Fundamental Units and Derived Units

The units which are independent of each other are known as Fundamental Units. Derived Units are derived from Fundamental Units. For example, meter is a fundamental unit of length and second is a fundamental unit of time. However, meter per second (ms^-1) is a derived unit of velocity. There are seven fundamental units as given in the below table:

Apart from the above seven, there are two supplementary fundamental units viz. Radian and Steradian. While Radian (Rad) is used to measure plane angle, Steradian (Sr) is used to measure Solid Angle.

The derived units are derived from fundamental units. Examples of derived units are velocity (meter/second), acceleration (meter /second²) etc.
Each fundamental unit has been defined as follows:

Kilogram

At present, a kilogram is defined by a cylindrical piece of platinum-iridium kept at the office of International Committee on Weights and Measures in Paris. However, that lump has lost 50 microgram since 1879 and that is why scientists are looking for ways of expressing a kilogram in terms of the fundamental constants of nature, rather than a man-made object.

Meter

Earlier meter was calibrated as the distance between two “Xs” on a platinum Iridium metal bar kept in Paris at a temperature of 0°C. Later, it was fixed as length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. Currently, one meter contains 1650763.73 wavelength of orange-red light of Kr-86.

Second

Earlier, second was the length of a mean solar day divided by 86,400. Since 1967, a second has been classified time in which caesium atom vibrates 9192631770 times in an atomic clock.

Kelvin

One Kelvin is the 1/273.16 part of the thermodynamics temperature of the triple point of water.

Candela

Candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10¹² hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian.

Ampere

1 ampere is the electric current which it maintained in two straight parallel conductor of infinite length and of negligible cross-section area placed one metre apart in vacuum will produce between them a force 2 x 10^-7 N per metre length.

Mole

One mole is the amount of substance of a system which contains a many elementary entities (may be atoms, molecules, ions, electrons or group of particles, as this and atoms in 0.012 kg of carbon isotope ₆C¹².