8th phy em.pdf - SCERT Telangana

SCERT TELA

NGANA

INSPIRE AWARDS

Inspire is a National level programme to strengthen theroots of our traditional and technological development.

The major aims of Innovations in Science Pursuit forInspired Research (INSPIRE) programme are...

• Attract intelligent students towards sciences

• Identifying intelligent students and encourage them tostudy science from early age

• Develop complex human resources to promote scientific, technologicaldevelopment and research

Inspire is a competitive examination. It is an innovative programme to makeyounger generation learn science interestingly. In 11th five year plan nearly TenLakhs of students were selected during 12th five year plan (2012-17) TwentyLakhs of students will be selected under this programme.

Two students from each high school (One student from 6 - 8 classes andone from 9 - 10 classes) and one student from each upper primary school areselected for this award.

Each selected student is awarded with Rs. 5000/-. One should utilize 50%of amount for making project or model remaining for display at district levelInspire programme. Selected students will be sent to State level as well asNational level.

Participate in Inspire programme - Develop our country.

Step Description A. Use Android mobile phone or tablet to view content linked to QR Code: 1. Click on Play Store on your mobile/ tablet. 2. In the search bar type DIKSHA. 3.

will appear on your screen. 4. Click Install 5. After successful download and installation, Click Open 6. Choose your prefered Language - Click English 7. Click Continue 8. Select Student/ Teacher (as the case may be) and Click on Continue 9. On the top right, click on the QR code scanner icon and scan a QR code

printed in your bookOR

Click on the search icon and type the code printed below the QR code,in the search bar.

10. A list of linked topics is displayed 11. Click on any link to view the desired contentB. Use Computer to view content linked to QR code: 1. Go to https://diksha.gov.in/telangana 2. Click on Explore Content 3. Enter the code printed below the QR code in the browser search bar 4. A list of linked topics is displayed 5. Click on any link to view the desired content

Let us know how to use QR Codes:In this textbook, you will see many printed QR (Quick Response) codes, such asUse your mobile phone or tablet or computer to see interesting lessons, videos, documents,etc. linked to the QR code.

SCERT TELA

NGANA

iTelangana Government Free Distribution 2019-20

Co-ordinators

Published by Government of Telangana, Hyderbad.

Respect the LawGet the Rights

Grow by EducationBehave Humbly

Editors

Academic Support

PHYSICAL PHYSICAL PHYSICAL PHYSICAL PHYSICAL SCIENCESCIENCESCIENCESCIENCESCIENCECLASS VIIICLASS VIIICLASS VIIICLASS VIIICLASS VIII

Sri M. Ramabrahmam, Lecturer,Govt. IASE, Masabtank, Hyderabad.

Dr. P. Shankar, Asst. Professor,IASE, O.U., Hyderabad.

Prof. Kamal Mahendroo,Vidya Bhavan Educational Resource Centre,

Udaipur, Rajastan.

Dr. TVS Ramesh,Co-ordinator, C&T Dept.,

SCERT, Hyderabad.

Miss. Preeti Misra,Vidya Bhavan Educational Resource Centre,

Udaipur, Rajastan.

Mr Kishore Darak,Vidya Bhavan Educational Resource Centre,

Udaipur, Rajastan.

Dr.B. Krishna rajulu Naidu,Retd., Professor of Physics

Osmania University, Hyderabad.

Dr.M. Adinarayana,Retd., Professor of Chemistry

Dr. Nannuru Upendar Reddy,Professor & Head C&T Dept.,

SCERT., Hyderabad.

Prof. V. SudhakarDept of Education, EFLU, Hyderabad.

SCERT TELA

NGANA

ii

© Government of Telangana, Hyderabad.

First Published 2013New Impressions 2014, 2015, 2016, 2017, 2018, 2019

All rights reserved.

No part of this publication may be reproduced, storedin a retrieval system, or transmitted, in any form or byany means without the prior permission in writing of thepublisher, nor be otherwise circulated in any form ofbinding or cover other than that in which it is publishedand without a similar condition including this conditionbeing imposed on the subsequent purchaser.

The copy right holder of this book is the Directorof School Education, Hyderabad, Telangana.We have used some photographs which are undercreative common licence. They are acknowledge atthe end of the book.

This Book has been printed on 70 G.S.M. Map litho,Title Page 200 G.S.M. White Art Card

Free Distribution by Government of Telangana 2019-20

Printed in Indiaat the Telangana Govt. Text Book Press,

Mint Compound, Hyderabad,Telangana.

SCERT TELA

NGANA

iiiTelangana Government Free Distribution 2019-20

Sri K. Sudhakara Chary, SGT,UPS Neelikurthy, Warangal.

Sri Kishan Thatoju, Computer Operator,C&T Dept., SCERT, Hyderabad.

Sri R. Ananda Kumar, SA,ZPHS Laxmipuram, Visakhapatnam.

Sri K.V.K. Srikanth, SA,GTWAHS S.L.Puram, Srikakulam.

Sri M. Eswara Rao, SA,GHS Sompeta, Srikakulam.

Sri Y. Guru Prasad, SA,ZPHS Chinnacherukuru, Nellore.

Sri K.L. Ganesh, SA,ZPHS M.D.Mangalam, Chittoor.

Dr. P. Shankar, Asst. Professor,IASE, O.U., Hyderabad.

Dr. K. Suresh, SA,ZPHS Pasaragonda, Warangal.

Sri Y. Venkat Reddy, SA,ZPHS Kudakuda, Nalgonda.

Sri Dr. S. Anjaneyulu, SA,ZPHS Veeraballi, YSR Kadapa.

Sri D. Madhusudhana Reddy, SA,ZPHS Munagala, Nalgonda.

Sri A. Satyanarayana Reddy, Director,S.C.E.R.T., Hyderabad

Sri B. Sudhakar, Director,Govt. Textbook printing press,

Hyderabad.

Dr.N. Upendar Reddy,Professor & Head C&T Dept.,

S.C.E.R.T., Hyderabad.

Text Book Development Committee

Writers

Graphics & Designing

Sri M. Ramabrahmam, Lecturer,Govt. IASE, Masabtank, Hyderabad.

Sri Kurra Suresh Babu, B.Tech., MA., MPhillMana Media Graphics, Hyderabad.

Sri C.V. Harikrishna, SA,ZPHS, Cheru Annaram, Nalgonda.

Sri A. Nagaraj Shekar, SA,ZPHS, Chatakonda, Bhadradri, Kothagudam.

Sri Md. Ayyub Ahmed, S.A.,Z.P. H.S U/M, Atmakur, Mahbubnagar.

SCERT TELA

NGANA

iv

Intro ...The nature is life source for all living organisms. Rocks, water, hills and

valleys, trees, animals etc. embedded in it… each of them are unique by themselves.Everything has its own prominence. Human being is only a part of the nature. The aspectwhich distinguishes the humans from all other organisms and exclusive for them is theirextraordinary thinking power. Thinking transforms a person as a unique entity from rest ofthe nature. Though it usually appears simple and normal, the intricacies of the very natureoften challenges us to untie the tough knots of its hidden secrets, day in and day out.

The human being intuitionally contemplates and searches solutions for all the criticalchallenges, all around,relentlessly. Curiously, the questions and answers are concealed inthe nature itself. The role of science, in fact, is to find them out. For this sake, somequestions, some more thoughts, and some other investigations are quite necessary. Scientificstudy is to move on systematically in different ways, until discovering concrete solutions.Essence of the investigations lies in inquiring i.e. identifying questions, asking them andderiving adequate and apt answers. That is why, Galileo Galilei, the Italianastronomer,emphasized that scientific learning is nothing but improving the ability ofquestioning. The teaching of science has to encourage children to think and work scientifically.Also, it must enhance their love towards the nature. Even it should enable them tocomprehend and appreciate the laws governing the nature in designing tremendous diversityfound around here and everywhere. Scientific learning is not just disclosing new things. It isalso essential to go ahead with deep understanding of the nature’s intrinsic principles;withoutinterrupting the harmony of interrelation and interdependence in the nature.

It is also necessary to step forward without interrupting the interrelationship andinterdependency along with understanding of the nature’s intrinsic principles.High Schoolchildren possess cognitive capacity of comprehending the nature and characteristics of thetransforming world surrounding them. And they are able to analyze abstract concepts.

At this level, we cannot quench their sharp thinking capability with the dry teaching ofmere equations and theoretic principles. For that, we should create a learning environmentin the classroom which provides an opportunity for them to apply the scientific knowledge,explore multiple alternatives in solving problems and establish new relations. Scientificlearning is not just confined to the four walls of classroom. It has a definite connection tolab and field as well. Therefore, there is a lot of importance to field experience/ experimentsin science teaching.

There is a great need for compulsory implementation of instructions of the NationalCurriculum Framework- 2005 which emphasizes linking of the science teaching with local

SCERT TELA

NGANA

vTelangana Government Free Distribution 2019-20

environment. The Right to Education Act- 2009 also suggested that priority should begiven to the achievement of learning competencies among children. Likewise, scienceteaching should be in such a way that it would help cultivate a new generation with scientificthinking.The key aspect of science teaching is to make the children understand the thinkingprocess of scientists and their efforts behind each and every discovery. The State CurriculumFramework- 2011 stated that children should be able to express their own ideas andopinions on various aspects.All the genuine concepts should culminate into efficaciousscience teaching, make the teaching-learning interactions in the classroom, laboratory andfield very effective and really become useful for the children to face the life challengesefficiently.

We thank the VidyaBhavan Society, Rajasthan, Dr. Desh Panday Rtd Prof. Collegeof Engineering Osmania University and Sri D.R. Varaprasad former Lecturer ELTCHyderabad for their cooperation in developing these new text books,the writers for preparingthe lessons, the editors for checking the textual matters and the DTP group for cutelycomposing the text book.

Teachers play a pivotal role in children’s comprehensive use of the text book. Wehope, teachers will exert their consistent efforts in proper utilization of the text book so asto inculcate scientific thinking process and inspire scientific approach in the children.

Energized Text Books facilitate the students in understanding the concepts clearly,accurately and effectively, this book has been “Energized” with QR (Quick Response)Codes as a pilot project. Content in the QR Codes can be read with the help of any smartphone or can as well be presented on the Screen with LCD projector/K-Yan projector.The content in the QR Codes is mostly in the form of videos, animations and slides, and isan additional information to what is already there in the text books.

This additional content will help the students understand the concepts clearly and willalso help the teachers in making their interaction with the students more meaningful.

At the end of each chapter, questions are provided in a separate QR Code which canassess the level of learning outcomes achieved by the students.

We expect the students and the teachers to use the content available in the QR Codesoptimally and make their class room interaction more enjoyable and educative.

25.02.2019Hyderabad

Director, SCERT,Hyderabad.

SCERT TELA

NGANA

vi

Dear teachers...New Science Text Books are prepared in such a way that they develop children’s observation

power and research enthusiasm. It is a primary duty of teachers to devise teaching- learningprocesses which arouse children’s natural interest of learning things. The official documents ofNational& State Curriculum Frameworks and Right to Education Act are aspiring to bring grassroot changes in science teaching. These textbooks are adopted in accordance with such anaspiration. Hence, science teachers need to adapt to the new approach in their teaching. In viewof this, let us observe certain Dos and Don’ts:

• Read the whole text book and analyze each and every concept in it in depth.

• In the text book, at the beginning and ending of an activity, a few questions are given.Teacher need to initiate discussion while dealing with them in the classroom, attempt toderive answers; irrespective of right or wrong responses, and so try to explain concept.

• Develop/Plan activities for children which help them to understand concepts presented intext.

• Textual concepts are presented in two ways: one as the classroom teaching and the otheras the laboratory performance.

• Lab activities are part and parcel of a lesson. So, teachers must make the children conductall such activities during the lesson itself, but not separately.

• Children have to be instructed to follow scientific steps while performing lab activities andrelevant reports can be prepared and displayed.

• In the text some special activities as boxed items- ‘think and discuss, let us do, conductinterview, prepare report, display in wall magazine, participate in Theatre Day, do fieldobservation, organize special days’ are presented. To perform all of them is compulsory.

• ‘Ask your teacher, collect information from library or internet’- such items must also beconsidered as compulsory.

• If any concept from any other subject got into this text, the concerned subject teacher hasto be invited into the classroom to elucidate it.

• Collect info of relevant website addresses and pass on to students so that they can utilizeinternet services for learning science.

• Let there be science magazines and science books in the school library.

• Motivate every student to go through each lesson before it is being actually taught andencourage everyone to understand and learn independently, with the help of activities suchas Mind Mapping and exciting discussions.

• Plan and execute activities like science club, elocution, drawing, writing poetry on science,making models etc.to develop positive attitude among children environment, biodiversity,ecological balance etc.

• As a part of continuous comprehensive evaluation, observe and record children’s learningabilities during various activities conducted in classroom, laboratory and field.

SCERT TELA

NGANA

viiTelangana Government Free Distribution 2019-20

We believe, you must have realizedthat the learning of science and scientific thinking arenot mere drilling of the lessons but, in fact, a valuable exercise in motivating the children toexplore solutions to problems all around by themselves systematically and preparing them tomeet life challenges properly.

Dear Students...

Learning science does not mean scoring good marks in the subject. Competencies likethinking logically and working systematically, learned through it,have to be practiced in dailylife. To achieve this, instead of memorizing the scientific theories by rote, one must be able tostudy them analytically. That means, in order to understand the concepts of science, you needto proceed by discussing, describing, conducting experiments to verify, making observations,confirming with your own ideas and drawing conclusions. This text helps you to learn in thatway.

What you need to do to achieve such things:• Thoroughly go through each lesson before the teacher actually deals with it.• Note down the points you came across so that you can grasp the lesson better.• Think of the principles in the lesson. Identify the concepts you need to know further,

to understand the lesson in depth.• Do not hesitate to discuss analytically about the questions given under the sub-heading

‘Think and Discuss’ with your friends or teachers.• You may get some doubts while conducting an experiment or discussing about a lesson.

Express them freely and clearly.• Plan to implement experiment/lab periods together with teachers, to understand the

concepts clearly. While learning through the experiments you may come to knowmany more things.

• Find out alternatives based on your own thoughts.• Relate each lesson to daily life situations.• Observe how each lesson is helpful to conserve nature. Try to do so.• Work as a group during interviews and field trips. Preparing reports and displaying

them is a must.• List out the observations regarding each lesson to be carried through internet, school

library and laboratory.• Whether in note book or exams, write analytically,expressing your own opinions.• Read books related to your text book, as many as you can.• You organize yourself the Science Club programs in your school.• Observe problems faced by the people in your locality and find out what solutions you

can suggest through your science classroom.• Discuss the things you learned in your science class with farmers, artisans etc.

SCERT TELA

NGANA

viii

ACADEMIC STANDARDS

S.No. Academic Standard Explanation

1.

2.

3.

4.

5.

6.

7.

Conceptual understanding

Asking questions andmaking hypothesis

Experimentation and fieldinvestigation.

Information skills andProjects

Communication throughdrawing, model making

Appreciation and aestheticsense, values

Application to daily life,concern to bio diversity.

Children are able to explain, cite examples, give reasons,and give comparison and differences, explain the processof given concepts in the textbook. Children are able todevelop their own brain mappings.

Children are able to ask questions to understand concepts,to clarify doubts about the concepts and to participate indiscussions. They are able to guess the results of on issuewith proper reasoning, able to predict the results ofexperiments.

Children are able to do the experiments given in the textbook and developed on their own. Able to arrange theapparatus, record the observational findings, suggestalternative apparatus, takes necessary precautions whiledoing the experiments, able to do to alternate experimentsby changing variables. They are able to participate in fieldinvestigation and prepare reports.

Children are able to collect information related to theconcepts given in the text book by using various methods(interviews, checklist questionnaire) analyse the informationand interpret it. Able to conduct project works.

Children are able to communicate their conceptualunderstanding by the way of drawing pictures labellingthe parts of the diagram by drawing graphs, flow chartsand making models.

Children are able to appreciate the nature and effortsof scientists and human beings in the development of sience and have aesthetic sense towards nature. They arealso able to follow constitutional values

.

Children are able to apply the knowlegde of scientificconcept they learned, to solve the problem faced in dailylife situations. Recognise the importance of biodiversityand takes measures to protect the biodiversity.

SCERT TELA

NGANA

ixTelangana Government Free Distribution 2019-20

12 Graphs of motion

1INDEXINDEXINDEXINDEXINDEX

2

3

4

5

6

7

8

9

10

Page No.Periods Month

Force

Friction

Synthetic fibres and plastics

Metals and non-metals

Reflection of light at plane surfaces

Coal and petroleum

Combustion, fuels and flame

Electric conductivity of liquids

Some natural phenomena

Sound

11 Stars and solar system

Revision

8 June 1

8 June/July 19

11 July 32

12 August 52

12 Aug./Sept. 65

6 September 83

12 October 93

10 November 107

11 November 119

11 December 131

13 January 147

4 February 171

March

SCERT TELA

NGANA

x

OUR NATIONAL ANTHEM

- Rabindranath Tagore

Jana gana mana adhinayaka Jaya heBharatha bhagya-vidhata

Punjab Sindh Gujaratha MarathaDravida Utkala Banga.

Vindhya Himachala Jamuna GangaUchchala Jaladhi taranga,

Tava shubha name jageTava shubha asisha mage

Gahe tava jaya gatha

Jana gana mangala-dayaka jaya he,Bharatha bhagya –vidhatha,

Jaya he, jaha he, jaya he,Jaya jaya jaya jaya he

PLEDGE

- Paydimarri Venkata Subba Rao

“India is my country; all Indians are my brothers and sisters.I love my country, and I am proud of its rich and varied heritage.

I shall always strive to be worthy of it.

I shall give my parents, teachers and all elders respect,and treat everyone with courtesy. I shall be kind to animals.

To my country and my people, I pledge my devotion.

In their well-being and prosperity alone lies my happiness.”SCERT TELA

NGANA

Telangana Government Free Distribution 2019-20 1

FORCEChapter

1We can observe many changes around

us, like changes in seasons, change duringsun rise and sun set, changes in tides of seaetc. Have you ever thought about the causebehind these changes? In ancient days,people thought that an invisible force wasresponsible for the changes occurring innature. Even now many people believe thatan invisible force exists that causeswhatever is happening in the world.

Later, the concept of force wasdeveloped, but it was limited to explainingour efforts and actions. The words force,effort, strength and power had almost thesame meaning at that time. Have you everwondered what forces are? What are thedifferent types of forces and how do theyact? Let’s find out the answers to suchquestions in this chapter.

For instance, when you ride a bicycle,most of the time your legs are pushingdown on the pedals. To push you have tomake an effort. When you pick up yourschool bag you have to make an effort inorder to lift or pull the bag upwards and offthe ground. When you open a door youmake an effort on the door knob with yourhands either to push the door forward orpull it backward.

What is force?Have you ever picked the stones A paper

boy throws a newspaper. Have you everwondered about this action. Actions likestretching a rubber band, pulling a rickshaw,rowing a boat etc., are some more exampleswhere our efforts help to change of theobjects. Such actions like picking,squeezing, twisting, stretching, loweringand lifting etc., cause a change in the stateof an object. Now let us try to group thesetasks as a pull or a push.

Activity-1

Identifying push or pullTable-1 gives some examples involving

the actions like digging, sucking, erasing,falling, attracting, raising etc. Classify theseactions in terms of a push or a pull or both.Write pull or push in the blank boxes. Ifyou feel that the action involves both pushand pull, write ''both'' in the box.

SCERT TELA

NGANA

2 Force

Table 1: Identify tasks as Push or Pull or Both

S. No Action Diagram Push/Pull/Both

1 Digging

bore well

2 Sipping Juice

with a straw

3 Erasing blackboard

with duster

4 A magnet

attracting nails

5 Fruits falling

from tree

6 Hoisting a flag

List three more activities where weexert force which appears as a push.

List three more activities where weexert a force as a pull.

State three actions which involve bothpush and pull.

Based on this activity, can you explainwhat is a force?

Shall we call the effort done on anobject by means of pushing or pulling as aforce exerted on the object?

We cannot directly see the forcesacting on a body, but we can see the effectscaused due to the forces.

SCERT TELA

NGANA


Forces acting at a distance (fieldforces)

1. Magnetic forceYou must have done some experiments

with magnets in class VI. Let us recall someof your experiences.

Activity-2

Observing the magnetic force.Take a sewing needle. Rub it with a bar

magnet several times always moving themagnet in the same direction. Does theneedle get magnetised ? You may find thatthe needle always acts like a magnet. Withthe help of a magnetic compass you canidentify the north and south poles of theneedle. Pin a red coloured cork ball toSouth Pole and white ball to North Pole ofthe needle; then drop it in a bowl of water,it floats. (Fig-2)

Make another needle in the same way.Float both the needles side by side such thatlike ends facing each other (either red orwhite balls). What happens to the needles?How do they move? Now place the needlesin such a way that unlike ends (White ballof first needles and red ball of secondneedle) face each other. Now observe whathappens this time. How do they push or pulleach other?

Fig-2: Making needle magnets andfloating them in a bowl of water.

When an object slips off your hand, italways falls down. What pulls it down? Ifyou roll a ball on a level ground, it slowsdown and after sometime it will come to astop. What makes the ball stop? Whatforces acting on objects, change their stateor state of motion?

Types of forcesContact forces and forces at adistance (Field Forces)Observe the following figures.

Fig-1 (a), Fig-1(b)

Why does the toothpaste come out whenwe press the tube? Why does the needle ofa magnetic compass move when we place abar magnet near it? Have you observed thedifference between the force you applied onthe tube and the force applied by a magneton the needle of a compass?In Fig.1 (a) you observe that there is directphysical contact (or interaction) betweenyour hand and the tube. Force, whichresults when there is a direct physicalcontact between two interacting objects, isknown as contact force.

In Fig.1 (b) the needle of the compasschanges its direction without any physicalcontact with the bar magnet. But a forcemust be acting on the needle. The forcewhich occurs without any physical contactbetween two objects is known as a forceat a distance or field force.

SCERT TELA

NGANA

4 Force

You have learnt in class VI that likepoles of two magnets repel each other andunlike poles attract each other. You canobserve the red end of one needle and whiteend of another needle attract each other,and ends with same colour repel.

Now, you know that like poles repel orpush each other away and unlike polesattract or pull each other. This action ofpull or push arises due to a magneticforce. A magnet can attract or repel anothermagnet without contact. So magnetic forceis a field force.

2. Electrostatic force

Activity-3

Observing electrostatic forces

Take a balloon. Inflate it and tie up theopen end. Now cut a paper into small piecesand place them on the floor. Rub the balloonwith a paper and bring the balloon near thepieces of papers. What happens now? Arethe bits of paper pulled towards theballoon? (Fig-3) Why does the balloon pullor attract the pieces of paper? Try to usepepper and salt in the place of pieces ofpaper. What do you observe?

Fig-3: Charged balloon

We can say that when the balloon isrubbed with a paper, it acquires anelectrostatic charge on its surface. Theballoon is now said to be a charged body.When it is brought near the bits of paper,the pieces acquire opposite charge and willrise and cling to the balloon.

The force exerted by a charged bodyon another charged body is known aselectrostatic force.

This force comes into play even whenthe bodies are not in contact. It is anexample of a force at a distance.

3. Gravitational forceIt is our common experience that if a

pen slips off from our hands it falls downto the floor.

Why does the pen fall down?

What is the force which pulls thepen down?

If we keep the same pen on a table, itdoes not fall down. Why?

Generally our answer would be that thetable supports the pen. If the table does notsupports the pen it would fall down until itis supported by another object, like thefloor.

Why does a stone thrown up into thesky fall back to the earth?Why do rivers flow down to thesea?How does the earth hold theatmosphere?Is there any force pulling the objectstowards earth?

SCERT TELA

NGANA


If an object is thrown upwards, thereexists a force which pulls it down towardsthe earth, because of this it falls down tothe ground. We call this force as agravitational force.

Every object on the Earth or close toEarth, will experience a gravitational pull.The force of gravity is not just due to theattraction of the Earth. It is a force ofattraction that exists between any two bodies(or masses) everywhere in the universe.

As the earth is so massive and huge,all the other objects close to the earth areattracted or pulled towards it. When yousit in your class room, there will be agravitational force between you and yourteacher, and a similar force exists betweenyou and the black board.

You cannot experience the gravitationalforce that exists between you and your teacheror between you and the block board becauseit is very small when compared to thegravitational force exerted by the earth onthese objects. You will learn more about thisin the lesson ''Gravitation'' in higher classes.

Gravitational force works even theobjects are not in contact. So, this is anexample of field force.

Think and discuss

A cricket ball of mass ‘m’ is thrownupward with some initial speed. If the airresistance is neglected, what forces areacting on the ball when it reaches (a) halfits maximum height and (b) its maximumheight?

Explaining of force acting at adistance: concept of field

The force which acts between twobodies, when the bodies are not directlytouching each other is called force at adistance. We can explain the forces at adistance by using the concept of field.

Activity-4

Visualizing magnetic field.

Fig-4 : Magnetic field

Take a bar magnet and place it on a table.Place a thick white paper over it (Whitedrawing sheet).On the paper, sprinkle fine powder of iron(iron filings) as shown in the fig -4.Tab the table or the paper gently withpen/pencil.What do you observe? Do you find anypattern of iron filings there?Rotate the magnet in differentdirections and do the same. How hasthe pattern changed?

You can see that in a small spacearound the magnet, iron filings setthemselves in a pattern because they areaffected by the magnetic force of the fieldcreated by the bar magnet. The patternrepresents the magnetic field. The spacearound the magnet where its influence canbe detected is called the magnetic field.This field is three dimensional.

SCERT TELA

NGANA

6 Force

Sl.No

Thus, a field is a region in which a forcecan be experienced by another magneticobject placed at any point in that region.

A body creates a field and another bodyexperiences the force by the field when itis placed in that field.

A magnetic field surrounds a magnet,an electric field surrounds electriccharges and a gravitational field surroundsmasses.

The strength of a field in a particularregion can be represented by field lines;the greater the density of lines, the strongerthe forces in that part of the field.

Think and discuss

Two identical bars, one which is steeland the other a magnet, are painted withthe same colour. How can you tell whichone is the magnet using only these twobars? (don’t break the bars)

Contact Forces1. Muscular Force

In all the actions that we perform in our

daily life like brushing, bathing, eating,writing, driving and walking; we have toexert a force. Do you know from wherethe force comes? The force which we exertby using our body muscles is known asmuscular force. Even when we smile ourmuscles exert force to bring changes in ourface. Human beings and animals usemuscular force to carry out their regularphysical activities. Muscular forces can beexerted only through contact.

Activity-5

Preparing a list of examples formuscular force

List at least ten activities where weapply muscular force to perform varioustasks, in table - 2.

Table-2

List of activitieswhere we exert force

1 Lowering a basket

2

3

4

5

6

Usually we are unaware of the muscularforces that are responsible for the variousactions taking place inside our body, likeblood circulation, expansion andcontraction of lungs during breathing, heartbeat etc.Fig-5

SCERT TELA

NGANA


Does the change in speed of the balland bicycle depend on roughness andsmoothness of the surface on which theymove? Let us find out.

