Question

What is meant by the unit of physical quantity?

Answer 1

A unit is a term which always completes the physical quantity, a unit can be expressed in many forms and it can also be convertible from one system to another. Unit has some properties that are rational, they are coherent and they also follow the metric system of the unit. Units can be expressed as Fundamental unit, derived unit and supplementary unit.

Complete step-by-step solution:
A quantity which can be measured and by which various physical things can be explained and expressed in form of laws is called a physical quantity. For example length, mass, time, force etc.
A physical quantity is always represented by its magnitude and a unit representing a physical quantity without magnitude or without unit is invalid .For example, 10 metre means a length which is ten times the length of 1 m. In this example 10 represents the numerical value of the given quantity and metre represents the unit of quantity. So expressing a physical quantity as:-
Physical quantity (Q) = Magnitude × Unit $=n\times u$
Where, n represents the numerical value of the physical quantity and u represents the unit of that physical quantity. So this expression defines the physical quantity, it is clear that when unit (u) changes, then the magnitude (n) will also change but product ‘nu’ will remain the same which means nu is always constant.
$n\times u=cons\tan t$
\Rightarrow $$$${{n}_{1}}{{u}_{1}}={{n}_{2}}{{u}_{2}}=\text{constant};
\therefore $$$$n\propto \dfrac{1}{u}
This expression signifies that the magnitude of a physical quantity and units are always inversely proportional to each other which mean the Larger the unit, smaller will be the magnitude.
Unit can be classified as:-
1. Fundamental quantities: There are few quantities which are independent of all other quantities and do not require the help of any other physical quantity hence these quantities are also called absolute quantities. These physical quantities are also called fundamental or base quantities, as all other quantities are based upon and can be expressed in terms of these quantities. Example: - Length, Mass, Time, Temperature, Current, Luminous Intensity, Quantity of Matter.
2. Derived quantities: All other physical quantities that are derived by doing the multiplication or division of different powers of fundamental or base quantities. These are therefore called derived quantities.
Example: - Speed, Acceleration etc.

Note: One more category of unit is present is called supplementary unit, in this category two quantities come: - Plane angle and Solid angle. Plane angle is measured in 2D figures like circles and solid angle is measured in 3D figure like sphere. Unit of plane angle is always taken as radian and the unit of solid angle is taken as steradian.