Dhristi JEE 2022-24

Introduction to SI Units List

Physical quantities were measured in terms of standardized values called units.

 

For Example - In this picture below, 


Length of a road- a physical quantity


Expressed in kilometers or meters- km/m are units.


If units are not clearly mentioned, then it could be difficult to compare the measured values elaborately.  


Units are measured in two ways- SI units (metric system) and English units (imperial system) 


The English system is widely used in the US. In the present times, every country has accepted SI units as standardized units, so mathematicians and scientists are accepting SI units as the universal method. 


The International System of Units (SI) is defined as a metric system that is used as a measurement standard accepted globally. SI unit is derived from the French word Systeme International. It plays a vital role in developing scientific and technical research to avoid confusion within units. It consists of 7 base units that define 22 derived units. The quantities are defined with the power of 10, ranging from 10-24 to 1024. It is expressed as fractional or standard quantities. 


Listed Below is The Table Which Represents SI Units

Sl. No.

Quantity Name

SI Unit

SI Unit System

1

Length (L)

Meter

M

2

Time (T)

Second

S

3

Mass (M)

Kilogram

Kg

4

Electric Current (I)

Ampere

A

5

Amount Of Substance (N)

Mole

Mol

6

Thermodynamics Temperature (Θ)

Kelvin

K

7

Luminous Intensity (J)

Candela

cd


List of SI Units

SI units are used in physics to express different quantities which are classified to: 

Basic units and Derived units.


  1. SI Basic Units

Basic units are known as the fundamental units which are mostly used as the building block of the system of units and then the other units can be derived from SI basic units. 


List of SI Basic units

There are 7 SI basic units. These units are interdependent with each other mutually stating Base Units.  Each unit along with its SI units, symbol, and expressions are given below:


Unit of Measurment

SI Unit

Description

Unit of length.

Meter (m).

It is the base unit of length and equals roughly 39.37 inches. It is from this unit that smaller units like the centimetre and millimetre have been derived

Unit of time

Second (s)

It is defined as the constant value of cesium frequency and is expressed as s. 


Unit of the mass

Kilogram (kg)


It is the basic SI Unit of Mass. Originally, a kilogram was meant to be exactly equal to the volume of 1000 cubic cm of water, but it is actually very slightly different


Unit of electric current

Ampere (A)

It is defined by considering the constant value of the elementary charge. 


Unit of luminous intensity

Candela (cd)

The luminous intensity is defined by the constant value of luminous efficacy.

Unit of the amount of substance

Mole (mol)

It is defined by the constant value of Avogadro constant (NA) fair one mole is 6.02214076 × 1023 elementary entities. It is expressed as mol-1


Unit of thermodynamics temperature

Kelvin (k)

Thermodynamics temperature is defined by taking the constant value of Boltzmann constant where k = 1.38 0649 ×10-23



  1. SI Derived Units:

There are only limited derived units that form different operations over the base units. In the case of derived units, the dimensions are expressed related to the dimensions of the base units. It can also be expressed by combining base and derived units.


List of SI Derived Units

Sl no

Unit names

SI Unit

Symbols of SI Units

Expressions in SI Base unit

Other SI Units

1

Force Or Weight

Newton

N

kg.g.s-2

-

2

Electric Charge

Coulomb

C

s.A

-

3

Frequency

Hertz

Hz

s-1

-

4

Electric Potential

Volt

V

kg.m2.s-3.A-1

W/A

5

Capacitance

Farad

F

kg.m2.s-2.A-2

C/V

6

Inductance

Henry

H

kg−1.m−2.s4.A2

Wb/A

7

Resistance, Impedance, Reactance

Ohm 

kg−1.m−2.s4.A2

V/A

8

Electric Conductance

Siemens

S

kg−1.m−2.s3.A2

Ω-1

9

Magnetic Flux

Weber

Wb

kg.m2.s−2.A−1

V.s

10

Magnetic Flux Density

Tesla

T

kg.s−2.A−1

Wb/m2

11

Energy, Work, Heat

Joule

J

kg.m2.s−2

N.m

12

Power, Radiant Flux

Watt

W

kg.m2.s−3

J/s

13

Angle

Radian

Rad

m.m-1

-

14

Radio Activity

Becquerel

Bq

s-1

-

15

Luminous Flux

Lumen

Im

cd

cd.sr


These SI units are mostly used units. Other than these there are certain units feature mostly seen in physics such as



There are numerous numbers, terms and quantities studied in physics which are expressed in various units. One must know the CGS and the SI units frequently to ace in physics. 