Activity-7

Observing the motion of a ballon different surfaces

Try to roll a ball on different surfaceslike carpet, rough roads, smooth floor etc.See that surfaces are plane, exerted forceis same.

On which surface does the ball rollfarther?

The motion of the ball is different ineach case. The force of resistance to themotion seems to be more on the roughsurface than on the smooth surface. Therolling ball moves farther on a smoothmarble floor than on a rough sandy surface.

Activity-8

Observing the motion of objectson an inclined plane

Take a tray. Place a small ice cube,eraser and a rupee coin on a line at one endof the tray. Now slowly lift this end of thetray as shown in the figure-6.

Fig-6: Motion of objects on an inclined plane.

Do you feel your muscles get tightenedwhile performing any physical activity?What could be the reason for it?

Activity-6

Observing the changes in anymuscle while working

Take a dumbbell and lift it in differentways. Observe while doing this exercisewhich muscle is going to be shortened.

Ask your friends to do the same andobserve the movement of their muscles.

The term muscle refers to multiplebundles of muscle cells held together.Muscles are normally arranged in such away that as one group of muscles contractor shortens, another group relaxes orexpands. For example, if you throw a ballthe muscles in the chest and front side ofthe shoulder lengthen to pull the armforward, while the muscles in the back andrear of the shoulder undergo contractionto slow down the motion.

2. Force of FrictionWhen you roll a ball on a level ground

it invariably stops after sometime.

Why does the ball stop?

Is there any hidden force which bringsit to stop?

If you stop peddling your bicycle on alevel road you observe that its speeddecreases gradually.

Why does the speed of the bicycledecrease gradually?

Is there any force acting on it whichtends to reduce its speed?

SCERT TELA

NGANA

8 Force

there was no friction between the tyres

and the road?

The direction of friction is always

opposite to the direction of motion relative

to the surface. Let’s imagine a world

without friction. Can we walk on a road

without friction? Would it be possible to

write with a pen on a paper or with a piece

of chalk on the black board? You will learn

more about friction in the next chapter.

Think and discuss

A book placed on a table is at rest. Isthe force of friction acting on it or not?Explain?

3. Normal force

Fig-8: Force of gravitation and normalforce acting on the book.

Place your science book on the table.Is it in a state of rest? Is there any forceacting on that book? Imagine that the tablehas disappeared suddenly by magic. Whatwill happen then? The book will fall down

What do you observe?

Which one of these three objectsslides down first? Why?

Do all the objects experience thesame resistance to motion? If not why?

Which one of the objects experiencesmore resistance to motion? Why?

Why is there a change in resistanceexperienced by the objects thoughthey all slide down on the sameplane?

Do this activity with different objectslike a book, a ball, a pen, a stone etc., and

record your observations.

Friction is the resistance to the

movement of a body over the surface of

another body.

Did you ever experience slipping on a

floor? What conditions caused you to slip?

Did you experience slipping while you are

walking on wet mud? Why do most road

accidents happen during rainy days?

Would it be possible to drive a car if

Fig-7

SCERT TELA

NGANA


For a stone tied to thestring, gravity pulls down thestone all the time but it doesnot fall down because it issupported by the string.Thus, there exists a forcewhich supports the stoneagainst gravity by pulling itupward.

What do we call thisforce?

It is called 'tension' which always pullsthe body along the string. Tension is acontact force. When you try to stretch arope or a string the tightness of rope orstring is called tension. In the aboveexample, the gravitational force is balancedby the upward tension force as shown in thefigure-10.

Aim: To find the limiting force that can be borneby a string.Material used: Spring balance,weights, light strings are of 10cm length, and of equalthickness, weight hanger.Procedure:1. Arrange the system as shown infigure-11. Put some small weightslike 50 gm on the weight hanger andnote the readings of the spring balance. Now, addsome more weights to the hanger and note thereadings of spring balance. Do the same till thestring is broken. Note the reading of the balancein the following table when the string is broken.Find out the limiting force of different types ofstrings and mention the values in the given table.

due to the gravitational pull of the Earth.Even when a book is lying on the table, thegravity pulls the book down all the time butit does not fall down because it issupported by the table. Therefore, thereexists a force which supports the bookagainst gravity by pushing it upward. Whatdo we call this force?

Look at the fig.8 A direction which isperpendicular to the plane of a surface issaid to be normal. The force that a solidsurface exerts on any object in the normaldirection is called the normal force.

In above example the downwardgravitational force is balanced by theupward normal force. Since these twoforces are of equal magnitude and actingin opposite directions, we say that the netforce acting on the book is zero and thebook is in equilibrium.

4. TensionAs shown in the

figure-9 a stone issuspended with thehelp of a string and itsfree end is tied to theceiling

What is thestate of the stone?

What forces are acting on it?What will happen if the string isbroken?

We know that the stone would fall downdue to gravitational pull (weight) of theearth if the string is broken.

Fig-9

Lab Activity

Fig-10

Fig-11

SCERT TELA

NGANA

10 Force

Fig - 12(a)

a lift which is accelerating.

How many forces are acting on thisobject?

Is the net force acting on the objectzero? Why?

Note: When an object is in non uniformmotion, it is said to be in acceleration.

The net force acting on an object keptin a lift which is accelerating is not zero,as the object is in non-uniform motion.

When two forces act on a body, as inthe above case, one of the forces shouldbe greater than the other to set the body inmotion.

The strength of a force is usuallyrepresented by its magnitude. The directionof a force is as important as its magnitude.We represent the direction of force,magnitude using ‘arrows’ ( ).

Activity-9

Effects of net force acting on atable

Try to push a heavy wooden table(Fig-12a). Is it hard to push ?

2. Separate the whole system from the ceiling,and tie the string to weight hanger and now slowlypull up the whole system with your hand whenthere is a small weight on the hanger. Note downthe readings are they same when you bring it down.

What do you observe from the readingswhen it is pulled up and released to movedown?Is the string broken when the whole systemis pulled quickly up?

Think and discussA system of two bodies A andB are placed as shown infigure. How many forces areacting on A and Brespectively?Why is it necessary to separatecontact force into a normal force andfrictional force? Give at least two reasons.

Net forceIn reality, many forces can act

simultaneously on a body. For example,there exists two forces on an object placedon a horizontal floor. One is gravitationalforce (vertically down) and other is normalforce (vertically up).

Do you observe any change in the stateof rest of that object because of theseforces? Obviously your answer is ‘No’.

In this case two forces acting on theobject are equal and opposite in direction.Hence, there is no change in its state.

Technically, we say that the net force onthis object is zero.

Imagine that the same object is kept in

Sl.No. Type of String Limiting Force

Ask your friend to help you in pushingthe table in the same direction, as

SCERT TELA

NGANA


Fig-12(c)

Fig - 12 (b)

shown in the fig.12 (b). Do you find iteasier to move the table now? Why?

You may notice that it is easier to pushthe table when you take the help of yourfriend. The force applied by your friendadded to the force exerted by you, results inboth forces being applied on the table in thesame direction. The total force applied byboth of you made it easy to move the table.

Now ask your friend to push the tablefrom the opposite side as shown infig.12(c). Does it move? If it moves, thenin which direction does it move?

When you and your friend push the tablefrom opposite sides, the table doesn’t moveif both of you apply force with equalmagnitude. Let us assume that one of youexerts a larger force, what will happen?Why?

F2F1

All forces have both magnitude anddirection. While adding forces, thedirections of forces have to be taken intoaccount. When forces act on a body alonga straight line and they are in the samedirection the net force is taken as the sumof all forces acting on the body. To addforces, sign convention must be used.

As shown in the above figure the forceF

1 directed towards right could be taken as

positive and the force F2 acting towards left

could be taken as negative. Let the forcesF

1 and F

2 act on the table in opposite

directions as shown in the figure and F1>F

2,

Then Fnet

= F1 + (-F

2) = F

1 - F

2

When the forces on a body in astraight line are in opposite directions, thenet force is equal to difference between thetwo forces. The object at rest moves in thedirection of the net force acting on it.

Activity-10

Effects of stretched rubber bandson fingers

Take a rubber band, stretch it using yourfingers. When you stretch the rubber bandit exerts force on your fingers and you feelthe force of pull on your fingers. Whathappens if you add one more similar rubberband around your fingers and stretch bothtogether to the same length? Do you feelthe combination of two bands exerts alarger force than that of one? Increase the

Fig - 13

SCERT TELA

NGANA

12 Force

number of rubber bands around yourfingers and observe the force exerted onyour fingers by the rubber bands.

Fig-14 Stretching rubber bands

Let us say the force exerted by onerubber band is F units and the force exertedby the second rubber band is also F units.Then what will be the net force of tworubber bands? We can express it as:

Fnet

= F + F = 2F unitsThe unit of force in SI system is

newton(N).What is the net force acting on your

finger when three, four etc. rubber bandsare used?

How to calculate net force fromfree body diagrams

The diagram showing all the forcesacting on an object at a particular instant iscalled Free Body Diagram. It is denoted asFBD.Example:

Let a car be moving with a non uniformspeed along a road. What are the forcesacting on the car? What is the net forceacting along the vertical direction? Whatis the net force acting along horizontaldirection?

Draw all the relevant forces acting ona body (we called it a free body diagram,FBD)

Choose a coordinate system withX-axis and Y-axis as shown in figure-15.

Sign convention is to be taken alongX and Y axes directions.

Add forces algebraically with signconventions along X and Y axes separately.Then those values give net forces alongX and Y directions respectively.

Solution:

The forces acting on the car are shownin the FBD (Free body diagram): They are

Force applied by the engine = F

Friction applied by road = f

Normal forces are N1and N

2

Gravitational force (Fg) = W

Net force along x-axis or direction

Fnet, x

= f - F

Net force along y-axis or direction

Fnet, y

= N1 + N

2 - W

Think and discuss

Play arm wrestling with your friend.How can you explain the winning ofthe game by using the concept of netforce?Name forces acting on arm and theirdirection while playing the game.

Try to draw FBD for this situation.

F

N1 N2

Wf

Fig-15: Free Body Diagram(FBD)

y-axis

x-axis

SCERT TELA

NGANA


What Forces can do ?

Activity-11

Effect of force on change of thestate of motion of an object andit's direction.

Place a football on the ground. Theball will remain in a state of rest unlesssomeone kicks the ball. Now kick the ball(Fig-16a). What happens? Does the ballstart moving? Kick the moving ball againin the same direction (Fig-16b). What willbe the result? Place your hand or leg againstthe ball. Does the ball stop? Or does itchange its direction? Note yourobservations.

Fig-16: (a) Force applied on a ball at rest. (b) Force applied on a moving ball

in the direction of motion.

We can move the ball from its positionof rest by applying a force on it. We canstop the moving ball and bring it back torest by catching it. Give few more exampleswhere the state of motion of an objectchanges due to the application of force.

You might have seen children playingwith a rubber tyre by pushing it with a stick.They push the tyre again and again with thestick to increase its speed. Do youunderstand why the speed of the tyreincreases whenever it is pushed by the stick?

With every push they are applying a littlemore force on the moving tyre in thedirection of motion. Hence the speed ofthe tyre increases continuously.

If the net force acts in the direction ofmotion, the speed of an object moving withconstant speed also increase. If the netforce acts in a direction opposite to themotion, then it either slows down theobject or brings it to a rest or it may changethe direction of motion.

Give some more examples where theobject speeds up or slows down or a changemay occur in its direction of motion. Whenwe exert a force on it?

Activity-12

Effects of net force on directionof moving object

Hit a carrom coin with the striker. Ask yourfriends to do the same. Does the coin movein the same direction in each case? If not why?

You can observe that in each case thedirection of the coin changes.

16(a)

16(b)

Fig.17SCERT TELA

NGANA

14 Force

When you hit the coin with the striker,not only does the coin change its direction,but the striker changes its direction too.What might be the cause for that?

From these observations, we can saythat a net force stops a moving object ormakes a stationary object move and alsochanges the speed and direction of amoving object. Thus, a force can change

the state of motion of an object. Does theforce change only the state of motion? Are

there any other effects of force?

Other effects of forceActivity-13

Effects of force on the shape ofan object

In table-3 some situations are given inthe first column showing how the force isapplied on an object. Observe the shape ofthe objects carefully before and afterapplying the force. Mark 'T' for temporarychange and mark 'P' for permanent changein the second column.

Table 3

Give some more examples where forcethat can change the shape of an object.

From the above table we can understandthat a force not only changes the state ofmotion of an object but can also change theshape of an object. It may change the shapetemporarily or permanently, based on thenature of the object and the force appliedon it.

Pressure

Activity-14

Change in effect of force witharea of contact

Take a pencil. Just push its rounded endon your palm. Now push from the otherside of the pencil gently so that the sharpend is on your palm. What difference didyou experience? Why?

Fig-18

Why do the porters place turbans(talapaga) on their head when they have tocarry heavy loads? Why do School bags andshopping bags have broad straps as handles?Have you ever wondered why lorriescarrying heavy loads have a larger number

Stretching rubber band

Squeezing sponge

Tearing paper

Breaking piece of chalk

Making chapathi

Breaking glass

Change in shape(temporary/permanent)

Actionof force

SCERT TELA

NGANA


Since the masses of both bricks aresimilar the force applied on lime powderby them is the same on both the trays. Thedifference lies in the surface area of thebrick in contact with the lime powder and

this is responsible for the change in the

extent to which the brick sinks in the limepowder.

In above activity, the contact area onwhich force is acting is different in eachcase. The depth to which the brick sinks inthe first tray (Fig 19 a) is deeper than thatin the second tray (Fig 19 b).This is becausein Fig 19(a), the contact area or the surfacearea on which force is acting is smaller andhence, the pressure exerted by the brick ismore. In Fig 19(b), the contact area or thesurface area on which force acting islarger. Hence the pressure exerted by thebrick is less.

Why does the sharper side of a knifecuts more easily than the blunt side of it? Asharp side of knife has a smaller contactarea. Therefore, for the same amount offorce applied on it, the sharp side of knifeexerts more pressure than the blunt side andhence cuts more easily. Can you give somemore examples of pressure?

From the above examples, you cansay that for a given force, if the surface areais smaller, the pressure will be greater. Ifyou use a larger area, you are spreading out

the force, and the pressure (or force per

unit area) becomes smaller.

Think and discuss

Does pressure have direction? Explain.

of broader tyres? In these examples youmight have noticed that the effect of forcedepends on the area of contact on whichthe force is acting. When there is adecrease in the area of contact of the forceor load then the effect of force increasesand vice versa.

The force acting perpendicularly on unitarea of a surface is called pressure.

Pressure = Force/AreaThe unit of pressure in S.I. system is

Newton/meter2 or N/m2.

Activity 15

Identifying effects of force

Take two trays. Fill both the trayswith lime powder or fine sand. Now taketwo rectangular bricks of equal mass andsimilar shape. Keep one brick vertically inone tray and the other brick horizontally inthe second tray. What do you notice? Doboth bricks sink to the same depth in limepowder? If not why?

Fig-19

You may notice that the brick standingvertically sinks deeper in lime powder thanthe brick standing horizontally.

SCERT TELA

NGANA

16 Force

What we have learnt

Improve your learning

Key words

Force, Push, Pull, Contact force, Force at a distance, Field, Friction, Muscular

force, Gravitational force, Magnetic force, Electrostatic force, Net force, Magnitude,

Equilibrium, Normal force, Tension, State of motion, Pressure, Freebody diagram

• Force is a push or a pull.

• A force can act on an object with or without being in contact with it. A force acting

on body is either a contact force or force acting at a distance.

• Field is a three dimensional region influenced. If an object is kept at any point in

the region, is influenced by it.

• Friction is the force that opposes relative motion of surfaces in contact.

• The force which we exerts by using our body muscles is known as muscular force.

• The attractive force between any two massive objects is called gravitational force.

• The magnetic force attracts a magnetic material such as iron. But it either attracts

or repels another magnet.

• The force exerted by a charged body on other charged body is known as electrostatic

force.

• Force has magnitude as well as direction.

• The algebraic sum of all the forces acting on a body is known as net force, and is

denoted by Fnet

.

• A force can change the state of motion of an object.

• Force may cause a change in the shape of an object.

• The force acting perpendecularly on a unit area of a surface is called pressure.

Reflections on concepts1. What is a force? What changes can be produced by a Force? (AS

1)

2. Give two examples each for a contact force and a force at a distance. (AS1)

SCERT TELA

NGANA


3. Explain Gravitational Force by giving a suitable example. (AS1)

4. Draw and explain a free body diagram (FBD) to show all the forces acting on a car?(AS

5)

5. Why do tools meant for cutting always have sharp edges?? (AS1)

Application of concepts1. How can you differentiate between a contact force and a force at a distance? (AS

1)

2. Find the net forces from the following diagrams.

3. A man stand still on a level floor.What forces act on him? Draw a free body diagram(FBD) to show all forces acting on him? (AS

5)

4. The surface area of an object is 20 m2 and a force of 10 N is applied on it, then whatis the pressure? (AS

7)

5. How do you appreciate the role of friction in facilitating our various activities? (AS6)

Higher Order Thinking Questions

1. If you push a heavy box which is rest, you must exert some force to start its motion.However once the box is sliding you apply a lesser force to maintain that motion.Why? (AS

1)

2. How do you increase the pressure by keeping (AS1)

a) area unchanged b) force unchanged

3. Imagine that friction disappeared from the earth.What will happen? Explain. (AS2)

Multiple Choice Questions

1. Hoisting a flag is related to [ ]

a) push b) pull c) Push and pull both d) pressure

2. A person is pulling water from well. Which type of force it is [ ]

a) Muscular force b) Magnetic force

c) Friction force d) Electrostatic force

3. The force that a solid surface exerts on any object in the normal

direction is called [ ]

a) Muscular force b) Normal Forcec) Tension force d) Magnetic force

a) b) c) d)8N

10N12N 8N 8N

8N

6N

9N

8N

SCERT TELA

NGANA

18 Force

4. Let the forces F1 and F

2 act on the table in opposite directions,

F1>F

2 , the F net = [ ]

a) F1-F

2b) F

1+F

2c) 0 d) 2F

2-F

1

5. A situation for effect of force leads to a permanent change in shapeof object is [ ]a) Stretching Rubber band b) Squeezing spongec) Pressing the Spring d) Breaking glass

Suggested Experiments1. Conduct an experiment to find the limiting forces that can be borne by different

strings.2. Design and conduct experiment to test few ways how friction may be reduced.3. Conduct an experiment to determine the change in effect of force with an area of

contact.

Suggested Projects1. Collect pictures to illustrate contact forces, forces at a distance and prepare a report.2. Classify the actions in your daily life into

i. actions where we exert force which appears as a pushii. actions where we exert force which appears as a pulliii. actions which involve both push and pull

3. Observe the situations of electrostatic forces in your daily life and prepare a report.

SCERT TELA

NGANA

19Telangana Government Free Distribution 2019-20

FRICTIONChapter

2

However, the speed of the book graduallydecreases and after some time it stops.

Why does the book stop after coveringsome distance?

Is the book moving with uniform speed?

Why does the speed of the book changegradually?

You know that the book is in non-uniform motion with respect to thefloor. In the "Force" chapter we studiedthat non-uniform motion of a body takesplace only when a net force acts on it.

How many forces act on the book whenit is pushed on the floor?

Let us examine the forces acting on thebook. Two forces act on the book in thevertical direction as shown in fig-3.

We have learnt about the various typesof forces in the chapter 'Force'. We alsohave learnt about the 'force of friction'which plays an important role in daily life.Let us learn in detail about this force in thischapter.

Force of friction and its Types

Activity -1

Identifying forces acting on abody and effect of frictional force

Fig-1: Pushing the book

Gently push a book on a horizontal flooras shown in fig.1.


You may observe that the book acquiresa certain speed in the direction of push.

Direction of motion

Fig-2 : The book acquires a speed

SCERT TELA

NGANA

20 Friction

Fig-3: Forces acting on the book

They are(i) Weight of the book (W) or gravitationalforce acting vertically down.(ii) Normal force (N) or reaction force

applied by the floor vertically up.

As there is no change in motion of thebook along the vertical direction, the netforce acting on the book in the verticaldirection is zero.

That is, 0W N W N− = ⇒ =

In the horizontal direction, the speedof the book is changing continuously. Itsspeed is decreasing gradually in thehorizontal direction i.e., the book hasacceleration opposite to the direction ofmotion (which we call deceleration).

What are the forces acting on the bookin the horizontal direction?

What is the net force acting in thehorizontal direction?

When the speed of the body moving ina straight line changes continuously, we saythat the body has acquired an acceleration.

By close observation of this activity, we

can understand that the floor applies a force

on the book against its motion. Similarly the

book also applies the same amount of force

on the floor in the opposite direction. Here

it is clear that the floor is at rest. Hence the

net force acts in the direction of the force

applied by the floor on the book.

The force applied by the floor on the

book is called “frictional force” orfriction.

Aim: Understanding thenature of friction andthe concept of staticfriction.

Material required:Toy Trolley, small wooden block,inextensible string, weight hanger, weights,pulley and table.

Fig-4:The trolley accelerating towards left

Procedure: Take a small toy trolley

and keep a small wooden block on it as

shown in fig-4. Tie an inextensible string

to the trolley and pass it over a pulley. And

other end of the string is fixed to weighthanger. Take a small weight and keep it on

Lab Activity

SCERT TELA

NGANA


We can increase the trolley’sacceleration by increasing the weight onthe hanger. If we increase the accelerationof trolley gradually, at certain limitingacceleration or limiting weight, the blockcomes into motion in the reverse direction.This means that now there exists relativemotion between the surface of the trolleyand the block.

What happens when theexperiment is repeated by usingrock and iron blocks of the samemass and different masses?

Does the limiting weight change?If so, why?

Apply some grease to the bottomof the wooden block and keep it onthe trolley’s surface and do thesame experiment.

What happens to the limitingweight?

What should we do to increase thelimiting weight?

From the above activities we maydefine friction as follows,

The force which opposes the relative

motion of two surfaces of bodies in

contact, is called 'frictional force'.

In activity-1, the book moves withrespect to the floor. So, this friction iscalled sliding friction.

Sliding friction is the friction whichcomes into play when the surface of oneobject moves relative to the surface ofanother object.

weight hanger and observe the changes inmotions of block and trolley.

What happens to the position of theblock kept on the trolley?

Does it fall or move along with thetrolley?

What changes occur in the motion oftrolley and block?

You will notice that the trolley with theblock on it moves towards left with anacceleration. The block is at rest withrespect to the surface of the trolley, but itis in motion with respect to the surface ofthe table.

Now keep on increasing the weight onthe hanger. Observe the motions of bothtrolley and block.

The surface of the trolley tries to keepthe block at rest here with respect to itssurface.

Thus, the force of friction by thesurface of the trolley acts on the block inthe direction of motion. At the same timethe block also applies a force on the trolleyin opposite direction and tries to movetowards the right.

Fig-5: The direction of friction on the block.

Direction of motion

SCERT TELA

NGANA

22 Friction

accordingly and thus balances the

increased applied force.

From this experience we conclude thatstatic friction is a self adjusting force

Fig-7 : Pushing a heavy box with increasing

force.

But there is a limit to this static friction.As you keep on increasing the appliedforce, at some point the box starts moving.That is, when the applied force is more thanthe limit of the static friction, the bodystarts to move as shown in fig-8.

Fig-8: The heavy box starts moving

When the box is sliding on the floor,the fricition between them is called slidingfricition.

Think and Discuss

Does friction oppose motion orrelative motion of surfaces incontact? Disscuss.

What observations and experiments

In lab activity, the block is at restrelative to the surface of the trolley up to acertain limiting acceleration. The frictionexists at this stage is a static friction.

So, static friction is the friction which

comes into play when surfaces of the

objects are at rest relative to each other

even there is an external force acted upon.

In the above lab activity we observe thatthere exists two types of frictional force ata time. One is friction between surface ofthe table and trolley, and the other is the staticfriction that exists between the surface oftrolley and wooden block kept on it.

Activity-2

Observing the variation offriction

Push a heavy box which is kept on afloor with a small force to movehorizontally as shown in fig-6. The boxdoes not move because there is a frictionalforce which is equal and opposite to theapplied force on the box.

Now gradually increase the

applied force, the box still does not move,

because the frictional force also increases

Fig-6: Pushing a heavy box with small force

F

f

F

f

F

f (maximum)

SCERT TELA

NGANA


can you cite to show the existenceof friction?

When do we speak of slidingfriction?

Factors affecting friciton

Activity-3

Effect of roughness on frictionalforce

Fig-9: Motion of a Ball on an inclined plane.

Set up an inclined plane on the

horizontal floor. Use a wooden board as

inclined plane. Put a mark at any point “A”

on the inclined plane. Now let a glass

marble or ball move down from this point.

Note the distance covered by the glass

marble from the bottom of the inclined

plane to point where it comes to a stop.

Now, spread a cloth over the floor.

Make sure that there are no wrinkles in the

cloth. Try again with glass marble. Now

note down the distance.

What are your observations from these

experiments?

In which case is the distance covered

maximum?

In which case is the distance covered

minimum?

Why is the distance covered by the

pencil cell different on different

surfaces? Discuss the result.

If you do the above activity by replacing

the cloth with white marble surface or glass

surface, can you predict the distance

covered by the pencil cell?

You can conclude that smoothness /

roughness of the surfaces of both the floor

and the glass marble could affect the

distance travelled by it.

Though many surfaces look like

perfect planes, there exists many ups and

downs (irregularities of surface) on them.

Friction is caused by the irregularities

on the two surfaces which are in contact.

Irregularities on the two surfaces lock into

one another, when we attempt to move on

any surface. We have to apply a force to

overcome interlocking. On rough surfaces,

there exist a large number of irregularities

(ups and downs). Hence, the force of friction

is greater if a rough surface is involved.

SCERT TELA

NGANA

24 Friction

Activity-4

Effect of area of contact onfrictional force

Fig -10: Pulling a brick with spring balance

Tie a string around a brick and pull thebrick by using a spring balance as shown infig-10. We use spring balance as a deviceto measure the applied force.

In a spring balance the spring isstretched by the applied force. The changein the length of the spring is proportionalto the applied force. So the scale of thespring balance directly gives the appliedforce in Newtons and in some springbalances the force is given in kilogram-weight also.

Pull it to move the brick. Note downthe reading of spring balance when the brickjust begins to move.

How many forces do act on the brickin the horizontal direction?

Two forces act on the brick in thehorizontal direction as shown in fig-11 .

Fig-11: Horizontal forces on the brick

One is force of friction (f) and the otheris the force applied (F) by you. The appliedforce is equal to the maximum limit of thefrictional force at the instant when the brickjust begins to move. But they act in oppositdirection. You can note down its value byobserving the reading of the spring balance.In this way we can measure the maximumfrictional force offered by the surface.

