Unit: A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement.
For example, length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre".
The SI or International System of Units is universally adopted system to measure physical quantities.
SI base units: There exists 7 SI base units, definition of these units given below. All units called ‘derived units’ are derived from SI base units.
The meter (abbreviation, m)  The meter is the SI unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1/299,792,458 (3.33564095 x 10 9) second. The meter was originally defined as one tenmillionth (0.0000001 or 10 7 ) of the distance, as measured over the earth's surface in a great circle passing through Paris, France, from the geographic north pole to the equator.
The kilogram (abbreviation, kg) The Kilogram is the SI unit of mass. It is defined as the mass of a particular international prototype made of platinumiridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10 3 cubic meter) of pure water.
The second (abbreviation, s or sec) The Second is the SI unit of time. One second is the time that elapses during 9.192631770 x 10 9 cycles of the radiation produced by the transition between two levels of Cesium133 or Cs133. It is also the time required for an EM field to propagate 299,792,458 (2.99792458 x 10 8 ) meters through a vacuum.
The kelvin (abbreviation K) The kelvin also called the degree Kelvin (abbreviation, o K), is the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 x 10 3 ) of the thermodynamic temperature of the triple point of pure water (H2O).
The ampere (abbreviation, A) The ampere is the SI unit of electric current. One ampere is the current that would produce a force of 0.0000002 (2 x 107 ) Newton between two straight, parallel, perfectly conducting wires having infinite length and zero diameter, separated by one meter in a vacuum. One ampere represents 6.24 x 10 18 unit electric charge carriers, such as electrons, passing a specified fixed point in one second.
The candela (abbreviation, cd) The Candela is the SI unit of luminous intensity. It is the electromagnetic radiation, in a specified direction, that has an intensity of 1/683 (1.46 x 10 3 ) watt per steradian at a frequency of 540 terahertz (5.40 x 10 14 hertz).
The mole (abbreviation, mol) The mole is the SI unit of material quantity. One mole is the number of atoms in 0.012 kilogram of the most common isotope of elemental carbon (C12). This is approximately 6.022169 x 10 23 , and is also called the Avogadro constant.
SI derived units include the hertz , the newton , the pascal (unit of pressure or stress) , the ohm , the farad , the joule , the coulomb , the tesla , the lumen , the becquerel , the siemen , the volt , and the watt .
Following are links to the definitions of these 22 units. The third column shows how each unit is derived from the preceding units, and the fourth column gives the formal equivalence of the unit in terms of the base units.
The term derived unit covers any algebraic combination of the base units, but it is only the 22 combinations listed above that have approved special names. For example, the SI derived unit of momentum (mass times velocity) has no special name; momentum is stated in kilogram meters per second (kg•m/s) or in newton seconds (N•s). A few SI derived units do have special names that have been defined but not approved. Here are some examples:
Length:
• 1 light year = distance traveled by light in year in vacuum
• 1 ly = 9.46x10 15 m
• 1 Parsec = 3.26 ly
• 1 seamile or nautical mile = 6020 ft
• 1 micron (m) = 106m
• 1 Angstrom (A) = 10 15 m
Mass
• 1 quintal = 100 kg
• 1 metric ton = 1000 kg
• 1 atomic mass unit or amu or Dalton = 1.66 x 10 27 kg
• 1 slug = 14.59 kg
• 1 pound = 0.4537 kg
• 1 Chandrashekhar Limit = 1.4times the mass of Sun = 208 x 10 30 kg
Time
• 1 Solar day = 86400 sec
• 1 year = 365.5 days
• 1 lunar month = 27.3 solar days
• Tropical Year = This is the year when Solar eclipse occurs
• Leap Year = It is the year when month of February happens to be 29 days.
There are twenty prefixes officially specified by SI.
For example, length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre".
The SI or International System of Units is universally adopted system to measure physical quantities.
SI base units: There exists 7 SI base units, definition of these units given below. All units called ‘derived units’ are derived from SI base units.
The meter (abbreviation, m)  The meter is the SI unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1/299,792,458 (3.33564095 x 10 9) second. The meter was originally defined as one tenmillionth (0.0000001 or 10 7 ) of the distance, as measured over the earth's surface in a great circle passing through Paris, France, from the geographic north pole to the equator.
The kilogram (abbreviation, kg) The Kilogram is the SI unit of mass. It is defined as the mass of a particular international prototype made of platinumiridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10 3 cubic meter) of pure water.
The second (abbreviation, s or sec) The Second is the SI unit of time. One second is the time that elapses during 9.192631770 x 10 9 cycles of the radiation produced by the transition between two levels of Cesium133 or Cs133. It is also the time required for an EM field to propagate 299,792,458 (2.99792458 x 10 8 ) meters through a vacuum.
The kelvin (abbreviation K) The kelvin also called the degree Kelvin (abbreviation, o K), is the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 x 10 3 ) of the thermodynamic temperature of the triple point of pure water (H2O).
The ampere (abbreviation, A) The ampere is the SI unit of electric current. One ampere is the current that would produce a force of 0.0000002 (2 x 107 ) Newton between two straight, parallel, perfectly conducting wires having infinite length and zero diameter, separated by one meter in a vacuum. One ampere represents 6.24 x 10 18 unit electric charge carriers, such as electrons, passing a specified fixed point in one second.
The candela (abbreviation, cd) The Candela is the SI unit of luminous intensity. It is the electromagnetic radiation, in a specified direction, that has an intensity of 1/683 (1.46 x 10 3 ) watt per steradian at a frequency of 540 terahertz (5.40 x 10 14 hertz).
The mole (abbreviation, mol) The mole is the SI unit of material quantity. One mole is the number of atoms in 0.012 kilogram of the most common isotope of elemental carbon (C12). This is approximately 6.022169 x 10 23 , and is also called the Avogadro constant.
SI derived units include the hertz , the newton , the pascal (unit of pressure or stress) , the ohm , the farad , the joule , the coulomb , the tesla , the lumen , the becquerel , the siemen , the volt , and the watt .
Derived Units of the International System (SI)
Following are links to the definitions of these 22 units. The third column shows how each unit is derived from the preceding units, and the fourth column gives the formal equivalence of the unit in terms of the base units.
Derived Unit