Advantages of CGS Units and SI Units

  • SI units and CGS units questions are very common in physics examinations

  • The units and explanations can help to get into the depth of different topics.

  • These are provided in an interactive manner to understand easily.

  • These units are provided in points so that the concept will remain clear for a longer time.


Units of Measurement for Length, Time, Mass And Volume

1.Length

The SI unit of length is a meter. It is defined as the length traveled by night in a path is 1/299792458 seconds. 


(Image to be added soon)


Some other units of length include nanometres, millimeters, decimeters, centimeters, meters, kilometers.


Non-SI units of length include yard, inch, foot.


2. Time 

The SI unit of time is second. It can be considered as the time-taken, which is equal to 9192631770 vibrations of radiation to the distance between two hyperfine intervals of the cesium-133 atom. Time is a continuous process of existence to the future, present, and past. To know about the existence of any being, units of time are considered. The units of time are an hour, minute, and second. 


Units of time are


Minute = 60 seconds


Hour = 60 minutes


Day = 24 hours


Week = 7 days


Month = 28 to 31 days


Year = 365 /366 days


3.Mass

One thing you must keep in mind is that mass is different from weight. Mass is defined as the quantity of matter in an object. The most used units for the measurement of mass are gram and kilogram.


SI unit of mass is the kilogram (kg)


Other units of mass are 


Milligram = 1/1000g


Decigram = 1/10g


Centigram = 1/100g


Gram = 1000 milligrams


Decagram= 10 grams


Hectogram = 100 grams


Metric ton =1000 kilograms


Kilogram = 1000 grams


4.Volume 

Capacity is measured by volume. To measure the capacity of an object the unit of volume is required. The SI unit of volume is measured in cubic meters. 

1m3= 1m × 1m × 1m


Some Other Units Of Volume Are

Unit of Measure

Cubic Meter

1 cubic metre 

1

1 barrel

0.1589

1 cubic foot

0. 0283

1 cubic decimeter

0.001

1 litre

0.001

1 gallon

0.0037 8

1 pint

0.0004 73

1 cubic inch

0.00001 6

1 cubic centimetre

0.000001


What does SI Unit mean?

The International System of Units, abbreviated as SI Units, is a system of units of measurement that has been internationally recognised. It is the only system that has been recognised in this manner and is therefore used wherever the metric system is adopted. It is also used worldwide in scientific research.


The system is based on seven units that are known as the Base SI Units. From these Base SI Units, there have been 22 more units derived, giving them the name Coherent Derived SI Units, or simply Derived SI Units.


The seven Base SI Units are the Second for Time, the Metre for Length, the Ampere for Electric Current, the Candela for Luminous Intensity, the Mole for Amount of Substance, the Kilogram for Mass, and the Kelvin for Thermodynamic Temperature.


The 22 Coherent Derived SI Units are as follows:


  • Hertz for Frequency

  • Joule for Energy, Work, Heat

  • Pascal for Pressure, Stress

  • Radian for Angle

  • Steradian for Solid Angle

  • Coulomb for Electric Charge

  • Newton for Force

  • Ohm for Electric Resistance

  • Volt for Voltage

  • Farad for Electrical Capacitance

  • Siemens for Electrical Conductance

  • Tesla for Magnetic Induction

  • Henry for Electrical Inductance

  • Degree Celsius for Temperature relative to degree Kelvin

  • Weber for Magnetic Flux

  • Gray for Absorbed Dose of Ionising Radiation

  • Lumen for Luminous Flux

  • Lux for Illuminance

  • Becquerel for Radioactivity

  • Katal for Catalytic Activity

  • Sievert for Equivalent Dose of Ionising Radiation


As you can see, the Derived SI Units and the quantities they represent are connected to or based on the seven Base SI Units. 