Now turn the brick upright as shown infig-12 so that the contact area with thefloor becomes small. Repeat the sameexperiment and measure the friction usingthe spring balance.

Fig-12: Pulling same brick with another

orientation

How does the frictional force vary withthe change in the area of contact?

The frictional force appears to be thesame in both cases irrespective of area ofcontact of the surfaces. So, the frictionalforce is independent of the area of contact.

Activity-5

Effect of normal force onfriction

As in the activity-4, keep a brick on thehorizontal floor and pull it with the springbalance attached to it and measure thefrictional force.

F

fSCERT TELA

NGANA


Now put another brick over the bricktied to the spring balance or press itvertically with your hand and then measurethe force of friction as described above.

Is there any difference betweenfrictional forces in two cases? If yes,why?

From the above activity you canunderstand that when we add a brick to theexisting brick or apply a force by pressingit vertically, the normal force increases andhence, we find there is an increase in thefrictional force.

So, Friction is proportional to theNormal force i.e.,

Friction ∝ Normal Force

where ∝ is the symbol representing“proportional to”

Think and Discuss

Does friction act on a table resting onthe floor?

If normal force is doubled, whathappens to friction? Discuss.

Your friend says, “Friction depends onthe area of contact”. How do youcorrect your friend through someexperiments?

“Friction is independent of weight, butdepends on normal force betweensurfaces of contact where frictionexists”. Do you agree with thisstatement? Discuss.

Is friction necessary?Try to walk on muddy or slippery

surface. Why do you find it difficult to walkon slippery surfaces?

We cannot walk or run without friction.Let us see the things that will not happen inthe world if friction was not present. Wewill not have any cars, bicycles or scooters.All of them move only because of friction.

Even if somebody pushes a car, we willnot be able to stop it by applying brakes.Carpenters will not be able to smoothensurfaces. You will not be able to hold anyappliances such as hammer, soap etc.

It will not be possible to write with penor pencil if there is no friction. You wouldnot be able to fix a nail on the wall. No buildingcould be constructed if there were no friction.

All the above examples tells theimportance of friction.

On the other hand friction is undesirabletoo in the machinery. For instance, frictionis responsible for overheating and wearingout of moving parts. You need to apply oil orgrease to your bicycle parts in order to makeit move smoothly.

Make a list of few more examples ofthe situations where we need to reducefriction for efficient functioning of tools.

Activity-6

Friction produces heat

Fig-13: Rubbing the hands

SCERT TELA

NGANA

26 Friction

Increasing and decreasingfriction

Activity-7

How to reduce friction ?

Take a spoon and hold its head (broaderportion) in the left hand and hold the midportion of the spoon by the right hand andpull it towards the other end of the spoon.

What do you notice?

Now dip your right hand fingers inwater, do it again as said above.

In which case it is easy to pull? Why?

Repeat the activity with other liquidssuch as coconut oil, grease etc and observethe difference.

Friction can be advantageous in somecases and disadvantageous in other cases.In the former case of activity 7 you willfind that the friction is more, in the lattercase, friction is reduced.

Let us try some examples.

Fig-15: Bottom of the shoe

Have you ever thought why the sole ofyour shoes is grooved as shown infig-15?

Fig-14: Striking a matchstick againstthe surface of matchbox

Rub your palms against each otherfor a few minutes. How do you feel?

Strike a match stick against the roughsurface of match box. What happens?

In both the activities we observebecause of the friction, temperature of thesurfaces increases. Matchstick catches firebecause of increase in temperature byfriction.

Thus, we can conclude that friction canproduce heat.

Give some more examples wherefriction produces heat.

You have probably heard that space craftreturning to the earth has to be protectedby a heat shield covering it. Find out why?What is the material used as the heatshield?

Think and Discuss

What important role does friction

play in the life of human being andanimals?

Why is friction important fortransport?

SCERT TELA

NGANA


It is done to provide the shoes bettergrip on the floor, so that you can movesafely. Similarly, the tires of cars, trucksand bulldozers are threaded (fig-16). Why?

Why do you need to change the tyreswhen it’s threading is worn out?

Fig-16: Pattern of tyre

Gymnasts apply some coarse substanceon their hands to increase friction forbetter grip.

In some situations, however, friction isundesirable and we would like to minimizeit.

Fig-17: The carom board

Play carom board without powder andthen play with fine powder sprinkled on theboard.

In which case is the movement of thestriker and the coins easy? Why?

Why do we pour a few drops of oil onthe hinges of a door?

Why do we use grease between themoving parts of motor vehicles?

In all the above cases, we want to reducefriction in order to increase efficiency.

When oil, grease or any other lubricantsare applied between the moving parts of amachine, a thin layer is formed between themoving surfaces and hence they do notdirectly rub against each other. Interlockingof irregularities is avoided to a great extentby the application of lubricants. Hencemovement becomes smooth.

The substances which reduce frictionare called “Lubricants”

Activity -8

Effect of rollers on friction

Fig-18: Pulling suitcase with rollers

Pull a suitcase without rollers and pulla suit case which has rollers.

In which case is pulling easy? Why?

SCERT TELA

NGANA

28 Friction

F F

Fig-19: Pushing a book on pencils

Try to push a book lying on the table.Now place the book on two to three pencilsor pens without caps. Push the book again.

What do you notice? Why?

In which case is it easy to pull the book?Why?

It is always easier to roll a body than toslide it over a surface. So it is convenientto pull the luggage fitted with rollers.

When one body rolls over the surfaceof another body, the friction offered iscalled rolling friction.

Activity-9Understanding the principle ofball bearings

Fig-20: Rotating the lids

Take two lids and rotate them by puttingone on the top of the other. What do youobserve? Now place four to five marbleson one lid and place the other lid on thetop of the marbles. Then try to rotate thetop lid. What do you observe?

This is the principle of ball bearings.

To reduce friction between rotatingshafts of machine tools, we use “ballbearings”.

Think and Discuss

Can we reduce friction to zero?Explain.

What purposes are served bybearings in machines? Explain withdaily life situations.

Fluid friction

Activity -10

Observing fluid friction

Take a glass of water and stir it with aspoon. You know that water whirls aroundan axis. Stop stirring and see what happens.You may notice that whirling speed of watergradually decreases and after some time,the water stops whirling and comes to astable state.

Which force is responsible forstopping the rotation of the water?The frictional forces between the liquid

layers and between the liquid surfaces thatis in contact with glass surface areresponsible for stopping the rotation of thewater.

Similarly water and other liquids exertforce of friction when objects move

Fig-21: Stirring water

SCERT TELA

NGANA


Key words

depends on its speed with respect to fluid,on the shape of the object and on the natureof the fluid.

Fig-22: Bird and Aeroplane

It is obvious that when objects movethrough fluids, they have to overcomefriction acting on them. So efforts are madeto minimize friction. Objects are givenspecial shapes. Where do you thinkscientists get hints for these specialshapes? From nature, of course.

Birds and fishes have to move about influids all the time. Their bodies must haveevolved to shapes which would make themnot to lose much energy in overcomingfriction.

Do you find any similarity in the shapeof an aeroplane and a bird? In fact allvehicles are designed to have shapes whichreduce fluid friction.

Friction, Static Friction, Sliding friction, Lubricants, Rolling friction, Ball bearings,

Drag, Fluid Friction.

Friction opposes the relative motion between two surfaces in contact. It acts on boththe surfaces.

Static friction comes into play when we try to move an object at rest relative to anothersurface or object.

through them. You can observe fluidfriction when you travel in a boat.

Not only liquids, gasses and air alsooffer friction to bodies like aeroplanes,jets when they move through air.

In science the common name for gasesand liquids is 'fluids'. So, we can say thatfluids exert force of friction on objects inmotion through them.

The frictional force exerted by fluidsis also called "drag".

Activity-11

Identifying factors influencingthe fluid friction

Take a tub of water. Try to move yourhand in water in the direction of stretchedfingers (up and down).Now try to moveyour hand in the direction perpendicular tothe plane of the hand (to and fro).

In which orientation of your hand, doyou experience more drag? Why?

Frictional force on an object in a fluid

What we have learnt

SCERT TELA

NGANA

30 Friction

Sliding friction comes into play when an object is sliding over another.

Friction depends on the nature of surface and the normal force with which the bodypresses the other surface.

Friction is independent of area of contact.

Friction can be reduced by using lubricants and ball bearings in many machines.

When bodies move through fluids, fluids exert a frictional force called drag.

Reflections on concepts1. What kinds of friction do you know and explain with suitable example? (AS

1)

2. A book placed on the surface of a table.The book is pushed in one direction.Draw theforces acting on the book and explain . (AS

5)

3. Give few examples for Sliding friction. (AS1)

4. Explain how does lubrication reduces friction? (AS1)

5. Explain with one example that Frictional force is proportional to the Normal force?(AS

1)

Application of concepts

1. Explain why do sportsmen use shoes with spikes?(AS1)

2. Would it be easier or difficult for you to walk on a marble floor with soapy water.Why? (AS

1)

3. What happens if we do not reduce friction in machines? (AS2)

4. What purposes are surved by using bearings in machines? Explain with daily lifeexamples. (AS

7)

5. Draw a free body diagram (FBD) to show various forces acting on a body which issliding on an inclined plane. (AS

5)


1. Do you agree with the statement. “ friction is both good and evil.” Explain withexamples. (AS

1)

2. Reducing friction to the lowest possible level in machine tools solves the problemof energy crisis and conserve biodiversity. How do you support the statement?Explain? (AS

7)

3. Scientists designed the shape of aeroplane using the technique of reducing fluidfriction by observing birds shape. How would you appreciate. (AS

6)


SCERT TELA

NGANA



1. Which material do gymnasts apply on their hands to increase friction

for better grip [ ]

a) Oils b) Soap c) Coarse substance d) water

2. The relation between the change the length of spring and the applied force [ ]

a) Inversely proportional b) Equal

c) Directly proportional d) Not depends on the force

3. When the speed of the body moving in a straight line changes

continuously then the body acquires [ ]

a) Acceleration b) Speed c) Velocity d) Displacement

4. The bodies of birds and fishes must have evolved to shapes which would make

them not to loose much energy in overcoming [ ]

a) Rolling friction b) Static friction

c) Sliding friction d) Fluid friction

5. Which of the following friction has self adjusting force [ ]

a) Sliding friction b) Static friction

c) Rolling friction d) Fluid friction

Suggested Experiments

1. Conduct an experiment to understand the nature of friction and the concept of Staticfriction.

2. Conduct an experiment to findout the effect of roughness on frictional force.

Suggested Projects

1. Collect information about various new techniques being adopted by human beings toreduce energy losses due to friction.Prepare a report on it.

2. Collect information about the substances using to reduce the friction in differentmachines and prepare a report on it.

SCERT TELA

NGANA

32 Synthetic Fibres and Plastics

In previous classes, you werefamiliar with different types of fibres.

You know that clothes can be made fromnatural fibres such as cotton, silk and wool(figure 1). You also learnt how they areprepared.

Clothes made of natural fibres:

We wear a wide variety of fabrics inour daily life.

Are all of our clothes made ofnatural fibres?

Fig-1: Clothes from Natural Resources

You might have heard about fabrics suchas polyester, nylon and acrylic etc. They areexamples of synthetic fibres.

Why do we call them synthetic?

Synthetic fibres are not obtained fromplant or animal source. They are made bythe raw materials obtained from petroleum.Synthetic fibres are used not only forfabrics but also in preparing many household articles.

Chapter

3SYNTHETIC FIBRES AND

PLASTICS

Activity-1

Identify household articles made up of natural and synthetic fibres.Identify the articles around you in your surroundings and write their names against relevantcategory in table-1.

Table -1

Source ArticlesNatural fibres from plants cotton saree, ......Natural fibres from animals silk clothes,.....Synthetic fibres ..........................

SCERT TELA

NGANA


What do you observe from the listmade by you? You notice that the list ofhousehold articles made of synthetic fibresis very long. It encompasses all our day today activities.

What do you know about syntheticfibres? How are they manufactured? Letus discuss synthetic fibres in detail.

What is synthetic fibre?Have you seen a bead necklace? Do

you observe any pattern in arrangement ofbeads ? see (Figure 2 a) Can you describethe pattern?

Activity-2

Beads and paper clips patternTake a few paper clips (Figure 2b)

and join them together as shown in figure2(c). Observe the pattern of clips. Do youfind any similarity between the pattern ofbeads in necklace and the pattern of paperclips in the chain?

Each unit of paper clip or bead canbe joined together to form a long chain likestructure. Each bead (or clip) is a separate

unit but when many such units are joinedtogether, they form a new, differentstructure.

Similarly, synthetic fibres are made ofchain like structure which are themselvesformed by joining small units together.Each small unit in the chain is a is knownas monomer units. Many small, identicalunits monomers combine to form a largeunit called polymer. Synthetic fibres aremade of polymers.

Unlike natural fibres, synthetic fibresare made from petroleum based chemicalsor petro chemicals. Petrochemicals aresubjected to various chemical processes toobtain synthetic fibres. Due to this, theyare called Synthetic fibres or artificialfibres.

Do you know?What does Polymer mean?

‘Poly’ means many and ‘mer’ means part/unit. The word is derived from Greek. Sowe can say polymer is a structure madeup of many small repeating units.

Think and Discuss

What made the human beings tosearch for the alternative fornatural fibres?

Which fibre source is notexhaustible? Why?

Fig-2(a):bead necklace

Fig-2(b):single paper clip

Fig-2(c):paper clips chain

SCERT TELA

NGANA


a days Nylon replaced silk in most hosieryarticles such as stockings.

How is nylon made?Nylon is a polymer made of chemical

units called polyamides. Polyamides aremade with monomers, hexamethylenediamine and adipic acid. Solid chips ofthese polyamides are melted and forcedthrough a heated spinneret which has very,very tiny holes.

Have you ever heard a crackling soundwhen you take off certain types of clothes?Or did you see sparks in them when it isdark? This is due to static electricity. Learnfrom your teacher about it.

The size and shape of the holes change,the characteristics of the resulting fibre.The fibre solidifies as it cools and can bespun or woven.

Nylon fibre is strong, elastic and lightin weight. Clothes made of Nylon arelustrous and easy to wash. So it has becomevery popular. Nylon does not absorb water- this fact explains many of its uses. Butstatic electricity is easily created in nylonfabrics. It also easily catches fire. So weshould not wear nylon clothes whilecooking, welding, working near a fire orusing heavy machineries. Why do we wearapron during cooking or working near thefire?

Many articles that we use in our dailylife are made up of nylon.

Can you name a few things made ofnylon? Tooth brush bristles, ropes, fishingnets, tents, sarees, stockings and socks, carseat belts, sleeping bags, curtains etc, aremade of nylon. Can you add some more tothis list?

Some synthetic fibresHow do you find whether a fabric is

synthetic or not ? Take a piece of cloth andpull out a thread. Unravel this thread (yarn).How long is the fibre? Is it lustrous or not?Do these characterstics help in identifyinga fibre?

You can’t identify the fibres by simplevisual inspection alone in the absence ofbrand label. Then how to identify them? Letus see.

Activity–3Identifying fibres – burning test

You can do a fibre burning test. Unravelseveral warp and weft yarns. Using tweezershold the yarn to the edge of a flame.Observe the changes.

If it smells like burning hair, theyarn is wool or silk.

If it smells like burning paper, theyarn may be cotton, or rayon.

If the yarn melts in the flame, it is asynthetic fibre such as nylon and acrylic.

You had learnt in the previous classeshow natural fibres are obtained and fabricsare made from them. Do you know howvarious synthetic fibres are obtained? Letus learn about it.

Think and Discuss

How synthetic fibres evolved to thepresent position?

NylonCan you name few synthetic fabrics that

you wear? Have you ever heard of nylon?

Nylon is a synthetic fibre. It is preparedfrom coal, water and air. It was the first fullyprocessed synthetic fibre. It becamepopular during the Second World War now

SCERT TELA

NGANA


Fig-3: Articles made of nylon

Nylon has replaced wool as the fibreused in making carpets. Nylon is also usedin making parachutes. Rock climbers usenylon ropes to climb mountains. It is usedin making of swim suits, sheer hosiery,sails, umbrella cloth, dress materials, cartyres, etc,

Fig-5: An iron stand with a thread hanging from the clamp

Fig-4: Use of Nylon Fibres

Why is nylon is prefered to make agood parachute or a climbing rope? Let usfind out!

Table 2Type of Total weight

S.No. thread/ in gr / kgfibre required to

break the thread1 Cotton2 Wool3 Silk4 Nylon

nylon. Fill the data in table 2. (Precaution:Note that all the threads should be of thesame length and almost of the samethickness.)

Arrange the threads in order of theirincreasing strength. What do you observefrom the above activity?

Do you know that if you compare anylon thread with a steel wire of samethickness, nylon will be as strong as steelwire?

Activity -4

How strong is nylon?Take an iron stand with a clamp. Take

cotton, wool, nylon and silk threads about50cm in length. Tie cotton thread to standso that it hangs freely from it. At the freeend, attach a pan so that a weight can beplaced on it (Figure 5). Add weight startingfrom 10 grams one by one, till the threadbreaks down. Note down the total weightrequired to break the thread. Repeat thesame activity with threads of wool, silk and

SCERT TELA

NGANA


synthetic fibres. But the source materialfor rayon is wood pulp. It is the onlysynthetic fibre obtained from plant’scellulose and so it is called cellulose fibre.

If rayon is obtained from plantcellulose then why don’t we call it as anatural fibre? Let us find out the processof preparing rayon to decide whether rayonis natural or artificial.

The cellulose that was collected fromwood or bamboo pulp, is treated withseveral chemicals. First sodium hydroxideis added and then carbon disulphide to thecellulose. The cellulose dissolves inchemicals, added to it and gives a syrupcalled viscose. Viscose is forced througha Spinneret (Metal plates with very, verytiny holes) into a solution of dilute sulfuricacid. This gives us silk like threads. Thethreads are cleaned with soap and dried.This new fibre is called rayon. What do yousay now is rayon natural fibre or artificial?Some kinds of rayons are made from theshort cotton fibres left on cotton seedsafter ginning.

Rayon is cheaper than silk and can bewoven like silk fibre. It can be dyed in awide variety of colours.

Is rayon used only for clothes? Whatare the other things that are made fromrayon fabric?

Rayon is mixed with cotton to make bedsheets. Rayon is mixed with wool inmaking of carpets. Rayon is often used infashion and home furnishings. Rayon is alsofound in sanitary products, diapers andbandages and lints for dressing wounds(Fig-6).

Think and discuss

If we use cotton cloth and cottonropes in preparing a parachute, whatwill happen?

Traditionally fishermen used cottonnets. Now they are using nylon nets.What is the advantage of usingnylon nets?

Nylon sarees are better than thecotton sarees. It is better to useonly nylon sarees. Do you agreewith this ? Why?

RayonWe know about natural silk which is

obtained from silkworms. Fabrics made ofsilk are very costly. It is not even accessibleto all. Production and maintenance of silkclothes are also very difficult and timetaking process. But it’s texture fascinateseveryone.

For many years scientists attempted tomake silk artificially and their efforts weresuccessful towards the end of the nineteenthcentury. The first commercial production ofartificial silk was achived in USA in 1911.But this fibre was named as rayon only in1924. The first rayon factory in India wasestablished in Kerala in 1946.

Where do we find rayon factories in ourstate? Discuss with your teacher.

Let us now discuss how rayon isprepared.

How rayon is prepared?We noted that petrochemicals are the

source material for many polymers and

SCERT TELA

NGANA


wool. Even Nylon is mixed with polyester.Why do we combine fibres? What is thisprocess called?

Any synthetic fibre can be combinedwith two or more other fibres. This is calledblending. Natural and synthetic fibres areoften blended for preparing better fabric.

A blend does not simply meanalternating threads of cotton and polyester.Unique and different textures can becreated through blending. When a fibre iscombined with other fibre, certain qualitiesof first fibre are combined with thequalities of other fibers and give us ablended fabric which possess the bestqualities of both. Blending helps us toreduce the limitations of both fibres. Forexample, if you consider cotton andpolyester blend, cotton is comfortable towear but it forms wrinkles. But polyesteris wrinkle free. Therefore when these twoare combined, the end result is acomfortable and wrinkle free fabric. Cotton

Rayon is not a perfect fibre to prepareall fabrics because it is made up from plantcellulose, it absorbs water easily.Absorbing water makes rayon weak andcauses the fibre to break easily.

Think and Discuss

What characteristics make rayonbetter than natural silk?

If you want to purchase a door matmade of synthetic fibre, whichsynthetic fibre door mat will youselect? Why?

If sanitary diapers and bandages aremade of nylon, what will happen?

Activity-5

Why do we combine fibres?Whenever you buy clothes or

readymade garments observe the brandlabel. What do you notice? Or, visit a nearbygarment shop and look at the labels on thecloth (Figure 7).

Record the percentage of differentfibres mentioned on the labels.

You may find rayon mixed with wooland cotton, polyester mixed with cotton and

Fig-7:Labels showing different percentages of blend

Fig-6: Articles made from rayon.

SCERT TELA

NGANA


shrinks in wash while polyester generallydoes not. So when these two are combinedin a proper ratio, the resultant material doesnot shrink to a large extent when comparedwith a pure cotton fabric.

The more the percentage of naturalfibre in blending of fibres, the more wouldbe the comfort to the skin.

When natural fibres contribute to a fabric,it allows the skin to breathe easily. Alsonatural fibres are generally free fromirritating chemicals.

AcrylicWhat type of clothes do you wear in

winter? All of us wear clothes whichkeep our body warm. We use sweaters,shawls or blankets. People feel that allthese are made up of natural wool. Woolis very expensive and generally is notaffordable and accessible to everyone.Think! How many sheep would be neededto obtain the wool to prepare sweatersfor everyone in India?

All the winter wear nowadays is madeof a synthetic fibre called acrylic. Itbecame commercially available in 1941.It looks like natural wool. It can beconsidered as artificial wool. It isgenerally called ‘fake fur’.

It is made from a combination of coal,air, water, oil and limestone. It is spun byeither dry spinning, or wet spinning. In dryspinning the dissolved polymers areextruded into warm air. The fibres solidifyby evaporation. In wet spinning, the

polymers are dissolved and extruded into abath and then driedout. The wool obtainedfrom natural sources is quite expensive,whereas clothes made from acrylic arerelatively cheap.

Can you list out the things made up ofacrylic? It is used in knitted apparels suchas fleece, socks, sportswear and sweaters.It is also used in craft yarns, upholsteryfabric, carpets, luggage awnings and vehiclecovers. Do you have any of them in yourhome?

(Here, we just introduced a new word

‘polyester’. You will learn about this in

detail in later part of this chapter).

Think and Discuss

Which type of blended fabrics doyou find more comfortable inwinter? Why?

The fabrics namely natural, syntheticand blended are available forgarments. Which fabrics will youprefer to wear for rare occasions likefunctions and in routine? Why?

Why synthetic fibres?What kind of umbrella would you use

on a rainy day? Would you use an umbrellamade of cotton or wool?

We have learnt about three differentsynthetic fibres. How are they differentfrom natural fibres? Synthetic fibres absorbless water and dry at a faster rate. Some areeven water proof. Most of them possess

SCERT TELA

NGANA


several unique characteristics. They aredurable, less expensive, readily available,affordable and are easy for maintenance.

In activity 3, when you burn syntheticfibre you find that their behaviour isdifferent from that of natural fibres.Synthetic fibres melt on heating. This is thedisadvantage of synthetic fibres. If thesynthetic fabric catches fire, it can bedisastrous. The fabric melts and sticks tothe body of the person who wears it. So weshouldn’t wear the synthetic clothes whileworking in the kitchen or in a laboratory.Washing and ironing also different forsynthetic clothes. Have you ever observedthe labels given in the figure-8 on thecollars of your shirts or inside the frocksand trousers? What does it indicate? collectsome labels and discuss with your teacherabout the symbols they represent.

Think and Discuss

Which fabrics do you prefer?Natural or synthetic? Why?Discuss comparatively.

What is the difference betweenwashing of clothes at home andwashing by dry cleaning at laundry?

PolyestersYou might have heard about polyester

clothes. You might have also heard aboutterylene. What type of fabrics are they?

Polyester is the most commonlyused synthetic fibre. Polyester fibresrevolutionalized the fabric industry andchanged the costume culture. Fabric madefrom polyester fibre doesn’t get wrinkledeasily. It remains crisp and is easilywashable. So it is quite suitable for makingdress material. Terylene is a popularpolyester. It can be drawn into very finefibres. That can be woven like any otherfibres. This fibre blends well with naturalfibres. Terylene is often mixed with cottonto make terricot and with wool to giveterriwool. Like nylon, pure polyester orterylene easily catches fire.

Polyester is made by reacting,terephthalic acid to di-methyl ether, thenwith dihydric alcohol. Polyester can bemelted and spun. This property allows thefibre to convert into different shapes andsizes. Polyester fibres of these days areultra thin microfibers which gives them asmoother, softer feel than the polyester oftwenty years ago. This base material can beused to make not just fibres for fabric butmany other things: from soda bottles toboats.

Have you seen or heard of PETbottles? PET is a very familiar form ofpolyester. It is used for making bottles,utensils, films, wires and many other usefulthings.

Fig-8: Laundry label codes

SCERT TELA

NGANA


Activity-6

How can you say a bottle is PETbottle?

Collect different kinds of waterbottles of your classmates and look at themcarefully. Do you observe any triangleshaped symbol at the bottom of the bottleor on the brand label sticker? What numberis marked in the center of the triangle?Look at figure 9. You will find that manyof the bottles will have 1 in the center ofthe triangle. If it is 1, then it is a PET bottle.

Code Numbers:

1. Polyethylene Terephthalate (PET,PETE)

2. High Density Polyethylene (HDPE)3. Vinyl (Polyvinyl Chloride or PVC)4. Low Density Polyethylene (LDPE)5. Polypropylene (PP)6. Polystyrene (PS)7. Other (The category of “Other” includesany resin not specifically numbered 1, 2,3, 4, 5, or 6, or combinations of one ormore of these resins.)

Fig-9 : Resin identification codes

Activity -7

Identification of various articleswith recycling codes

Collect bottles of soft drinks (500 ml ormore), bottles of juice, and containers of fruitjam, Ketchup, shampoo, coffee powder and

try to look for the triangle. What do younotice? What are these codes? What is theuse of these codes? Are all bottles found withcodes?

First look at the soft drinks and juices.You may observe that irrespective of thebrand name, the number 1 is marked in themiddle of the triangle. It indicates that it isa PET bottle. What about other bottles?

There are other numbers as shownin figure-9.

Where do you find these codes?

Explore from various sources and tryto know more about coded articles.

Do you know?Why are soft drinks stored in these

PET bottles? Soft drinks are carbonatedso they should be stored in containerswith which they will not react.

