Unit
System of Units:- Units depend on choice.each choice of units leads to a new system(set) of units. The Internationally accepted unit systems are:-
(i) CGS System (ii) MKS System (iii) FPS System (iv) SI Units
In SI units there are seven fundamental units as given in the table below:-
Physical Quantity
|
SI Unit
|
Symbol
|
Length
|
Meter
|
m
|
Mass
|
Length
|
kg
|
Time
|
Second
|
s
|
Electric Current
|
Ampere
|
A
|
Temperature
|
Kelvin
|
K
|
Luminous Intensity
|
Candela
|
Cd
|
Amount of substance
|
Mol
|
mol
|
Besides these two 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 the terms of fundamental units are called derived units.
Some important derived units are:-
Physical Quantity
|
CGS units
|
SI unit
|
Relation
|
Force
|
Dyne
|
Newton
|
1 Newton=105dyne
|
Work
|
erg
|
Joule
|
1 Joule= 107erg
|
Some practical units of length mass and time:-
Length
|
Mass
|
Time
|
Light year= distance traveled by light in one year in vaccum .
1 Ly = 9.46*1015m
1 Parsec= 3.26ly =2.06*1016m
1 Nautical mile or sea mile
= 6020 ft.
1 micron = 1µm =10-6m.
1 Angestron (A0) =10-15m.
|
1 Quintol = 102kg
1 Metric ton =103kg
1 Atomic mass unit(amu) or dalton =1.66*10-27kg
1Slug =14.59 kg
1 Pound = 0.4537 kg
1 Chandrashekhar limit = 1.4 times the mass of the Sun
= 2.8*1030kg.
|
1 Solar year = 86400 seconds.
1 year 365/2 solar days.
1 Lunar month =27.3 solar days
Tropical year = It is the year in which total solar eclipse occurs.
Leap year = It is the year in which the month of February is of 29 days.
|
Prefixes used in metric system:
|
Prefix
Name |
Symbol |
Power-of-Ten |
|
yocto |
y |
10-24 |
|
zepto |
z |
10-21 |
|
atto |
a |
10-18 |
|
femto |
f |
10-15 |
|
pico |
p |
10-12 |
|
nano |
n |
10-9 |
|
micro |
µ |
10-6 |
|
milli |
m |
10-3 |
|
centi |
c |
10-2 |
|
deci |
d |
10-1 |
|
[unity] |
[none] |
100 |
|
deka |
da |
10+1 |
|
hecto |
h |
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 |
|
zetta |
Z |
10+21 |
|
yotta |
Y |
10+24 |
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