Physical World - Revised NCERT based notes Class 11 Physics Chapter 1

1. PHYSICAL WORLD

1.1 WHAT IS PHYSICS?

Science: A systematic study of natural phenomena and use the knowledge so gained to predict, modify and control the phenomena is called science.



Scientific Method: Scientific Method is a series of steps to gather knowledge about the natural phenomena. The several involved interconnected steps are: systematic observations, controlled experiments, qualitative and quantitative reasoning, mathematical modelling, prediction and verification or falsifying of theories.



Physics: A branch of science that deals with the study of the basic laws of nature and their manifestation in different natural phenomena is called physics.



Principal Thrusts in Physics:

(i) Unification: Attempting to explain diverse physical phenomena in terms of a few concepts and laws is known as unification. For example, same law of motion and law of gravitation describes the mechanics of both celestial and terrestrial domains.

(ii) Reduction: Attempting to derive the properties of bigger, more complex system from the properties and interactions of its constituents simpler parts is known as reduction. For example, the temperature of a bulk system is related to the average kinetic energy of its molecules.

1.2 SCOPE AND EXCITEMENT OF PHYSICS

SCOPE:

➤ The scope of physics is wide, covering a tremendous range of magnitude of physical quantities. In terms of length scale : from order of $10^{-14}$ m or even less (electrons, protons, etc.) to the order of $10^{26}$ m (galaxies or even the entire universe); Time scales : $10^{-22}$ s to $10^{18}$ s (obtained by dividing the length scales by the speed of light); Mass scales : say $10^{-30}$ kg (mass of electron) to $10^{55}$ kg (mass of known observable universe). Terrestrial phenomena lie somewhere in the middle of this range.



There are two domain of interest in Physics:

(i) Macroscopic domain: The domain which deals with the phenomena at the laboratory, terrestrial and astronomical scales is called macroscopic domain.

(ii) Microscopic domain: The domain which deals with the constitution and structure of matter at the minute scales of atoms and nuclei (and even lower scales of length) and their interaction with different probes such as electrons, protons and other elementary particles is called microscopic domain.

NOTE : Recently the domain intermediate between the macroscopic and the microscopic (so-called mesoscopic), dealing with a few tens or hundreds of atoms, has emerged as an exciting field of research.

Theories related to macroscopic and microscopic domain :

(i) Classical Physics: Classical physics deals mainly with macroscopic phenomena and includes subjects like

• Mechanics: A branch of physics that deals with the motion (or equilibrium) of particles, rigid and deformable bodies and general system of particles in space and time.


• Electrodynamics: A branch of physics that deals with electric and magnetic phenomena associated with charged and magnetic bodies.


• Optics: A branch of physics that deals with the phenomena involving light.


• Thermodynamics: A branch of physics that deals with systems in macroscopic equilibrium and is concered with changes in internal energy, temperature, entropy, etc., of the system through external work and transfer of heat.

(ii) Quantum theory: The theory which is accepted as the proper framework for explaining microscopic phenomena.

EXCITEMENT:

➤ Physics is exciting in many ways. To some people, the excitement comes from the universility of its basic theories, from the fact that a few basic concepts and laws can explain phenomena covering a large range of magnitude of physical quantities. To some other people, it comes from carrying out imaginative new experiments to unlock the secrets of nature, to verify or refute the theories.

Hypothesis, axioms and models

Hypothesis: A hypothesis is a supposition without assuming that it is true. Example : universal law of gravitation



Axioms: An axiom is a self-evident true that is accepted without controversy or question. Example : If equals be subtracted from equals, the remainders are equal.



Model: A model is a theory proposed to explain observed phenomena. Example : Bohr's atomic model.

1.3 PHYSICS, TECHNOLOGY AND SOCIETY

➤Physics and technology are related to each other. Sometimes technology gives rise to new physics, at other times physics generates new technology. Both have direct impact on society. An example of the first is the discipline of thermodynamics arose from the need to understand and improve the working of heat engines. An example of the second is the discovery of the basic laws of electricity and magnetism gives rise to the wireless communication technology.

Table 1.1 : Link between technology and physics (More rows are available on detailed notes. click here...)