Plastics around us

Observe various articles around you, inyour house, kitchen, rooms, andbathrooms. What is the most commonmaterial used in making these objects? Milkand oil pouches, containers to store picklesand rice, buckets to store water, chairs,water pipes, electric appliances, television,radio and computers, mobile phones–everything seems to be made of plastic.

Talk to the elders in your family aboutthe materials they used in the past.Particularly, what do they think about buying

SCERT TELA

NGANA


Fig-11(a): Linear arrangement ofmonomers

Fig-11(b): Cross-linked arrangement

water in polythene pouches? What did theyuse to bring milk, oil, other liquids fromshop to home in the past? What were thecontainers, buckets, mugs, chairs and tablesused in the past made of? What do we useto make these articles?

Plastic has taken over the place thatoccupied by metals and wood earlier.Plastics have also replaced glass items. Ifwe continue to write the list, it will beendless. Plastics completely occupied ourlife because of their characteristicproperties.

Fig-10: Articles made of plastics

What is plastic?

Plastic is also a polymer like manysynthetic fibers we have studied so far. Butthe monomers in plastic can be arranged intwo ways. Some are in linear chains (Figure11 a) and others cross linked (Figure 11b).

Plastics are available in differentshapes and sizes. They have a wide varietyof uses. We also observed that they havedifferent code numbers as mentioned inactivity-6. Does the arrangement ofmonomer units in plastics has any relationfor this diversity in plastics?

Do you know?

Alexander Parkes(1813-1890)- Creatorof first Plastic -“Parkesine”

One of the manytriumphs of modern

science which is completelyencompassed the day to day life acrossthe world is the invention of plastics.The synthetically produced materialwas first presented in 1862 in Londonby Alexander Parkes. To prepare thismaterial Parkes heated nitrated cottonwhich previously soaked in sulfuric acidand made fabric soft and elastic with oiland camphor. The end product was anivory – colored material that becamedistorted when subjected to heat. Thisis named as Parkesine. Based on theseinitial efforts plastic evolved as mainbase for many modern materials. Butat that time people were not interestedon this alternative to natural substances.Now it is the basis for most of themodern materials.

SCERT TELA

NGANA


Activity-8

Types of PlasticsLet’s take two bottles made up of plastic.

One is PP bottle and another ordinary. (PET)

Pour some hot water in both. What changes

do you notice ? Have you experienced such

instances in your daily life (Figure 12). See

the code of the bottle that gets deformed.

Plastics which get

deformed easily on

heating and can be bent

are known as

thermoplastics. Some

of the thermoplastics

are polythene and PVC.

These are used in

manufacturing toys,

combs and various

types of containers.

There are some

plastics which moulded once can’t be

softened by heating. They are called

thermosetting plastics. Bakelite and

melamine are examples for this type. Now

can you tell whether Tupperware is

thermosetting plastic or thermoplastic?

Thus we can conclude that different

types of plastics have different properties.

Plastics are easily mouldable, can be

converted into any shape, can be recycled,

reused, coloured, melted and rolled into

sheets and wires. Do all kinds of plasticsshow the above properties? Let us see.

Fig-12deformed bottle

Aim: Identifying thermoplastic andthermosetting plastics by flame test.

Materials required: Pair of Tongs,spirit lamp, samples of plastics (Collectsmall pieces of plastics from the objects likecomb, tooth brush handle, plastic bucket,handle of utensil, electric switch, piece ofmelamine of meals plate and coffee mug)

Procedure:Take a spirit lamp and light it.

Clamp one piece of plastic samplesay piece of tooth brush with tongs.

Place the sample on spirit lampflame. See fig. 13.

Observe the changes during theburning of sample.

Note your observations like,whether sample is being softened orburnt with smell or become hard etc.

Repeat the procedure with othersamples.

Record your observations samplewise in the following table - 3.

Note: While doing this activity, if needed,cover your nose and mouth with a mask toavoid breathing of fumes. And also keepyourself away from spirit lamp flame whenyou put sample on the flame by stretchingyour hand.

Lab Activity

Fig-13: Conducting flame test

SCERT TELA

NGANA


In the above activity, we have taken theknown samples and tested. If unknownsamples are given, how do you decide whetherit is thermoplastic or thermosetting plastic?

If you burn a piece of wood, paper,cloth, steel rod, do you notice similarobservations like plastic material? Howare they different from plastics?

Based on the observations can you writea note on the properties of thermoplasticsand thermosetting plastics?

Thermoplastics This is a plastic which will soften when

heated and harden when cooled. Athermoplastic is a polymer that turns intoa liquid when heated and freezes to a glassystate when cooled sufficiently.

Table-3Sl.no. Name of the Softened/burnt with Thermoplastic/

plastic sample burning smell and Thermosetting plasticbecome hard

1. Tooth brush handle2. Comb3. Piece of bucket4. Handle of utensil5. Electric Switch6. Meals plate7. Coffee mug

Do you know?Hermann Staudinger (1881-1965) :

Hermann Staudinger, aGerman Chemist , in 1920made an important stepforward in the processing ofsynthetic materials when hedeveloped thermoplastics which are stablein themselves. Hermann Staudinger wonNobel Prize in 1953 for demonstrating thatPolymers are long–chain molecules.

Thermosetting PlasticsThermosetting plastics are simply

plastics, when moulded into a shape andallowed to cool down, will remain inmoulded form and will not change theirshape. When heated again they will char orburn. Thermosetting plastics are syntheticmaterials which gain strength duringmoulding by heating, but cannot be remoulded or reheated after their initial heat,moulding. Thermosetting plastics are notre-mouldable. Strong cross links areformed during the initial moulding processthat gives the material a stable structure.They are more likely to be used in situationswhere thermal stability is required. Wheredo we use this type of plastics?

Observe the kitchenware in your houseor visit any home appliances shop. Whatdo you notice? Is all kitchenware totallymade up of metal? With what materials arethe handles of utensils made? Utensils aremade of metals like aluminum, copper orsteel. Then what is the additional material?Bakelite is used for making handles ofvarious utensils due to its poorconductivity of heat and electricity. It isalso used for making electrical appliancesincluding switch boards. Bakelite is used

SCERT TELA

NGANA


as an alternative for pearl and jade. You cansee the Bakelite made articles in figure 14.

The other thermosetting plastic,Melamine used for making kitchenware.Utensils and other items of kitchen aremade by melamine. It is also used formaking of floor and dress material for theirnature of fire resistance. Computer and TVcabinets are made of melamine. Now Listout the melamine products in your house.You can see few melamine articles infigure15.

Why do we prefer plastics?Plastics are non- reactive: You mighthave experienced that iron gates and nailsexposed to atmosphere, soon get rusted.But does this happen to plastic articles?Plastic material will not corrode easily.That’s why they are used to store variouskinds of materials including chemicals.

Plastic is very light, strong, durable andcan be moulded into different shapes andsizes. Plastics are generally cheaper thanmetals

Father of Plastic IndustryA Belgian scientist

named Dr. Leo HendrikBakeland wasresponsible for theinvention of Bakelite.In 1907 he wasworking as chemist, byaccident he discoveredthe compound of carbonic acid andformaldehyde. When he tried to reheatthe solidified compound, he discovered.It would not melt-no matter how highthe temperature was. Dr. Baekeland isconsidered as the father of the presentplastics industry.

Hot Pin TestTo test if a piece is Bakelite, get a

very very hot pin from an open flamesource, then touch the pin to the item. Ifit is Bakelite it will not penetrate. Itmay give off the acid smell and it mayleave a purple burn mark. If the pinpenetrates or melts the plastic, then itis not genuine Bakelite.

Do you know?

Fig-14: Articles made of Bakelite

Why do electrical wires have plasticcovering? You might have seen pans,pressure cookers and screw driver handles.Why are they made of plastic? Plastics arepoor conductors of heat and electricity.

Think and Discuss

Certain fry pans are said to be non-stick. What made them non-stick?Firemen wear dress which does notcatch fire. What type of fabric isit made of ?

Fig-15: Articles of Melamine

SCERT TELA

NGANA


Plastics and environmentYou must have seen garbage (waste)

dumps. Some articles there seem to remainfor a long time while some other disappear.You may notice that most of the materialthat does not disappear are the polythenebags. Polythene is a plastic. It is mainlyused for making carry bags.

The polythene bags thrown around areresponsible for clogging drains. Animals inurban areas, particularly cows, eatpolythene bags containing food material.Can you imagine the consequences? Seethe Fig-16 and read the comments ofSupreme Court of India about the effect ofplastic on environment.

With the direction of the Supreme Courtof India, very thin polythene bags have beenbanned to stop indiscriminate usage ofpolythene bags. What is the differencebetween the banned polythene bags and thebags using presently in place of previousones? What do you notice on that bags?

Some efforts are taking place to reducenegative consequences of plastics on theenvironment.

If we burn plastic, it creates a lot of airpollution.

“Plastic bags are more danger thanatom bomb for future generations”:Supreme Court of India.

“Excessive use of plastic bagsand their unregulated disposal hasbeen choking lakes, ponds and urbansewerage systems, the Supreme Courtsaid while warning that it posed athreat more serious than the atom bombfor the next generation .AndhraPradesh-based NGOs drawing thecourt’s attention to 30-60 kg of plasticbags recovered from the stomachs ofcows because of irresponsible disposalof plastic bags and defunct municipalwaste collection system. But the benchwanted to address the larger questionsarising from indiscriminate use ofplastic bags, which not only posed agrave threat to nature and environmentbut also to the human race itself. “Allof us are watching how our lakes, pondsand urban sewerage systems aregetting choked by plastic bags. Unlesswe examine a total ban on plastic bagsor put in place a system formanufacturers mandating them tocollect back all plastic bags, the nextgeneration will be threatened withsomething more serious than the atombomb”, Justices Singhvi andMukhopadhaya said. Large quantitiesof water packed in plastic pouches,which were thrown around inundisciplined and uncivil manneracross the country every day. “A roughestimate shows more than 100 millionwater pouches are thrown all over thecities and towns in a day” the benchsaid.

Fig-16: Plastic bags found in the stomach of acow after a postmortem

SCERT TELA

NGANA


Plastics take several years todecompose as compared to other materialslike peels of fruits, vegetables and wastefood stuff which decompose within a shortperiod. Slow decomposition causesenvironmental pollution. The burningprocess of synthetic material is also veryslow and can’t be burnt completely. Theprocess of burning, releases a lot ofpoisonous fumes into air causing airpollution. So it is better to avoid orminimize the use of plastics. Make a listof some occasions where you can avoid theplastics and use alternatives.

Reduce, Recycle, Reuse andRecover – 4R principle

Can we avoid using plastics altogether?The entire civilization is enveloped withthem. Every walk of life is linked withplastics. Now you can understand that youcannot avoid use of plastics completely butwe can reduce, recycle, and reuse plasticsand avoid indiscriminate use of plastics. Wecan also adopt recovery mechanisms toconvert waste into a usable resource. Letus discus the 4R principle for creating aneco friendly environment.

ReduceWhat do you observe when you attend

a marriage or function? What plates theyuse to serve food? By what type of glassesfor the drinking water is supplied? Whatcontainers do they use to serve sweets andice cream? What are the spoons made upof? Everything is made of plastic. Imaginehow much plastic garbage will beaccumulated in this single function. Isthere a way to stop this ‘Use and throw’

Activity-9

Biodegradable – Non-biodegradable

Let’s take peels of fruits andvegetables, left over food stuff, waste paper,cotton cloth and plastic bag. Keep thismaterial in a pit. Open the pit after somedays and list the material which remain along time and those that disappear quickly.

Table-4

Type of waste Approximate Change

time to disappear

Peels of fruitsand vegetables

Left over foodstuffWaste paperCotton clothPlastic bag

You observe that certain materials canbreak down into smaller fragments in thepresence of water, sunlight and oxygen.These fragments get further broken downby bacteria. This is called decomposition.A material which is easily decomposed bynatural process is called bio-degradable,and one which is not decomposed bynatural processes, is called non-biodegradable.

The time taken for decompositiondepends upon whether a material is bio-degradable or not. Explore various sourcesof material to know the time required forthem to decompose.SCERT T

ELANGANA


culture? Discuss and write the ways andmeans where we can reduce the usage ofplastic, to avoid its negative consequenceson environment. Land filling of plastics andburning of plastics in incinerators is theother way of disposal of plastics. They toohave negative consequences. Therefore, weshould reduce it’s usage whenever it ispossible.

Reuse Have you observed that when we go to

market to buy something we go without anybag? With the result that we come backwith lot of plastic bags. Every time we goto market, we accumulate lot of plasticbags. Some of these can be used again andagain. Are we doing so? Articles made ofplastics may be used again and again for itsoptimum utilization. This should be everybody’s concern for an eco friendlyenvironment. If anything you feel now outof fashion and you want to buy a new one,give it to others instead of putting it idle athome or throwing it out .Can you givecertain instances where we reuse plastics?

Recycle

You might have noticed your motherselling old plastic articles which are brokenand not useful, to the local vendor. Whatwill they do with that? They collect allplastics from the households and send itfor recycling. From this recycled plasticnew products are prepared, after giving it aproper treatment.

Are all types of plastics recyclable?

There are 60,000 types of plasticsavailable in the world as on today (thesynthetic fibers that we discussed are alsoplastics). There are only 6 from these60,000 that we use regularly. We have seenthat each of these six plastics is given acode. These codes help us in recycling.

PET (code 1) and HDPE ( code -2) arecommonly recycled. But LDPE (code – 4used for carrier bags) is not recycledcommonly. Similarly PVC (code-3) whichis used for pipes are currently not recycled.The PS (code -6), used for making coffeecups, egg boxes, packing peanuts and ‘takeout’ food packing can be recycled.Recycling can be used to obtain materialsfrom which the original products weremade.

Recycling codeIn activity-7 we raised certain questions

regarding recycling symbol code and itsuse. Now let us discuss them in this section.

The Society of the Plastics Industries,Inc. (SPI) introduced its voluntary resinidentification coding system in 1988 toassist recycling programs. The SPI codingsystem offered a way to identify and sortthe resin content of containers commonlyfound in the household waste stream.Plastic household containers are markedwith a number that indicates the type ofresin, or plastic as shown in fig-9.

To identify the plastic, look at therecycling icon, the chasing arrows. Insidethe arrows, there will be a number that

SCERT TELA

NGANA


Key words

identifies the polymer. When the numberis omitted as seen in the figure 17, thesymbol is known as the UniversalRecycling Symbol, indicating genericrecyclable materials.

What is role of codes in recyclingprocess?

Each plastic is manufactured withdifferent processes. During recyclingprocess, if same codes are not sorted outseparately the whole lot meant for recyclewill spoil the process. Therefore, it isessential to recycle the same codes in onelot. If you add a simple PET bottle in the lotof other plastics during recycling process,the entire lot will be spoiled. Think why ?

Uncoded plasticsDo all plastics have resin

indentification code on them? Plastic tarps,toys, computer key boards and many other

products simply do not fit in the codingsystem adopted for recycling. Wide varietyof plastic materials are made to suit theneeds of consumer by using differentvarieties of plastic resins or mixtures ofresins. Is it possible to code thousands ofplastic varieties? Recycling of varioustypes of plastics is not commercially viablebecause their production when comparedto coded plastics is less. Code 1 and 2plastics occupy major share in recyclingplastics.

RecoverSupreme Court gave a judgment on

ways and means of solid waste managementand gave order to implement this in all thecities of India by 2003. In the solid wastegarbage of municipalities, plastic occupiesmajor share. The principle of recover playsmajor role in this solid waste management.The solid waste should be converted intoresources such as electricity, heat,compost and fuel through thermal andbiological means. Is any such effort forsolid waste management taking place inyour village/town.? How do you appreciate4R principle?

Fig-17: Universal recycling symbol

Acrylic, synthetic fibre, Bakelite, biodegradable, blend, cellulose, incineratorsmelamine, natural fibres, non-biodegradable , nylon, petro chemicals, plastics, polymer,polyamide, polythene, polyester, rayon, recycling, spinneret, terricot, terylene,terriwool, thermoplastics, thermosetting plastics, universal recycling symbol.SCERT T

ELANGANA


Synthetic fibres are made of very large units called polymers.

Rayon is an artificial silk made up of cellulose fibre.

Nylon is made artificially by using raw material like coal, water and air

Polyester is a synthetic fibre.

Acrylic is artificial wool made from coal, air, water, oil and limestone.

Petro chemicals are used to manufacture synthetic fibres.

Commonly used synthetic fibres are rayon, nylon, polyester and acrylic.

The different types of fibres differ from one another in their strength, water absorbing

capacity, nature of burning, cost, and durability.

Synthetic fibres and plastics enveloped our life.

The waste created by plastics is not environment friendly.

Plastics take years to decompose.

Enjoy the good qualities of synthetic fibres and plastics and reduce the indiscriminate

use of plastics to minimize environmental hazards.

Synthetic fibres find uses ranging from house hold articles to healthcare.

Synthetic fibres blend with natural and artificial fibres.

Reflections on concepts

1. Why some fibres are called Synthetic?Explain. (AS1)

2. List out the objects made up of Acrylic . (AS1)

3. Draw and explain the diagram of Universal reclycling symbol. (AS5)

4. What are thermosetting plastics.Give two examples. (AS1)

5. Give reasons. “Why plastic containers are used as storing devices.” (AS1)


1. How synthetic fibres have changed our daily life?(AS1)

2. If we make electric switches with thermo plastics.What would happen?(AS2)

3. What could be the consequences? If plastics are not properly disposed. (AS7)

What we have learnt?


SCERT TELA

NGANA


4. Is there any such effort for solid waste management taking place in your village/

Town? How do you appreciate 4R principle? (AS6)

5. Where do we use the process of recycling? How is it useful? Give examples. (AS1)

6. Rani wants to buy clothes to her parents for winter wear.What types of clothes would

you suggest? Specify reasons.(AS7)


1. What made the human beings to search for the alternative for natural fibres? (AS2)

2. Imagine what would happen if we do not discover plastics? (AS2)

3. Thermo plastics are eco friendly than thermo setting plastics. Do you agree with

statement? Why?(AS1)

4. Introduction of synthetic fibres in the textiles industry brought revolutionary change

across the world in the dressing patterns irrespective of culture and customs.How

do you appreciate this. (AS6)

5. Indiscriminate usage of plastic is a serious threat to bio diversity.What are the efforts

of Government and Non government organizations in this regard. (AS7)


1. Rayon is prepared by [ ]

a) Coal b) Oxygen c) Fibre d) Cellulose

2. Necessity of labels on Clothes [ ]

a) Required by law b) To identify fabric content

c) Both A and B d) They do not decompose

3. The material which is not decomposed by natural process is called [ ]

a) Non bio-degradable material

b) Bio-degradable material

c) Polyster

d) Nylon

4. The symbol represents [ ]

a) PET b) HDPE c) LDPE d) Others

SCERT TELA

NGANA


5. Which is a Natural fibre among the following [ ]

a) Rayon b) Nylon c) Polyster d) Silk


1. Conduct a flame test to identify Thermo plastics and Thermosetting plastics.

2. Take a wool, silk, cotton thread, bandage, piece of umbrella cloth, thread of sweater,

piece of rope and carefully conduct a flame test. Based on smell and type of melting

Classify them as natural and artificial fibres.

Suggested Projects

1. Prepare a table of various synthetic fibres which are used to make household articles

from them.

2. Collect the figures made up of thermosetting, thermo plastics used in your daily life

and make a poster.

3. Prepare a chart which can explain recycling codes, full names and acronym of plastic

and its usage for various household articles, recycled or not, if recycled what will be

made from that.

SCERT TELA

NGANA

52 Metals and Non-Metals

In the previous classes, you have studiedabout some materials that are usedin our daily life.You studied about naturalfibres and their properties. You also studiedthe soil and its properties. You learnt acids,bases and salts. You also studied changesaround us like rusting etc. In this chapteryou learn about the properties of metals andnon-metals

You are familiar with a number ofmaterials like aluminium, copper, gold,iron, etc.

Can you name some objects made ofmetals?

Observe the fig-1. Try to name themetals from which the objects are made.Add names of more metals that you knowto the list.

Your first answer is gold. Some of youmay also have added aluminium, silver,lead, iron, copper, tin, mercury etc.

Did any of your friends add steel to thelist of metals?

Do you think that steel is a metal?

Let us learn the properties of metalsso that you are able to answer thesequestions at the end of the chapter. You alsolearn about another type of materials, callednon-metals, which may be new to you.

Now observe carefully all the materialsthat you have listed above as metals.

Do all these look alike?

Do all of them shine?

Are they hard or soft?

Do they break easily?

Can you group materials into twocategories by looking at theirproperties?

We try to find two groups, then discussand compare them in detail in this chapter.

Physical Properties

Before we start this section, you willneed to collect pieces of iron (iron nails),copper, zinc, sulphur powder, aluminium,carbon, magnesium and iodine for carryingout the activities.

Chapter

4METALS AND NON -METALS

Fig-1

SCERT TELA

NGANA


AppearanceIn previous classes, you learnt that

the materials which have a bright surfaceand reflect light are called lustrousmaterials and materials that do not shineare non-lustrous.

Activity-1

Observing appearance andcolour of some materials

Observe the appearance of yoursamples. Look at their colour. Decidewhether they appear shining or dull andrecord your observations in table - 1. If thesurface seems dirty, clean it with sandpaper.

Table–1

Your observations in the table showsthat some materials are shining and someare dull.

Which of the samples did not shine evenafter you cleaned them with sandpaper?

Generally metals are lustrous. Do alllustrous materials are metals?

We all know that mirror reflects light.

Can a mirror be called metal?

No, so you need to look at severalproperties to decide if a given material ismetal or not.

SonorityWhile Aryan was carrying a geometry

box he slipped and fell down. He noticedthat the box made a ringing sound when ithit the hard floor. It was similar to thesound that of ringing bell.

Have you observed material used tomake school bell or bells in temple?

Why are wooden bells not used inschools?

Do all materials produce sound whenthey dropped on hard surface?

Let us find

Activity-2

Listening the sound produced bysome material

Fig-2

Drop a piece of coal on the floor andlisten to the sound.

Do you think coal is sonorous?

Take the pieces of zinc, copper,aluminium, magnesium and tightly packedpackets of sulphur, carbon and iodine. Dropthem one by one, on a hard surface. Listencarefully to the sound produced and recordyour observations in table-2.

IronZincCopperSulphurAluminiumCarbonMagnesiumIodine

SampleAppearance

Shining/not shining Colour

SCERT TELA

NGANA


Table-2

What similarity do you notice amongmaterials which produce sound?

You may notice that some of thematerials produce sound and some of themdo not. Materials which produce ringingsound are called sonorous materials.Generally, most of the metals are sonorous.The materials other than metals are notsonorous.

Lustre and sonority are the propertiesassociated with the metals. But there is noneed that all metals should posses thisproperty. For example, though mercury isa metal, it is not sonorous.

Which property of metals firstattracted the attention of human beings?

The story of early tools will give you a hint.

Story of Early Tools

Do you think tools were always made of metals? Earlyhuman beings made their tools from which were easilyavailable - stone and wood. Later, they used the bones ofanimals.

Then they discovered metals like copper and iron.Tools made of copper and iron are much stronger thantools made of stone and wood. Metals had the advantageof not just being harder but they can be heated in a fireand moulded or cast into different shapes. So it becamepossible to make a wider range of tools with such metals.

Fig-3 : hammers made ofdifferent materials being used

to hammer nail

Material Sample thatProduce sound

Material Sample thatdo not Produce sound

MalleabilityHave you ever noticed the thin silver

foil used to decorate sweets or the thinaluminium foil used for packing food?

Try to observe a blacksmith at work. Hebeats a hot iron piece repeatedly till itsshape changes.

Do you bring a similar change in theshape of a clay material by beating it? Not all materials can be converted into

thin sheets to make the desirable objects.

Activity-3Identifying malleability ofmaterial

Take a hammer and beat the materialsamples which are used in Activity-2 andobserve the changes in material samples.Record your observations in the table-3.

Observing thechange

Name of sample

Flattens Iron, …..Breaks/ convertsinto powder

No change

Table-3

SCERT TELA

NGANA


Some of the samples, when beatenhard, were flattened whereas somematerials broke into pieces or became apowder. The materials which can beflattened into thin sheets are calledmalleable materials. Malleability is one ofthe properties associated with metals.

What did you observe in the case ofiron? You may not be able to flatten it butthe blacksmith can do it. He heats it beforebeating. So we can say that materials differin the range of their malleability. Metalslike aluminium, silver and gold are highlymalleable.

DuctilityWe use wires in different situations in

our daily life. Look at the samples given inthe table-4.

Have you ever seen the wires made up

of materials mentioned in table-4.

Write yes, if you have seen wires made

of them.

Take help from your teacher, friends and

elders to decide which of the material can

be used to make wires.

From the above table you may infer that

some materials can be drawn into wires and

some materials cannot be drawn into wires.

Fig-5The property of drawing a material to

make fine wires is called ductility. Mostmetals are ductile.

Is ductility the only property ofmetal to use them as connecting wires inelectric circuits?

Let us explore another property ofmetals.

Electrical Conductivity

You might have seen an electricianusing the screwdriver.

What materials does it contain?

Why does not a screwdriver used byelectricians have metal handle?

Fig-4

IronZincCopperSulphurAluminiumCarbonMagnesiumIodine

Sample Can we convert itinto Wires (Yes/No)

Table-4

yes

SCERT TELA

NGANA


Activity-4

Identifying electricconductivity of a material

Arrange an electric circuit with abattery and bulb (remember the simpleelectric circuits from 7th classes). Closethe circuit using an iron nail, as shown infigure 6.

Observe whether the bulb glows or not.Record your observation in table-5.

Repeat the same experiment using the othersamples and record your observations in thesame table.

Table-5

Iron

Zinc

Copper

Sulphur

Aluminium

Carbon

Magnesium

Iodine

Sample Does the bulb glow?(Yes/No)

Think and discuss

Do all the samples allow the bulb toglow?

Materials that allow electricity to passthrough them and make the bulb to gloware called electric conductors. Most metalslike iron, copper and aluminium are goodconductors of electricity.

Talk to an electrician. Look at thehandles of his tools.

Are the handles made of the samematerial? If not, why?

Note the precautions to be taken whileworking with such tools.

The handles of both electricalappliances and cooking utensils are notmade of metals. Electrical appliancesconduct electricity.

What do cooking appliances conduct?

How will you close the circuit usingsulphur, carbon or iodine? They may bein powder form. Try to tightly pack thepowder in a straw and use it. Think ofother ways!

Activity-5Observing heat conduction bymetals

Take an iron rod. Stick pins on it withthe help of wax (see fig-7). Now fix therod to a stand as shown in the fig-7. Heatone end of the rod with a spirit lamp andsee how the pins fall off?

Why did the pins fall from iron rod?

Pin of which end fell first?

What could be the reason for this?

Fig-6

SCERT TELA

NGANA


You know that the pin fell because ofthe heat supplied to the iron rod makes thewax to melt at one end. The wax closer tothe flame melted first. This activity clearlyshows that heat moves from one end of theiron rod to the other. This property of amaterial is known as conductivity of heat.All metals conduct heat.

