# Physical World Class 11 notes Physics Chapter 1

Objective Physics, Fundamental Forces in Nature, Role of Mathematics in Physics, Scope and Excitement of Physics, Conservation Laws in Physics

## ▶What is Physics?

Physics is the study of nature and its laws. There are so many different events in nature which are taking place and we expect that all these different events in nature are taking place according to some basic law and revealing these laws of nature from the observed events is physics.

Humans have always been curious about the world around them. The night sky with its bright celestial objects has fascinated humans since time immemorial. The regular repetitions of the day and night, the annual cycle of seasons, the eclipses, the tides, the volcanoes, the rainbow have always been a source of wonder.

The word Science originates from the Latin verb Scientia meaning ‘to know’. The Sanskrit word Vigyan and the Arabic word ilm convey similar meaning, namely ‘knowledge’. Science is a systematic attempt to understand natural phenomena in as much detail and depth as possible, and use the knowledge so gained to predict, modify and control phenomena. Science is exploring, experimenting and predicting from what we see around us.

## ▶Role of Mathematics in Physics

Description of all natural phenomenon is made easy by the help of mathematics. Thus, we can say that mathematics is the language of physics, by the help of mathematics we explain and understand basic law of physics in more better way. For example, gravitational force of attraction between two point masses m1 and m2 can be written by mathematical equation

F=G\frac{m_{1}m_{2}}{r^{2}}

## Scope and Expansion of Physics

Various branches of physics are mainly divided into two parts- (A) Classical Physics and (B) Modern Physics.

### (A). Classical Physics

(i). Mechanics:- Under this subject the systematic motion of objects is studied. One of its branches is Fluid Mechanics, in which the dynamic behavior of liquids is studied.

(ii). Thermodynamics:- Under this subject the motion in a system made of heat, heat and fine particles is studied.

(iii). Electromagnetism:- Under this subject the theory of electromagnetism and electromagnetic waves is studied.

(iv). Classical Wave Mechanics and Sound:- Under this subject, vibrations and progressive and progressive waves are studied.

(v). Optics:- Under this subject the nature and transmission of light is studied. To understand the Images, refraction, reflection, interference, diffraction and polarization formed through lenses and mirrors, it is necessary to have knowledge of this subject.

### (B). Modern Physics

(i). Relativity:- Under this subject the motion of those bodies is studied which move with a velocity equal to the speed of light. In fact it is a theory of relativism in nature.

(ii). Quantum Mechanics:- Under this subject the principles of modern physics, the dual nature of light and matter are studied. It acts as a bridge between classical physics and modern physics.

(iii). Atomic Physics:- Under this subject, atomic structure and properties of atoms are studied.

(iv). Nuclear Physics:- Under this subject, the nucleus of an atom and its properties are studied, apart from this some other subjects are as follows-

1. Solid State Physics
2. Plasma Physics
3. High Energy Physics
4. Electronics
5. Engineering Physics
6. Medical Physics
7. Cosmology
8. Bio Physics
9. Chemical Physics
10. Geo Physics

## Contribution of Physics in Technology and Society

Physics is an important branch of science, without whose knowledge the development of other branches of science is not possible. Physics has an important contribution in the development of all branches of science and the upliftment of society.

(i). Importance of Physics in Chemistry:- The study of the chemical composition of matter, types of bonds, etc. has become possible on the basis of intermolecular forces found between molecules. On the basis of the diffraction of X-rays, the structure of the atom, the radioactivity, the detailed study of the structures of many solids has become possible.

(ii). Importance of Physics in Biology:- Many biological specimens are studied with the help of light microscope. The study of many physical structures became possible with the creation of the electron microscope.

(iii). Importance of Physics in Astronomy:- With the help of optical telescope, the study of motion of various planets and celestial bodies has become possible.

(iv). Importance of Physics in Mathematics:- The development of many activities has been made possible by the principles of physics. Technological development is particularly concerned with the application of physics. Examples of some new techniques based on the application of physics are as follows-

1. Power generation is based on the principle of electromagnetic induction.
2. Diesel engine, petrol engine, steam engine etc. are based on the laws of thermodynamics.
3. Radio, Television, S.T.D., I.S.D., Fax, Wireless etc. are based on the principle of electromagnetic waves.
4. The development of the atomic furnace and atomic bomb is based on nuclear fission.
5. Rocket propulsion is based on Newton's second and third laws of motion.
6. The flight of air vehicles is based on the Bernoulli principle.

