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CBSE Class 11 Physics Notes

Physical World

Science and its origin

Science is a systematic understanding of natural phenomena in detail so that it can be predicted, controlled and modified. Science involves exploring, experimenting and speculating phenomena happening around us.

  • The word Science is derived from a latin verb Scientiameaning 'to know'.
  • Scientific method is a way to gain knowledge in a systematic and in-depth way. It involves:
    • Systematic observations
    • Controlled experiments
    • Qualitative and Quantitative reasoning
    • Mathematical modeling
    • Prediction and verification (or falsification) of theories
    • Speculation or Prediction
  • Science does not have any final theory. The improved observations, accurate tools keep improving the knowledge and perspective. Johannes Kepler used Tycho Brahe's research on planetary motion to improve Nicolas Copernicus theory.
  • Quantum mechanics was developed to deal with atomic and nuclear phenomena. Work of Ernest Rutherford on nuclear model of atom became basis of quantum theory given by Niels Bohr. Antiparticle theory of Paul Dirac led to the discovery of antielectron (positron) by Carl Anderson.

Natural Sciences

Natural science is a branch of science concerned with the description, prediction, and understanding of natural phenomena, based on observational and empirical evidence. It consists of following disciplines:

  • Physics
  • Chemistry
  • Biology


Physics is a study of basic laws of nature and their manifestation in different natural phenomena. Physics is the study of physical world and matter and its motion through space and time, along with related concepts such as energy and force.

  • Word Physics is derived from a Greek word phusikḗmeaning nature.
  • Two principal types of approaches in Physics are:
    • Unification: This approach considers all of the world's phenomena as a collection of universal laws in different domains and conditions. Example, law of gravitation applies both to a falling apple from a tree as well as motion of planets around the sun. Electromagnetism laws govern all electric and magnetic phenomena.
    • Reduction: This approach is to derive properties of complex systems from the properties and interaction of its constituent parts. Example, temperature studied under thermodynamics is also related to average kinetic energy of molecules in a system (kinetic theory).

Impactand uses of Physics

  • It can explain a phenomena happening over a large magnitude with a simple theory.
  • Experiments and observations are used to develop new theories for unidentified phenomena and improve old theories for existing phenomena.
  • Development of devices using laws of physics.

Scope of Physics

Scope of Physics is vast as it covers quantities with length magnitude as high as 1040m or more (astronomical studies of universe) and as low as 10-14m or less (study of electrons, protons etc). Similarly the range of time scale goes from 10-22s to 1018s and mass from 10-30kg to 1055kg.

Physics is broadly divided into two types based on its scope - Classical Physicsand Modern Physics. Classical physics deal with the macroscopic phenomena while the modern physics deals with the microscopic phenomena.


Macroscopic Domain

Macroscopic domain includes phenomena at large scales like laboratory, terrestrial and astronomical. It includes following subjects:

  • Mechanics It is based on Newton's laws on motion and the laws of gravitation. It is concerned with motion/equilibrium of particles, rigid and deformable bodies and general system of particles. Examples,
    • Propulsion of rocket by ejecting gases
    • Water/Sound waves
    • Equilibrium of bent rod under a load

  • Electrodynamics It deals with electric and magnetic phenomena associated with charged and magnetic bodies. Examples,
    • motion of a current-carrying conductor in a magnetic field
    • the response of a circuit to an ac voltage (signal)
    • the propagation of radio waves in the ionosphere

  • Optics It deals with phenomena involving light. Examples,
    • Reflection and refraction of light
    • Dispersion of light through a prism
    • Colour exhibited by thin films

  • Thermodynamics It deals with systems in macroscopic equilibrium and changes in internal energy, temperature, entropy etc. of systems under application of external force or heat. Examples,
    • Efficiency of heat engines
    • Direction of physical and chemical process

Microscopic Domain

Microscopic domain includes phenomena at minuscule scales like atomic, molecular and nuclear. It also deals with interaction of probes like electrons, photons and other elementary particles. Quantum theory has been developed to handle these phenomena.


Factors responsible for progress of Physics

  • Quantitative analysis along with qualitative analysis.
  • Application of universal laws in different contexts.
  • Approximation approach (complex phenomena broken down into collection of basic laws).
  • Extracting and focusing on essential features of a phenomenon.


Hypothesis, Axiom and Models

Hypothesis is a supposition without assuming that it is true. It may not be proved but can be verified through a series of experiments.

Axiom is a self-evident truth that it is accepted without controversy or question.

Model is a theory proposed to explain observed phenomena.