All materials do not have equalconductivity. Iron, copper and aluminiumcooking vessels are preferred due to theirhigh heat conductivity.

Do it!

On the basis of all the activities carriedout, fill in the table 6.

We find that it is the metal whichpossess most of the properties of lustre,malleability, ductility, sonority, conductionof heat and electricity. Non-metalsgenerally do not show these properties.

Chemical properties

The properties youhave studied so far are allphysical properties.Though these propertiesare quite reliable,chemical properties are

better indicators of determining as towhether a given material is metallic or not.Let us try to see what happens when metalsand non-metals react with other substances.

Reaction with Oxygen

Aim: To know the reaction of oxygen withmetals and non-metals

Material required: One metal sample(magnesium) and one non-metal sample(sulphur), spirit lamp or Bunsen burner andlitmus papers, etc.

Lab Activity

Table-6

Lustrous Sonorous Conductsheat

Conductselectricity ductileMalleableMaterial sample

Iron

Zinc

Copper

Sulphur

Aluminium

Carbon

Magnesium

Iodine

Fig-7

SCERT TELA

NGANA


Procedure: Take a small strip of magnesium and

note its appearance. Burn it. Note theappearance after burning.

Take a small quantity of powderedsulphur in a deflagrating spoon and heat iton a spirit lamp. (You can make your ownspoon using a metal bottle cap and wrappinga wire around it).

(Be cautious, do not inhale fumes, theyare harmful).

Collect the ashes of magnesium ina test tube and add some distilled water toit. Test the solution with red and blue litmuspapers. Note the colour change in table-7.

As soon as sulphur starts burning,introduce the spoon into a gas jar/tumbler.Cover the tumbler with a lid to ensure thatthe gas produced does not escape.Remove the spoon after some time buttry to keep the jar covered. Add a smallquantity of water into the tumbler andquickly replace the lid. Shake the tumblerwell. Check the solution with red and bluelitmus papers. Record the changes intable7.


When the samples are burnt, theyreact with oxygen in air to give differentproducts. See how.

Fig-8

Table-7

Sample

Magnesium

Sulphur

Physical Appearancebefore Reaction

PhysicalAppearance After

Reaction

Effect on LitmusPaper

SCERT TELA

NGANA


Think and discuss

Something similar happens with othermetals also. Magnesium ribbon is dullwhen exposed to air and shiny if we cut it.Silver objects and jewellery becomes black.Copper statues and vessels become dullgreen. All these metals react withcomponents in air. But gold jewellery doesnot become dull.

What could be the reason?Thus, different metals react with the

components of air in a different manner atdifferent rates and conditions. There aresome metals which do not react with thecomponents of air. Gold and platinum aresuch metals which do not rust.

Reaction with waterNote:- This is a demonstration to be

carried out by the teacher. Sodium isextremely reactive and dangerous andstudents should see the demonstration froma distance.

Take a 500 ml beaker or a big glasstrough and fill half of it with water. Takethe sodium which is stored in kerosene andkeep it on a sheet of filter paper to blot thekerosene and cut a very small piece ofsodium from it.

Put the remaining sodium back againin kerosene. Standing away from the troughput the sodium piece in water using forceps.

Magnesium+Oxygen → Magnesium oxide (s) (g) (s)

Sulphur + Oxygen → Sulphur dioxide (s) (g) (s)

All the products in the above reactionsare oxides, but are they same in nature? Theoxide of magnesium turns red litmus toblue. The oxide of sulphur turns blue litmusto red. From this knowledge, you can saythat magnesium oxide is basic and Sulphurdi oxide is acidic.

You can also infer that non-metals reactwith oxygen to give oxides which areacidic, while metals react with oxygen togive oxides that are basic in nature.

Is our body a metal or non-metal?Most of the human body is made up

of water (H2O). It isn’t surprising that

majority of a human body’s mass isoxygen. Carbon, the basic unit of organicmolecules is the second. 99% of themass of the human body is made up ofjust six elements. Oxygen (65%),carbon(18%), hydrogen (10%), Nitrogen(3%), calcium (1.5%), phosphorus(1.0%). Can we decide wether our bodyis metal or non metal?

Rusting of MetalsIn class VII we studied rusting of iron

in detail. Recall that iron rusts when it is incontact with air that contains oxygen andmoisture. When it is covered with paint, itcannot come in contact with air, hence itdoes not rust. If paint is gone on iron thenthe rust forms on it.

Fig-9SCERT TELA

NGANA


The piece of sodium floats on thesurface of the water with a ‘hissing’ sound.This shows that sodium is reactingextremely fast with water. After thereaction is complete, test the solution withlitmus paper.

Repeat the same experiment usingaluminium or iron. You will not see anychange even after five minutes. This isbecause these metals react extremelyslowly with water.

Does non-metals react with water?

Generally they do not react with water.

Reaction with Acids

Take the samples given in the table 8in separate test tubes. Add 5ml of dilutehydrochloric acid to each of the test tubeswith the help of a dropper.

Observe the reactions. If you find noreaction, heat the test tube gently. If youstill see no reaction, add 5 drops of conc.Hydrochloric acid. Now bring a burningMatch stick near the mouth of the test tubeand observe what happens. Record yourobservations in the table-8.

Do you find any difference in thesereactions?

When do you notice a pop sound with aburning match stick?

This sound indicates the presence ofhydrogen.

You found that some metals react withdilute hydrochloric acid liberatinghydrogen but non-metals usually do notreact with acids.

Repeat the same experiment withsulphuric acid and record your observationsin table-8.

Reactivity of metalsYou have already seen that some metals

react with air and others do not. The reactionis fast in some cases like Magnesium andslow in case of silver and copper. Similarly,different metals react with water and acidsunder different conditions. Let us explorethis reactivity further.

Table-8

Iron

Zinc

Copper

Sulphur

Aluminium

Carbon

Magnesium

Iodine

Reaction withdilute

hydrochloricacid

Reactionwith dilutesulphuric

acidSample

Fig:10

SCERT TELA

NGANA


Take five beakers and label them a, b,c, d and e. Take 50ml of water in each beakerand dissolve a spatulaful of copper sulphatein beakers 'a' and 'b'. Dissolve a spatulafulof zinc sulphate in beakers c, d and ironsulphate in beakers e and f Now put:

* Zinc granules in beakers 'a', 'e'

* Iron nails in beakers 'b', 'd'

* Copper turnings in beakers 'c', 'f'Leave the beakers undisturbed. Record thechanges in the colour of the solutions inthe table-9.

The blue colour of copper sulphatedisappears and a powdery red mass ofcopper is deposited at the bottom of thebeaker ’a’.

Also notice that in beaker ’b’ redcopper is found at the bottom of the beakerand on the nail leaving light green ironsulphate solution.

What could be the reasons behind thesechanges?

In beaker ‘a’ zinc displaces copper fromcopper sulphate giving rise to colourless

zinc sulphate. Iron displaces copper fromcopper sulphate in beaker ’b’ leaving lightgreen colour of Iron sulphate.

Copper sulphate+Zinc → Zinc sulphate + Copper

Copper sulphate + Iron→Iron sulphate + Copper

Similar to the reactions in beakers ’a’and ‘b’, is there displacement of zinc fromZinc sulphate by Copper in beaker ‘c’ andIron from iron suphate by Copper in beaker‘d’and Zinc from Zinc sulphate by Iron in‘e’?

Do you find any changes in beakers c,dand e?

Since we do not see any change in theother three beakers (c,d and e) we can inferthat.i) Copper is unable to displace zinc from

zinc sulphate.ii) Copper is unable to displace iron from

iron sulphate.iii) Iron is unable to displace zinc from

zinc sulphate.We can understand with the above

reactions that a more reactive metal canreplace a less reactive metal, but a lessreactive one cannot replace a more reactivemetal. That is why there are no displacementreactions in the beakers c, d and e.Some uses of non-metals

You are all aware of the uses of metals.Non-metals are also useful. The three non-metals studied by us are sulphur, carbon andiodine. Let us know about their uses.

a b c d e f

Fig:11

Table-9

Beaker 'a'

Beaker 'b'

Beaker 'c'

Beaker 'd'

Beaker 'e'

Beaker 'f'

Solutions Observations

SCERT TELA

NGANA


Sulphur is used in making fireworks,crackers, gun powder, matchsticks andantiseptic ointments. It is found in onions,garlic, eggs, hair and nails.

Activated carbon is used as adecolourising agent and also in waterpurification systems.

Tincture iodine is used in medicalpurposes.

Uses of metals

Have you ever noticed a thin silver foildecorated on sweets and thin aluminium foilused in inner packing of food materials andtoffees. Aluminium and copper mixture isused in currency coins, medals and statues.Zinc and iron mixture used in making ofiron sheet. Most of the agriculturalinstruments are made by iron. Electricalappliances, automobiles, satellites,

aeroplanes ,cooking utensils, machinery,decorative materials made by metals dueto their malleability, ductility.

Try thisRecall the names of the some of the

laboratory acids and bases that you know.Write down their names in (table -10) andidentify metal/non-metal present in them,which form oxides when react with oxygen.Take the help of your teacher (table -10).

Have you seen a periodic table?

Try to find the metals and non-metalsthat you come across in the periodic table.

Key words

Metals, non-metals, lustrous, sonority, malleability, ductility, good conductorsof heat and electricity, oxides of metals and non-metals, displacement reaction.

The materials which show brightness on surface and reflect the light are called lustrousand which do not shine are non-lustrous material.

The property of materials by which they can be beaten into thin sheets is calledmalleability.

What we have learnt

S.No. Name of the Base Metalpresent in it

Name of the Acid

Non- Metal presentin it

1. Calcium hydroxide Calcium Sulphuric acid Sulphur

Table-10

SCERT TELA

NGANA


The property of drawing material to make fine wires is called ductility.

The ability of materials to produce a particular sound when it is dropped on the hardsurface is termed as sonorous.

Metals often possess all of the following properties. They are lustrous, hard, malleable,ductile, good conductors of heat and electricity and sonorous Ex: copper, magnesium,aluminium, iron, zinc etc.

Most of the metals exist in solid state.

Some metals react with the components of air in different manner with different ratesand in different conditions.

Gold and platinum are the metals which do not react with air.

Metals react with acids and liberate hydrogen gas.

Metals can displace each other according to their reactivity.

Oxides of non-metals are usually acidic in nature.

Oxides of metals are usually basic in nature.


1. Explain about ductility. (AS1)

2. Explain the physical properties of metals with suitable examples. (AS1)

3. Gold jewellery does not become dull.Why?(AS1)

4. Draw the diagram of identifying electric conductivity of a material. (AS5)

5. Which gas makes a Pop Sound when it exposed to lighted match stick?(AS1)


1. If you are given two samples, how do you distinguish which one is metal and which is

non metal? (AS7)

2. Which metals are used in making jewellary?Why?(AS2)

3. Why cooking pans don’t have metal handles? (AS7)

4. How do you appreciate the efforts of man to use the Metals in making tools? (AS6)

5. Why the bells are made by metals instead of wood? (AS1)


SCERT TELA

NGANA


Higher Order Thinking Questions1. You came to know that the Diamond is a hardest material and it is a non metal and

similarly Mercury is a soft material and it is a metal.Write down the questions raisedin your mind. (AS

2)

2. Dumping of waste material made up of metals and non metals leads to environmentpollution. Do you support the statement? Give your justification with suitableexamples. (AS

7)

3. In a chemical reaction iron is unable to displace Zinc from Zinc Sulphate.Why?(AS1)

4. How do you appreciate wide range utility of aluminium right from utensils to spacecrafts? (AS

6)

5. How the property malleability of metals is used in our daily life?(AS7)


1. Sulphur dioxide , by nature is [ ]a) Basic oxide b) Acidic Oxidec) Nuetral Oxide d) Dual Nature oxide

2. Maximum metals are obtained in the state of [ ]

a) Liquid b) Solid c) Gaseous d) Plasma

3. Some metals react with acids and evolve [ ]

a) Hydrogen b) Oxygen c) Carbon dioxide d) Nitrogen

4. Generally non metallic oxides are by nature [ ]

a) Basic b) Acidic c) Nuetral d) Dual

5. The property of changing the metals into sheets is known as [ ]

a) Malleability b) Ductility

c) Conductivity of heat d) Electric conductivity

Suggested Experiments1. Conduct an experiment to know the reaction of Oxygen with metals and non metals.2. The nature of oxides helps to identify the metals and non metals conduct an experiment

to prove this and record the observations.3. Identify the electric conductivity of materials with an experiment.

Suggested Projects1. Collect the information about metals which we use in our daily life, and their uses.

Write a report on it.2. Imagine the human life without metals. Write a report.

* * * * * *

SCERT TELA

NGANA


SOUNDChapter

5

to sounds of objects, animals. Prepare a listof sounds that we hear and the sources fromwhich they might have originated. Writethem in table-1.

Activity-2

Identifying different sounds

Make a student to stand at the black

board such that his face is turned towards

the black board and ask other student in the

class to make different sounds. The student

at the board should tabulate the sounds he

heard and sources of those sounds as shownin table-2.

Table 1

Sound heard Source of Sound

Feeble barking Dog from some distance

Bell ringing

Throughout the day we are constantlyexposed to different sounds. For example,people talk, birds chirping, cries ofanimals, sounds of autos, motorbikes,buses, lorries, tractors, trains and musicfrom loudspeakers at public places,television. Sound is an unavoidable andintegral part of our lives. We are alwayssurrounded by sound. It is almostomnipresent. Sound plays an importantrole in our lives. It helps us to easilycommunicate with each other.

Make a list of sounds that you get tohear in your surroundings.

How are these sounds produced?

How do sounds travel from one placeto another?

Are we able to hear all sounds in nature?

We will try to seek answers to someof these and similar questions in thischapter.

Production of soundActivity-1

Listening to sound

and predictingits source

Let us sit quietly for a while and listen

SCERT TELA

NGANA

66 Sound

Table 2

S.No. Sound heard way of producing Sound

1. Gala Gala A few stones rattling in a metal box2. Eela (whistling sound) A student has produced the sound

from her/his mouth3 Some one striking the table tab with a

scale4 Someone thumping the ground with

shoes56

Is there any change in the soundproduced in the two situations?

How do you feel when you touch theringing bell? Remove your hand and ringit again. Do you hear a different sound?Why?

Fig-1: Observing vibrations of bell when itis producing sound

How does the student at the black boardguess the source of sound withoutactually seeing the source?

You might have observed many othersources of sound in your daily life. Try tolisten and identify some more sources ofsound and prepare a list.

How do objects produce sound?

What happens when objects made ofmetals are hit by a hammer or fall downfrom a height on a concrete floor?How does a flute or a whistle producesound?What would you feel if you touch a bodywhile it is producing sound?

Activity-3

Vibrating body produces sound

Take a brass bell (bell used in poojaroom or in your school). Ring the bell andlisten to the sound carefully. Now hold thebell tightly with your hand as shown in fig-1 and ring it again.

Do you hear sound from the bell?

SCERT TELA

NGANA


Let us do following activities

1. Fix a rubber band tightly on an emptymatchbox. (See fig-2). Pluck the rubberband and keep it close to your ear.

Fig-2: Listening to sound from match boxtied with rubber band

Do you hear any sound? Do you feelany vibration in your hands?

2. Blow air into papers of yournotebook as shown in fig-3. What happens?Does the action produce any sound? Do youfind any vibrations in the note book?

Fig-3: Blowing air into papers

3. Fill a plate with water and let thewater settle. Strike the rim of the plate witha spoon as shown in fig-4. What do youobserve? What do you hear? Where do youfind vibrations in this case?

Fig-4: Striking the rim of a plate with a spoon

4. Put a hack-saw blade in between atable and a brick as shown in the figure-5and press it and leave it abruptly. Whathappens? Does it produce sound? What isthe state of the hack-saw blade while it isproducing sound?

Fig-5:Sound produced by a vibrating hack-saw blade

What have you observed while doing theabove activities?

What changes took place in thoseobjects?

You must have noticed that objectsvibrate while they are producing sound. Youcan feel these vibrations if you touch theobjects. We are able to observe thevibrations of the plate and papers of thebook. We have also observed vibrations ofthe hack-saw blade. Thus, we can concludethat a vibrating body produces sound.

SCERT TELA

NGANA

68 Sound

Fig-6: Sound has energy

But there are certain instruments whichproduce sound though we cannot see anyvibrations in the instruments like in fluteand clarinet etc.

How do they produce sound?

Is it possible to produce sound withoutvibrations?

Does every vibrating body producesound?

Does sound have energy?

Let us find out:

Activity-4

Sound has energy

Take a plastic bottle and a cell phone.Cut the top of the bottle so that it lookslike a glass. Play songs on the mobile phonein high volume and place it in the bottle.Close the mouth of bottle with a balloonusing rubber band so that it covers thebottle as shown in the fig-6 and stretch ittightly so as to behave like a diaphragm.Place some sugar crystals or small size ofsand particles on the balloon diaphragmand observe the movement of particles. Dothe same activity after removing the phonefrom the bottle. What do you notice?

Fig-7

When there is no cell phone inside thebottle, sand particles on the diaphragmremain stationary. While the cell phoneplays songs inside the bottle, the diaphragmvibrates which can be seen through dancingof sand particles. The sound produced bycell phone inside the bottle is responsiblefor these vibrations. Thus, sound has energyto make sand particles vibrate on thediaphragm.

Musicalinstruments

You might have observed manymusical instruments like Tabala, Flute,Harmonium and Gitar. The sounds producedby these instruments are distinct. It is easyfor us to identify sounds separately fromdifferent instruments.

How do they produce sound?

Why is there a difference between thesounds produced by various musicalinstruments?

Which part of these instruments isresponsible for production of sound?

Let us do

List out the names of musicalinstruments and mention the vibrating partof each instrument, write them in table-3.

SCERT TELA

NGANA


Table 3

Name of instrument Vibrating part of it

Tabala Membrane, air insidehollow body

Have you named all the vibrating partsfor each musical instrument? For example,in tabala, not only the stretched membranebut the air inside the hollow body alsovibrates.

Can you name the instrument for whichmore than one part is responsible forthe production of sound?

How do you compare the productionof sound in a flute and sound producedin a water tap when it is turned on, justbefore the water flows out of it.

Activity-5

Producing a sound thatresembles sound of rainfall

Start clapping with fore finger on lefthand palm, add the middle finger and clapagain, then ring finger and lastly smallfinger successively and reverse the processgradually. If all the students in your classdo it simultaneously the sounds producedwould resemble the onset and stopping ofrainfall.

Activity-6

Observing the changes in sound

Take 4 or 5 metal glass bowls ortumblers of same size. Fill them withdifferent amount of water. Arrange them inthe order of decreasing water levels. Strikegently each bowl or tumbler with a spoon.What do you hear? (This is a jalatarang) (seefigure-8)

Fill the bowls or tumblers with equalamounts of water, strike each bowl like inabove case and listen to the sound.

What difference do you notice in thesound produced?

Why is there a variation in the soundproduced due to change in the waterlevel of a bowl?

Thus we conclude that sounds areproduced by vibrating bodies and the airthat passes through orifices of theinstruments.

Fig-8 : JalatarangSCERT TELA

NGANA

70 Sound

Do you know?

Bismillah Khan, the most outstanding and world-famous shehnai player, had attainedastonishing mastery over the instrument. He was born in a small village in Bihar about 80years ago. He spent his childhood in the holy city of Varanasi, on the banks of the Ganga,where his uncle was the official shehnai player in the famous Kasi Viswanath temple.

Bismilla Khan Chitti babuChitti Babu (October 13, 1936 - February 9, 1996) was a renowned classical musician

from India, and arguably one of the greatest Veena artists, in the field of Karnatic Music aspeciality of South India. He became a legend in his own lifetime. His name wassynonymous with the musical instrument Veena, and he was and still is known in theKarnatic Music world, simply as Veena Chitti Babu.

Sounds produced by humanbeings

We know that all animals producesound to communicate with other animals.Human beings use these sounds moreeffectively. Sounds produced in particularorder and manner constitutes our speech.This order in production of sound isdifferent for different languages orcommunication processes.

Honeybee makes sounds on seeingflowers to communicate to the other beeswho are at a distance. Do they produce thispeculiar sound through their mouth or someother organ used for that purpose?

Majority of communication in humanbeings is through speech. Which organs dogive human beings the ability to talk?

Try this :

Imitate different sounds made byanimals. Try to mimic your friends. Whilemaking these sounds, place your fingers onyour throat. What do you feel? Do yourfingers sense any vibrations? Are thevibrations same for all the sounds that youmake?

Structrure of larynx or vioce box

Larynx is the important organ in humanbody to produce sound.SCERT T

ELANGANA


Nasopharynx

OropharynxTongue

Epiglottis

Vocal cord

TracheaEsophagus

Larynx

Laryngopharynx

Fig-9: Anatomy of the larynx

Larynx has two muscular ligamentscalled vocal cords. They are stretchedacross voice box; it leads to a narrow slitbetween them, to allow passage of air, toproduce sounds.

Fig- 10 (a) and 10 (b) : Opened vocal cordsand Closed vocal cords

Obseve Fig- 10(a), Vocal cords openduring breathing to allow air into lungs.

Observe Fig- 10(b), Vocal cords closewhen we speak. The air from the lungspresses between them to cause vibrationand produce sound.

Do you know?Length of vocal cords in men is

nearly 20mm, in women it is about5mm less, whereas in children it isshorter. Can we say that the length ofvocal cords plays any role in decidingthe quality of sound produced by men,women and children?

Activity-7

Observing the movements ofvocal cords during speech

Ask a friend to raise his neck up.Stretch a chocolate wrapper across hismouth and ask him to blow air on thewrapper forcibly. Observe the changes inmovement at his throat. Ask him to blowagain slowly and observe the difference inmovements.

What changes do you observe in themovements at the throat on the twooccasions?During the first trial the voice box gets

tensed and produces high sound whileduring the second trial it is close to normalposition of throat and produces lowersound. The sound produced in the aboveactivity is due to a combination ofvibrations produced in the wrapper and thevocal cord.

If the sound is a kind of vibration, howdoes this vibration reach us from thesource? How are we able to hear the soundsproduced at a distance?

Propagation of soundSound needs a medium topropagate

The sound produced by the school bellwill be heard by all of us irrespective ofwhether we are in a room opposite to thebell or in a room at the back of the bell.Obviously, the sound produced by theschool bell travels in all directions andreaches us, propagating through the airpresent between source of the sound andour ears. That is the air surrounding us acts

SCERT TELA

NGANA

72 Sound

as a medium which allows the sound to passthrough it.

Does sound travel only in air?

Does it travel in any other gaseousmedium?

Does sound also travel in other medialike solids and liquids?

Propagation of sound in different media

Let us try to know how vibration canpropagate through different media!

Do you know?

Can we talk without moving of lips?Ventriloquists make sounds or talk with hardly any movement of lips. Lips are slightlyseparated. The various words are spoken quickly and it is difficult for listeners to noticethe difference. They have a very good control over breathing and movement of lips, throatand the muscles of mouth which aids in pronunciation and delivery of speech withoutmuch movement of lips and throat. They let the breath out of the mouth by vibrating theirlips in accordance to the air supply and relax muscles while doing this. This is one of themost effective vocal activities. In Telangana, Chinchapattana Gomatham Srinivas fromWarangal district is a famous ventriloquist. He has performed more than 6000 showsaround the world. He created a sensational world record by performing a 32 hour non-stop mimicry show in 1990.

MimicryThe mimicry artists maintain a very good control over their voices. The magic they createis solely with their voice. They exercise their vocal parts to keep them fit to be able toenthrall the audience through their voice. Dr.Nerella Venu Madhav is a world famousmimicry artist. He belongs to Warangal District of Telangana State. Govt of India honouredhim with Padma Shree in 2001.Try these yourself and you can acquire a hobby.

Nerella VenumadhavGomatham Srinivas

Activity-8Observing sound propagation insolids1) Strike one end of the table with a pen and

ask your friend to listen to the soundproduced keeping her ear touching thetable at other end and also ask her to listento the sound by lifting her head slightlyfrom the table (fig-11). Ask your friendwhat difference she noticed while hearingthe sounds when her ears were away fromthe table and touching the table.

SCERT TELA

NGANA


Fig-11: Propagation of sound in solids

2) Take a metal or wooden strip. Strike itat one end and ask your friend to hearthe sound by keeping his ears at theother end of the strip. Ask your friendwhat difference he noticed whilehearing the sounds when his ears areaway from the strip and touching thestrip (fig-12).

3) Do you know how to make a toytelephone using tea cups?

Take two paper-cups. Make small holesat the bottom of these cups. The holes shouldbe very small so that only a thin string canpass through them. Take a long string. Makesure that the string does not have any knotsin it. Push the string through the hole in oneof the cups. Fix the string by putting a knotat the end. Similarly fix the string to the othercup. Our phone is ready.

You and your friend can communicatewith this phone now. Stand away from eachother so that the string is tightly held. Oneof you can speak in the cup while the othercan listen by putting the cup on his ear.

Are you able to hear the sound?What is a medium between you andyour friend which is responsible forpropagation of sound? In the above activities you observed

that sound travels in solid medium likewood, metal, thread, etc.

Does sound travel in liquids?Can we hear the sound produced inwater?Let us find out.

Acivity-9

Propagation of sound throughliquids

Take a bucket fill it with water. Take twostones and strike them against each otherkeeping your hands inside the water. Askyour friend to listen to the sound by keepinghis / her ears touching walls of the bucket.

Fig-13

Fig-12

Fig-14

SCERT TELA

NGANA

74 Sound

Fig-15: Sound does not travel (propagate)through vaccum

When the tumbler is covered with aplate, the volume reduces but you can hearthe ringtone. As you start sucking more andmore air, you can notice that the volumedecreases gradually. If the air is suckedcompletely, you will not hear the sound atall. But practically it is not so easy. Thisactivity gives an idea about the need of amedium for propagation of sound.

We can demonstrate that sound doesnot propagate through vacuum and itrequires a medium, if we use perfectvacuum pumps to create vacuum which isnot easy with the glass tumbler.

How do we hear sound?We hear the sound

produced in oursurroundings with thehelp of our ears. Thestructure of ears play animportant role in hearingthe sound. Let us peep into our ear and tryto understand how we hear sound.

Structure and functioning of theeardrum

Our ear consists of three sections, theouter ear, the middle ear and the inner earas shown in the figure-16. Pinna of externalear collects the sound vibrations. They

Ask your friend about the differencebetween sounds produced by striking thestones against each other in water andstriking them in air.

Thus the conclusion is that soundpropagates through matter in all the threestates – solid, liquid and gas.

Think and Discuss

What is the effect of humidity onquality of sound propagation? Is thereany difference in propagation of soundin air during the summer and winterseasons? Discuss with your friends.

Activity-10

Does sound travel if there is nomedium?

