Physical World - Detailed 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 constituent simpler parts is known as reduction. For example, the temperature of a bulk system is related to the average kinetic energy of its molecules.

➤ Some Collected Important Facts :

♞ The word Science orginates from the Latin verb Scientia meaning to 'to know'. The Sanskrit word Vijnan and The Arabic word Ilm convey similar meaning, namely, knowledge.

♞ Heliocentric Theory (sun at the centre and all the planets revolving around it in circular orbits in the solar system) is given by Nicolas Copernicus (1473-1543).

♞ The planetary circular orbits in Heliocentric Theory is replaced with elliptical orbits by Johannes Kepler (1571-1630) after examing the extensive data collected by Tyco Brahe (1546-1601).

♞ The result of the experiment of scattering $\alpha$ particles by gold foil, in 1911 by Ernest Rutherford (1871-1937) established the nuclear model of atom.

♞ Quantum Theory of Hydrogen atom is given in 1913 by Niels Bohr (1885-1962).

♞ The concept of antiparticle was first introduced theorectically by Paul Dirac(1902-1984) in 1930 and confirmed two years later by the experimental discovery of positron(antielectron) by Carl Aderson.

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.

➤ Some Collected Important Facts :

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

♞ 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.

♞ 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

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
Lasers	Light amplification by stimulated emission of radiation
Production of ultra high magnetic fields	Superconductivity
Rocket propulsion	Newton's laws of motion
Electric Generator	Faraday's laws of electromagnetic induction
Hydroelectric power	Conversion of gravitational potential energy into electrical energy
Aeroplane	Bernoulli's principle in fluid dynamics
Particle accelerators	Motion of charged particles in electromagnetic fields
Sonar	Reflection of ultrasonic waves
Optical fibres	Total internal reflection of light
Non-reflecting coatings	Thin film optical interference
Electron microscope	Wave nature of electrons
Photocell	Photoelectric effect
Fusion test reactor(Tokamak)	Magnetic confinement of plasma
Giant Metrewave Radio Telescope (GMRT)	Detection of cosmic radio waves
Bose-Einstein condensate	Trapping and cooling of atoms by laser beams and magnetic fields.

Table 1.2 : Some physicists from different countries of the world and their major contributions

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.
Michael Faraday	Law of electromagnetic Induction	U.K.
James Clerk Maxwell	Electromagnetic Theory; Light-an electromagnetic wave	U.K.
Henrich Rudolf Hertz	Generation of electromagnetic waves	Germany
J.C. Bose	Ultra short radio waves	India
W.K. Roentgen	X-rays	Germany
J.J. Thomson	Electron	U.K.
Marie Sklodowska Curie	Discovery of radium and polonium; Studies on natural radioactivity	poland
Albert Einstein	Explanation of photoelectric effect; Theory of relativity	Germany
Victor Francis Hess	Cosmic radiation	Austria
R.A. Millikan	Measurement of electronic charge	U.S.A.
Ernest Rutherford	Nuclear Model of Atom	New Zealand
Niels Bohr	Quantum model of hydrogen atom	Denmark
C.V. Raman	Inelastic scattering of light by molecules	India
Louis Victor de Borglie	Wave nature of matter	France
M.N. Saha	Thermal ionisation	India
S.N. Bose	Quantum statistics	India
Wolfgang Pauli	Exclusion principle	Austria
Enrico Fermi	Controlled nuclear fission	Italy
Werner Heinsenberg	Quantum mechanics; Uncertainity principle	Germany
Paul Dirac	Relativistic theory of electron; Quantum statistics	U.K.
Edwin Hubble	Expanding universe	U.S.A.
Ernest Orlando Lawrence	Cyclotron	U.S.A.
James Chadwick	Neutron	U.K.
Hideki Yukawa	Theory of nuclear forces	Japan
Homi Jehangir Bhabha	Cascade process of cosmic radiation	India
Lev Davidovich Landau	Theory of condensed matter; Liquid helium	Russia
S. Chandrasekhar	Chandrasekhar limit, structure and evolution of stars	India
John Bardeen	Transistors; Theory of super conductivity	U.S.A.
C.H.Townes	Maser; Laser	U.S.A.
Abdus Salam	Unification of weak and electromagnetic interactions	Pakistan

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.


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


(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.

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 :

♞ 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.

♞ 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.

♞ 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.

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

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

1.5 NATURE OF PHYSICAL LAWS

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



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.

➤ Some Collected Important Facts :

♞ 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.

♞ The acceleration due to gravity at the moon is one-sixth of that of at the earth.