## 1. Introduction

• Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen internationally accepted reference standard called unit.
• The units for the fundamental or base quantities are called fundamental or base units.
• The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units.
• A complete set of these units, both the base units and derived units, is known as the system of units.

## 2. The International System of Units

• The whole sets of units is basically known as the system of units.
• International system of units is of four types:

a. C.G.S: it is also known as centimetre-gram-second system. In this system the length, mass and time is measured in centimetre, gram and seconds                    respectively. It is a French metric system of unit.

b. M.K.S: it is also known as metre-kilogram-second system. In this system the length, mass and time is measured in metre, kilogram and second                         respectively.

c. F.P.S: it is also known as foot-pound-second system. In this system the length, mass and time is measured in foot, pound and seconds respectively.

d. S.I: it stands for standard international system. This system is used over all the above systems and it is universally accepted.

## 3. Prefixes used in SI system

 MULTIPLE PREFIX SYMBOL 10-24 Yocto y 10-21 Zepto z 10-18 Atto a 10-16 Femto f 10-12 Pico p 10-9 Nano n 10-6 Micro 10-3 Milli m 10-2 Centi c 10-1 Deci d 10 Deca da 102 Hector h 103 Kilo k 106 Mega M 109 Giga G 1012 Tera T 1015 Peta P 1018 Exa E 1021 Zeta Z 1024 yotta Y

## 4. S.I units

• With the use of different systems of measurement, it became difficult to exchange scientific information all over the world.
• Hence a common system known as SI unit system came into existence.
• The Eleventh International General Conference of weights and measures in Paris, a common system of units was accepted in October 1960.
• SI unit consists of fundamentalsupplementary and derived units.

## 5. Fundamental units

• It is defined as the units used to measure the fundamental quantities.
 Fundamental quantity Unit Symbol Definition Unit of length metre m The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of as second. Unit of mass Kilogram kg It is equal to the mass of the international prototype of a kilogram. Unit of time seconds s The second is the duration of 9192631770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caestum-133 atom Unit of electric current ampere A The ampere is the constant current, which if maintained in two straight parallel conductors of infinite length of negligible circular cross section and placed 1 metre apart in vacuum Unit of temperature kelvin K It is fraction 1/273.16 of the thermodynamic temperature of the triple point of water (279.16) K. Unit of amount of substance mole mol It is the amount of substance of any system, which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12 Unit of luminous intensity candela cd It is the luminous intensity, in a given direction of a source that emits monochromatic radiation of frequency in hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

## 6. Derived units

• It is the unit of derived quantities which depends on fundamental quantities for their measurements.
 Derived quantities SI unit C.G.S unit speed m/s Cm/s force newton dyne density Kg/m3 g/m3 acceleration m/s2 Cm/s2

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