SI Units

 

SI Units in Physics

The answer to what is SI unit is that it is an abbreviation of the French word Système International. The International System Of Units (SI) is the metric system that is used universally as a standard for measurements. SI units play a vital role in scientific and technological research and development. It is made up of 7 base units which are used for defining 22 derived units. The SI units can be expressed either as standard multiple or as fractional quantities. These quantities are defined with the help of prefix multipliers with powers of 10 that range from 10-24 to 1024.

The Full Form of SI Unit is Système International

What is the SI Unit?

SI unit is an international system of measurements that are used universally in technical and scientific research to avoid the confusion with the units. Having a standard unit system is important because it helps the entire world to understand the measurements in one set of unit systems. Following is the table with base 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

What is SI Units List?

There are several SI units used in physics that are used to express the different quantities. The quantities can be classified into two groups i.e. base units and derived units.

SI Base Units

These are the fundamental units and are considered as the building blocks of the system. All the other units are derived from the SI Base units. One of the examples is that the SI unit of mass is kilogram. This is often confused with grams.

SI Base Units List

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

1. 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.
2. Unit of mass, kilogram (kg): 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.
3. 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 s1.
4. 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.
5. 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.
6. Unit of the amount of substance, mole (mol): Mole is the SI unit of the 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.
7. 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.

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.	Force, Weight	Newton	N	kg⋅m⋅s-2	–
2.	Frequency	Hertz	Hz	s-1	–
3.	Electric charge	Coulomb	C	s⋅A	–
4.	Electric potential (Voltage)	Volt	V	kg.m2.s-3.A-1	W/A
5.	Inductance	Henry	H	kg.m2.s-2.A-2	Wb/A
6.	Capacitance	Farad	F	kg−1.m−2.s4.A2	C/V
7.	Resistance, Impedance, Reactance	Ohm	Ω	kg.m2.s−3.A−2	V/A
8.	Electrical 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 = Pa⋅m3
12.	Power, Radiant flux	Watt	W	kg.m2.s−3	J/s
13.	Angle	Radian	rad	m.m−1	–
14.	Radioactivity	Becquerel	Bq	s-1	–
15.	Luminous flux	Lumen	lm	cd	cd⋅sr

These were a few widely used units along with their SI units. Apart from these units, there are certain additional units that are commonly seen in physics. Some of such units are:

• The SI unit of momentum (P) is kilogram meter per second (kg⋅ m/s)
• The SI unit of the magnetic field (B) is Tesla
• The SI unit of heat is the unit joule
• The SI unit of velocity is m/s