Definitions of Units of Physical quantities

There are 7 fundamental physical quantities that we use in our daily life. They are

1. length(m)
2. mass(kg)
3. time(s)
4. electric current(A)
5. temperature(K)
6. amount of substance(n)
7. luminous intensity(L)

A system called as the System International(SI) decided to define these fundamental quantities.

The definitions are as follows

1. Meter: The meter is the distance travelled by light in 1/29979248 seconds.
2. Kilogram: The Kilogram is the mass of a cylinder made of platinum iridium alloy kept at the International Bureau of Weights and Measures at Sevres, near Paris
3. Second: The second is the time taken by light of specified wavelength emitted by a cesium-133 atom to execute 9,192,631,770 vibrations.
4. Ampere: The Ampere is that current which when flowing in each of two infinitely long straight parallel conductors of negligible cross-section placed one meter apart in vacuum produces between the conductors, a force of 2 x 10^-7 newton per meter length of each conductor.
5. Kelvin: The kelvin is 1/273.16 of the thermodynamic temperature of triple point of water.
6. Mole: The mole is the amount of substance that contains as many elementary entities(atoms or molecules) as there are number of atoms in 0.012kg of carbon-12. This number is called Avogadro constant and its value is 6.02214199 x 10^23.
7. Candela: The candela is the luminous intensity of a black body of surface area 1/600 m² placed at the temperature of freezing platinum and at a pressure of 101,325 newton per square meter, in the direction perpendicular to its surface.
8. Radian: The radian is the angle subtended at the center of a circle by an arc whose length is equal to the radius of the circle.
9. Steradian: The steradian is the solid angle subtended at the center of a sphere by an area of its surface equal to the square of the radius of the sphere.

	Fundamental quantity	Symbol of the quantity	Name of the SI Unit	Symbol of the unit
1	Length	l	meter	m
2	Mass	m	kilogram	kg
3	Time	t	second	s
4	Electric Current	I	ampere	A
5	Temperature	T	kelvin	K
6	Amount of substance	n	mole	mol
7	Luminous Intensity	L	candela	cd