Measures

Derivation

Formal Definition

Hertz (Hz)

frequency

/s

s^{1}

newton (N)

force

kg·(m/s^{2})

kg·m·s^{2}

pascal (Pa)

pressure

N/m^{2}

kg·m^{1}·s^{2}

joule (J)

energy or work

N·m

kg·m^{2}·s^{2}

watt (W)

power

J/s

kg·m^{2}·s^{3}

coulomb (C)

electric charge

A·s

A·s

volt (V)

electric potential

W/A

kg·m^{2}·s^{3}·A^{1}

farad (F)

electric capacitance

C/V

kg^{1}·m^{2}·s^{4}·A^{2}

ohm (omega)

electric resistance

V/A

kg·m^{2}·s^{3}·A^{2}

siemens (S)

electric conductance

A/V

kg^{1}·m^{2}·s^{3}·A^{2}

weber (Wb)

magnetic flux

V·s

kg·m^{2}·s^{2}·A^{1}

tesla (T)

magnetic flux density

Wb/m^{2}

kg·s^{2}·A^{1}

henry (H)

inductance

Wb/A

kg·m^{2}·s^{2}·A^{2}

degree Celsius (°C)

temperature

K  273.15

K

radian (rad)

plane angle


m·m^{1}

steradian (sr)

solid angle


m^{2}·m^{2}

lumen (lm)

luminous flux

cd·sr

cd·sr

lux (lx)

illuminance

lm/m^{2}

cd·sr·m^{2}

becquerel (Bq)

activity

/s

s^{1}

gray (Gy)

absorbed dose

J/kg

m^{2}·s^{2}

sievert (Sv)

dose equivalent

Gy·(multiplier)

m^{2}·s^{2}

katal (kat)

catalytic activity

mol/s

mol·s^{1}

The term derived unit covers any algebraic combination of the base units, but it is only the 22 combinations listed above that have approved special names. For example, the SI derived unit of momentum (mass times velocity) has no special name; momentum is stated in kilogram meters per second (kg•m/s) or in newton seconds (N•s). A few SI derived units do have special names that have been defined but not approved. Here are some examples:
Derived Unit

