UNIT I - PHYSICAL WORLD AND MEASUREMENT

CHAPTER 1 – INTRODUCTION

1. What is science ? Define the terms scientific method and theoryAns. : The word science has been derived from a Latin word Scientia, which means “to know”. The main aim of science is to know the nature and the natural phenomena. Science is organized , systematic and formulated knowledge obtained through observations, experiments and verifications. According to Einstein, “Science is an attempt to make chaotic diversity of our sense experiences correspond to logically uniform system of thought”.

Scientific method : The systematic observations, reasoning, model making and theoretical prediction form the scientific method.

Theory : The behavior of physical system is explained in terms of a set of minimum number of laws which is called theory. A theory can be modified in the light of new observations.

2. What is physics? Mention various branches of physics.

Ans. : The study of nature is divided into two main branches, namely physical science and biological science.Physics is that branch of science which deals with nature and natural physical phenomena. In physics , attempts are made to measure the quantities with the best possible accuracy to explain the properties of matter.The main branches are : (1) Mechanics (it deals with the motion of objects in macroscopic world. It is based on Newtons’ Laws of Motion and the law of gravitation).(2) Thermodynamics ( It deals with the disorderly motion of large number of particles on heating. It is concerned with the change in temperature, change in internal energy and change in entropy of a systems through the transfer of heat and external work).

3. Electrodynamics : It deals with the theory of electricity, magnetism and electromagnetic waves. The basic Laws of electrodynamics were given by Coulomb, Oersted, Ampere, Faraday and Maxwell.

4. Acoustics : It deals with nature of propagation of sound waves.5. Optics : It deals with the nature and propagation of light, reflection, refraction,

interference, diffraction, polarization etc.6. Relativity : It delas with the theory of invariance in nature and the motion of object

moving with speed comparable to speed of light.

7. Nuclear physics : It deals with the atomic nuclei and their properties.8. Particle physics : It deals with the ultimate particles of which matter is made up of.9. Quantum mechanics : It deals with mechanical behavior of particles of sub-microscopic

world.

3. What is the scope of and excitement of physics?

Ans : Physics deals with the study of various natural phenomenon and is considered as most fundamental branch of science. It includes the macroscopic world like galaxies and universe as well as microscopic world like nucleus of atom and fundamental particles like electrons, protons, neutrons etc. The study of physics is quite interesting and exciting. We come across of wide ranges of mass, length and time and other physical quantities. This is the excitement in physics. The measurement of all small and big quantities can be done with high degree of accuracy.

4. Discuss the relation between physics and technology.

Ans : The technological development is closely related to the application of science and physics in particular. Following are a few advancements in technology based on physics : (a) the study of heat and thermodynamics has helped us to design the various heat engines like diesel engine, petrol engine, steam engine and refrigerator and airconditioner. (b) the study of propagation of electromagnetic waves has helped us to design radio, television, and wireless communication. (c) a launching of satellite has brought a revolution in weather forecast, geographical survey and long distance telecommunication. (d) the study of nuclear fission has helped us in the production of electricity. Other outcomes of physics are designing of electric motors, dynamos, radar, spacecraft, computer and so on.

5. Name the great scientists and their discoveries.

6. Name the various devices and their scientific principles.

7. Name the basic forces in nature.

Ans: There are four basic forces in nature. (a) gravitational force (b) electromagnetic forces (c) strong nuclear force (d) weak nuclear force.

8. Write the important properties of the basic forces.

Gravitational force

Ans : According to Newton, the force of attraction between every pair of particles or bodies is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between these particles.

F α m1 m 2/d 2 F= G m 1 m 2/d 2 where G is called the universal gravitational constant.

Properties:

(a) Gravitational forces between two bodies form an action and reaction pair. i.e they are equal in magnitude but opposite in direction. (b) it is central force , i.e it acts along the line joining the centres of the two interacting bodies. (c) it is independent of the nature of the medium. (d) gravitational force is negligible in case of light bodies but appreciable in case of massive bodies like stars and planets. (e) it is long range force i.e the force is effective even if the distance between the two bodies is very large. (f) the gravitational force between two bodies is due to the exchange of particles known as gravitons between them. (g) it acts without making any physical contacts with each other. (h) gravitational force is conservative

Electromagnetic forceElectrostatic force between two charges Q1 and Q2 separated by a distance (r} in air is directly proportional to the magnitude of charges and universally proportional to the square of distance between them.F α Q1 Q 2/r 2 F= k Q 1 Q 2/r 2 where k is the proportionality constantProperties : (a) it is both attractive as well as repulsive (b) it is long range force (c) electromagnetic force is a central force (d) it obeys inverse square laws. (e) It is 10 36

times stronger than the gravitational force. (f) It is a conservative force. (G) Electromagnetic force between two charged particle is due to the exchange of photons between them.

STRONG NEUCLEAR FORCE

The force holding the protons and neutrons inside a nucleus is known as strong nuclear force.Properties : (a) it is attractive in nature (b) it acts over a very short distance so it is a short range force (c) it is hundred times stronger than electromagnetic force and 1038

times stronger than the gravitational force. (d) it is a non-central force. (e) it does not obey inverse square law. (f) it is due to the exchange of π mesons

WEAK NUCLEAR FORCE

The force between the elementary particles emitted during radio active decay of a substance is known as weak nuclear force. In B decay, an electron and a neutrino are emitted . The force between them is called weak nuclear force.

Q. No. 9 Discuss the unification of forces in nature.

Ans.: The main thrust in physics is to unify the basic forces in nature. In 1687 Issac Newton showed that the motion of terrestrial and celestial bodies could be explained with the same laws of motion and the gravitational law. This is the unification of terrestrial and celestial mechanics. Then Oersted and Faraday unified electric and magnetic phenomena leading to electromagnetism. In 1873 Maxwell unified electromagnetism and optics . Also in 1979 the unification of electromagnetic force and weak nuclear force were unified. (Abdus Salam, Glashow and Steven Weinberg)

10. What do you mean by conservation laws in physics?Ans. : The quantities like charge, mass, energy, linear momentum, angular momentum etc of a body or a system are conserved under certain conditions . The laws governing the conservation of these quantities in nature are called conservation laws.

11. Explain the different laws of conservation.a. Law of conservation of electric charge: According to this the total electric charge on

an isolated system remains constant. Example when a glass rod is rubbed with silk, the glass rod looses electrons and becomes positively charged and silk receives electrons and becomes negatively charged. But the total amount of charge of both together remains constant.

b. Law of conservation of mass energy : According to Einstein, mass and energy are interconvertible if a mass of m is converted into energy it is seen that the energy E = mc2 where c is the speed of light in air/vacuum.

c. Law of conservation of energy : According to this energy can neither be created nor be destroyed, but can be changed from one form to another form. For example, the total energy (kinetic energy and potential energy) of a freely falling body remains constant.

d. Law of conservation of linear moment : According to this if no external force acts on a system its total linear momentum remains constant. Example, a gun recoils when a bullet is fired from it. Because before firing the system has zero linear momentum. After firing the bullet moves forward with high velocity and to conserve the linear momentum the gun moves backward.

e. Conservation of angular momentum : According to this the total angular momentum of a system remains constant if no external torque acts on it.

12. What is meant by reductionism?Ans .: A related effort is to derive the properties of a bigger, more complex, system from the properties and interactions of its constituent simpler parts. This approach is called reductionism and is at the heart of physics.