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The selected standard used for measuring a physical quantity is named a unit. It should be well defined, easy to reproduce, easy to compare, internationally accepted and independent of changes in physical conditions. There are two types of units.

- Fundamental units- independent of any other units

The seven fundamental units are the units used to measure the physical quantities length, mass, time, electric current, temperature, luminous intensity and amount of substance. Besides these seven, two supplementary units are given to measure the physical quantities angle and solid angle

- Derived units – obtained by mathematical operations between two fundamental units.These units are used to measure physical quantities like speed, velocity, density, force, pressure, work, energy etc.

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**System of units**

Below stated are some of the internationally accepted systems of units

- CGS System -centimetre gram second system
- MKS Sysytem- Meter-Kilogram-Second System
- FPS System- Foot-Pound System
- SI Units

**SI Units**

The term SI unit is the condensation of the French term Système International. The Systeme International d’Unites (SI) was implemented in 1960 after an international convention of 50 member states which consisted of all the main industrialized nations. SI is the International System of Units and have existed since 1960. Founded and sustained by the General Conference on Weights and Measures (CGPM). This has substituted the variety of measurement units across the world. Unifying the unit system has made it easier to express the units universally. They are the standard units used in science, technology, industry, and everyday commerce all over the world. The SI is envisioned to be an evolving structure; units and prefixes are formed and unit definitions are altered through the international agreement as to the technology of measurement advances and the accuracy of measurements improves. SI unit is also called the metric system.

**SI Units of seven fundamental quantities**

physical quantity |
unit |
abbreviation |

mass | kilogram | kg |

length | meter | m |

time | second | s |

temperature | Kelvin | K |

amount of substance | mole | mol |

electric current | ampere | A |

luminous intensity | candela | cd |

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**SI Units of two supplementary units**

Physical quantity |
SI Unit |
Symbol |

Angle | Radian | rad |

Solid angle | Steradian | sr |

**Table of important physical quantities and their SI Units**

Quantity |
Units (S.I.) |

Radioactivity | Becquerel |

Luminous flux | Lumen |

Magnetic flux density | Tesla |

Magnetic flux | Weber |

Length | Metre |

Time | Second |

Mass | Kilogram |

Area | Square metre |

Volume | Cubic metre |

Velocity | Metre/second |

Acceleration | Metre/second square |

Density | Kilogram/metre Cube |

Work | Joule |

Energy | Joule |

Force | Newton |

Pressure | Pascal or Newton/sq. metre Charge |

Frequency | Hertz |

Power | Watt |

Weight | Newton or Kilogram |

Impulse | Newton-second |

Angular velocity | Radian /second |

Viscosity | Pascal |

Surface tension | Newton/square metre |

Heat | Joule |

Temperature | Kelvin |

Absolute temperature | Kelvin |

Resistance | Ohm |

Electric current | Ampere |

Electromotive force | Volt |

Electrical conductivity | Ohm/metre |

Electric energy | Kilo watt-hour |

Electric power | Kilowatt or watt |

Magnetic intensity | Oersted |

Charge | Coulomb |

Magnetic induction | Gauss |

Luminous flux | Candela |

Intensity of sound | Decibel |

Power of lens | Dioptre |

Depth of sea | Fathom |

Inductance | Henry |

Electrical conductance | Siemens |

**Study Notes PDF for Physical Quantities and their SI Units**

Given below is in the table is a PDF of the notes for the topic

Topic |
PDF |

Physical quantities and their SI Units | physical quantities and their SI units |

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**Important quantities and their CGS units**

Quantity |
CGS unit name |
Unit symbol |

length, position | centimetre | cm |

mass | gram | g |

time | second | s |

velocity | centimetre per second | cm/s |

acceleration | galileo | Gal |

force | dyne | dyne |

energy | erg | erg |

power | erg per second | erg/s |

pressure | barye | Ba |

dynamic viscosity | poise | P |

electric charge | franklin | Fr |

kinematic viscosity | stokes | St |

electric current | biot | Bi |

wavenumber | kayser (K) | cm^{−1} |

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**Dimensions of a physical quantity**

Dimension of a physical quantity is the powers to which the fundamental qualities have to be raised to define that quantity. Some examples are given in the following table.

No |
Physical Quantity |
Dimensional Formula |

1 | Area (A) | [M^{0}L^{2}T^{0}] |

2 | Volume (V) | [M^{0}L^{3}T^{0}] |

3 | Density (d) | [M^{1}L^{-3}T^{0}] |

4 | Speed (s) | [M^{0}L^{1}T^{-1}] |

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