## What is Units?◘ The internationally accepted ◘ To measure a physical quantity, a widely accepted standard value of the same physical quantity is used. ◘ It Indicates that how many times the standard physical quantity is used to measure that whole physical quantity.The standard physical quantity known as the ◘ Consider the following example, Suppose the mass of an object is 20 kg. • The unit of a physical quantity always written in small letter even if the unit is named after the scientist. For example – Unit of Force ‘newton’, Unit of electric current ‘ampere’, Unit of temperature ‘kelvin’, unit of luminous intensity ‘candela’. • If the unit represented by a single letter, then it must be written in Capital letters. For example: Unit of force ‘newton’ (N), Unit of temperature ‘kelvin’ (K). |

## Classification of Units♦ Units are also divided into the following categories _ ◘ • These limited number of units for expressing all the physical quantities are called fundamental or base units. • In another way we can say that the units of fundamental physical quantities are called fundamental units or base units. ♦ Fundamental Units or Base units are independent of each other.There are 7 fundamental units, They are ◘ or • The units of all other physical quantities except fundamental physical quantities which are obtained with the help of base units are called derived units. ♦ Some Examples_ ◘ |

## ✪ System of Units• The common systems of units are classified into four categories_ ♦ ♦ ♦ ♦ • For the international usage in scientific, technical, industrial and commercial field the standard SI symbols, units and abbreviations, was developed by the General Conference on Weights and Measures held at Geneva in 1971 • Conversions of SI unit within the system are quite simple and convenient, because the SI unit system used decimal systems. • SI system is extended and modified form of MKS system. |

## ✪ The 7 Fundamental SI units and their symbol |
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Base Quantity | Name | Symbol | Definition |
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Length | meter | m | Presently the meter is defined as the length of the path traveled by light in a vacuum in 1/299 792 458 of a second. |

Mass | kilogram | kg | The kilogram is defined as the equal mass of the platinum-iridium alloy cylinder (International Prototype) kept at the International Bureau of Weights and Measures, at Paris, France. |

Time | second | s | The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.(1967) |

Electric current | ampere | A | The ampere is defined by the force produced by constant current in two parallel conductors which is equal to 2×10^{–7} newton per meter of length between the two conductors.
• The two straight parallel conductor should be infinite in length • Also negligible circular cross-section • And both the conductor placed 1 meter apart in vacuum |

Thermodynamic temperature | kelvin | K | The Kelvin, is the fraction 1/273.16 of the thermodynamic of the triple point of water. (1967) |

Luminous intensity | Candela | cd | It is defined by the luminous intensity, in a given direction, of a source.
• The source of radiation should be monochromatic • Frequency of the radiation 540×10 • Also it has a radiant intensity in that direction of 1/683 watt per steradian. |

Amount of substance | mole | mol | The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon – 12. (1971) |

## Supplementary Units and their Symbols in SI unit System |
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Name of the Quantity | Name of the Unit | Symbol |
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Plane angle | Radian | rad |

• ‘Radian’ is the angle subtended at the center of a circle by an arc whose length is equal to the radius of the circle. • π radian = 180° • 1 radian = 180° / π. • Length of the Arc AB equal to the radius of the circle. |
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Solid angle | Steradian | Sr |

## Some units retained for general use (Though outside SI) |
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Name of Unit | Symbol | Value in SI unit |
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Minute | min | 60 s |

Hour | h | 60 min = 3600 s |

Day | d | 24 h= 86400 s |

Year | y | 365.25 d=3.156 x 10^7 |

Degree | ^{o} |
1^{o} = ( π / 180) rad |

Litre | L | 1 dm^{3}= 10^{-3}m^{3} |

Tonne | t | 10^{3}Kg |

Carat | c | 200mg |

Bar | bar | 0.1 Mpa = 10^{5 }Pa |

curie | Ci | 3.7 x 10^{10} S^{-1} |

Rontgen | R | 2.58 x 10^{-4} C/Kg |

Quintal | q | 100 Kg |

Barn | b | 100 fm^{2} = 10^{-28}m^{2} |

Are | a | 1 dam^{2} =10^{2}m^{2} |

Hectare | ha | 1 hm^{2} = 10^{4} m^{2} |

Standard atmospheric pressure | atm | 101325 Pa = 1.013 10^{5} Pa |