## ▶Purpose and Excitement of Physics

We can get some idea of the scope of physics by looking at its various sub-disciplines. Basically, there are two domains of interest : macroscopic and microscopic. The macroscopic domain includes phenomena at the laboratory, terrestrial and astronomical scales. The microscopic domain includes atomic, molecular and nuclear phenomena. Classical Physics deals mainly with macroscopic phenomena and includes subjects like Mechanics, Electrodynamics, Optics and Thermodynamics.

You can now see that the scope of physics is truly vast. It covers a tremendous range of magnitude of physical quantities like length, mass, time, energy, etc. At one end, it studies phenomena at the very small scale of length (10-14 m or even less) involving electrons, protons, etc.; at the other end, it deals with astronomical phenomena at the scale of galaxies or even the entire universe whose extent is of the order of 1026 m.

## ▶Fundamental Forces in Nature

Mainly, four types of force are exist in our nature that are described bellow:

### ▶(i) Gravitational Force

It is the force of mutual attraction between any two objects due to their masses. All bodies on the Earth experience this force due to the Earth. Gravity governs the motion of moons around the Earth, motion of planets around the sun. It plays a main role in formation and evolution of stars, galaxies and galactic clusters. Newton's law of gravitation gives the magnitude of force exerted by a particle of point mass m1 on another particle of point mass m2 at a distance r from it as

F=G\frac{m_{1}m_{2}}{r^{2}}

where G is universal gravitational constant G = 6.67 × 10–11 Nm2/kg2. This force acts along the line joining the two particles.

### ▶(ii) Electromagnetic Force

It is the force between charged particles. It includes electric and magnetic forces. If two static point charges q1 and q2 are kept at a distance r, then the electrostatic force between them is given as

F=\frac{1}{4\pi \epsilon_{0}}\frac{q_{1}q_{2}}{r^{2}}

This is known as the Coulomb force.

### ▶(iii) Strong Nuclear Force

It is the force that binds nucleons (protons and neutrons) in a nucleus. The nucleus contains positively charged protons and electrically neutral neutrons. The repulsive electrostatic force between protons should make a nucleus unstable. There should be a strong attractive force that counteracts the repulsive force to keep a nucleus stable.

We know that gravitational force is negligible as compared to electrostatic force. So, we have a new basic force, i.e., strong nuclear force, which is the strongest of all fundamental forces, about 100 times the electromagnetic force. It is the same between a proton and a neutron, a proton and a proton, a neutron and a neutron. It is an extremely short-ranged force (≈ 10–15 m). It keeps the nucleus stable. It does not depend upon charge. An electron does not experience this force.

### ▶(iv) Weak Nuclear Force

It appears in some nuclear processes like β-decay of the nucleus, in which the nucleus emits an electron and an uncharged particle called the neutrino. It is also responsible for the decay of many unstable particles (muons into electrons, pions into muons, and so on). It is not as weak as the gravitational force, but much weaker than the strong nuclear and electromagnetic force. They are exceedingly short-ranged forces, of the order of 10–16 m.

## ▶Conservation Laws in Physics

A remarkable fact about any physical phenomenon is the invariance of some special physical quantities. They are the conserved quantities of nature, i.e., energy, mass, charge, linear momentum, and angular momentum. In classical physics, we have the following conservation laws:

### ▶(i) Law of Conservation of Energy

According to this law, the sum of energy of all kinds in this universe or of an ideal isolated system remains constant. Energy can neither be created nor be destroyed. It can only be transformed from one form to the other.

### ▶(ii) Law of Conservation of Linear Momentum

According to this law, the linear momentum of a system remains unchanged in the absence of an external force. It is denoted by P and expression is given by P = mv.

### ▶(iii) Law of Conservation of Angular Momentum

A rotating body has inertia, so it also possesses momentum associated with its rotation. This momentum is called 'angular momentum'.

Angular momentum = Moment of inertia × Angular speed

L = I × ω

According to this law, the angular momentum of the system remains constant if the total external torque acting on it is zero. e.g., Planets revolving around the sun in an elliptical orbit with constant angular momentum.

### ▶(iv) Law of Conservation of Charge

According to this law, charges (in the form of electrons) are neither created nor destroyed but are simply transferred from one body to another.

## ▶Summary

• Domains in physics are macroscopic, microscopic and mesoscopic.

• Physics and technology are interrelated. Sometimes technology gives rise to new physics; at other times physics gives rise to new technology.

• Physics deals with the study of the basic laws of nature and their manifestation in different phenomena.

• There are four fundamental forces in nature. These are the ‘gravitational force’, the ‘electromagnetic force’, the ‘strong nuclear force’, and the ‘weak nuclear force’.

• The physical quantities that remain unchanged in a process are called conserved quantities.

• The scope of physics is wide, covering a tremendous range of magnitude of physical quantities.