CBSE CLASS XI PHYSICS UNIT I PHYSICAL WORLD & MEASUREMENT NOTES

CBSE CLASS XI 

SUBJECT-PHYSICS (042)

SYLLABUS FOR SESSION 2020-21(Theory)

SYLLABUS CLASS XI PHYSICS FOR SESSION 2020-21

UNIT I

PHYSICAL WORLD AND MEASUREMENT

CHAPTER 1

PHYSICAL WORLD

What is Science ? Click Here. 

Branches of Science: Click Here.

THE SCIENTIFIC METHOD AND THE SCIENTIFIC THEORY:

The scientific method involves the following steps:

1. Taking a large number of systematic observations through controlled experiments.
2. Studying these observation and looking for their logical behaviour based on qualitative and quantitative reasoning.
3. Mathematical modelling , i e, suggesting some model to account for the observed behaviour.
4.  Theoretical prediction of what is not actually observed on the basis of the suggested model.
5. Verification or falsification of the model.

There is no final theory in science, it can be changed if any discrepancies found.

e g Geocentric theory of earth changed to Heliocentric theory sun at the center of solar system.

What is Physics?

Physics: Physics is the branch of science which deals with the study of nature and natural phenomena.

In study of Physics , there are two main principal thrusts: Unification and Reduction.

Unification means attempting to explain diverse physical phenomena in terms of a few concepts and laws.

As we have done in class 10th Magnetic effect of electric current .

Reduction means attempting to derive the properties to derive the properties of a bigger , more complex system from the properties of its constituent simpler parts.

SCOPE AND EXCITEMENT OF PHYSICS:

All branches of physics comes in the scope of Physics.

Branches of Physics:

1. Classical Mechanics:The branch of physics which deals with the study of motion of the objects(mainly visible objects) on the application of force(including object at rest).
2. Quantum Mechanics:deals with the study of motion of microscopic particles.
3. Electromagnetism: deals with study of electric charges at rest and motion,magnetic fields,inter conversion and interaction with light.
4. Thermodynamics:deals with inter conversion of thermal energy and mechanical work.
5. Relativistic mechanics:Related with study of theory of relativity.
6. Optics: Optics is the study of phenomena associated to light and optical instruments.
7. Condensed matter physics:deals with the study of physical properties of matter in condensed phase(when material mixed with each other).
8. Nuclear Physics: deals with study of atomic nucleus.
9. Particle Physics: study of particle at high energy.

From above we can realize that the scope of Physics is very wide.
Moreover,it covers wide range of magnitude of physical quantities like length,mass,time,energy etc.
Physics include the study of very small size particles like electron , proton and nuclei etc of size 10^-14 m to size of universe 10²⁶m.

The study of physics is exciting in many ways:

• A few basic concepts and laws can explain diverse physical phenomena.
• Carrying out imaginative new experiments to unlock secrets of nature by verifying or falsifying the existing theories.
• The most interesting part is designing useful devices based on the physical laws.e g television, telephone , computers,buses,laser,weapons, planes most of the material things we see around us pen , paper, refrigerator etc.

 PHYSICS IN RELATION TO OTHER SCIENCES :

1. Physics in relation to chemistry: The study of structure of atoms, radioactivity, X-ray diffraction etc. in Physics have enabled chemists to rearrange elements in periodic table on the basis of atomic number and complex chemical structures.
2. Physics in relation to Biological sciences: The optical microscope developed in Physics are extensively used in study of biological samples.The x-ray and radio-isotopes are used in medical treatment.
3. Physics in relation to Mathematics: Theories in Physics often make use of mathematical concept. 

PHYSICS IN RELATION TO SOCIETY

Most of the development in Physics have a direct impact on society. e g:

• The development of telephone , telegraph, telex enable us to transmit message instantly.
• The development of satellites revolutionized the means of communication.
• Rapid mode of transport.
• Exploration of the new sources of energy is of great significance to the society.

and so on list goes on .

PHYSICS IN RELATION TO TECHNOLOGY:

Technology is the application of the principles of Physics for practical purposes.

S N	Technology	Basic Principle(s)
1	Steam Engine	Laws of thermodynamics
2	Rocket propulsion	Newton’s Laws of motion
3	Aeroplane	Bernoulli’s principal in fluid dynamics.
4	Sonar	Reflection of ultrasonic waves
5	Optical fibres	Total internal reflection
6	Photocell	Photoelectric effect
7	Computers	Digital logic
8	Nuclear reactor	Controlled nuclear chain reaction

