Units

Edited: 17/8/2025

 

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 bar 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

Chemistry Fun and Facts

• The only elements that are liquid at room temperature are bromine and mercury.
• Water expands as it freezes. An ice cube takes up about 9% more volume than the water used to make it.
• If you pour a handful of salt into a full glass of water, the water level will actually go down rather than overflowing the glass.

Non Metal

In modern periodic table there are 24 non metal elements. These are electronegative elements and bad conductor of heat and electricity. Here are some of the commonly occurring non metals and their properties-

Metallurgy

What is Metallurgy?

The process of extracting metals from ores is called metallurgy.

Ores - An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable elements.

Fuels

Fuel is a substance that can supply energy alone or upon reaction with some other substance. Fuel when provide energy also produced heat which is measured in Calories. An ideal fuel should posses the following properties –

Carbon and Its Compounds

Carbon is a non metal, its atomic number is 6 and mass number 12. It is placed in Group 14 or IV A of Periodic Table.

Allotropy- The substances which shows similar chemical characteristics but different physical characteristics are called allotropes and this property of material is called allotropy. Carbon has three allotropes – Charcoal, Graphite and Diamond

Electrolysis

Electrolytes- Electrolytes are the materials which allow electric current to pass through them when in molten state and goes for chemical decomposition. For example; acids, bases, and salts.

Behavior of Gases

Boyle’s Law: Boyle's lawstates that the absolute pressure and volume of a given mass of confined gas are inversely proportional, if the temperature remains unchanged within a closed system.

Mathematically, V ∝ 1/p; when temperature T is constant
Or V = K./p
Thus PV = K where K is constant
P1V1 = P1V1

Acids, Bases and Salts

Acid – An acid is a chemical material which exhibits following properties –

Sour in taste; when sprinkled on a blue litmus paper, turns it red; contains replaceable hydrogen; gives out hydrogen ion H+ in Aqueous Solution according to Arrhenius Theorem; can donate a proton According to Bronsted Lowry concept and can accept electrons according to Lewis Concept

Oxidation and Reduction

Oxidation and Reduction - The earliest view of oxidation and reduction is that of adding oxygen to form an oxide (oxidation) or removing oxygen (reduction). They always occur together. For example, in the burning of hydrogen the hydrogen is oxidized and the oxygen is reduced. The combination of nitrogen and oxygen which occurs at high temperatures follows the same pattern.