Take a glass or plastic tumbler. Makesure that the tumbler is dry. It should belong enough to accommodate a cellphone vertically. Place a cell phone in theglass and play the ring tone of the mobile.Listen to the ringtone and it’s volume level.Cover the glass with a small plate and againlisten to the ringtone and note thedifference in volume of the sound. Nowsuck the air from the glass keeping it closeto your mouth as shown in the figure15. Ifyou suck air quickly the rim of the glasswill stick around your mouth due to airlock. Listen to the volume of the ringtoneat this stage. And also ask your friend tolisten to the sound for comparing variationin its volume. Is there any change in thevolume of sound observed by you or yourfriend?

SCERT TELA

NGANA


enter into the ear canal. We have learnt thatsound travels in the form of vibrations.These vibrations strike the tympanum (ear-drum) and make it to vibrate.

Fig-16: Structure of the ear drum

The vibrations from the tympanicmembrane reach the middle ear (earossicles), contains three small bonesmalleus (hammer shaped), incus (anvilshaped) and stapes (stirrup shaped). Theymagnify the sound vibrations. The stapestransmits the vibrations to the membraneof oval window. The oval window has thesurface area 1/20th of the ear-drum. Bythis the vibrations increase 30 to 60 times.The vibrations from the oval windowtransmit to the cochlea which is the innerpart of the ear. The cochlea is filled withthick fluids which transmits the vibrations.The motion of the vibrations in the cochleais detected by tiny hairs connected tonerves at this point. The vibrations aretransformed into electrical signals andcarried by the nerves to the brain where thesensation of the sound is realized. Thesound vibrations can also reach the innerear by travelling directly through the bonesof the skull, as you tap your head.

Are there any harmful sounds that wehear? What happens when we hear suchsounds?

Charecteristics of soundWe learnt how sound

is produced and what avibration is. Now, let uslook at differentcharacteristics of sound.

Loudness, feebleness andamplitude

Sometimes people talk loudly andsometimes softly. If we hit a table hard weget a loud sound. If we tap the same tablegently, we hear a feeble sound.

In our daily life we hear many loudsounds and feeble sounds on differentoccasions. For example, sounds emanatingfrom drums during the prayer of assemblyin the school are very loud. But the soundproduced while we are taking pledge in theassembly is a feeble sound. We know thatdiwali is a festival of sound and light. Whenfire crackers they make loud sounds.

Why are some sounds loud?Why are some sounds feeble?Is there any relation between theintensity of sound and vibrations of thebody which produces sound?

Think and Discuss

"Vibrations produce sound and soundproduces vibrations". Which is true inthis? Discuss."Our ear has the three media throughwhich sound propagates." Discuss withyour friends as to whether the abovestatement is true.

ear drum

cochlea

outer ear (pinne)

auditory canalsmiddle bone

SCERT TELA

NGANA

76 Sound

Aim: To know the relation between theintensity of sound produced by a body andthe vibrations of the body.

Material required: Wooden table, 30cm metal scale or nearly 30 cm hack-sawblade and a brick.

Procedure :

1. Place the blade/scale on the table,with 10cm of the blade on the surface ofthe table and rest of it in air. Keep a heavybrick on one end of the 10 cm blade/scalekept on table (fig-17).

2. Vibrate the blade gently and observethe vibrations and simultaneously listen tothe sounds. Repeat the same for 2-3 timesand record your observations in thetable-4.

3. Vibrate the blade using greaterforce. Observe the vibrations and listen tothe sound. Repeat this for 2-3 times andrecord your observations in the table-4.

Lab Activity -1Table 4

Force Vibrations of Intensity of

the blade / scale sound

Small

Large

When do you hear a loud sound?

When do you hear a feeble sound?

What difference do you notice invibrations of blade / scale during loudand feeble sounds?

The initial position of the scale at restalong the surface of the table, is calledmean position.

As shown in the figure 18(scale) OA isthe mean position of the vibrating body. OBand OC are the vibrations occurring in thebody.

Fig-18

The to and fro motion of a body fromits mean position is known as onevibration.

The body vibrates from the meanposition OA to OC and comes back to OAand then moves from OA to OB and comesback. The maximum displacement ofvibrating body from its mean position iscalled amplitude. In the picture givenabove the maximum displacement is A toB or A to C.

O

B

A

C

Fig-17:vibrations of the body andintensity of sound.

SCERT TELA

NGANA


What difference do you find in theamplitude of vibration for a feeble anda loud sound in the above experiment?

Do you Know?

Decibel is the unit for measuringthe intensity of sound. It is denoted asdB.

The unit expressed as decibel in thename of Alexander Graham Bell(1847 – 1942), whose research insound is famous.

The smallest audible sound (nearerto total silence) is 0 dB. A sound 10times more powerful than this is 10 dB.A sound 100 times more powerful thanthat of total silence is 20 dB. A sound1,000 times more powerful than thesound nearer to total silence is 30 dB.Some common sounds and their decibelratings are given below.

Near total silence - 0 dB

A whisper - 15 dB

Normal conversation - 60 dB

A lawnmower - 90 dB

A car horn - 110 dB

A jet engine - 120 dB

A gunshot or firecracker - 140 dB

Problems: From the above data,compute the following.

How many times is a car horn morepowerful than normal conversation?

How many times is a jet enginemore powerful than a whispering sound?

Lab Activity-2

Aim: Identifying pitch or shrillness ofa sound.

Material required: A wooden table,two hack-saw blades or metal scales of30cm length and a brick.

Procedure:

1. Place the first blade/scale on the table,with 10cm portion of the blade on thetable and rest of it in air. Keep a brickas weight on the 10 cm portion of theblade/scale kept on the table.

2. Place the second blade/scale on the table(see that the gap between these twoblades is 10cms), with 25cm on thetable and 5cm in air. Keep a brick asweight on the scale/blade (fig-19).

3. Vibrate both blades with same force.Observe the vibrations and listen to thesounds produced.

Fig-19: Vibrations of hack - saw blades

Repeat the same 2-3 times and recordobservations in the table-5.

Table 5

Blade length Vibrations Soundin Air

Blade 1 : 20cm

Blade 2 : 5cm

SCERT TELA

NGANA

78 Sound

What difference do you notice innumber of vibrations of two blades?

What difference do you notice in thequality of sound produced by them?

You would notice that the number ofvibrations produced are less in the 20cmlong blade when compared to the vibrationsof the 5cm long blade. The sound producedby the 5cm blade is more shrill whencompared to that of the 20cm blade.

The shrillness of a sound is known aspitch.

The number of vibrations per second(vib/sec) is called frequency.

The pitch of the sound depends uponits frequency.

In the above experiment number ofvibrations per second in 20cm blade is less.It denotes that it has low frequency. Whereasthe number of vibrations per second are morein 5cm blade so it has high frequency.

Thus the sound produced by a shortblade (5cm) has high pitch and soundproduced by long blade (20cm) has lowpitch.

Conduct the above experiment withdifferent lengths of the blades or metalscales and note your observations.

Think of lengths of male, female andchildren vocal cards and try to decide theirvoice pitch.

A bird makes high pitch sound and a lionmakes low pitch roar. Which one doesproduces the sound of more frequency?

Give some more examples of naturalsounds that you come across in your daily

life and differentiate them as low pitchsounds and high pitch sounds.

Do you know?

Pitch of the voices of the followingis in ascending order.

Lion<adult man<adult women<child<infant<insect

Can you guess the reason why?

Do you find any difference in blowingthe whistle and striking the drum?

Normal sound consists of mixedfrequencies

In our daily activities, we hear manydifferent sounds. But we generaly do notconcentrate on every sound that we hear.All these different sounds are producedwith different frequencies and withdifferent amplitudes. For example, thedifferent sounds produced in a classroombefore the commencement of the class. Ifwe keenly observe these sounds, we canfind that no two persons produce the samesound of equal pitch and amplitude.

Why do we produce sounds withdifferent pitches or amplitudes whilespeaking?

The variation in pitch and amplitude ofsound during our speech helps us tocommunicate with others in the form oflanguage.

Primitive man had no developedlanguage, so he used to communicate withsigns and some sounds which had nowritten equivalent. Later in the process of

SCERT TELA

NGANA


evolution these sounds became a source ofa meaningful communication and eventuallywere converted into written forms. This ledto the development of the full form oflanguage which we all use now tocommunicate. Not only human beings butanimals too have a mechanism ofcommunication by producing sounds withdifferent frequencies and amplitudesaccording to their need.

The parts of the speech organ which areinvolved in producing sounds are

Vocal cords

Lips

Teeth & tongue

Nose & throat

Naturally, the words that we utter donot have a single sound but are acombination of sounds with differentfrequencies and amplitudes. The soundproduced for each letter possess aparticular frequency. Thus, the word is acombination of different letters ofdifferent frequencies i.e., a word is soundof mixed frequencies. Sometimes thesame word is uttered in different ways toexpress different emotions. For example,the word ‘NO’ is uttered in different pitchesto express negativity, anger andfrustration.

Noise and musicHow do you feel when you hear soundsin busy traffic?

How do you feel while listening tosongs from a radio?

Which of the above sounds are morepleasant to hear?

We enjoy the sounds in a musicconcert. They are pleasant to hear. But thereare some sounds which are unbearable tohear like the sounds produced when a steelplate or utensil is dropped on the floor.

The sounds which are pleasant to hearare called music. The sounds which are notpleasant to hear are called noise. Noise isan irregular combination of sounds whichare ‘unpleasant’ to hear. Music is acombination of sounds that are producedin an order and pleasant to hear.

Give some more examples for pleasantand unpleasant sounds.Audible range

One of our sense organs, ear, enable usto hear a number of sounds. Are we able tohear all sounds produced in oursurroundings?

Do we hear the sounds produced bybats?The sounds that a normal human being can

hear are called audible sounds. The soundsthat a normal human being cannot hear arecalled inaudible sounds. Frequency of theaudible sound ranges from 20vibrations/sec-20000vibrations/second. Frequency ofinaudible sounds are less than 20vibrations/sec or greater than 20000vibrations/sec.

Sound pollution

Sound pollution is a serious problemlike air, water pollution. It is harmful tohuman beings. We express the loudness ofthe sound in decibels (dB), which we have

SCERT TELA

NGANA

80 Sound

already learnt in this chapter. The soundproduced in our normal conversation isabout 60dB. If the loudness exceeds 80dB,the sound becomes physically painful. If aperson is being exposed to the sound of80dB continuously it may lead to hearingproblems.

Let us observe the sounds that areresponsible for sound pollution.

In our surroundings there are manysounds causing sound pollution, like soundsof traffic, their horns, sounds inconstruction sites, sounds at industries,sounds at mines, sounds during explosionsand firing of crackers, etc.

The unwanted sound in oursurroundings leads to sound pollution.There are some more sources of soundpollution in our homes like mixer/ grinder,washing machines and motors etc.

Write some more sources of soundpollution in your surrounding.

Effects of sound pollutionWhat are the harmful effects of sound

pollution?The first harmful effect is loss of

hearing. It also leads to several healthrelated problems. eg: sleeplessness, hypertension, increase in blood pressure, etc…,

Discuss and list some more effects ofsound pollution.

Measures to control soundpollution

We cannot stop production of soundbut we can reduce sound pollution by somemeasures.

Let us list the steps which can be takento reduce sound pollution:

Attach silencers to bikes and othermachines to reduce sounds.

Do you know?

M.S. Subbulakshmi was famous forher melodious music. It would bedifficult to overstate the talent and theimpact made by Smt. M.S.

Subbulakshmi,not just in thefield of Karnaticmusic, but also asa philanthropistand a person who

Manufacture machines that work withless noiseDuring the use of TVs and musicplayers tone down volume of sound.Plant trees to reduce sound pollution.

Discuss with your friends about someother measures to limit sound pollution andtabulate them.

placed her life at the service to thecountry and people. She rendered hervoice to devotional songs.

Ghantasala Venkateswar Rao wasfamous playback singer. He wasfamous for his melodious voice. He

sang more than10,000 songs inTelugu, Tamil,Kannada, andMalayalam andworked as a musicdirector for over

100 films. His private songs wereequally popular and his devotionalsongs are popular even today.

SCERT TELA

NGANA


Key words

Vibration, vocal cords, medium, vacuum, ear drum, loudness, feeble,amplitude, decibel, pitch (shrillness), frequency, noise, music.

Vibrating body produces sound.

Human beings are able to produce sound with the help of vocal cords.Sounds travel through solids, liquids, and gases. It cannot travel through vacuum.The vibration of the ear drum caused by the sound produced by a vibrating body gives ussense of hearing.Loudness and feebleness of a sound are determined by amplitude of vibration.The intensity of sound is measured in dB (decibels)Pitch or shrillness is determined by the frequency.The number of vibrations per second is called frequency.Normal sounds consist of mixed frequencies. The hearing limit of sounds by human beings is called audible range.

Sounds pleasant to listen are called music and unpleasant to listen are called noise.


1. How can you explain the sound has energy. (AS1)

2. What is the unit to measure the sound intensity. (AS1)

3. What are the differences between the noise and music? Explain. (AS1)

4. Explain the sources which produce sound pollution in your surroundings. (AS1)

5. Write your suggestions about reducing sound pollution. (AS7)


1. The sounds of crickets (insects) make us close our ears. Why? (AS1)

2. Write the names of any three musical instruments that you know and explain howthey produce sound.(AS

1)

3. Write any two slogans to reduce sound pollution . (AS7)

4. Draw the figures depicting low amplitude and high amplitude. (AS5)

5. “Vibrations in a body produce sounds”. How do you prove it? (AS3)



SCERT TELA

NGANA

82 Sound


1. What is the effect of humidity on quality of sound propagation?Is there any differencein propagation of sound in air during the summer and winter seasons? Discuss. (AS

1)

2. How does sound pollution effect Bio diversity? Explain. (AS7)

III.Multiple Choice Questions1. An object which moves to and fro motion from rest known as......[ ]

a) Linear motion b) vibration c) simple motion d) Angular motion2. The number of vibrations produced per second is known as…… [ ]

a) Frequency b) Shrillness c) Vibration limit d) Decibels3. The audible sound range of man is ……………. [ ]

a) 10-10000 Vibrations/sec b) 20-20000 Vibrations/secc) 30-30000 Vibrations/sec d) 40-40000 Vibrations/sec

4. The oval window has the surface area……….of the eardrum [ ]a) 1/10 b) 2/10 c) 2/20 d) 1/20

5. The shrillness of a sound is known as ………….. [ ]a) Pitch b) Vibration c) Intensity d) decibel

Suggested Experiments1. Conduct an experiment to know the relationship between the intensity of sound

produced by a body and its amplitude.2. Conduct an experiment to Identify Pitch or Shrillness of different sounds.

Suggested project works1. Collect photographs showing various situations of sound pollution and prepare a

report.2. Collect the photographs of different musical instruments and scrap book.

3. Collect the photographs of local musicians and exhibit them in your class.

Do you know?

Golconda Fort - HyderabadIt is a famous fort in India. It is famous for many

engineering and architectural marvels. If you clapyour hands at a particular point under the dome itreverberates and can be heard at the highest point ofthe fort which is about 1km away.SCERT T

ELANGANA

83Free distribution by T.S. Government 2019-20

REFLECTION OF LIGHTBY PLANE SURFACES

Chapter

6In class 6, we have learnt about

shadows and we carried out manyexperiments with light rays and alsodiscussed the rectilinear propagation oflight i.e., light travels in a straight line. Inclass 7 we learnt the laws of reflection.

Let us recall some of them.

− A source of light, an opaque object anda screen are needed to form a shadow.

− Light travels in a straight line.

− When light gets reflected from asurface, the angle of reflection is equalto the angle of incidence.

− The incident ray, the normal at the pointof incidence and the reflected ray alllie in the same plane.

You must have observed shadows andimages in your daily life. Some questionsmight have come to your mind whileobserving these shadows or images.

• How can we get the image of a bigbuilding in a small mirror?

• Can we get the image formed by a planemirror on a screen?

• Why is there right-left inversion(lateral inversion) when we look into amirror?

• Why is the angle of reflection equal tothe angle of incidence when a light raygets reflected from a surface?

In this lesson we are going to learnabout reflection of light on plane surfacesin detail so that we can answer the abovequestions. Let’s start with some activitiesbased on your previous knowledge.

Activity-1Formation of image by a pinholecamera

Recall how an image forms in apinhole camera that you have learnt in class6. Draw a ray diagram of the formation ofan image in a pinhole camera.

What whould happen if we increasethe size of the hole of the pinhole camera.Observe the flame of a candle with apinhole camera making a big hole. Try todraw a ray diagram of the formation of animage in a pinhole camera with a big hole.Look at figure 1.

fig-1

SCERT TELA

NGANA

84 Reflection of light by plane surfaces

By observing the figure we canunderstand that the light rays coming fromthe top of the candle flame fall at differentpoints on the screen. Similarly the rayscoming from bottom of the candle flamealso fall at different points on the screen.Thus we get blurred image on the screendue to the big hole of the camera as shownfigure 1.

Think and discuss

• Does the explanation match yourobservation?

• What happens if the hole is muchbigger i.e. equal to the size of theflame?

• If so, can we get an image of a flameon the screen of the pinhole camera?Why?

• What happens if we observe the sameflame with the same pinhole camerafrom a long distance?

• What happens if we arrange two holesto the pin hole camera?

Think and answer. Do the experimentand check your answer.

Now think about reflection of light,and solve the task given below.

Activity-2

A smart crow is on a tree at point‘A’ as shown in figure-2. Some grains areon the ground [M N]. If the crow wants totake a grain and reach the point ‘B’ on theother tree as early as possible(in leasttime), from where should the crow pick upthe grain?

With the mathematical knowledgeyou have about angles and triangles can youguess the path that the crow selects? If youcan’t, read the following.

The crow can pick the grain fromany point on the ground but the conditionis; selecting a point on the ground to reachpoint ‘B’ from point ‘A’ in least possibletime. If we assume that the speed of the crowis constant the path that the crow selectsshould be the shortest. Let us find theshortest path.

Observe some of the paths in figure-3.

Which among the paths ACB, ADB,AEB and AFB is the shortest path?

To compare the lengths of thesepaths, we make duplicates of them in sucha way that all the duplicate paths meet atpoint G as shown in figure-4.

fig-2

AB

M N

AB

C D E Ffig-3

M N

G

AB

C D E F

fig-4

M NSCERT TELA

NGANA


surface, it selects the path that takes theleast time. That is why the angle ofincidence is equal to the angle of reflectionas shown in figure-5.

• Which line segment can be consideredas a mirror in the above activity?

• We know that plane mirror is a surface.How can we denote it in ray diagrams?

Let us know about plane mirror,before going to discuss reflection of lighton plane surfaces and to know how to drawray diagrams.

Plane mirror

Plane mirrors are made bydepositing a Silver layer on one side of aglass plate and then the Silver layer isprotected by a paint. Silver metal is one ofthe best reflectors of light. But now-a-daysinstead of Silver, a thin layer of Aluminiumis used to doposit on glass plates, becauseAluminium is much cheaper than Silver andit also reflects light very well. Hence thethin layer of Silver or Aluminium is thereflecting surface at the backside of themirror.

In the figure, CB = CG. The lengthof path ACB = AC+CB = AC+CG = ACG.Thus the length of the path ACG is equal tothe length of the path ACB. similarly ,

length of the path ADB = length of the path ADG

length of the path AEB = length of the path AEG

length of the path AFB = length of the path AFG

If you observe Fig-4 carefully, youwill notice that, among the paths ACG, ADG,AEG and AFG, the shortest path is AEG,because it is the straight line distancebetween points A and G. You can measureand check this using a scale. As AEG=AEB,path AEB is the shortest path to reach pointB from point A. It would take the least time.So the smart crow will pick the grain frompoint E.

Observe the path AEB once again infigure-5.

If we draw a normal EEI (E, E-dash)at point E, we can easily find that angle AEEI

(angle 1) is equal to angle EIEB (angle 2).

Like the crow in the above situation,light also selects the path which takes theleast time to travel. This principle was firstgiven by Pierre de Fermat, a French lawyerand an amateur mathematician.

It is also applicable to reflection oflight. When light gets reflected from a

A

fig-5

B

21

E I

EM

N

front side ofplane mirror

Fig-6(a)Plane Mirror

Fig-6(b)This is how we representplane mirror in diagrams

back side ofplane mirror

(Silver /Aluminiumcoating onglass plate

and coveredby a paint)

front side of

plane mirrorSCERT T

ELANGANA


Activity-3

Check your understanding ofreflection

Look at figures 7a and 7b. Let ussuppose that you have been given a planemirror strip.

• What will you do to obtain figures thatare shown in figure 7b using mirrorstrip and figure 7a?

Place the plane mirror strip on thefigure shown in 7a in such a manner thatyou see one of the figures shown in figure7b. The procedure is shown in figure 7c.

• Are you able to obtain all figures shownin 7b ?

Take the help of your friends tocomplete the task.

Let us begin the detailed discussionon reflection of light by plane surfaces.

Reflection of light by plane mirrors

Aim: Verification of laws of reflection

Required material: mirror strip, drawingboard, white paper, pins, clamps, scale andpencilProcedure: Take a drawing board and fix awhite paper on it with the help of clamps.Draw a straight line AB at the centre of thepaper and also a normal (ON) to AB at thepoint ‘O’. Draw a straight line PQ makingcertain angle (angle i) with ON as shown infigure 8. Fix two pins at the points P and Qon the paper vertically. Observe the imagePI of the pin P and QI of the pin Q, in themirror kept along the line AB. Fix two morepins R and S such that they are in the sameline as that of PI and QI. Join R, S and O asshown in figure 8.

fig-7a

fig-7b

fig-7c

Mirror

Lab Activity

P1

A B

P

Q1

Q

N

S

R

O

i r

fig-8

SCERT TELA

NGANA


Measure the angle between RS andON (angle of reflection). You will find thatangle of incidence = angle of reflection.Repeat the experiment for different anglesof incidence and measure thecorresponding angles of reflection (r).

Record your observations in table - 1

• Is the angle of reflection equal to theangle of incidence in all cases ?

In which plane does the incident ray,reflected ray and the normal lie (2nd law ofreflection of light) ? Let us discuss this.

Plane of reflection

In the above activity, the incident ray isthe ray which passes through the points Pand Q touching the paper. The reflected rayis the ray which passes through the pointsR and S touching the same paper, and ONis the normal to the mirror at point O.• Do the two rays and the normal lie in

the same plane? If yes, which is thatplane?

If the incident ray, reflected ray andnormal are in the plane parallel to the planeof the paper, where will that plane be?

Assume that the heads of all pinspierced at points P,Q,R and S in the aboveactivity are at the same height. The incidentray is the ray which passes through theheads of pins which are located at points Pand Q, and reflected ray is the ray whichpasses through the heads of pins which arelocated at points R and S.

• Where will the normal be?• In which plane will the incident ray,

reflected ray and the normal lie?

The plane in which the incident ray,reflected ray and normal lie is the plane ofreflection.

Assume that the heads of the pins whichare located at the points P and Q are not ofthe same height.• How will the incident ray be?• How will the reflected ray be?• How will the normal be?• How will the plane of reflection be?

Arrange two pins with different heights.Arrange the incident ray, reflected ray andthe normal with the help of spokes of acycle. Then think of the plane of reflection.• How does a mirror form the image of

a pin or any object? Let us discuss.

Formation of an image by a planemirrorCase - I: Point Object

In figure 9, O is a point object. Somerays from O reach the mirror and getreflected. When we look into the mirror,the reflected rays seem to be coming fromthe point I. So point I is the image of pointobject O.

S.No. (i) rIs i = rY/N

fig-9

OI

SCERT TELA

NGANA


Observe the distances of object O andimage I from the surface of the mirror andtry to compare these distances byapproximate estimation in figure 9. We findthat these distances are equal.

Case II: Object with certain height

Let us assume that an object (OOI) is

kept in front of a mirror as shown in figure

10. Draw a few incident rays from the object

to the mirror and reflected rays from the

mirror using laws of reflection. Your

drawing may look like that shown in figure10.

In the figure, the rays coming fromthe point O get reflected from the mirrorand seem to be coming from the point I. Sowe say I is the image of O.

The rays coming from the point OI getreflected from the mirror and seem to becoming from the point II. So we say II is theimage of OI.

The rays coming from the middle partof the O and OI will form their own imagesbetween I and II.

Thus, III is the image of the object OOI.• What is the size of the image compared

to the size of the object?

Let us discuss some of the characteristicslike size, distance and right-left inversionof an image formed by a plane mirror.

Characteristics of an image formed bya plane mirror

Take an object, say pen or pencil. Putit in front of a plane mirror, touching thesurface of the mirror.

• What do you say about the size of theimage compared to the size of theobject?

Move the object towards your eye. Whatdo you observe?

• Is the size of the image decreasing orincreasing?

Figure 10 shows the formation ofan image by a plane mirror. In that figure,you might have noticed that the size of theimage is equal to the size of the object. Whydoes the size of the image seem to bedecreased when you move the objecttowards your eye?

To understand this see figure 11,which shows how our eyes judge the sizeof an object.

Observers 1 and 2 are looking at theobject which is at point O. It looks smallerto observer 2 than to observer 1, becausethe light rays coming from the object makea smaller angle at the eye of observer 2 whois at a larger distance compared to observer1. (Confirm your self by measuring theseangles in Fig-11). The angle plays a role injudging the size of the object.

2

O

1

fig-11

fig-10

O

OI

I

II

SCERT TELA

NGANA


The lightrays which comefrom our right earget reflected fromthe plane mirrorand reach our eye.Our brain feelsthat the ray(reflected ray) iscoming from theinside of themirror (shown by dotted line in the figure-12). That is why our right ear looks like leftear in the image.

Now observe the lateral inversion ofa letter with a ray diagram in figure-13.

Think of the process of imageformation by a plane mirror and explainlateral inversion by observing figure-13.

In the same way when we move theobject from the mirror to our eye, theimage in the mirror seems to move back inthe mirror. Then the distance from theimage to our eye increases. The angle madeby image at our eye is smaller than the anglemade by the object. That is why the imagelooks smaller than the object.

When you stand in front of a mirroryou might have observed that the distanceof your image in a plane mirror seems tobe equal to the distance between the mirrorand yourself. What you observe is generallytrue. You can verify this by observing figure-10.

You also might have observed theright-left inversion of your image in a planemirror.

• Why does an image suffer lateral (right-left) inversion?

See figure-12.

• What do you understand from thefigure 12?

fig-12

fig-13

As shown in figure - 14 arrange some Telugu, English alphabets infront of a planemirror and observe the images.

fig-14

SCERT TELA

NGANA


Uses of plane mirrors in our daily life

1. Plane mirrors are used for dressing purpose of ourselves.

2. Plane mirrors are used for decoration on walls of some

shops like Jewellerey shops, Sweet shops, Barber shops

etc. to observe the shop in all directions, and to get

multiple images of things and persons.

