Unit and Dimensions (Part 1)

UNIT & DIMENSION (PART I)

1. INTRODUCTION:

Physics is an experimental science and experiments require measurement of physical quantities. Measuring a physical quantity involves comparing the quantity with a reference standard called the unit of the quantity. Some physical quantities are taken as base quantities and other quantities are expressed in terms of the base quantities called derived quantities. This forms a system of base quantities and their units.

 

2. UNITS 

To measure a physical quantity we need some standard unit of that quantity. The measurement of the quantity is mentioned in two parts, the first part gives magnitude and the second part gives the name of the unit. Thus, suppose I say that length of this wire is 5 meters. The numeric part 5 says that it is 5 times of the unit of length and the second part meter says that unit chosen here is meter. 

 

2.1 Fundamental and Derived Quantities 

The basic physical quantities, which are independent of other quantities, are known as the fundamental quantities. For example, mass, length and time are considered to be the fundamental quantities. In the same manner, the units which can be derived from the fundamental units are known as derived units. In mechanics, virtually all quantities can be expressed in terms of mass, length and time. The main systems of units are given as follows: 

(a) CGS or Centimetre, Gram, Second System 

(b) FPS or Foot, Pound, Second System 

(c) MKS or Metre, Kilogram, Second System 

(d) SI system: Totally, there are seven basic or fundamental quantities in the international system of units called the SI system which can express all physical quantities including heat, optics and electricity and magnetism. We now provide these basic seven quantities with their units and symbols:
July 07, 2020

UNIT & DIMENSION (PART I)

1. INTRODUCTION:

Physics is an experimental science and experiments require measurement of physical quantities. Measuring a physical quantity involves comparing the quantity with a reference standard called the unit of the quantity. Some physical quantities are taken as base quantities and other quantities are expressed in terms of the base quantities called derived quantities. This forms a system of base quantities and their units.

 

2. UNITS 

To measure a physical quantity we need some standard unit of that quantity. The measurement of the quantity is mentioned in two parts, the first part gives magnitude and the second part gives the name of the unit. Thus, suppose I say that length of this wire is 5 meters. The numeric part 5 says that it is 5 times of the unit of length and the second part meter says that unit chosen here is meter. 

 

2.1 Fundamental and Derived Quantities 

The basic physical quantities, which are independent of other quantities, are known as the fundamental quantities. For example, mass, length and time are considered to be the fundamental quantities. In the same manner, the units which can be derived from the fundamental units are known as derived units. In mechanics, virtually all quantities can be expressed in terms of mass, length and time. The main systems of units are given as follows: 

(a) CGS or Centimetre, Gram, Second System 

(b) FPS or Foot, Pound, Second System 

(c) MKS or Metre, Kilogram, Second System 

(d) SI system: Totally, there are seven basic or fundamental quantities in the international system of units called the SI system which can express all physical quantities including heat, optics and electricity and magnetism. We now provide these basic seven quantities with their units and symbols:

S. No.	Physical Quantity	SI Unit	Symbol
1	Mass	Kilogram	kg
2	Length	Metre	m
3	Time	second	s
4	Temperature	Kelvin	K
5	Luminous intensity	Candela	cd
6	Electric current	Ampere	A
7	Amount of substance	mole	mol

 

There are also two supplementary units used as radian (rad) for plane angle and steradian (sr) for solid angle. The above mentioned International System of Units (SI) is now extensively used in scientific measurements. However, the following practical units of length are also conveniently used and are expressed in terms of SI system of units.

 

(a) Micron is a small unit for measurement of length. 1 micron =10^-6 m

(b) Angstrom is a unit of length in which the size of an atom is measured and is used in atomic physics. 1 Angstrom=1Å=10^-10m

 

(c) Light year is a unit of distance travelled by light in 1 year free space and is used in astrophysics. 1 Light year = 3*10⁸m/s*365*24*60*60=9.5*10¹⁵m

 

(d) Fermi is a unit of distance in which the size of a nucleus is measured. 1 Fermi=10^-15 m

 

(e) Atomic mass unit: It is a unit of mass equal to 1/12th of mass of carbon-12 atom. 

1 atomic mass unit ≅ 1.67*10^-27 kg 

Note: There are only seven fundamental units. Apart from these, there are two supplementary units—plane angle (radian) and solid angle (steradian). By using these units, all other units can be derived. However, we need to know the fact that both radian and steradian have no dimensions.