UPENDRA KUMAR
9:23 AM

Upendra Kumar

In 1960, the general conference on weight and measures recommended that SI (System International d' Units) should replace all metric system now in use. The SI system of the unit for use in all aspects of life, including international trade, manufacturing, security, health, and safety, protection of the environment, and the basic science.

SI system has an edge over other versions of the metric system.

**There are seven base units:-**

__Definition__

**SECOND:**-Second is the SI Unit of the Time and its unit symbol **s**. One second is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground-state of the 133 Cs atom.

__METER:__- Meter is the SI Unit of the length and its unit symbol **m**. The one-meter equal to the length of the path traveled by light in vacuum during a time interval with a duration of 1/299792 of a second.

__KILOGRAM:-__The kilogram is the SI unit of the mass and its unit symbol **kg**. It is fixed numerical value of the plank constant h to be 6.62607015 X 10-34 when expressed in the unit j s, which is equal to kg m2 s−1, where the meter and the second are defined in terms of c and ΔνCs.

__AMPERE:-__ The ampere is the SI unit of the electric current and its unit symbol **A**. It defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 ×10-19. When expressed in unit C, which is equal to As where the second is defined in terms of ΔνCs.

__KELVIN__:- The kelvin is the SI unit of thermodynamic temperature and its unit symbol **K**, It is defined by taking the numerical value of Boltzmann constant k to be 1.380 649 ×10-23 when expressed in the unit J K-1 which is equal to kg m2 s-2 K-1, where the kilogram, meter and second are defined in terms of h, c, and ΔνCs.

__MOLE__:- The mole is the SI unit of amount of substance and its unit symbol mol.One mole contains exactly 6.022 140 76 × 1023elementary entities.

__CANDELLA:__-Candella is the SI unit of luminous intensity and its symbol **cd**. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 ×1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, meter and second are defined in terms of h, c, and ΔνCs

Derived Units:- Units that are a combination of two or more quantities and which require a compound word to name them are called compound or derived units.

**DERIVED UNITS**

**Size of Units: **The choice of meter and kilogram as the base units of length and mass yield coherently derived units of more practical value in many cases. The meter, for instance, is a convenient unit for building site plans but it is too large for precision engineering purposes and too small for the large distance between the town and astronomical distance.

BASE UNIT SYMBOL |

In 1960, the general conference on weight and measures recommended that SI (System International d' Units) should replace all metric system now in use. The SI system of the unit for use in all aspects of life, including international trade, manufacturing, security, health, and safety, protection of the environment, and the basic science.

SI system has an edge over other versions of the metric system.

Physical Quantity | SI Units | Unit Symbol |

Length | meter | m |

Mass | kilogram | kg |

Time | second | s |

Electric current | ampere | A |

Luminous intensity | Candella | cd |

Thermodynamic temperature | kelvin | K |

Amount of substance | mole | mol |

There are supplementary Units.

Physical Quantity | SI Units | Unit Symbol |

Plan angle | radian | rad |

Solid angle | steradian | sr |

Derived Units:- Units that are a combination of two or more quantities and which require a compound word to name them are called compound or derived units.

Physical quantity | Name of the SI Unit | SI Unit symbol |

Force, weight | newton | N=kg m/s2 |

Work, energy, the quantity of heat | joule | J=Nm |

Power | watt | W=J/s=Nm/s=Kg m2/s2 |

Pressure, stress | pascal | Pa=N/m2=kg/ms2 |

Frequency | hertz | Hz=s-1 |

Potential | volt | V=W/A |

Resistance | ohm | 𝝮=V/A=WA2 |

Customary temperature | Degree celsius | 0c |

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