3. Plane mirrors are used in making some optical instruments like periscopes etc.

4. Some type of Solar cookers are made by using plane mirrors.

• Light selects the path which takes the least time to travel (Fermat's Principle). It is

also applicable to reflection of light.

• Image of a real object in a plane mirror is virtual, errect and the same size of the

object.

• The image in a plane mirror appears to be small because of the small angle subtended

at our eye.

• Image in a plane mirror suffers lateral inversion.

I. Reflections on concepts.1. If a ray incidents normally on a plane mirror, what will be the angle of reflection?(AS

1)

2. Explain the laws of reflection? (AS1)

3. Explain the process of formation of an image with a pinhole camera? Draw a ray diagram

to show this. (AS1)

4. Why does the image in plane mirror suffers lateral inversion? (AS1)

5. Draw a ray diagram to understand the formation of image for a pointed object by a

plane mirror? Explain it. (AS1)

What we have learnt


Key wordsreflection, incident ray, reflected ray, normal, angle of incidence, angle of reflection,plane of reflection, lateral inversion, object distance, image distance, virtual image,real image

SCERT TELA

NGANA


II. Application of concepts

1. In the adjacent figure, AO and OB are incident and

reflected rays respectively. ∠AOB = 900. Find the

values of angle of incidence and angle of reflection?

(AS4)

2. Bharath stands in fornt of a plane mirror at a distance of 5m. from the mirror andobserves his image in the mirror. If he moves 2m. towards the plane mirror, thenwhat will be the distance between Bharath and his image? (AS

4)

3. Explain diagramatically the image of letter 'B' in a plane mirror. (AS5)

4. Why can't we see our image in a white sheet of paper though it reflects light? (AS2)

5. Discuss the merits and demerits of using mirrors in building elevation? (AS1)

III. Heigher Order Thinking Questions.

1. Observe the adjacent figure. AB and BC are two

plane mirrors arranged at 120o. A ray incidents at

and angle 55o on AB. Find the value of 'x' ? (AS6)

2. Niharika holds a clock in her hand, which shows thetime 3'O clock. If she observes the clock in aplane mirror, what will be the time thatthe clock show in the plane mirror? (AS

1)

3. Two plane mirrors are fixed at right angles to each other

and an object is placed between them as shown in the

figure. Trace the path of the rays by which an observer

see the final image in one of the mirrors. (AS4)

4. Two divergent rays originationg from the same point

have and angle of 100 between them. If they strike a plane mirror with same incident

angle, what will be the angle of reflection? (AS1)

5. The size of the image in the mirror seem to bedecreased when you move the object

towards your eye from the mirror. Draw the diagram showing angles depicting the

situation.

55o

120o

xoC

BA

A B

90o

fig-II-1

fig-III-1

fig-III-3

SCERT TELA

NGANA


Multipule choice questions

1. Angle of incidence = Angle of reflection. This rule is explained by __________

principle. [ ]

(A) Fermat (B) Newton (C) Archemedes (D) Pascal

2. Which of the following letters doesn't suffer lateral inversion. [ ]

(A) C (B) O (C) B (D) N

3. A ray of light incidents on a plane mirror at an angle of 90o to its surface. What will

be the angle of reflection. [ ]

(A) 0o (B) 90o (C) 45o (D) 180o

4. If we move an object away from the plane mirror the size of images seems to be

[ ]

(A) Increaces (B) decreased

(C) of the same size (D) Image can't be seen

5. Which of the following is incorrect with respect to the image in a plane mirror.

[ ]

(A) Image is erect (B) Size of the image is same as the size of object

(C) Larerally Inverted (D) Image is real


1. Verify laws of reflection experimentally.

2. Find the plane of reflection experimantally for the incident ray which passes through

the heads of the pins pierced infornt of the mirror as shown in figure 8.

Suggested Project Works

1. Make a solar heater / cooker with plane mirrors and write a report on the process of

making.

2. Generally periscopes are made in the shape of "Z" make your own periscope in the

shape of "C". Discuss the problems faced while using the periscope you made in

shape "C". Draw the ray diagram to explain the formation of image in 'C' shaped

periscope.

SCERT TELA

NGANA


COAL AND PETROLEUMChapter

7Deekshita is struggling to take down a

pickle jar which is at the top most sectionof a shelf.

Grandfather: Be careful, it may falldown and break.

Deekshita: No, I am taking down aplastic jar. It won’t break.

Deekshita realizes that Grandfather wasthinking of the clay jars that were used inolden days. Nowadays plastic jars are usedand they are unbreakable.

What else has changed, she wondered?Help Deekshita to find out.

Articles/ Activities (A) Articles/ Materials used30-40 years ago (B)

Articles/ Materials usedToday (C)

Activity-1

Identifying articles and materials used for various purposesHave a look at table 1. Column (A) gives the names of some activities and items. Ask

your grandparents or other older people about the names of the materials which is used forthe items given in column (A), and write them in column B. Then in column C, write thenames of materials being used at present. Few examples are given to guide you.

Table-1

Containers for storing pickle clay jars

Packing food for a journey plastic tiffin boxes

Water pipes in the houseHair combsVessels for cooking food Copper vessels Steel vesselsFuels used to cook foodFuel used in a train engine CoalLuggage for carrying clothes Metal trunksWater buckets, pailsWater storages in houses Plastic bucketsConstruction materialJewellery

Furniture (chairs, cots)

SCERT TELA

NGANA

94 Coal and Petroleum

From activity 1, you have a long list ofmaterials. Some of them like wood, metals(iron, silver, gold etc) were used ten years,fifty years and even hundred years ago. Doyour grandparents remember using plasticwhen they were young?

Now think and write the answers:

• How many of these materials wereavailable 10 years ago?



In case you have any doubt, you cantake the help of your social teacher and yourelders too.

you find that some materials such aswood and gold that are used today, were alsoused even thousands of years ago. Butothers like plastics are of recent origin.

The progress in science and technologygives us new products every day. The branchof science which deals with materials is

called material science.

Sources of materials

We know that we get wood from trees.Do you know that we get metals like iron,copper from their ores? Which material isused to make plastic or glass?

Do you remember reading aboutpetrochemicals in the chapter on syntheticfibres and plastics. How do we obtainvarious materials?

Table 2 gives some answers.

After looking at table 2, you find thatthe materials used earlier were obtainedfrom soil (clay, sand), water, ores, etc. Soil,water and air were not only sources ofmaterials used earlier but also sources ofenergy. Now, plastic has replaced manymaterials used earlier. Petrochemicals areused for making plastics. Many materialsthat we use for different purposes todaycome from various sources present innature. Therefore, soil, water, air, petroletc. are called natural resources.

Table 2

Material How is it obtained?Glass By melting sand with other materials and then, cooling it rapidly

Clay By mixing the mineral kaolinite with water

Wood From dried up trees

Plastics From petrochemicals

Metals From their respective ores

SCERT TELA

NGANA


Exhaustible and InexhaustibleResources

Now, we have to think about fewquestions to understand about need andimportance of natural resources.• Will the above resources be available

forever?• Won't we ever run out of air?• Won't we ever run out of water?• Can these resources be exhausted by

human activities?• Do we have unlimited supply of coal

and petroleum?

• How long do you think it would taketo grow the forests again?There are many resources like treeswhich will be depleted if we don’t makejudicious use of them.

• How long the fossil fuel like petroleumwill be available? Is it inexhaustible?

We need petroleum not just as a fuelbut also for several other uses as we willsee later in this chapter.

Resources therefore can be classifiedas inexhaustible and exhaustible dependingon whether they are expected to last foreveror not.

Activity-2List out the natural resources which are

limited and which are abundant and recordthem in table-3.

Table -3Resources Abundant Resources Limited

............................ ............................

Is petroleum exhaustible?

If we see the history of petroleumproduction, from 1859 to 1969, the totalproduction of oil was 227 billion barrels.(In the oil industry the barrel is the unitfor measuring petroleum volume andone barrel equals 159 litres). 50 percentof this total was produced during the first100 years (1859-1959), while the next50 percent was extracted in just ten years(1959-1969). Today our consumptionrate of oil is far excess than that of therate of its formation. Earth takes morethan one thousand years to form the oilthat we consume in one day. By about2015, we would have consumed half ofthe total reserves of the oil. It wouldbecome more and more difficult toextract oil in future.

Wind millsWind is an important natural

resource. Sail boats and sailing shipshave been using wind power sincethousands of years. Wind mills wereused to grind corn and to pump seawater to make salt.

Do you know?

Some energy sources like air and waterare not exhaustible on usage. Hence theseare called Inexhaustible resources. Coal andpetroleum etc, are the energy sourceswhich are exhaustible on usage. If thissituation continues, in future we might haveto depend on solar energy.

Is wood an inexhaustible resource?Think about using wood for makingfurniture.• What will happen if all the forests are

cut down and the wood is used forvarious purposes?

SCERT TELA

NGANA


Fig-

• What will happen if fossil fuel likecoal and petroleum are completelyexhausted?

• What would be our future energyresources?

The entire Research and Development(R&D) in the field of sources of energyshows that at the present rate of use ofthe conventional energy sources like

fossil fuels will not last for long. Presentlyonly 10% of non-conventional energyresources like solar energy, wind energy,tidal energy, etc, are used. These areinexhaustible resources and abundantlyavailable in nature.• Are the fossil fuel resources available

sufficiently to meet the future energyneeds?

• What actions are required to meet thefuture energy needs?

Coal, petroleum and natural gas as fuelsEarlier people used kiln (poyyi) for cooking food

but now people use gas stoves or kerosene stoves.Earlier food was cooked using wood as fuel, then coal,now kerosene and LPG are being used.

Fuel is needed not only for cooking but also fortransport. Different vehicles (train, bus, cars, two-wheelers) are used for travelling long distances and theyuse different fuels. People also travel by ships. Whatfuel is used there? Fuel is also required for electricitygeneration.

Bio -diesel an alternative fuel sourceBio-fuels are one of the major non-

conventional energy resources. They arenon-toxic and renewable. Bio-diesel isone of the bio-fuel which is an alternativeor additive fuel source to the standarddiesel fuel which is used now. It is madefrom the biological ingredients insteadof petroleum or crude oil. Bio-diesel

usually made from the plant oils or animal fat through a series of chemical reactions.It is safe and can be used in diesel engines.(But bio-diesel requires more cultivatableland which may affect shortage of food production in future.)

Do you know?

SCERT TELA

NGANA


Do you know?

Earlier you read that wind power wasused in wind mills. Then the steam enginewhich used coal was invented during theindustrial revolution.

During this period steam engine wasused to power everything from cloth loomsto vehicles on land and water.

Coal was the most important fuel in the19th Century. It is an exhaustible resourcebut we can meet our needs for another 250-300 years at the current rate of use. Till1950, coal accounted for half of theelectricity generation in the world.

Coal was replaced by petroleum withthe invention of more efficient engines indifferent vehicles. Now, coal is mostlyused to produce electricity in thermalpower plants.

Coal is an ancient gift serving

modern society.

Coal has been known and used for

thousands of years. It is one of our earliest

source of heat and light. But it became

a fuel of importance only after the

industrial revolution which led to an

increase in demand as fuel.

Do you know that the coal obtained

from the fire wood and the coal used in

vehicles and factories are different? Coal

used in factories is mined from the earth’s

crust. The coal obtained from the fire wood

is usually charcoal.

Petrol and diesel which are used invehicles today are obtained from mineralcalled petroleum. Petroleum has beenknown almost since prehistoric times.More than 4000 years ago, asphalt(Petroleum Product) was used in theconstruction of walls and towers ofBabylon. There are also records fromancient China about shallow wells beingdug to get petroleum. But what did ourancestors do with this petroleum? Theymainly used for making their woodenboats waterproof, as a fuel for lamps and in some traditional remedies. We realized theimportance of petroleum only after science and technology progressed to give uspetrochemicals and petrol to run engines.

Drilling Rig

Earth's crust

Natural gas

Petroleum

Water

Imperviousrock

Natural gas is another importantfuel

Natural gas is found trapped betweenimpervious rocks, sometimes along with

petroleum and sometimes withoutpetroleum. Previously, this gas was allowedto escape or even burnt while pumping outthe petroleum.

SCERT TELA

NGANA


(Below 400 c) Fuel gas LPG Fertilizer Syntheticrubber

To makechemicals

Shoepolish

cleaningsolution

petrol dry cleaning fluid Heating torch

Kerosene oil Wick stove Lantern

Diesel Oil Diesel generator Diesel inginewater

Lubricating Oil Lubricating oilgenerator

Fuel Oil IndustrialBoiler

Paints Makingroads

Asphalt

Paraffinwax

Ointment Facecream

Grease MatchBox

candle washpaper

vaseline

Residue

6000c

4000c

1700c

700c

1200c

2700c

and natural gas are going on under the supervision of ONGC (Oil and Natural GasCorporation ) throughout India. In India gasfields have been discovered in Tripura ,Mumbai High, Krishna, Godavari delta andJaisalmer.

Now, the natural gas is treated as equallyprecious because it is safer for theenvironment. Natural gas is stored underhigh pressure as compressed natural gas(CNG).

The exploration for more petroleum

Other uses of coal, petroleum and natural gas

Most of us think petroleum is a source of fuel. But advances in our understanding ofvarious chemical processes has led to the use of both coal and petroleum as the startingmaterials for a wide variety of products.

Petroleum is a complex mixture. It is separated into various components by aseparation technique known as fractional distillation

Look at the figure-2. We cansee the various productswhich are now being obtainedfrom petroleum. Initially, theseparation techniquesavailable were not advancedand could separate only a fewcomponents from thepetroleum mixture. One ofthe first fractions to beseparated from petroleum

Fig-2 : Uses of coal andpetroleum

was kerosenewhich wasfound to bebetter fuel thanthe petroleum.Now, we canseparate many

more components. Fuel gas, petrol, dieseletc. are all obtained from petroleummixture. These components of petroleumare then used to obtain other products.

Refining of Petroleum

SCERT TELA

NGANA


Activity-3

Identifying various uses of petroleum.Look at the figure-2 and find the other uses of petroleum and its products. Fill in the table.

Table -4Name of thepetroleum product Uses

Petrol

Fuel Oil

Kerosene

Diesel OilParaffin Wax

Coal:Coal is not as versatile as petroleum but

it is also very useful. Look at the figure-3.

We can see that coal gives us coke, coaltar and coal gas. Each of these componentshas several uses.

When coal is heated in air, it burns andproduces mainly carbon dioxide gas.

Coal is processed in industry to getsome useful products such as coke, coaltar and coal gas.

CokeIt is a tough, porous and black

substance. It is an almost pure form ofcarbon. Coke is used in the manufacture ofsteel and in the extraction of many metals.

Manufactureof steel

Extractionin metals

COKE

COAL

Roofing Materials

PerfumesNapthalene In Roads

Synthetic dyes

Pesticides

Medicines

ExplosivesPaints

Photographic Metals

Syenthetic Fibres

Fuel

Lighting

COAL

GAS

COALTAR

Fig-3: uses of coal and its products

SCERT TELA

NGANA


Aim: To show that when we heat highquality coal (carbon content is more) a gasevolves which can burn.

Material required: Two boilingtubes, Rubber corks, iron stands, deliverytube, jet tube, Bunsen burner.

Procedure: Take a spoon of powderedcoal into a hard boiling tube and fix it to astand as shown in the figure. Close the testtube with a rubber cork and connect it toanother boiling tube which is partially filledwith water and has fixed to other stand,with the help of ‘U’ shaped delivery tubeas shown in the figure. Arrange a jet tubeto the cork of second boiling tube. Heatthe boiling tube containing coal stronglywith the help of Bunsen burner.

Coalpowder

Boilingtubes

Delivery tube

water

Bunsen burner

Jettube

Stand

Fig-4: Burning the coal

Lab Activity

Coal tarIt is a black coloured thick liquid with

an unpleasant smell. It is a mixture of about200 substances. Products obtained from thecoal tar are used as starting material formanufacture of various substances likeSynthetic dyes, drugs, explosives,perfumes, plastics, paints, and roofingmaterials etc. Interestingly naphthaleneballs used to repel moths and other insectsare also obtained from coal tar.

Coal gasCoal gas is obtained during the

processing of coal to get coke. It is usedas a fuel in many industries situated nearthe coal processing unit.

Activity-4

Identifying uses of coalproducts.

Observe the figure-3 and list out the usesof coal products in the followingtable-5.You can collect more information bydiscussion with elders and with your friends.

Table-5

Coke Coal Tar Coal Gas

SCERT TELA

NGANA


• What do you observe?

You may notice that the brownish blackvapours evolved in the first boiling tube.

These vapours are passed through waterin the second boiling tube. Colourless gasbubbles evolve from the water. If you burnthe gas that is evolving through the jet tube,you can witness a bright flame at the nozzle.

Repeat the above experiment by usinglime water, soap water instead of waterused in second boiling tube.

• What do you observe? Give reasonsfor the changes you observed in twoexperiments.

• What inference can you draw fromthese two experiments?

Natural gas and

petrochemicalsNatural gas is not just an important

domestic and industrial fuel but also usedin the manufacture of fertilizers.

Some petrochemical products

Petrochemicals are used inpreparation of various articles assubstitutes for materials like wood andmetals which are limited. They are alsoused to manufacture several new productsthat cannot be obtained from wood, soil,metals etc. The table below shows howpetrochemicals are now used in all walksof life.

Look at the table given above. Think ofall materials which you use in a day. Canyou imagine spending a day without usingpetrochemical products? List how oftenyou use products obtained frompetrochemicals. Can you imagine life 100years ago when these are not available?

We use so many products now, whichwe did not have 100 or even 50 years ago.Increased consumption has lead to

Medical equipments,apparels like clothes,bedding, socks, furniture,paints, washing liquids, fibre,cosmetics, medicines,polishing liquids etc.

Cars, motor boats,communication devices,construction materials,paper industry, belt andstraps, tyres.

Plastic tubes,baskets, storing box,cultivation implements,fertilizers

Agricultural Sector Industrial sector Domestic and other sectors

Table - 6

Do you know?

Petrochemicals: The usefulsubstances which are obtained frompetroleum and natural gas are called petrochemicals. These are used in themanufacture of detergents, syntheticfibres (Polyester, Nylon , Acrylicpolythene etc). (Due to its greatcommercial importance petroleum isalso called as liquid gold).

SCERT TELA

NGANA


increased production of waste materialwhich created disposal problems. Forexample, for the last 3 years the use ofmaterials obtained from coal and petroleumto make a wide variety of cheap plastics hasrevolutionised the manner in whichhundreds of materials are manufactured.These materials also created a majorpollution to the environment.

••••• How are coal and petroleum formed?To understand this, explore the

processes by which coal and petroleum areformed.

Formation of coal

The plants in large and dense forests inlow lying wetlands got burried under thesoil due to the natural processes like floodsand earthquakes. As more soil depositedover them, the organic matter wascompressed. The temperature also rose. Asthe organic matter sank deeper and deeper,under high pressure and high temperature,these dead plants slowly converted to coal.As coal contains mainly carbon, the slowprocess of conversion of dead vegetationinto coal is called corbonisation. Sincecoal was formed from the remains ofvegetation, it is called fossil fuel.

Formation of petroleumPetroleum was formed from the

remains of tiny organisms called planktonthat were found in the bottom of seas andoceans. Plankton have tiny droplets of oilinside their bodies.

As these organisms died, their bodies

settled at the bottom of the sea or oceanand covered with layers of sand and clay.Over millions of years, due to absence ofair, high temperature and high pressurethese dead organisms transformed intopetroleum and natural gas.

Like coal, petroleum and natural gaswere also formed from the dead remains(fossils) of living organisms. Hence theyare also known as fossil fuels.

Why are coal and petroleum soversatile?

Coal consists of mainly carbon whilepetroleum consists of a mixture ofcompounds called hydrocarbons (theycontain mainly hydrogen and carbon).These compounds make good startingmaterials for other compounds based oncarbon. Carbon is very versatile and isthe basis for most of the materials.

Do you know?

Alternatives to natural gas

The gas resources which are notconventional like natural gas are knownas non-conventional gas resources . Ourcountry has enormous non-conventionalgas resources like coal bed methane andgas hydrates. These are not in commercialproduction phase due to the lack inproper technology. In future, when the oilera is expected to end, the only way tomeet the energy demands will be byproducing this non-conventional gas.SCERT T

ELANGANA


Therefore, coal and petroleum are veryimportant starting materials forsynthesising variety of useful compounds.

Conserving coal and petroleum

Why does the price of petrol go up allthe time? In simple terms, the price ofanything depends on how much of it isavailable and how essential it is. Both coaland petroleum are exhaustible resources,but we need them, both as fuel and asstarting materials for synthesising newcompounds. Since supplies are limited theyare becoming more expensive as thedemand for them increases. We have toconserve these resources as much aspossible, and also look for alternatives forthese resources.

Let us look at the issue of conservation.We can reduce the consumption of bothcoal and petroleum by either opting for adifferent model of development which doesnot depend on these resources or reducethe wastage of these resources. Since thefirst option is right now impracticable,second option of reducing wastage is to bepracticed. The governments of manycountries are working on this problem offinding alternative technologies to reducethe use of fossil fuels.

Misuse of energy resources andconsequences

In our daily life while doing variousday to day activities we use many typesof energy resources and fuels withoutgiving a thought about the exhaustibilityof these resources and consequences. Forexample in urban areas dryers are used in

washing machines which consume electricenergy, eventhough abundant source of heatenergy in the form of sunlight is availablearound us. Similarly motor bikes are usedeven for shorter distances. Walking shorterdistances or using bicycles saves not onlyfuel but also keeps good health.

• Can you give some more exampleswhere energy or fuel resources aremisused in our daily life?

• Can you suggest alternate ways to savethe fuel resources?

• How is biodiversity effected byexcessive use of fossil fuels?

Group discussion on misuse offuel resources and itsconsequences.

Discuss in small groups how fuelresources are being misused in our dailylife while doing various activities like...

a) transportation,

b) cooking

c) industrial use

Fig-5

Activity-5

only one litre of kerosenefor each person

SCERT TELA

NGANA


• What are the consequences of misuseof fuels? Prepare a report.

• What methods could you suggest toprevent the misuse of fuels?

Harmful effects causedduring use of fuels

Most of the harmful effects are due tocarelessness of handling petroleumproducts. For example, crude oil andrefined fuel spills into sea from tanker shipby accident causes damage to natural ecosystem and kill sea birds, mammals, shellfish and other organisms.

Utilisation of these fuels leads to some

more harmful effects. Let us see that

• Burning fuels releases carbon dioxide,a green house gas, which causesclimate changes and leads to globalwarming.

• Coal fired power plants emits mercury,selenium, arsenic, lead in addition togreen house gases which are harmfulto human health and environment.

Many paints made from petroleumand heavy matels release toxic productsinto air. These toxic products cause avariety of health problems including heart,lungs damage, nausea and dizziness.

Key words

Natural resources, exhaustible resources, inexhaustible resources, petroleum,fractional distillation, natural gas, CNG, coke, coal gas, coal tar, carbonisation,plankton, fossil fuels, petrochemicals.

• Natural resources can be classified into Exhaustible and inexhaustible resources.

• Fossil fuels are formed from the dead remains of living organism under the earth overmillions of years.

• Coal, petroleum and natural gas are fossil fuels.

• Coke, coal tar and coal gas are the products of coal.

• Petroleum is formed at the bottom from the remains of tiny organism called plankton.

• Petroleum gas, Petrol, Diesel, Kerosene, paraffin wax, lubricating oil are obtained byrefining of petroleum.

• The natural gas is found sometimes along with petroleum and sometimes withoutpetroleum.

What we have learnt

SCERT TELA

NGANA


• The useful substances which are obtained from petroleum and natural gas are calledpetrochemicals.

• Excessive use of fossil fuels causes air pollution, greenhouse effect, global warmingand many health problems.

• Fossil fuel resources are very limited. We should think for the alternatives.

• Advances in science and technology have changed our lives.

Reflections on concepts1. Draw the diagram of uses of coal and its products. (AS

5)

2. Explain why petroleum is also called as liquid gold? (AS1)

3. Name the petrochemical products which are used in agriculture industry. (AS1)

4. Explain the process of formation of petroleum in Earth. (AS1)

5. Why should people look for alternative sources of fossil fuel? (AS7)

Application of concepts1. Name the petroleum products used for surfacing of roads. (AS

1)

2. What will happen if fossil fuels like coal and petroleum are completely exhausted?(AS

2)

3. Collect information about places where we get coal, petroleum and natural gas inIndia and mark the places on outline map of India. (AS

5)

4. Assume that you are a driver, what measures do you take to save petrol and diesel?(AS

7)

5. How do you appreciate the efforts of human beings to discover an alternate energysources to Coal and Petroleum for their daily use? (AS

7)

6. Suggest some alternative ways to save the fuel resources. (AS7)

Higher Order Thinking Questions1. How is biodiversity effected by the excessive use of fossil fuels? (AS

7)

2. “Crude oil and refined fuel when spills into the sea from tanker of ships by accident”.Discuss the consequences of this on environment. (AS

7)

Multiple Choice Questions1. Which one of the following is less polluting fuel [ ]

a) Natural gas b) coal gas c) Kerosene d) Petrol

2. The main constituent of coal is [ ]

a) Carbon b) Oxygen c) Air d) Water


SCERT TELA

NGANA


3. Which one of the following material is used for making shoe polish [ ]

a) Paraffinwax b) Petrol c) Diesel d) Lubricating oil

4. Which of the following is not a fossil fuel [ ]

a) Coal b) Petroleum c) Charcoal d) Natural gas

5. Which of the following is known as liquid gold [ ]

a) Kerosene b) Diesel c) Petroleum d) ParaffinwaxSuggested Experimetns1. Conduct an experiment to show that when we heat high quality coal,a gas evolves

which can be burnt.Suggested Projects1. Burning fuels releases carbon di oxide , a green house gas, which causes climate

changes and leads to global worming.Collect information about this throughnewspapers,magazines etc. and prepare report.

2. Compare a CNG run vehicle with that of a diesel run vehicle. What difference doyou notice in both cases with respect to pollutants released, level of pollution andcost of fuel.

Prepare a report on your findings.

3. Choose five families of your neighbourhood, collect the information about themeasures that they adopt to conserve energy resources in transport and cooking.Make a report on your observation

Name of the fuel Cost of the fuel as on today Pollutants liberated

Diesel/Petrol.

CNG

the Head ofthe family

Name of Number of No of vehicles Amount spent for Amount spent for

members in family using Fuel in one month Cooking purpose

SCERT TELA

NGANA

107Telangana Government Free Distribution2019-20

COMBUSTION, FUELS ANDFLAME

Chapter

8

We use different kinds ofmaterials as fuels for various purposes athome. You might have observed or heardabout how people used to burn wood, coal,cakes of cow dung, kerosene etc., forcooking food at home. Blacksmiths invillages also use them for heating metals.Both in urban and rural areas, now a day’s,LPG is used as fuel for cooking the food.We use the light from the burning candleor kerosene lamp, when there is no supplyof electricity. You might have also observedburning of a candle or coal. Whatdifference do you notice in the burningprocess?• Why does candle give flame when it is

burnt but why does coal burn withoutemitting a flame ?

