### Physical quantities

Physical quantity is a quantity that can be measured. Physical quantities can be classi ed into two: fundamental quantities and derived quantities. Quantities which cannot be expressed in terms of any other physical quantities are called fundamental quantities. Example: Length, mass, time, temperature. Quantities like area, volume and density can be expressed in terms of some other quantities. They are called derived quantities.

Physical quantities have a numerical value (a number) and a unit of measurement (say, 3 kilogram). Suppose you are buying 3 kilograms of vegetable in a shop. Here, 3 is the numerical value and kilogram is the unit. Let us see about units now.

### Unit

A unit is the standard quantity with which unknown quantities are compared. It is defined as a specific magnitude of a physical quantity that has been adopted by law or convention. For example, feet is the unit for measuring length. That means, 10 feet is equal to 10 times the definite predetermined length, called feet. Our forefathers used units like muzham, furlong (660 feet), mile (5280 feet) to measure length.

 Quantity Unit Symbol Mass Kilogram kg Time Second s Temperature Kelvin K Electric Current Ampere A Luminous Intensity Candela cd Length Meter m Amount of Substance Mole mol