Together, the seven Base SI Units plus the 22 Coherent Derived SI Units form the collective International System of Units. This system of units is used all over the world in scientific research and study. Most of these units are also used in most of the world for regular, day-to-day use. For example, most countries in the world use the metre and kilometre as measures of distance and length, while only a few countries use miles as the main measurement of distance and length.

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FAQs (Frequently Asked Questions)

1. How do you differentiate the SI unit names and symbols?

SI unit names and symbols are very original words and letters that are in a very constant form. Let us illustrate some examples.

Unit names

Unit Symbols

Unit names are written in the form of capital letters if it's the name of the inventor or their titles. For example- Watt is named after the inventor James Watt. So, while writing we write megawatt not megaWatt.

Symbols are case sensitive so we write MW for megawatt and mW for milliwatt.

While writing units it is singular and plural as well.  For example- one meter, five meters. But five hertz as the plural form of hertz is hertz, joule is joule, etc.

Unit symbols have no plural forms. We must write for 1m, 5m, etc.

Unit names are pronounced and written in different languages differently. For example, in the US it is written as “metre”, in French it's “metre”, in German it's “meter”, and in Spanish, it's “metro”.

Unit symbols are the same in different languages. Absolutely no changes in it. It's “m” for meters globally.

2. Why are SI units called a coherent system of units?

In the coherent unit system, we choose several base units for a set of fundamental quantities and derive other units by dividing or multiplying any constant into it resulting in the same form of numerical and physical units. So the term SI units are the total set of base units, derived units with decimal multiples and submultiples. For example, kilometres, meters, and millimetres are SI units, likewise meter per second and millimetre per second. From that group, “meter” and “meter per second” are termed as coherent SI units.

3. What are the Base SI units and what are they used for?

The Base SI Units refer to seven units as defined by the International System of Units. These SI Units are used for seven different Base Quantities, which form the basis of what is now known as the International System of Quantities.


The Base SI Units are as follows:


Second: This unit is the base measurement of Time.


Metre: This unit is used as the base measurement for Length.


Kilogram: This is the base measurement for Mass.


Ampere: This unit forms the base measurement of Electric Current.


Candela: This unit is used to measure luminous intensity.


Mole: This unit is the measurement for The Amount of a Substance.


Kelvin: This is the measurement for Thermodynamic Temperature.


The above seven units and the quantities they represent are the seven base SI Units and their base quantities.

4. What are the Coherent Derived SI units?


The Coherent Derived SI Units are a series of units and the quantities they represent that have been logically derived from the seven base units. Some of these represent new quantities that are connected to a base quantity, while some are simply a derivation of an existing base quantity.


The Derived SI Units are and their representative quantities are:


  • Hertz - Frequency

  • Joule - Energy, Work, Heat

  • Pascal - Pressure, Stress

  • Radian - Angle

  • Steradian - Solid Angle

  • Coulomb - Electric Charge

  • Newton - Force

  • Ohm - Electric Resistance

  • Volt - Voltage

  • Farad - Electrical Capacitance

  • Siemens - Electrical Conductance

  • Tesla - Magnetic Induction

  • Henry - Electrical Inductance

  • Degree Celsius - Temperature relative to degree Kelvin

  • Weber - Magnetic Flux

  • Gray - Absorbed Dose of Ionising Radiation

  • Lumen - Luminous Flux

  • Lux - Illuminance

  • Becquerel - Radioactivity

  • Katal - Catalytic Activity

  • Sievert - Equivalent Dose of Ionising Radiation


You can refer to the table on this page for a list of the symbols used for each Derived SI Unit.

5. Where can I find out more about SI Units?

There are two main resources through which you can find out more about SI Units. 


The first resource is your textbook and reference book. These have been written and prescribed by the relevant authorities because of their adherence to the board guidelines. Textbooks and reference books also contain in-depth information about the topic at hand. Therefore, you should begin your revision by going through your textbooks.


The next place to look is on the Vedantu website or app. Vedantu has provided a wide range of study materials for students to refer to while they prepare for their exams. These resources are available for free and can be downloaded as long as you have a Vedantu account. If you don't have one, you can sign up for a new account and immediately gain access to the free materials.

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