SOME GREAT PHYSICISTS AND THEIR CONTRIBUTIONS

S N	Name	Country	Discovery/ Major Contribution
1	J C Bose	India	Ultra short radio waves
2	C V Raman	India	Inelastic scattering of light by molecules
3	M N Saha	India	Thermal ionisation
4	S N Bose	India	Quantum statistics
5	S Chandra Shekhar	India	Structure and evolution of stars
6	Homi Jehangir Bhabha	India	Cascade process of cosmic radiation
7	Sir Isaac Newton	U K	Universal law of gravitation , laws of motion and more
8	J J Thomson	U K	Electron
9	W Roentgen	Germany	X-rays
10	Marie Curie	Poland	Studies of Natural radioactivity.
11	Albert Einstein	Germany	Photoelectric effect and theory of Relativity
12	James Chadwick	U K	Neutron
13	Paul Dirac	U K	Quantum statistics
14	Enrico Fermi	Italy	Controlled nuclear fission
15	W Pauli	Austria	Exclusion Principle

FUNDAMENTAL FORCES IN NATURE:

There are four main fundamental forces in nature:

1.    Gravitational Force

2.    Weak Nuclear Forces

3.    Electromagnetic Forces

4.    Strong Nuclear Forces

1. Gravitational Force : The gravitational force is the force of mutual attraction between any two objects by virtue of their masses.

The magnitude of the force exerted by a particle of mass 'M'  and 'm' at a distance 'r' is given by Newton's law of gravitation:

Statement: The magnitude of force of attraction between two masses is directly proportional to the product of their masses and  inversely proportional to the square of distance between them.

Mathematically,

 F ∝ m M , F ∝ 1/r²

, where G is universal gravitational constant.

G= 6.67 x 10^-11 N m²/kg²

Characteristics/Important properties of gravitational forces:

1. 1.      Always attractive in nature.
2. 2.      Acts on all masses in universe.
3. 3.      Weakest force in nature.
4. 4.      Gravitational forces are central in nature.
5. 5.      Obey inverse square law
6. 6.     The exchange particle in two masses have gravitational force is Graviton.

 2 Weak nuclear Forces:

Weak nuclear forces are the forces of interaction between elementary particles of short life time.

e g These forces were discovered during the study of the phenomenon of β-decay, in radioactivity.

Characteristics/Important properties of weak nuclear forces:

1. 1.      The weak nuclear forces are 10²⁵ times stronger than the gravitational force.
2. 2.      The exchange particle for weak nuclear forces is intermediate Boson.
3. 3.      It does not obey inverse square law.

 3 Electromagnetic Forces :

 The electromagnetic forces are the forces between charged particles.

The charges in motion produce magnetic effects and a magnetic field give rise to a force on a moving charge. As we have studied in class 10^th. So electric and magnetic forces , in general , are inseparable . Hence the name is electromagnetic forces.

The force we deal in day to day life all are electromagnetic in nature e g Frictional, Tension etc.

Characteristics/Important properties of Electromagnetic forces:

1. 1.      Attractive or repulsive in nature.
2. 2.      They obey inverse square law.
3. 3.      They are central in nature.
4. 4.      The exchange particle of electromagnetic force is Photon.
5. 5.      Conservative in nature.

4 Strong Nuclear Forces:

The forces that bind the neutrons and protons together in a nucleus are called the strong nuclear forces.

Characteristics/Important properties of nuclear forces:

1. 1.      Nuclear forces are the strongest forces in nature.
2. 2.      Nuclear forces are non-central forces.
3. 3.      They are also non-conservative forces.
4. 4.      The field particle for nuclear forces is the π-meson.

S N	Name	Relative Strength	Exchange particles	Range	Operates among
1	Gravitational Force	1	Graviton	Infinite	All objects in universe
2	Weak nuclear Force	1025	Intermediate Bosons	Very short sub-nuclear size	Some elementary particles(electron and neutrino)
3	Electromagnetic Force	1036	Photon	Not very large	Charged particles
4	Strong nuclear Force	1038	Π-mesons	Very Short, nuclear size	Nucleons

UNIFICATION OF FORCES:

Unification of forces means that there exist a relationship between all the forces in nature.
Issac Newton unified terrestrial and celestial mechanics.

Efforts are made to unify all the forces and made one theory known as grand unified theory.

NATURE OF PHYSICAL LAW:

Law of conservation of energy

Law of conservation of linear momentum

Law of conservation of angular momentum

Law of conservation of mass.

QUESTIONS BASED ON TOPICS:

1 What is Physics?

2 What are the five main branches of Physics?

3................discovered theory of relativity.

4 Nuclear reactors are based on the phenomena of ...........

5 How do science and technology differ?

6 Why Physics is called exact science?

7 What contribution has Physics made in the development of biological science?

8 Why was science called natural philosophy in earlier days?

9 Name any two Indian scientist and their major contribution in the field of science.

10 What are the scope in the study of Physics?
11 Name the fundamental forces in nature.
12 Name any two forces which obey inverse square law.
13 Arrange four type of basic forces in the order of increasing strength.
14 How do science and technology differ ?
15 Name the exchange particle in case of electromagnetic force.