Physical Quantity Unit and Dimension

Best Notes on Physical quantity | Scalar quantity | vector quantity | Fundamental quantity | Derived quantity | Unit | Importance of Unit | Property of Unit | Type of Unit | System of Unit | Definition of Basic and Supplementary S.I. units | Advantage of SI units | Some important derived units | Some practical units of length, mass and time | Prefixes used in metric system | Dimension | List of all type of Dimension | Principle of dimensional homogeneity | Uses of Dimensional analysis | Limitations of Dimensional analysis | FAQ

Notes on Physical quantity, Unit and Dimension

Hello everyone, In this post, we are providing best notes of Physical quantity Unit and Dimension – an important topic of Physics for India’s top government exams. This notes is completely free. And it will help students to crack government exams like UPSC CSE, State PCS, SSC CGL, CHSL, CPO, MTS, Railway, EPFO, APFC, CDS, NDA, UGC NET, State pcs, etc.

Physical Quantity

That quantity which can be measured by an instrument is called physical quantity. Example – length of any object, mass of body, time of any event, velocity, force etc.

Physical Quantity is divided into two parts-

Definition of Unit:

The chosen standard value used for measuring a physical quantity is called unit.

Why are units important?

To separate out one physical quantity from another we need units.

Basic properties of Unit –

1. well defined
2. easy to reproduce
3. easy to compare
4. internationally accepted
5. independent of changes in physical quantity

Types of units-

System of Unit-

Units depend on choice. Each choice of the unit leads to a new system (set) of units. The internationally accepted systems are-

In SI Units, there are seven fundamental units given in the following table-

CGS system also known as Metric or Decimal System

Definition of Basic and Supplementary S.I. units

1. Metre (m): It is defined as the distance occupied by 1650763.73 wavelengths in vacuum of the radiations emitted by Krypton-86 atom in the transition between 2p¹⁰ and ₅d⁵ states.

OR

One metre is the length of the path travelled by light in vacuum in 1/299,792458 second.

2. Kilogram (kg): It is the mass of a platinum-iridium cylinder (prototype cylinder) having its diameter equal to the height preserved at the International Bureau of Weights and Measures at Sevres near Paris in a Vault. (Height and diameter is 3.9 cm) [Pt +Ir = 90% + 10%]

3. Second (s): It is the duration of 9192, 631,770.0 periods of the radiations corresponding to the transition between two hyper fine levels of the ground state of cesium-133 (Cs¹¹³) atom.

OR

It is the time taken by cesium-133 atom to make 9, 192, 631, 770.0 vibrations.

4. Ampere: It is S.I. unit of electric current.It is that constant current which when flows through each of the two long straight parallel conductors of infinite length and negligible area of cross-section placed at one metre apart in vacuum experiences a force of 2 x 10^-7 N/m between them. Symbol is ‘A’.

5. Kelvin (K): S.I. unit of thermodynamic temperature. One kelvin (K) is defined as 1/273.16th of the thermodynamic temperature of the triple point of water.

6. Candela (cd): One candela is defined as luminous intensity in a direction at right angle to a surface of 1/600000 square metre area of a black body, at a temperature of freezing platinum under a pressure of 101325 newton per metre square.

In 1979 the new definition of candela was put forward as ‘The candela is the luminous intensity in a given direction of a source that emits monochromatic radiations of frequency 5.4 x 10¹⁴ Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

7. Mole (Mol): One mole is the amount of substance that contains as many elementary units as there are atoms in exactly 0.12 kg of pure carbon-12.

Supplementary Units.

1. Radian: It is the unit of plane angle. It is the measure of the angle subtended at the centre of circle by an arc having length equal to radius of circle.

2л radian = 360°

л radian = 180°

1 radian = 180°/π = 180°×7/22 = 630°/11 =57.3°

2. Steradian (Sr): Steradian is the unit of solid angle. It is the angle subtended at the centre of a sphere by its surface whose area is equal to the square of the radius of the sphere.

Advantages of S.I. units :

1. It is a coherent system of units. (A coherent system is one in which all the derived units can be obtained by multiplying or dividing the given basic units.)

2. It is a rational system of units. (It means it assigns only one unit to a particular physical quantity) e.g. Joule is unit of energy of all types where as in mks system mechanical energy is in Joule, Heat Energy is in calorie and electric energy is in watt hour.

3. It is internationally accepted system of units.

4. It is a metric system of units (i.e., all multiples and submultiples can be expressed in the power of ten)

5. It is absolute system of units, which means there are no gravitational units on the system.

6. It is closely related to cgs system (which means S.I. system can be converted to C.G.S. and vice-versa)

Some important derived units-

Some practical units of length, mass and time-

Prefixes used in metric system-

Dimension

Dimension of a physical quantity is defined as the process of representation with the different power of fundamental physical quantity.

For example Dimensional formula of momentum is MLT^-1

List of all type of Dimension

Principle of dimensional homogeneity:

According to this principle the dimensions of each term of a physical equation should be similar. This principle is based on the fact that physical quantities of same kind can be added, subtracted or compared. So force can be added to force not to pressure.

For example, let a physical relation in terms of fundamental quantities.

[M^aL^bT^c] = [M^xL^yT^z]

∴ by principle of homogeneity a = x, b = y, c = z

Uses of Dimensional analysis

The method of studying a physical phenomenon on the basis of dimensions is called dimensional analysis. The main uses of dimensional analysis are as

1. Convert value of a physical quantity from one unit system to another.
2. To check the correctness of a physical equation.
3. Determine the relation among different physical quantities.

Limitations of Dimensional analysis

1. This method cannot be used to determine relations between more than three physical quantities.
2. This method cannot be used to derive a relation containing trigonometric or exponential functions.
3. Many different physical quantities (e.g. modulus of elasticity, stress, pressure) which have the same dimensions, cannot be identified correctly with this method.

FAQ