## Reference Notes – System of Units

Unit: Units and Measurement

Science | Physics Class 11

Different systems of units are used to perform measurement. The different systems of units are explained below:

**MKS-System:**In this system, length is measured in Meter (M), mass is measured in Kilogram (K) and time is measured in Second (S).**CGS-System:**In this system, length is measured in centimeter (C), mass in gram (G) and time in second (S).**FPS-System:**In this system, length is measured in foot (F), mass in pound (P) and time in second (S).

**SI-Units:**

In this system, seven basic and two supplementary units are taken. The fundamental quantities adopted by the SI-units, their units and symbols are given below:

**Units of Fundamental Quantities:**

Fundamental Quantity | Unit | Symbol |

Length | Meter | M |

Mass | Kilogram | Kg |

Time | Second | s |

Temperature | Kelvin | K |

Current | ampere | A |

Luminous intensity | candela | cd |

Amount of substances | mole | mol |

In addition to the above seven fundamental units in SI-units, two supplementary units are also defined. These supplementary units are given in the table below:

Quantity | Unit | Symbol |

Plane Angle | radian | rad |

Solid angle | steradian | sr |

**Definitions of SI Base Units:**

**Metre:**The meter is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.**Second:**The second is the duration of 9,162,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atoms.**Kilogram:**The kilogram is the mass equal to the mass of standard platinum-iridium alloy cylinder (90% platinum and 10% iridium) kept at the International Bureau of Weights and Measures in Paris. This cylinder is about 4 cm high and 4 cm in diameter.

** Advantages of SI-Units:**

The advantages of SI-Units are given below:

- It is a rational system of units.
- It is a coherent system of units.
- It is a decimal system.
- It is highly useful in practical and theoretical work.