What is SI Unit? Notes on Definition, Facts and List of SI Units

SI Units an important topic that is asked in many competitive exams like CDS, AFCAT, Air Force Group X & Y, SSC, and other government exams.

What is the SI Unit?

SI Unit is a global measuring system that is widely used in technical and scientific research in order to prevent confusion with units. Having a universal system of units is necessary because it standardizes the units of measurement and it becomes easier to comprehend quantities. Below is a table that represents various SI Units:

Sl. No.	Name of the Quantity	SI Unit	SI Unit Symbol
1.	Length (l)	Meter	m
2.	Mass (M)	Kilogram	kg
3.	Time (T)	Second	s
4.	Electric current (I)	Ampere	A
5.	Thermodynamic temperature (Θ)	Kelvin	K
6.	Amount of substance (N)	Mole	mol
7.	Luminous intensity (J)	Candela	cd

SI Units List

In physics, there are several SI units used to convey the various amounts. It is possible to classify the amounts into two groups, i.e. base units and derived units.

SI Base Units

These are basic units and are considered as building blocks for various units. All the other units are obtained from SI base units. Eg – Kilograms. There are a total of 7 SI base units. Below is a table which has SI units along with the symbol:

SI Base Units List

There are 7 SI base units. The seven units along with their SI unit and symbol are given below:

Unit of Measurement	SI Unit	Description
Unit of length	meter (m)	Meter is the SI unit of length and is defined by taking the fixed value of the speed of light in vacuum. It is expressed as m.s-1
Unit of mass	kilogram (kg)	A kilogram is the SI unit of mass and is defined by taking the fixed value of the Planck constant. It is expressed as kg.m2.s-1
Unit of time	second (s)	Second is the SI unit of time and is defined by taking the fixed value of Cesium frequency. It is expressed as s.
Unit of electric current	ampere (A)	Ampere is the SI unit of electric current and is defined by taking the fixed value of the elementary charge.
Unit of thermodynamic temperature	Kelvin (K)	Kelvin is the SI unit of thermodynamic temperature and is defined by taking the fixed value of Boltzmann constant k = 1.380649×10-23.
Unit of the amount of substance	mole (mol)	Mole is the SI unit of amount of substance and is defined by the fixed value of Avogadro constant NA. One mole contains 6.02214076×1023 elementary entities and is expressed as mol-1
Unit of luminous intensity	Candela (cd)	Candela is the SI unit of luminous intensity and is defined by the fixed value of the luminous efficacy.

It should be noted that these 7 units are assumed to be mutually independent and hence are called base units.

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SI Derived Units

The derived units are unlimited as they are formed by different operations on the base units. For derived units, the dimensions are expressed in terms of the dimensions of the base units. The derived units might also be expressed with the combination of base and derived units.

SI Derived Units List

There are several derived units in physics. Some of the most widely used SI derived units in physics are given below.

Sl. No	Unit(s) Name	SI Unit	SI Unit Symbol	Expressed in SI Base Unit	Expressed in other SI units
1.	Frequency	Hertz	Hz	s-1	–
2.	Force, Weight	Newton	N	kg⋅m⋅s-2	–
3.	Electric potential (Voltage)	Volt	V	kg.m2.s-3.A-1	W/A
4.	Electric charge	Coulomb	C	s⋅A	–
5.	Capacitance	Farad	F	kg-1.m-2.s4.A2	C/V
6.	Inductance	Henry	H	kg.m2.s-2.A-2	Wb/A
7.	Electrical conductance	Siemens	S	kg-1.m-2.s3.A2	Ω-1
8.	Resistance, Impedance, Reactance	Ohm	Ω	kg.m2.s-3.A-2	V/A
9.	Magnetic flux density	Tesla	T	kg.s-2.A-1	Wb/m2
10.	Magnetic flux	Weber	Wb	kg.m2.s-2.A-1	Vs
11.	Power, Radiant flux	Watt	W	kg.m2.s-3	J/s
12.	Energy, Work, Heat	Joule	J	kg.m2.s-2	Nm = Pa⋅m3
13.	Radioactivity	Becquerel	Bq	s-1	–
14.	Angle	Radian	rad	m.m-1	–
15.	Luminous flux	Lumen	lm	cd	cd⋅sr

Preparing for AFCAT? Boost your prep

Here are some important questions that are asked in exams:

1. Ampere is the SI unit of ____

A. Electric current

B. Potential difference

C. Resistance

D. Capacitance

Answer ||| A

Solution

Ampere is the SI unit of the electric current.

A current of 1 ampere flowing through a conductor means 6.25×10-18 electrons are entering at one end and leaving at other ends every second.

 

2.SI unit of pressure is

A. Pascal

B. torr

C. atm

D. Atmosphere

Answer ||| A

Solution

The SI unit of pressure in pascal (Pa), defined as a force of one Newton per square meter. The conversion between atm, Pa, and the torr is as follows: 1 atm = 101325 Pa = 760 torr

 

3. What is the SI unit (measurement unit) of Impulse?

A. Radian

B. Newton Second

C. Lumen

D. Coulomb

Answer ||| B

Solution

SI unit of Impulse is Newton Second(Ns).

Impulse is basically the change in momentum and the dimensionally equivalent unit of momentum is the kilogram meter per second (kg⋅m/s)

 

4. The SI Unit of area is:

A. mm²

B. cm²

C. m²

D. km

Answer ||| C

Solution ||| The SI Unit of the area is sq. m or m²

 

5.SI unit of surface tension is _____.

A. N/m²

B. N/m

C. Ns/m

D. J/S

Answer ||| B

Solution

Surface tension is represented by the symbol γ, is measured in force per unit length.

Its SI unit is newton per meter but the cgs unit of dyne per cm is also used.

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