## Units

 Measurement is the comparison of an unknown quantity with a known quantity of same kind. For the measurement of any substance, we require a standard quantity called unit. Units can be defined as the standard quantities with which we carry out the measurement of any object or substance. For example kilogram, meter, second, candela, Newton, ampere etc.

Fundamental quantities
Fundamental quantities are the quantities which do not depend upon any other quantities for their existence. They can also be defined as such quantities which cannot be divided into same or different other units.  Some examples of fundamental quantities are mass (M), length (L), time (T), ampere (A), moles (mol), Kelvin (K), candela (cd) etc.

Derived units
Derived units are such units which are derived or extracted from fundamental quantities or units. They can also be defined as such physical quantities which are formed by the combination of same or different fundamental quantities. Some examples of derived units are area (m2), volume (m3), velocity (m/s), acceleration (m/s2), Newton (N) etc.

Derived units can be classified into four units. They are:

1. C.G.S. System: C.G.S. system is such system of units in which length is measured in centimeter (cm), mass is measured in gram (g) and time is measured in seconds (s).
2. F.P.S. System: F.P.S. system is such system of units in which length is measured in foot (ft), pound is measured in (lbs) and time is measured in seconds (s).
3. M.K.S System: M.K.S. system is such system of units in which length is measured in metre (m), mass is measured in kilogram (kg) and time is measured in seconds (s).
4. S.I. system: It is a system of units which was designed by scientists for scientific purposes. It is somewhat similar to M.K.S. system of units. It is also known as System International. In this system we use the following units:
• Metre (m): Metre is the S.I. unit of length. One metre is defined as the length of 1650763.73 wavelength of the orange-red radiation emitted by Krypton-86 atom at the triple point of nitrogen.
• Kilogram (kg): Kilogram is the S.I. unit of mass. One kilogram is the mass of the standard platinum-iridium alloy cylinder of diameter equal to its height kept at the International Bureau of Weights and Measures at Sevres, near Paris, France.
• Second (s): Second is the S.I. unit of time. One second is defined as the time in which a cesium-133 atom makes 9192631770 vibrations.
• Ampere (A): Ampere is the S.I. unit of electric current. One ampere is that current which flows in each of two infinitely long parallel straight wires of negligible cross-sectional area separated by a distance of one metre in vacuum producing a force of 2×10-7 N/m between them.
• Mole (mol): Mole is the S.I. unit of amount of substance. One mole of substance contains as many elementary entities as there are in 0.012 kg of carbon-12.
• Kelvin (K): Kelvin is also S.I. unit of temperature. One Kelvin is defined as 1/273.16 of the thermo-dynamical temperature of triple point of water.
• Candela (cd): It is the S.I. unit of luminous intensity. One candela is the luminous intensity of a black body having surface area 1/600000  placed at the temperature of freezing platinum and at the pressure of 1.1325  in the direction perpendicular to its surface.
• Radian (rad): It is defined as the plane angle between the two radii of circle which cut off an arc of length equal to the radius of that circle.
• Steradian (sr): It is defined as the angle subtended by the surface area at the centre of the sphere. It is equal to the ratio of surface area to the square of radius of the sphere.