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NCERT Solution: Basic Concepts of Chemistry

PHYSICAL QUANTITIES AND SI UNITS

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PHYSICAL QUANTITIES AND SI UNITS

The 11th general conference of weights and measures in 1960 recommended the use of international system of units. Abbreviated as SI Units (after the French expression La System International de units).

FUNDAMENTAL UNITS

The SI system has seven basic units of physical quantities as follows:

Physical Quantity	Abbreviation	Name of unit	Symbol
time	t	second	s
mass	m	kilogram	kg
length	l	metre	m
temperature	T	kelvin	K
electric current	I	ampere	A
light intensity	Iv	candela	Cd
amount of substance	n	mole	mol

DERIVED UNITS

The units obtained by combination of basic units are known as derived units e.g. velocity is expressed as distance/time. Hence unit is m/s or ms–1. Some common derived units are:

Physical Quantity	Definition	SI Unit
volume	length cube	m3
area	length square	m2
speed	distance travelled	ms–1 per unit time
acceleration	speed changed	ms–2 per unit time
density	mass per unit volume	kg m–3
pressure	force per unit area	kgm–1s–2 or Nm–2 (pressure = Pa)
force	mass times acceleration of object	kgms–2 (Newton N)
energy	force times distance travelled	kgm2s–2(Joule J)
frequency	cycles per second	s–1 (hertz = Hz)
power	energy per second	kgm2s–3 or Js–1 (Watt = W)
electric charge	ampere times second	As (coloumb = C)
electric potential	energy per unit	JA–1s–1 or kgm2s–3
difference	charge	A–1 (volt = V)

SOME NON-SI UNITS IN COMMON USE

Quantity	Unit	Symbol	SI definition	SI Name
Length	angstrom	Å	10–10 m	0.1 nanometers (nm)
Volume	litre	L	10–3 m3	1 decimeter (dm3)
Energy	calorie	cal	kg m2s–2	4.184 Joule (J)