Unit

 UNIT

[1] How A Unit Of  A Physical Quality Should  Be ?

• Most Of  Three Requirements For Selecting A Unit 

      1. The unit should be reproducible.

      2. It should be possible to define the unit without ambiguity.

      3. The value of the unit should not change with space and time

[2] Some System of  Units

- SI (International System of Units) (meter-kilogram-second-ampere-

  kelvin -mole-candela)

- FPS (foot-pound-second)

- MKS (meter-kilogram-second)

- CGS (centimeter-gram-second)

- EMU (Electromagnetic) (centimeter-gram-second-abampere)

- ESU (Electrostatic) (centimeter-gram-second-abcoulomb)

- Atomic (bohr-electron mass-atomic second-electron)

- MTS (meter-tonne-second)

[3] SI (International System of Units)

- 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.

 

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 :

- There are several SI units used in physics that are used to express 

  the different quantities. The quantities can be classified into two groups 

(1) Base units 

(2) Derived units.

SI Base Units List

- There are 7 SI base units.

(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 calledbase 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

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

[4] Supplementary Units 

- Throughout the International economy, dimensionless units have been

   combined alongside base units just to generate derived units are

   known as supplementary quantities.

- Supplementary quantities are of two types

(1) Plane angle - radian (rad)

(2) Solid angle - steradian (sr)

[5] Practical Norms For The Use of The SI System :

(1) The name of the unit should be written in small letters even if it is after

      the name of the scientist.

(2) Symbols are generally written in small letters, for example, metre (m).

(3) But the symbol of units named after scientists is written in capital letters, 

      for example, newton (N).

(4) Symbols for derived units which are obtained by multiplication of two 

     units are written with space,  for example, newton second as N s.

(5) Symbols for derived units which are obtained by division of two units are

      written with solidus (/) or as negative exponent, for example, metre per

      second as m/s or m s-1.

(6) Name of the unit may be written in plural form but the symbol is always

      written in singular form.

(7) Symbols do not end with a full stop unless they are at the end of the

     sentence.