PHYSICAL QUANTITIES AND UNITS

PHYSICAL QUANTITIES

1.   Physical quantities are quantities that can be measured.
2.   Examples : length, mass, time, weight, electric current, force, velocity and energy.
3.   To describe a physical quantity, two things must be specified: a numerical value and the unit. 

SI UNITS

1. The unit of a physical quantity is the standard used to compare different magnitudes of the same physical quantity.
2. In SI, each physical quantity has only one unit. Prefixes are attached to SI units.  

 

Prefixes	Factor	Symbol
Pico	10-12	p
Nano	10-9	n
Micro	10-6	m
Milli	10-3	m
Centi	10-2	c
Deci	10-1	d
Kilo	103	k
Mega	106	M
Giga	109	G
Tera	1012	T

BASE QUANTITY AND BASE UNITS

QUANTITY	SI UNIT	SYMBOL
Length	Metre	m
Mass	Kilogram	kg
Time	Second	s
Electric current	Ampere	A
Thermodynamic temperature	Kelvin	K
Amount of substance	Mole	mol
Light intensity	Candela	cd

 

DERIVED QUANTITY AND DERIVED UNITS

1.   A derived quantity is a combination of different base quantities.
2.   The unit for a derived quantity is known as derived unit.

Derived quantity	Derived unit
Area	m2
Volume	m3
Frequency	Hz @ s-1
Density	kg m-3
Velocity	m s-1
Acceleration	m s-2
Force	N  @ kg m s-2
Pressure	Pa  @ N m-2
Energy or work	J  @ N m
Power	W  @ J s-1
Electric charge	C  @ A s
Electric potential	V  @ J C-1
Electric intensity	V m-1
Electric resistance	Ω  @ V A-1
Capacitance	F  @ C V-1
Heat capacity	J K-1
Specific heat capacity	J kg-1 K-1

 

DIMENSIONS OF PHYSICAL QUANTITIES

1. The dimension of a physical quantity is the relation between the physical quantities and the base physical quantities.
2. The dimension of a physical quantity is represented by [ physical quantity ]

QUANTITY	SI UNIT	Dimension
Length	m	L
Mass	kg	M
Time	s	T
Electric current	A	A
Thermodynamic temperature	K	θ
Amount of substance	mol	N
Light intensity	cd	I

Derived quantity	Derived unit	Dimension
Area	m2	L2
Volume	m3	L3
Frequency	Hz @ s-1	T-1
Density	kg m-3	ML-3
Velocity	m s-1	LT-1
Acceleration	m s-2	LT-2
Force	N  @ kg m s-2	MLT-2
Pressure	Pa  @ N m-2	ML-1T-2
Energy or work	J  @ N m	ML2T-2
Power	W  @ J s-1	ML2T-3
Electric charge	C  @ A s	AT
Electric potential	V  @ J C-1	ML2TA-1
Electric intensity	V m-1	MLTA-1
Electric resistance	Ω  @ V A-1	ML2TA-2
Capacitance	F  @ C V-1	A2M-1L-2
Heat capacity	J K-1	ML2T-2K-1
Specific heat capacity	J kg-1 K-1	L2T-2K-1