Units and Measurements

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Units and Measurements

Without a standard unit system, there will not be any uniformity in measurement. Measurements are the only tools with which you can assert the validity of any theory. Hence, understanding the Units and Measurements is very important as it deals with the tool of comparison. Though a part of the school curriculum, particularly, the Physics syllabus for class 11, it comes into practice in our day-to-day life. A revision of the following class XI Physics chapter will give you an idea of the concepts dealt with in the chapter. 

What are Units and Measurements? 

When it comes to units and measurements, the very first thing is to understand what these terms denote. The comparison of any physical quantity in terms of the numerical value is called measurement. It provides a standard for everything in life. On the contrary, the standard by which quantities of similar nature are measured is called Units. The measurement is done based on internationally accepted units. 

Source: LearnoHub

Properties of Units

Listed below are some of the basic properties of a unit:

  • It must be well-defined
  • It should not vary with time
  • It should not alter with changes in physical quantities such as pressure, temperature, height, weight, etc.

Types of Units

There are two types of units- Fundamental Units and Derived Units.

Fundamental Units

Another important concept in units and measurements is Fundamental Unis. The units for fundamental quantities that are defined and denoted by international systems of units are called fundamental units. There are seven fundamental units which have been tabulated below:

Quantity Unit Symbol 
Distance or lengthMetreM
TimeSecond S
Electric currentAmpereA
Mass or weightKilogramKg
Luminous intensityCandelacd
Quantity or amount Molemol

Supplementary Fundamental Units

QuantityUnit Symbol 
Plane angleRadianrad
Solid angleSteradianSr

Derived Units

It is a SI unit of measurement that consists of the combination of two or more of the seven fundamental units. There are many derived units in Physics. The examples are area, volume, speed, force, surface tension, pressure, latent heat, etc. 

Unit(s) NameSI UnitSI Unit SymbolExpressed in SI Base UnitExpressed in other SI units
Force, WeightNewtonNkg⋅m⋅s-2
Electric chargeCoulombCs⋅A
Electric Potential (Voltage)VoltVkg.m2.s-3.A-1W/A
Resistance, Impedance, ReactanceOhmΩkg.m2.s−3.A−2V/A
Electrical ConductanceSiemensSkg−1.m−2.s3.A2Ω−1
Magnetic FluxWeberWbkg.m2.s−2.A−1V⋅s
Magnetic Flux DensityTeslaTkg.s−2.A−1Wb/m2
Energy, Work, HeatJouleJkg.m2.s−2N⋅m = Pa⋅m3
Power, Radiant FluxWattWkg.m2.s−3J/s
Luminous FluxLumenlmcdcd⋅sr

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Systems of Units

The units of measurement of physical quantities have evolved over centuries and decades. The old systems were not universal, owing to which there were discrepancies in measurement. These variations led to the evolution and adoption of a logical and universal International System of Units (SI) which is used in contemporary times. Here are the highly prevalent old and new systems of units:

Systems of UnitsUnit of LengthUnit of WeightUnit of Time
SIMetreKilogram Second

Units and Measurements: Definitions of SI Base Units 

Source: ProEdify

Now that you have understood the basic concepts related to units and measurements, let us go through the definition of different S.I units in detail. 


Metre is the length of the path travelled by light in a vacuum in a second and is expressed as m.s


It is the SI unit of time defined in terms of radiation frequency at which the atoms of caesium change from one state to another. It is expressed as s. 


It is defined as the magnitude of the electromagnetic field and is the unit of luminous intensity. 


It is the SI unit of electric current and represents the flow of one coulomb of electricity per second. 


It is the SI unit of mass. It is the quantitative measure of inertia and is expressed as kg.m2.s-1


It is defined as 1/273.16 (3.6609 x 10 -3) thermodynamic temperature of the triple point of water. 


It is defined as 6.02214076 × 1023 of a substance. 

Units and Measurement Class 11: NCERT Chapter PDF

Latest Changes in SI Units

In 2019, on World Meteorology Day, 4 out of the 7 redefined S.I units came into existence. They were redefined by the General Conference on Weights and Measures taking various factors into consideration. Here are the important details you need to know about the changes in units and measurements.

  • Kilogram will be based on Planck Constant. For this, a Kibble Balance will be used.
  • Ampere will be denoted based on the electron’s charge.
  • Avogadro Constant will be used to define Mole.
  • Kelvin, which is the unit of temperature, will now be based on Boltzmann Constant.

Other Important Units and Measurements 

Apart from the aforementioned units and measurements, here are some others which you must understand. 

Astronomical unit – It is the average distance from the centre of the earth to the sun. 1 A. U = 1.495 x 1011 m.

Light year – It refers to the distance travelled by light in one year. 1 light-year = 9.46 x 1015m

Parsec – It is used to measure large distances outside the solar system. 1 parsec = 3.3 light-years.


What is Dimensional Analysis in units and measurements?

The study of the relationship between physical quantities with dimensions and units of measurement is called dimensional analysis.

What are the applications of Dimensional Analysis?

1. To check the dimensional accuracy of the equation
2. To find the relation between physical quantities
3. To change units

What are the different types of errors which occur while measuring?

1. Constant errors 
2. Systematic errors
3. Random errors
4. Absolute errors
5. Relative errors
6. Percentage errors

What is the Parallax Method in units and measurement?

The shift between two objects when the eyes move side to side is called parallax. This is used to measure the distance of nearby stars, planets and other celestial objects from the Earth.
Parallax measurement can show an error when viewed from the wrong position. If the heavenly objects are at different distances from the eye error occurs in measurement, they are brought to one position by shifting. When the two objects are in the same position from the eye, then that is said to be the removal of parallax error. 

A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the Sun and the Earth in terms of the new unit if light takes 8 min and 20 s to cover this distance?

Distance between Sun and Earth = Speed of light x Time taken by light to cover the distance
Speed of light = 1 unit
Time taken by light to reach Earth is 8 minute 20 seconds
Time taken = (8\times 60) + 20 = 500 s
The distance between Sun and Earth = 1 x 500 = 500 units.

What is the S.I unit of luminous intensity and temperature?

The S.I unit of luminous intensity is Candela (cd) and of temperature is Kelvin (k).

Name physical quantities whose units are electron volt and pascal?

Energy and pressure.

Define S.I. unit of solid angle?

One steradian is defined as the angle made by a spherical plane of area 1 square meter at the centre of a sphere of radius 1m.

What is the difference between Ao and A.U.?

Ao and A.U. both are the units of distances but 1Ao = 10-10m and 1A.U. = 1.4961011m.

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Related Read: Sample Papers for Class 12th Science

We hope that this blog helped you understand units and measurements. To read about other related topics stay tuned to Leverage Edu.

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