**The fundamentals of physics form the basis for the study and the development of engineering and technology. Measurement consists of the comparison of an unknown quantity with a known fixed quantity. The quantity used as the standard of measurement is called ‘unit’.**

**Fundamental physical quantities**–

Fundamental quantities are the quantities which cannot be expressed in terms of any other physical quantity.

Example*– length, mass and time. *

**Derived quantities**–

A derived quantity is a quantity that is based on the result of a systematic equation that includes any of the seven basic quantities, which are the kilogram, meter, second, ampere, kelvin, mole and candela.

Examples*–Area (Square meter; M*^{2}), speed (meters per second; m/s) and Density (kg/m^{3}).

**Unit –**

Unit is the reference used as the standard measurement of a physical quantity. The unit in which the fundamental quantities are measured are called fundamental unit and the units used to measure derived quantities are called derived units.

**System of unit – **

Group of units of all physical quantities are system of unit.

- G.S. system – It is the centimetre gram second system which are units of length, mass and time.
- P.S. system – it is foot pound second system. Britishers used this system.
- K.S. System – it is metre kilogram second system. European countries like France use this system.
- S.I. System – It is the international system of unit. It is universally accepted and has seven fundamental units.

**There are seven fundamental units (base units) and two supplementary units.**

__SI system of units-__

__Quantity__ |
__Definition__ |
__Unit __ |
__Symbol__ |

*Fundamental quantities* |

Length |
The longest extent of anything as measured from end to end. |
Metre |
M |

Mass |
A fundamental measure of the amount of matter in the object |
Kilogram |
Kg |

Time |
Time is the dimension on which the evolution of state of a system is allowed to occur. |
Seconds |
S |

Electric current |
Electric current is defined as the rate at which charge flows through a surface. |
Ampere |
A |

Temperature |
A measure of the warmth or coldness of an object or substance with reference to some standard value. |
Kelvin |
K |

Luminous intensity |
Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian. |
Candela |
Cd |

Amount of substance |
Amount of substance, also called material quantity, is a dimensionless expression of the number of particles in a sample. |
Mole |
Mol |

*Supplementary quantities * |

Plane angle |
Plane angle is angle between two line meeting at the vertex. |
Radian |
__rad__ |

Solid angle |
Solid angle is the angle subtended by the object in three dimensional space from a point. |
Steradian |
__sr__ |

Wavelength |
Solid angle is the angle subtended by the object in three dimensional space from a point. |
Angstrom |
A^{0 }= 10^{-8} cm |