Measures

Derivation

stere (st)

volume

m^{3}

diopter (dpt)

refractive power

m^{1}

thermal ohm

thermal resistance

K/W

poiseuille (Pl)

dynamic viscosity

Pa·s

rayl

sound impedance

Pa·s/m

acoustic ohm

sound resistance

Pa·s/m^{3}

daraf

electric elastance

F^{1}

talbot

luminous energy

lm·s

nit (nt)

luminance

cd/m^{2}

molal

chemical concentration

mol/kg

Some practical units of Length, Mass and Time
Length:
• 1 light year = distance traveled by light in year in vacuum
• 1 ly = 9.46x10 15 m
• 1 Parsec = 3.26 ly
• 1 seamile or nautical mile = 6020 ft
• 1 micron (m) = 106m
• 1 Angstrom (A) = 10 15 m
Mass
• 1 quintal = 100 kg
• 1 metric ton = 1000 kg
• 1 atomic mass unit or amu or Dalton = 1.66 x 10 27 kg
• 1 slug = 14.59 kg
• 1 pound = 0.4537 kg
• 1 Chandrashekhar Limit = 1.4times the mass of Sun = 208 x 10 30 kg
Time
• 1 Solar day = 86400 sec
• 1 year = 365.5 days
• 1 lunar month = 27.3 solar days
• Tropical Year = This is the year when Solar eclipse occurs
• Leap Year = It is the year when month of February happens to be 29 days.
Prefixes Used in Metric System
There are twenty prefixes officially specified by SI.
Prefix

Symbol

1000^{m}

10^{n}

Decimal

Short scale

Long scale

Since

yotta

Y

1000^{8}

10^{24}

1000000000000000000000000

septillion

quadrillion

1991

zetta

Z

1000^{7}

10^{21}

1000000000000000000000

sextillion

trilliard

1991

exa

E

1000^{6}

10^{18}

1000000000000000000

quintillion

trillion

1975

peta

P

1000^{5}

10^{15}

1000000000000000

quadrillion

billiard

1975

tera

T

1000^{4}

10^{12}

1000000000000

trillion

billion

1960

giga

G

1000^{3}

10^{9}

1000000000

billion

milliard

1960

mega

M

1000^{2}

10^{6}

1000000

million

1960


kilo

k

1000^{1}

10^{3}

1000

thousand

1795


hecto

h

1000^{2/3}

10^{2}

100

hundred

1795


deca

da

1000^{1/3}

10^{1}

10

ten

1795



1000^{0}

10^{0}

1

one

–


deci

d

1000^{−1/3}

10^{−1}

0.1

tenth

1795


centi

c

1000^{−2/3}

10^{−2}

0.01

hundredth

1795


milli

m

1000^{−1}

10^{−3}

0.001

thousandth

1795


micro

Î¼

1000^{−2}

10^{−6}

0.000001

millionth

1960


nano

n

1000^{−3}

10^{−9}

0.000000001

billionth

milliardth

1960

pico

p

1000^{−4}

10^{−12}

0.000000000001

trillionth

billionth

1960

femto

f

1000^{−5}

10^{−15}

0.000000000000001

quadrillionth

billiardth

1964

atto

a

1000^{−6}

10^{−18}

0.000000000000000001

quintillionth

trillionth

1964

zepto

z

1000^{−7}

10^{−21}

0.000000000000000000001

sextillionth

trilliardth

1991


y

1000^{−8}

10^{−24}

0.000000000000000000000001

septillionth

quadrillionth

1991


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