# Units and measurement

# Units and measurement

In this topic we will discuss about, type of units, system of units , name and define of all fundamental units . Also the abbreviations in power of tens. one of the important thing given is chart of physicist and their discoveries.

**Units and measurement **

__Physical quantity- __

All those quantities which can be measured in terms of which the laws of physics can be expressed are called physical quantity. Example- mass , speed, force , power etc.

Physical quantities are of two types- fundamental and derived

–__Fundamental quantities__

The physical quantities which is independent of other physical quantities and are not usually defined in terms of other physical quantities are called fundamental quantities. These are , mass, length, time, electric current, temperature ,luminous intensity and amount of substance.

__Derived quantities-__

The physical quantities which can obtained using other physical quantities are called derived units. Example – velocity, force , power etc. or we can say all the physical quantities other than seven fundamental quantities are derived units.

**UNITS-**

*The standard amount of a physical quantity chosen to measure the physical quantity of the same kind are called physical unit.*

To represent a physical quantity we need numeric value and its units .i.e.

Physical quantity Q = n U ( where n represent numeric value and U is the unit) . So we can define unit is the thing used to identify and measure a physical quantity .

There are two types of unit .Fundamental and derived units .

**Fundamental units**–

The physical unit which neither be derived from one another and it can not be resolved into more simpler units called fundamental units. There are seven fundamental units which are the units of These are , mass, length, time, electric current, temperature ,luminous intensity and amount of substance .

**Derived units** – All the units can be expressed using fundamental units is called derived units .

Example unit of speed = distance/time =m/s …etc .

**System of units** – A complete set of units which is used to measure all kind of fundamental and derived quantities is called system of units.

(i) __cgs system-__ It is based on centimetre, gram and second as the fundamental unit of length, mass and time respectively.

(ii) __MKS system__ – – It is based on metre, kilogram and second as the fundamental unit of length, mass and time respectively.

(iii__) FPS system__ – – It is based on foot, pound and second as the fundamental unit of length, mass and time respectively.

(iv)SI ( the international system of units) system .

Definition of basics units –

- Meter (m) – It is the SI unit of length, One meter is defined as the path travelled by light in vacuum in 1/ 299,792,458 seconds ,
- kilogram (kg) – It is the SI unit of mass .- It is the mass of prototype cylinder of platinum-iridium alloy .
- Second(s)- It is the SI unit of time . On e second is the duration of 9,192,631,770 period of the radiation between two levels of the ground state of the Cesium-133 atom.
- Ampere (A) – It is the unit of electric current. It is the force 2 x 10
^{-7}Newton between two parallel current carrying wire placed 1m away is of unit length . To know more on Ampere click here- - Kelvin (K) – It is the SI unit of temperature. One kelvin is the fraction 1/273 of the thermodynamic temperature of the triple point of water.
- Candela(cd)- It is the SI unit of luminous intensity. It is the intensity of a source that emits monochromatic radiation of frequency 540 x 10
^{12 }Hz , and that has the radiant intensity 1/683 watt per steradian in that direction. - Mole (mol)- It is the unit of amount of substance . .It is the amount of substance which contain as many elementary entities as there are atom in 0.012 kg of C-12 isotopes.

Supplementary SI units –

(a). Radian (rad)- It is the plane angle subtended at the centre of a circle by an arc equal in length to the radius of the circle . ϴ= arc/radius .

(b). Steradian (sr)- It is defined as the solid angle subtended at the centre of the sphere by a surface of the sphere equal in area to that of a square , having each side equal to the radius of the sphere . sr= surface area / radius^{2} .

Some important thing ( multiple , prefix and symbol) in the power of ten.

**Some great physicist and their discoveries ( This topic is important for objective point of view)-**

Next topic after this topic units and measurement students have to learn DIMENSIONS AND USES OF DIMENSIONS

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