Units

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 ten-millionth (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 platinum-iridium 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 Cesium-133 or Cs-133. 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 10-7 ) 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 (C-12). 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/s2)	kg·m·s-2
pascal (Pa)	pressure	N/m2	kg·m-1·s-2
joule (J)	energy or work	N·m	kg·m2·s-2
watt (W)	power	J/s	kg·m2·s-3
coulomb (C)	electric charge	A·s	A·s
volt (V)	electric potential	W/A	kg·m2·s-3·A-1
farad (F)	electric capacitance	C/V	kg-1·m-2·s4·A2
ohm (omega)	electric resistance	V/A	kg·m2·s-3·A-2
siemens (S)	electric conductance	A/V	kg-1·m-2·s3·A2
weber (Wb)	magnetic flux	V·s	kg·m2·s-2·A-1
tesla (T)	magnetic flux density	Wb/m2	kg·s-2·A-1
henry (H)	inductance	Wb/A	kg·m2·s-2·A-2
degree Celsius (°C)	temperature	K - 273.15	K
radian (rad)	plane angle		m·m-1
steradian (sr)	solid angle		m2·m-2
lumen (lm)	luminous flux	cd·sr	cd·sr
lux (lx)	illuminance	lm/m2	cd·sr·m-2
becquerel (Bq)	activity	/s	s-1
gray (Gy)	absorbed dose	J/kg	m2·s-2
sievert (Sv)	dose equivalent	Gy·(multiplier)	m2·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	m3
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/m3
daraf	electric elastance	F-1
talbot	luminous energy	lm·s
nit (nt)	luminance	cd/m2
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) = 10-6m

•    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	1000m	10n	Decimal	Short scale	Long scale	Since
yotta	Y	10008	1024	1000000000000000000000000	septillion	quadrillion	1991
zetta	Z	10007	1021	1000000000000000000000	sextillion	trilliard	1991
exa	E	10006	1018	1000000000000000000	quintillion	trillion	1975
peta	P	10005	1015	1000000000000000	quadrillion	billiard	1975
tera	T	10004	1012	1000000000000	trillion	billion	1960
giga	G	10003	109	1000000000	billion	milliard	1960
mega	M	10002	106	1000000	million		1960
kilo	k	10001	103	1000	thousand		1795
hecto	h	10002/3	102	100	hundred		1795
deca	da	10001/3	101	10	ten		1795
		10000	100	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	μ	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