UNITS AND MEASUREMENTS

Physical Quantities

All the quantities which are used to describe the laws of physics are called the physical quantities, e.g. length, mass, area, volume etc.

All the physical quantities are measurable. When we measure a physical quantity, we express the result of the measurement by using two things, i.e.

(i) the unit in which the physical quantity is measured; and

(ii) the magnitude of the physical quantity.

For example, when we say that the mass of a block is 5 kg, it means:

(i) the mass of the block is measured by using the unit kg; and

(ii) the magnitude of the block is 5 times as big as a kg.

Thus, physical quantity = (magnitude) x (unit)

Units

The standard amount of a physical quantity which is chosen to measure the physical quantity of the same kind is called a unit.

The essential requirements of a unit are:

(i) It should be of suitable size.

(ii) It should be easily accessible.

(iii) It should not vary with time.

(iv) It should be easily reproducible.

(v) It should not depend upon physical conditions like pressure, volume etc.

Systems of Units

A complete set of units which is used to measure all kinds of fundamental and derived physical quantities is called a system of unit. The commonly used systems of units are:

1. CGS System: It is a system of units based on the fundamental units of centimeter (cm) for distance, gram (g) for mass and second (s) for time. In this system, the unit of force is dyne and the unit of work or energy is erg.

2. MKS System: It is a system of units based on the fundamental units of meter for distance (m), kilogram (kg) for mass and second (s) for time. In this system, the unit of force is newton (N) and the unit of work or energy is joule (J).

3. FPS System: It is a system of units in which the units of length, mass and time are foot, pound and second respectively . In this system, the unit of force is poundal (pl) and the unit of work or energy is poundal-foot.

4. International System (SI) of Units: SI units (Systeme International d’Unites) were adopted internationally in 1968. The system has seven fundamental (or basic) units, one for each of the fundamental quantities.

Fundamental Quantities and Fundamental Units:

The physical quantities which are independent of other physical quantities are and not defined in terms of other physical quantities are known as fundamental quantities or base quantities. The units of these quantities are called fundamental units.

The following table shows the seven fundamental quantities and their SI units:

Fundamental quantities

Unit

Name

Symbol

Mass

kilogram

kg

Length

meter

m

Time

second

s

Electric current

ampere

A

Temperature

kelvin

K

Luminous intensity

candela

cd

Amount of substance

mole

mol