In this post we will study all the fundamental Quantity like mass, time amount of substance and more. How we would define the length and its measured.

Many students are curious about how length has been measured, what is a fixed value of a Length. So in this post we will study all the quantities in detail.

## Length (m)

The meter symbol is the si unit of length. It is defined by taking the fixed numerical value of speed of light in vacuum c to be 299792458 when expressed in the unit m s^-1 where the second is defined in terms of the caesium frequency.

## Mass (Kg)

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 x 10^-34 when expressed in the unit J s, which is equal to kg m^2 s^-1 where the metre and the second are defined in terms of c.

## Time (Second)

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, the unperturbed ground state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s^-1.

## Electric Current (A)

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10^-19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of Delta v.

## Thermodynamic Temperature (K)

The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10^{-23} when expressed in the unit J K-1, which is equal to kg m^{2}s^{-2}k^{-1} where the kilogram, metre and second are defined in terms of h, c.

## Amount of substance (mole)

The mole, symbol mol, is the SI unit of amount of substance. One mole substance contains exactly 6.02214076×10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A} when expressed in the unit molĀ¹ and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.

## Luminous intensity (cd)

The candela, symbol cd, is the SI unit of luminous intensity in given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^{12} Hz, K_{cd}, to be 683 when expressed in the unit Im W^{-1}. which is equal to cd sr W^{-1} or cd sr kg^{-1} m^{-2}s^{3} where the kilogram, metre and second are defined in terms of h and c.