S.I. Units

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The value of any physical quantity is always expressed as its numerical value as well as a unit. One often hears the weight expressed as grams or kilograms; the distance as foot, yard or mile; the volume as litres or gallons. The use of a number of units cause confusion and complications. Hence, the need for a uniform system of measurement was felt as a necessity. In 1971, the French Academy of Science devised the metric system of measurement based on the decimal system. In October 1960, The XIth General Conference of Weights and Measures (Conference Generale des Poids et Mesures) adopted an International System of Units. This system is known as SI system of units after the French word, Systeme' International d'Unites.

The SI system has seven basic units from which all other units are derived.

The Seven Basic Units

Quantity	Unit	Symbol
Length	metre	m
mass	kilogram	kg
Time	second	S
Temperature	kelvin	K
Amount of Substance	mole	mol
Electric current	ampere	A
Luminous intensity	candela	Cd

Note:

Although the SI unit of temperature is Kelvin, Celsius scale (^oC) is commonly used. K=^oC + 273.15. Similarly Angstrom (Å) is the commonly used unit but nanometer or picometer is the SI unit.1nm = 10^-9m, 1pm = 10^-12m.

Prefixes in SI system

The SI units of some of the physical quantities are either too small or too large. To change the order of magnitude, the metric system, which is a decimal system, relates this in powers of ten. These powers are indicated by prefixes.

Standard prefixes for reducing or enlarging the size of any unit

 

Multiple	Prefix	Symbol
10 18	exa	E
10 15	peta	P
10 12	tera	T
10 9	giga	G
10 6	mega	M
10 3	kilo	k
10 2	hecto	h
10	deka	da
10 - 1	deci	d
10 - 2	centi	c
10 - 3	milli	m
10 - 6	micro	μ
10 - 9	nano	n
10 - 12	pico	P
10 - 15	femto	f
10 - 18	atto	a

 

Some rules on the use of SI units

• While writing a unit, only its singular form is used.

• The abbreviation does not have a full stop.

• The sign of degree (^o) is omitted when Kelvin scale is used.

• Unit combinations should be indicated by means of either a dot or leaving space in between.

Metre Kelvin = m.K or m K

• Words and symbols should not be used in mixed forms. J per mole should be either written as joule per mole or J mole^-1.

• A unit with a prefix is a power for the complete unit. cm³ means (centimetre)³ and not centi (metre)³

• Exponents also operate on prefixes. 1cm² = (10^-2 m)² = 10^-4 m²