Technology	Scientific principle(s)
Steam engine	Laws of thermodynamics
Nuclear reactor	Controlled nuclear fission
Radio and Television	Generation, propagation and detection of electromagnetic waves
Computers	Digital logic

Table 1.2 : Some physicists from different countries of the world and their major contributions (More rows are available on detailed notes. click here...)

Name	Major contribution/discovery	Country of Origin
Archimedes	Principle of buoyancy; Principle of lever	Greece
Galileo Galilei	Law of inertia	Italy
Christiaan Huygens	Wave theory of light	Holland
Isaac Newton	Universal law of gravitation; Laws of motion; Reflecting Telescope	U.K.

1.4 FUNDAMENTAL FORCES IN NATURE

Force: The one which is needed to push, pull, carry or throw objects, deform or break them is called force.



There are four fundametal forces in nature:

(i) Gravitational force: The force of mutual attraction between any two objects by virtue of their masses is called gravitational force. It is a universal force.

(ii) Electromagnetic force: The force between charged particles is called electromagnetic force.

➤When the charges are at rest, the electromagnetic force is given by Coulomb's law: attractive for unlike charges and repulsive for like charges. Charges in motion produces magnetic effects and gives rise to a force on moving charge.

➤Electric field and magnetic field are, in general, inseparable - hence the name electromagnetic force.

(iii) Strong nuclear force: The strong attractive force that binds protons and neutrons in a nucleus is called strong nuclear force.

➤Strong nuclear force is charge-independent and acts equally between two neucleons. It is responsible for the stability of nuclei.

➤Electron does not experience the strong nuclear force because this force has extremely small range.

(iv) Weak nuclear force: The force that appears only in cetain nuclear process such as the $\beta$ decay of a nucleus is called weak nuclear force.

➤In $\beta$ decay, the nucleus emits an electron and an uncharge particle called neutrino.

Table 1.3 : Fundamental forces in nature

Name	Relative strength	Range	Operates among
Gravitational force	$10^{-39}$	Infinite	All objects in the universe
Weak nuclear force	$10^{-13}$	Very short, Sub-nuclear size(~ $10^{-16}$m)	Some elementary particles, particularly electron and neutrino
Electromagnetic force	$10^{-2}$	Infinite	Charged Particles
Strong nuclear force	1	Short, nuclear size (~$10^{-15}$m.	nucleons, havier elementary particles

Table 1.4 : Progress in unification of different forces/domains in nature

Name of the physicist	Year	Achievement in unification
Isaac Newton	1687	Unified celestial and terrestrial mechanics; showed that the same laws of motion and the law of gravitation apply to both the domains.
Hans Christian Oersted, Michael Faraday	1820 1830	Showed that electric and magnetic phenomena are inseparable aspects of a unified domain: electromagnetism.
James Clerk Maxwell	1873	Unified electricity, magnetism and optics; showed that light is an electromagnetic wave.
Sheldon Glashow, Abdus Salam, Steven Weinberg	1979	Showed that the weak nuclear force and the electromagnetic force could be viewed as different aspects of a single electro-weak force.
Carlo Rubia, Simon Vander Meer	1984	Verified experimentally the predictions of the theory of electro-weak force.

➤ Some Collected Important Facts :

♞Like the gravitational force, electromagnetic force acts over large distance and does not need any intervening medium.

♞The electromagnetic force is much stronger than gravitational force and hence dominates all the phenomena at atomic and molecular scales.

♞Matter is mostly electrically neutral. So, Gravitational force dominates terrestrial phenomena.

♞Recent developments have indicated that protons and neutrons are built out of still more elementary particles constituents called quarks.

1.5 NATURE OF PHYSICAL LAWS

Conserved quantities: The physical quantities that remains unchanged in a process are called conserved quantities.

➤Some of the general conservation laws in nature include the laws of conservation of mass, energy, liner momentum, angular momentum, parity, etc. Some conservation laws are true for one fundamental force but not for the other.

➤Some of the conserved quantities that introduce in nuclear and particle physics are spin, baryon number, strangeness, hyperchange, etc.

➤A conservation law is a hypothesis, based on observations and experiments. It can be verified, or disproved by experiment but cannot be proved

Einstein Theory:

According to this theory, mass m is equivalent to energy E given by the relation E=mc$^2$, where c is the speed of light in vacuum.