Assumption is the basis of physics, where a number of phenomena can be explained. These assumptions are made from experiments, observation and a lot of statistical data.

Technological applications of Physics

Several examples where Physics and its concepts have led to discoveries/inventions are listed below.

  • Steam engine was developed from the industrial revolution in eighteenth century.
  • Wireless communication was developed after discovery of laws of electricity and magnetism.
  • Neuron-induced fission of uranium, done by Hahn and Meitner in 1938, led to the formation of nuclear power reactors and nuclear weapons.
  • Conversion of solar, wind, geothermal etc.energy into electricity.

Fundamental Forces in nature

The forces which we see in our day to day life like muscular, friction, forces due to compression and elongation of springs and strings, fluid and gas pressure, electric, magnetic, interatomic and intermolecular forces are derived forces as their originations are due to a few fundamental forces in nature.

A few fundamental forces are:

  1. Gravitational Force: It is the force of mutual attraction between any two objects by virtue of their masses. It is a universal force as every object experiences this force due to every other object in the universe.

  1. Electromagnetic Force: It is the force between charged particles. Charges at rest have electric attraction (between unlike charges) and repulsion (between like charges). Charges in motion produce magnetic force. Together they are called Electromagnetic Force.

  1. Strong Nuclear Force: It is the attractive force between protons and neutrons in a nucleus.It is charge-independent and acts equally between a proton and a proton, a neutron and a neutron, and a proton and a neutron. Recent discoveries show that protons and neutrons are built of elementary particles, quarks.

  1. Weak Nuclear Force: This force appears only in certain nuclear processes such as the β-decay of a nucleus. In β-decay, the nucleus emits an electron and an uncharged particle called neutrino.This particle was first predicted by Wolfgang Pauli in 1931.

Below table shows difference between the above forces.





Relative Strength


Operates among

Gravitational force



All objects in the universe

Weak nuclear force


Very short, Sub-nuclearsize (-10-16m)

Some elementary particles, particularly electron and neutrino

Electromagnetic force



Charged particles

Strong nuclear force


Short, nuclear size (-10-15m)

Nucleons, heavier

elementary particles


Unification of Forces: There have been physicists who have tried to combine a few of the above fundamental forces. These are listed in table below.

Name of Physicist


Achievement in Unification

Isaac Newton


Unified celestial and terrestrial mechanics.

Hans Christian Oersted and Michael Faraday

1820 and 1830 respectively

Unified electric and magnetic phenomena to give rise to electromagnetism.

James Clerk Maxwell


Unified electricity, magnetism and optics to show that light is an electromagnetic wave.

Sheldon Glashow, Abdus Salam, Steven Weinberg

Carlo Rubia, Simon Vander Meer





Gave the idea of electro-weak force which is a combination of electromagnetic and weak nuclear force.


Verified the theory of elctro-weak force.


Conserved Quantities

Physics gives laws to summarize the investigations and observations of the phenomena occurring in the universe.

  • Physical quantities that remain constant with time are called conserved quantities. Example, for a body under external force, the kinetic and potential energy change over time but the total mechanical energy (kinetic + potential) remains constant.
  • Conserved quantities can be scalar (Energy) or vector (Total linear momentum and total angular momentum).

Conservation Laws

A conservation law is a hypothesis based on observation and experiments which cannot be proved. These can be verified via experiments.

Law of conservation of Energy

  • According to the general Law of conservation of energy, the energies remain constant over time and convert from one form to another.
  • The law of conservation of energy applies to the whole universe and it is believed that the total energy of the universe remains unchanged.
  • Under identical conditions, the nature produces symmetric results at different time.

Law of conservation of Mass

This is a principle used in analysis of chemical reactions.

  • A chemical reaction is basically a rearrangement of atoms among different molecules.
  • If the total binding energy of the reacting molecules is less than the total binding energy of the product molecules, the difference appears as heat and the reaction is exothermic.
  • The opposite is true for energy absorbing (endothermic) reactions.
  • Since the atoms are merely rearranged but not destroyed, the total mass of the reactants is the same as the total mass of the products in a chemical reaction.
  • Mass is related to energy through Einstein theory,

E = mc2, c speed of light in vacuum

Law of conservation of linear momentum

  • Symmetry of laws of nature with respect to translation in space is termed as law of conservation of linear momentum.
  • Example law of gravitation is same on earth and moon even if the acceleration due to gravity at moon is 1/6th than that at earth.

Law of conservation of angular momentum

  • Isotropy of space (no intrinsically preferred direction in space) underlies the law of conservation of angular momentum.
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