**S.I. Units of Physical Quantity**

**Unit**-The chosen standard used for measuring a physical quantity is called unit.

Unit should be:

- Well defined
- Easy to reproduce
- Easy to compare
- Internationally accepted
- Independent of changes in physical conditions

**Units are of two types:**

**Fundamental Unit :**- Are expressed in terms of standard units.
- There are seven (7) basic physical quantities.
- Examples- mass, length, time, electric current, temperature, luminous intensity and amount of substance.
- All other quantities are expressed in terms of these quantities.

**Derived Unit :**- When a physical quantity expresses itself in terms of two or more fundamental physical quantities.
- It is derived by multiplying or dividing one basic physical quantity with another basic physical quantity.
- Examples- density, volume, force, power, velocity, acceleration etc.

**System of Units** –Units depends on choice .each choice of units leads to a new system of units. The internationally accepted systems are

i) CGS System ii) MKS System iii) FPS System iv) SI Units.

**In SI Units(International System of Units)-** There are seven fundamental units and 2 supplementary units.All the units which are defined/ expressed in terms of fundamental units are called derived units.such as force work etc.

**Here list of most often used physical quantities their unit and symbols.**

Quantity |
SI |
Symbol |

Length | Meter | M |

Mass | Kilogram | Kg |

Time | Second | S |

Work and Energy | Joule | J |

Electric Current | Ampere | A |

Temperature | Kelvin | K |

Intensity of flame | Candela | Cd |

Angle | Radian | Rad |

Solid angle | Steredian | Sr |

Force | Newton | N |

Area | Square meter | M2 |

Volume | Cubic meter | M3 |

Speed | Meter per second | Ms-1 |

Angle Velocity | Radian per second | Rad s-1 |

Frequency | Hertz | Hz |

Moment of inertia | Kilogram square meter | Kgm2 |

Momentum | Kilogram meter per second | Kg ms-1 |

Impulse | Newton second | Ns |

Angular Momentum | Kilogram square meter per second | Kgm2s-1 |

Pressure | Pascal | Pa |

Power | Watt | W |

Surface tension | Newton per meter | Nm-1 |

Viscosity | Newton second per square m. | N.s.M-2 |

Thermal Conductivity | Watt per meter per degree celcius | Wm-1c-1 |

Specific Heat capacity | Joule per kilogram per Kelvin | Jkg-1K-1 |

Electric charge | Coulomb | C |

Potential Difference | Volt | V |

Electric Resistance | Ohm | O |

Electrical Capacity | Farad | F |

Magnetic Induction | Henry | H |

Magnetic Flux | Weber | Wb |

Or photometric power | Lumen | Lm |

Intensity of illumination | Lux | Lx |

Wave length | Angstrom | A0 |

Astronomical distance | Light year | ly |

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