Class 11 Physics Chapter 2 Units and Measurement

## Physical Quantities and Units

**[Watch Video for Detailed Explanation]**

### Physical Quantities

A physical quantity is a property of a material or system that can be quantified by measurement. For an example, length of a pen.

Based on the dependency we can divide physical quantities into two, they are fundamental and derived quantities. Fundamental quantities are also called base quantities.

#### Fundamental or Base Quantities and Derived Quantities

Fundamental or base quantities are those physical quantities which are independent of each other and all other quantities can be expressed in terms of these quantities.

All other quantities which can be expressed in terms of fundamental quantities are called derived quantities.

### Units

To compare and measure a physical quantity we require a base or standard of that quantity. This name of base or standard used for measurement are simply units. A physical quantity can be expressed as a combination of a magnitude and a unit. In other words, standard = One unit of the quantity.

To measure a physical quantity we need some standard internationally accepted, and the standard for a unit should have the following characteristics:

1. It should be well defined.

2. It should be invariable (should not change with time)

3. It should be convenient to use.

4. It should be easily accessible

Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. Based on physical quantity it measures, the units are also divided into two, fundamental or base units and derived units. That is, the units defined for the fundamental or base quantities are called fundamental or base units and those obtained for the derived quantities are called the derived units. A complete set of these units are known as the system of units. Some system of units are:

**CGS System:** In this system, the units of length, mass and time are centimeter (cm), gram (g) and second (s) respectively.

**FPS System:** In this system, the units of length, mass and time are foot, pound and second respectively.

**MKS System:** In this system, the units of length, mass and time are metre, kilogram and second respectively.

**SI Units:** International System of Units, consists of seven fundamental units and two supplementary units.