• Do all fuels produce same amount ofheat when they are burnt?

• What do we need to burn a material?• Have you ever tried burning a piece of

paper or wood or coal, a small rock ora pebble?

• Do all of them burn?Let us do an activity to know which of

these materials burn and which do not.

Activity-1

Do all materials burn ?You will need a pair of tongs, some

metal or clay dishes and a candle or a spiritlamp.

Using tongs, pick up a small piece of paperand bring it close to the lighted spirit lampand keep it on flame as shown in figure-1.

Record your observation in table 1.

Fig-1

Carry out this experiment with a pieceof charcoal, magnesium ribbon, straw,cotton cloth, nylon cloth, dry wood, pebble,wax, plastic piece etc, and record yourobservations.

You can also try to burn liquids.Take 2ml of water in small plate. Bring

lighted stick near to water in the plate (seefigure 2).

SCERT TELA

NGANA

108 Combustion, Fuels and Flame

• What do you observe in your attemptof burning water?

• Is there any difference in flame oflighted stick?

• What happened to the lighted stick whenit is brought closer to water in the plate?

Carry out this activity using coconutoil, mustard oil, kerosene, spirit, petroletc. (sticks should be long enough toprevent any fire accident if the materialcatches fire)

Record your observation in table 1.• What can we conclude from this

activity?We can conclude that some materials

burn and others don’t. In the aboveactivity you observed that when materialsburn in air, heat and light are produced.

Fig- 2

A chemical process in which a materialreacts with oxygen present in the air togenerate heat is called combustion. Thematerials which burn when brought near aflame are combustible materials. Some ofthem can also be used as fuels. Thematerials which do not burn are called non-combustible materials.

• Which of the material in the aboveactivity are combustible?

Think and discuss

• Why some material burn and whysome do not? Give reasons.

• Why some materials which do not burnat normal temperature burn at highertemperatures?

What is needed for the processof combustion?

We know that we need a match stick ora lighter to burn a material.• How will you prove that air is needed

to burn a material?• Can we burn a material in the absence

of air?

S.No. Name of the material How does it burn burnt Burns immediately Burns slowly Does not burn

1 Magnesium ribbon2 Pebble

3 Petrol

4

5.

Table 1

SCERT TELA

NGANA


We can carry out an experiment todemonstrate that oxygen helps in burning.

Aim: To prove that oxygen helps in burningMaterial required: Test tube, test tubeholder, spirit lamp, match box, inscencestick (agarbatti), potassium permanganatecrystals.

ProcedureLight a scented / incense stick (agarbatti),and let it burn for 10 s, then put out theflame and keep it aside.

Take potassium permanganate in a testtube. Hold the test tube with a test tubeholder and heat it over the flame of spiritlamp. Oxygen is released on heating ofpotassium permanganate.

Insert the agarbatti with the burningstub, in to the test tube as shown infigure 4.

Fig-4

Observations• How does scented stick started burning?• Why does not it catch again fire when

it is kept aside in air after putting itsflame off?You observe that stick burns with a

flame. Here the oxygen supportscombustion by helping Agarbathi to burnwith bright flame.

Activity-2

Testing the necessity of air forburning

Take a small burning candle and put iton a table. Invert a glass tumbler over it.The candle continues to burn for some time.Then flickers and finally flame goes off.(See figure 3)

Remove the tumbler and again light thecandle. Put the tumbler back over thecandle. When the candle flame begins toflicker, remove the tumbler. What happensto the candle? Notice carefully.

We find that putting the glass tumblerover the candle cuts off the supply of airand the candle flame goes off.

This experiment proves that air isneeded to burn a material. Some otherexperiments need to be conducted to provethat the oxygen present in air supports thecombustion.

If you lift the glass tumbler (Whichis placed over a burning candle) to 1cmheight what happens? Why?

Think and discuss

Lab Activity

Fig-3

SCERT TELA

NGANA


Think and discuss

• How do you say that the gas releasedin the above experiment is oxygen?

• Can we replace potassium permanganatewith any other substance to releaseoxygen?

• Is there any other procedure to provethat oxygen is needed for burning?

A few more examples of combustionare given below. Can you explain thereasons for the changes taking place ?

• A slow fire bursts into a flame whenair is blown on it, but a candle burningwith flame goes off when air is blownon it. Why?

• If a large quantity of dry grass is seton fire in forests then it is verydifficult to put off the fire. Why?

• When an object catches fire, the fireis put off by covering with sand or ablanket. Why?

The examples and activities discussedabove shows that combustion takes placeonly in the presence of air. We know thatsome materials catch fire as soon as theyare brought near a flame, but some materialstake a long time to start burning though theyare kept near the flame.• What could be the reason for the

difference in burning amongcombustible materials?Let us explore this in the following

activities.

Ignition Temperature

In activity 1, a candle is used to burn apiece of paper. Can we burn paper withoutthe help of flame ?

Activity-3

Burning a paper with sun rays

On a sunny day, go out and focus thesun rays on a piece of paper using amagnifying lens (Figure-5). Touch the spotafter some time. How do you feel?

Fig-5

You must have heard about people inancient times rubbing pieces of stonestogether to produce sparks. Have you triedit? Rub two stones hard together and touchthem. What do you feel?

Now recall some of your experiences:• Does a matchstick burn by itself?• Why do you rub the match stick on the

side of the match box to burn it?• Can you burn a piece of wood by bringing

it close to a lighted matchstick?• Why do we use paper or kerosene oil

to start fire in wood or coal?On the basis of above observations and

previous experiences, we can conclude thata combustible substance has to be initiallyheated to catch fire or burn. The lowest

SCERT TELA

NGANA


temperature at which a substance catchesfire is called its ignition temperature.When a substance starts burning heat isproduced and it helps to burn the substancecontinuously. The ignition temperature isdifferent for different substances. Theignition temperature of a substance decidesquickness of catching fire.

The substances which have very lowignition temperature and easily catch fireare called Inflammable Substances.Examples of inflammable substances arepetrol, alcohol, liquified petroleum gas(LPG) etc.,

• Can you make a list of some moreinflammable substances?

Activity - 4

Understanding ignition temperature

Take two small paper cups. Pour waterin one of the cups. Put the two cups ondifferent tripod stands and heat both ofthem using a candle as shown in thefigure-6.

Fig-6

• Which cup burns first?• Does the water in the cup become hot?

Why?Explain why one cup burns quickly but

the other does not, on the basis of ignitiontemperature. When heat is supplied to

cups, the heat received by second cup istransferred to water in it. The water in thiscup prevents the paper to reach its ignitiontemperature and hence it does not burn.• When does the second cup start burning?

Make a guess and discuss with yourteacher.Types of Combustion

During summer time, dry grass catchesfire in the forests. It may spread to the treesand very soon the whole forest will be onfire. It is very difficult to control such forestfires.

If the head of a match stick is rubbed onthe side of a match box, it starts burning.

• What makes match sticks to catch fire?A mixture of antimony trisulphide,

potassium chlorate and white phosphoruswith some glue and starch was applied onthe head of a match stick made of suitablewood. When it struck against a rough surfacewhite phosphorus got ignited due to the heatof friction. This starts the combustion of thematch stick. However, white phosphorusproved to be dangerous both for the workersinvolved in the manufacturing of matchboxes and for the users.

These days the head of the safety matchstick contains only antimony trisulphide andpotassium chlorate. The rubbing surface haspowdered glass and a little red phosphorus(which is much less dangerous). When thematch stick is struck against the rubbingsurface, some red phosphorus gets convertedinto white phosphorus. This immediatelyreacts with potassium chlorate in thematchstick head to produce enough heat toignite antimony trisulphide and start thecombustion.

empty cup cup with water

SCERT TELA

NGANA


FuelsWe know that combustion gives heat

and light. The sources of heat for domestic,

automobile and industrial purposes are

mainly wood, charcoal, petrol, kerosene,

LPG, CNG etc. These substances are fuels.

In the previous chapter, we studied about

the fossil fuels and their use in various

ways. We not only use fossil fuels but alsoother kinds of fuels for different purposesat home, in automobiles and in industries.Tabulate different fuels that are used for !

Domestic Transportation Industrial

purpose purpose purpose

Classify the above fuels into solid, liquid,gases and write them in table 2.

Table - 2

Solid Liquid Gas

Look at the fuels in the table 2.

• Can you decide the best fuel amongthem?

• What is the criteria to decide a bestfuel?

Discuss with your friends.

Deciding the best fuel depends uponthe purpose of its use. A best fuel forcooking may not be a best fuel for runningan automobile.

The type of combustion in whichmaterial suddenly burns into flameswithout the application of any externalagent is called spontaneous combustion.

Turn on the knob of the gas stove in thekitchen and bring a burning match stick ora gas lighter near it. The gas burns rapidlyand produces heat and light.

Such combustion is known as rapidcombustion. Materials such as spirit,petrol and camphor burn even with a sparkfrom a gas lighter.

Do you know?

1. You might have noticed wordswritten as highly inflammable on petroltankers. This is a warning to the publicto keep flame away from the inflammablematerial.

2. We generally enjoy sound andlight from fire works on festival days.When a cracker is ignited a suddenreaction takes place with the evolutionof heat, light and sound. A large amountof gas is also liberated in this reaction.Such a reaction is called explosion.Explosions can also take place ifpressure is applied on the crackers.

Think and discuss

• Why is phosphorus preserved inwater? (Hint: think about the role ofignition temperature in combustion)

• Why Kerosene stoves and Bunsenburners have small holes in them?(Hint; Think about the role of aircombustion)

• It is hard to ignite match stick in rainydays. Why ?

SCERT TELA

NGANA


In general, there are several factors

that have to be kept in mind while choosing

a fuel, like purpose of use, fuel efficiency

and availability, reasonable price, easy to

handle and safe to store, easy to ignite and

put off etc. The fuel should also burn at a

moderate rate and cause less pollution. In

addition, it should have a high calorific

value.

• What is calorific value ?

Suppose 1 kg of coal and 1 kg of cow

dung are burnt. Which one produces more

heat? Different substances produce

different amounts of heat on burning. Heat

is measured in kilo joules. Calorific value

of a fuel is the amount of heat energy

produced on complete combustion of 1 kg

of that fuel. It is measured in kilo joules

per kg (kj / kg).

Fire control

You must have seen or heard about firebreaking out in houses, shops, factories,etc.• How can we put off the fire if it breaks

out ?We use many methods to extinguish a

fire but they all follow one principle, whichis the principle of elimination of factors

which support the combustion.Let us recall the factors which support

the combustion:(a) Presence of a combustible material or

the fuel(b) Supply of air or oxygen(c) High temperature (More than the

ignition temperature)So, elimination of any one of the three

factors will help in controlling the fire. Letus see some examples.Example

If a fire breaks out in a house or in anybusiness establishment the fire brigade willimmmediately put off the electric mainsand then start spraying water on the fire.• Why the fire brigade start the work by

putting of the electric mains?• How water helps in eliminating the

factors, which support the combustion?Initially, the water spray cools the

combustible material below its ignitiontemperature. This prevents the fire fromspreading.

Fuel Calorific value (kj/kg)

Cow dung 6000-8000

Wood 17000-22000

Coal 25000-33000

Petrol 45000

Kerosene 45000

Diesel 45000

CNG 50000

LPG 55000

Biogas 35000-40000

Hydrogen 150000

Do you know?

SCERT TELA

NGANA


Then the heat turns the water intovapours which surround the burningmaterial and prevent supply of oxygen tothe burning materials. So, the fire isextinguished.

The most common fire extinguisher iswater. But water works only when thingslike wood cloth and paper are on fire. Ifelectrical equipment is on fire water mayconduct electricity and harm those tryingto douse the fire.

Water is also not suitable for firesinvolving oil and petrol, because water isheavier than the oil, it sinks below the oiland oil keeps burning on the top.

Since it is difficult to remove thecombustible material from a fire, cuttingof air supply and lowering the temperatureare better methods.

In cases where water cannot be used,carbon dioxide gas is the best choice usedwhich is heavier than oxygen. It can bestored as a liquid in cylinders under highpressure. When released from the cylinder,it expands and brings down the temperature.

It also covers the fire like a blanket.Since the contact between the fuel andoxygen is cut off, the fire is controlled.

That is why it is an excellent fireextinguisher. The added advantage of carbondioxide is that in most cases it does notharm the electrical equipment. It ismandatory for offices, educationalinstitutions and multistoried buildings toinstall fire extinguishers.

Flame

Activity - 5

Observing the behaviour ofdifferent solid fuels

Collect some fuels like candle, coal,domestic gas, charcoal, magnesium ribbon,wood, cakes of cow-dung, camphor, wickof the oil lamp, wick of kerosene stove, etc.Burn each of them one by one with the helpof spirit lamp and note the time they taketo catch fire. Also observe how do theyburn?

• Do all of them burn in the samemanner? If not, what difference do younotice?

• Do all of them form a flame while theyare burning?Record your observation in the

following tableTable - 3

Material Forms Does not flame form flame

Candle

Magnesium

Wick ofKerosene stoveCharcoal

Domestic gasCamphor

Cow dung cake You may observe that a candle burnswith flame where as charcoal does not.Some materials burn with flame, some donot. Kerosene oil and molten wax risethrough the wick become gas and formflames. But charcoal cannot be vapourised.

SCERT TELA

NGANA


So it does not produce a flame. A fuelcatches fire immediately if it is in the formof gas. Cooking gas catches fireimmediately. Spirit and petrol turn into gasat room temperature. Hence, they catchfire quickly.

Think and discuss

A wax candle burns with a yellowflame. The domestic gas burns with ablue flame. Why?

Structure of a flame

Activity-6

Observingthe structure of the flame

Light a wax candle and watch the flame.Carefully note the different coloured zonesin the flame. How many colours are therein the flame?

Starting from the base of the flame,how many flame zones do you observe?What is the colour of the outer most zoneof the flame?

Fig-7

hottest part

moderatelyhot

least hotdark zone

middle zone

outer most zone

blue zone

Observe the innermost zone which isdark. What do you observe there? Is therecombustion takes place? In this zone wax getsvapourized. This is a dark zone. See figure 7.

Observe near the base of the flame.Vapourized wax gets completely oxidizedand burns with a blue flame. It is blue zone.

Do you know?

A candle is mainly a source of lightbut heat is also released. A candle is madeof wax in which a thick thread is inserted.Wax in the candle melts when it is lightedby a match stick. A little of the wax formsvapour. This vapour combines withoxygen in the air to form flame. The heatof the flame melts more wax from thetop of the candle. The liquid wax movesupward through the thread. It also changesto vapour when it reaches the top of thewick and burns with the flame.

Activity-7

Observing what happens in differentzones of candle flame

Light a candle. Hold a glass tube with apair of tongs and introduce its one end in thedark zone of a non flickering candle flame.Keep lighted match stick near the other endof the glass tube as shown in figure-8. Whatdo you observe? Do you see a flame? If sowhat is it that produces a flame? Notice thatthe wax near the heated wick melts quickly.SCERT T

ELANGANA


Fig-8

When the candle's flame is steady,introduce a clean glass slide into theluminous zone (yellow zone) of the flameand hold for 10 seconds. See figure 9. Whatdo you observe?

Fig-9

A blackish circular ring is formed onthe glass slide. What is it? It indicates thedeposition of un-burnt carbon particlespresent in the luminous zone of the flame.Incomplete combustion takes place in thiszone.

Hold a thin long copper wire just insidethe flame for about half a minute as shownin figure-10. What do you observe? Thecopper wire just outside the flame gets redhot. It indicates that the non-luminous zoneof the flame has high temperature. It is thehottest part of the flame. It is blue in colourand complete combustion takes place dueto good supply of oxygen.

Fig-10

Key words

Combustion, combustible and non - combustible materials, ignition temperature,

inflammable material, spontaneous combustion, rapid combustion, explosion, fuels,

calorific value.

• Burning a material in the presence of air (oxygen) is called combustion.

• Oxygen or air is needed for combustion to take place.

• The lowest temperature at which a substance catches fire is called its ignitiontemperature.

What we have learnt

SCERT TELA

NGANA


• The type of combustion in which material suddenly burns into flames without theapplication of any external agent is called spontaneous combustion.

• The type of combustion in which material burns rapidly and produces heat and light iscalled rapid combustion.

• The amount of heat energy produced on complete combustion of 1kg of fuel is calledthe calorific value of that fuel.

• Wax does not burn in the dark zone of the candle flame.

• In the blue zone of the candle flame, vaporized wax burns completly due to goodsupply of oxygen.

I. Reflections on Concepts

1. Give four examples of combustible materials. (AS1)

2. Why should not we store spirit or petrol near our living place? (AS1)

3. The oil fires should not be sprayed with water. Why? (AS2)

4. Water is not used to control fires involving electrical equipment. Why? (AS1)

5. List the ways adopted by fire fighters to combat fires. (AS1)

6. Draw the diagram of candle flame and label all the zones. (AS5)


1. What precautions are to be taken while pouring water on fire? (AS1)

2. Give an example of a good fuel. How do you choose that fuel? Explain. (AS1)

3. It is difficult to burn a heap of green leaves but not a heap of dry leaves. Explain why?(AS

2)

4. Where do you find spontaneous combustion and rapid combustion in your dailylife? (AS

7)

5. Why the fire brigade start the work by putting off the electric mains? (AS1)

6. Explain giving reasons: In which of the following situations water will get heatedin a shorter time? (AS

3)

a) Srikar kept water beaker near the wick in the yellow part of a candle flame.

b) Sonu kept water beaker in the outer most part of the flame.


SCERT TELA

NGANA



1. Why Phosphorus preserved in water? (AS1)

2. How do you feel about “Fuels have become a part of human life”? (AS7)

3. Is there any other procedure to prove that oxygen is needed for burning? (AS2)

4. In a few years the fuels on earth will be exhausted. Think,what would happen tohuman civilization? (AS

2)


1. The gas needed for combustion among the following [ ]

a) Argon b) Oxygen c) Carbon dioxide d) Hydrogen

2. The lowest temperature at which a substance catches fire is called its [ ]

a) Ignition temperature b) Maximum temperature

c) Room temperature d) Normal temperature

3. The units of calorific value is [ ]

a) Newtons/grams b) Newtons/Kg c) Kilojoules/kg d) Kilojoules/gram

4. Spirit and petroleum turns into gas at [ ]

a) Room temperature b) Ignition temperature

c) Maximum temperature d) Normal temperature

5. The type of combustion in which material suddenly burns into flames without theapplication of any external agent is called [ ]

a) Rapid combustion b) Slow combustion

c) Spontaneous combustion d) Explosion

Suggested Experiments1. Conduct an experiment for testing the necessity of air for burning.

2. Conduct an experiment to prove that Oxygen helps in burning.

3. Can you heat water in a paper vessel? How is it possible?Conduct an experimentto understand the Ignition temperature.

Suggested Projects1. List out the different fuels that are used in your daily life and classify them into

solids,liquids and gases.

2. Collect information available on different fuels. Find out the cost per kg.Compare the cost with calorific value. Prepare report on that.

3. Collect the information about annual fuel consumption in different parts of the world.How many years more the fossil fuels last? Make a poster with this information andissue an appeal to save fuel.

SCERT TELA

NGANA


ELECTRICAL CONDUCTIVITYOF LIQUIDS

Chapter

9

pieces of connecting wires. Set up theelectric circuit as shown in the figure-1.

Fig-1 : Testing conductivity of material

Place the key on drawing pin. The bulb

begins to glow as soon as the key touches

the drawing pin. Now replace the key by a

nail. Does the bulb glow?

Repeat the activity using different types

of materials instead of the nail, say a strip

of paper, a piece of chalk, a drinking straw,

a piece of plastic, a paper clip, a rubber

eraser, pencil graphite etc.

Note in each case whether the bulb

glows or not and enter your observations

in Table-1.

Sometimes we read in newspapersabout farmers getting electricshocks while starting water pumps of thewells, especially because of contacts madewith switches or starters by wet hands. Doyou know the reason behind getting electricshock while working with wet hands?

Our elders caution us about touchingelectric heater immersed in water. Why dothey instruct us to stay away from it? Howdoes the electric current flow throughwater?

In the earlier class you have learnt thatelectric current can pass easily throughmetals like copper, aluminium, etc. Do yourecall some other materials which conductelectric current? Let us revisit the activityand do it now.

Activity-1

Testing the material to knowwhich allows electric currentto pass through it

Take a torchbulb or LED (LightEmitting Diode), a drycell, Wooden sheet,two drawing pins, akey (safety pin) andSCERT T

ELANGANA

120 Electrical Conductivity of Liquids

Take care that whenever the bulb glows,it should not be kept in the 'ON' positionfor a long time to avoid the early dischargeof dry cell.

In the above activity, we observed thatsome materials allow electric current topass through them. We call them as goodconductors of electricity.

In general, all metals are goodconductors of electricity. On the otherhand, the materials that do not allow currentto pass through them are called bad or poorconductors of electricity.

Think and discuss

Why some material allows electriccurrent to pass through them and whysome do not?

Electric conductivity is a property ofany given material. We can say that amaterial has good electrical conductivity

Table 1

S.No. Object Material Bulb glows Good conductor/ Bad or

Yes/No Poor conductor

1 Nail Iron Yes Good conductor

2 Eraser Rubber No Bad/Poor Conductor

if it allows electric current to pass throughit easily.

Electrical conductivity of liquids

In the activity-1, we have testedconductivity of objects like nail, paperstrip, chalk, etc. All of these are solids.What about liquids? Do the liquids allowelectric current to pass through them?

Let us do another activity to find outwhether a given liquid allows electriccurrent to pass through it or not.

Activity-2

Testing the electric conductivityof liquids

Take a LED, dry cell, metal pins, rubbercap of injection bottle and wires for makingconnections. Set up an electric circuit asshown in the figure 2.

SCERT TELA

NGANA


LED

cell

Fill the rubber cap with differentliquids, one after another and in each case,check whether the LED glows or not. Startwith distilled water (you can get distilledwater from battery stores or from medicalshop). Pour distilled water in the rubber captill the two metal pins come in contact withit. Check whether the LED glows or not.

Then take water that you drink in schooland repeat the procedure. Do this activitywith liquids like coconut oil, kerosene,lemon juice, mustard oil, sugar solution,etc. After testing each of the liquids,carefully wipe and dry the cap and the pinsbefore filling in the next liquid. In eachcase, note your observations in Table2.

From your observations, decide whichliquids are good conductors of electricityand which are poor or bad conductors. Noteyour inference also in Table 2.

Fig. 2: Testing conductivity of liquids

See that the two metal pins, passthrough the cap should have a very smallgap (around 2 mm) between them so thatthe pins are fairly closer but not touchingeach other. The LED should not glow whenpins are separated by the small distance.

Now, join the free ends of the pinstogether by pressing them for a momentand make sure that the LED glows. Releasethe pins, they get separated and LED shouldnot glow. This becomes our tester. We willuse this tester to check the conductivityof liquids.

Rubbercap

Liquid

Table 2

S.No. Liquid LED glows Good conductors/ poor orYes/No bad conductors

1 Distilled Water No bad conductor

2 Drinking water Yes good conductor

3 Coconut oil

4 Lemon juice

5 Vinegar

6 Kerosene

7 Vegetable Oil

8 Sugar solution

9

10

Let us think about the above table.

SCERT TELA

NGANA


Why doesn’t the LED glow in all thecases? Or why doesn’t the LED remainoff in all the cases?

In activity 1, we saw that when currentflows through the object inserted in the gap,the bulb glows. Similarly, we can see thatwhen the liquid between the two pins ofthe tester allows electric current to passthrough, the circuit is completed (closed)and the LED glows. Then we say, the liquidis a good conductor.

On the other hand, when the liquid doesnot allow the current to pass through, thecircuit is incomplete (Open) and the LEDdoes not glow. Then we say the liquid is abad conductor.

List out the good conductors from table 2.

In the above activity, you may haveobserved that in all those cases where theLED glows, its brightness (intensity) is notthe same. Sometimes it may be brighter andsometimes it may be relatively dimmer.Why is that so?

The intensity of the glow of the LEDdepends on the flow of electric currentthrough the circuit. Although a liquid maybe a conductor, it may not allow currentthrough it as easily as a metal does.

As result, although the circuit iscompleted and the LED glows, due to weakcurrent in cases of some of the liquids, theintensity of glow would be lower comparedto other liquids.

Do you know?

Why do we use LED in the tester

instead of a bulb?

LED glows even when a very weak

current is passing through the circuit.

Thus, it helps in testing flow of

electricity in conductors when meager

current is passing through the circuit.

Since LEDs glow even with a very

little current passing through them, they

are used as ‘indicators’ in electrical

appliances like mobile phones,

televisions, transformers, etc. to indicate

whether the device is working or not.

There are two wires called leadsattached to an LED. One lead is slightlylonger than the other. See figure 3.

Fig. 3 (LED)

While connecting to the LED to thecircuit, the longer lead is alwaysconnected to positive terminal of thebattery and the short lead is connectedto the negative terminal of the battery.

SCERT TELA

NGANA


three different containers. Dissolve smallquantity of common salt in the water of firstcontainer. Dissolve the Copper Sulphate(Mylatuttam), lemon juice in 2nd and 3rd

containers respectively.

Use the tester that we used in activity 2,and repeat the activity 2. Note yourobservations in Table 3.

(Caution: Wash and wipe the pins oftester to dry after testing with each liquid.)

Table 3

S.No. Material Does the LED glow? Good conductor/ bad

Yes/No or poor conductor

1 Distilled water No Bad conductor

2 Dist. Water + salt

3 Dist. Water +

4 Dist. Water + lemon juice

From the above observations, what canwe infer? Distilled water does not allow theelectric current to pass. Water in its pure(distilled) form is a bad conductor ofelectricity. But when water contains salts oracids, it allows passage of electric currentand turns into a good conductor ofelectricity.

The water that we get from sources suchas taps, hand pumps, wells and ponds is notpure like distilled water. It contains somesalts and minerals dissolved in it. Some ofthese minerals are useful for our health. Thiswater is a good conductor of electricity.

On the other hand distilled water is freeof all salts, minerals, acids, etc. and is apoor conductor of electricity.

Do you now understand why you areadvised not to touch electric applianceswith wet hands?

Water with salts is a good conductorof electricity and the current flowingthrough house hold electric appliances isvery high. Therefore, we should never touchthe electrical appliances with wet hands.

Like water in the above activity, mostliquids that conduct electricity aresolutions of acids, bases or salts.

When do liquids conductelectricity?

In the last activity we observed thatdistilled water does not conduct electricity.Can we make poor conductors like distilledwater to conduct electricity? Let us try itout.

Activity-3

Electric conductivity ofelectrolyte

Take same amount of distilled water in

coppersulphate

SCERT TELA

NGANA


LED

Chemical effect of electriccurrent

If solutions of different salts and acidsconduct electricity, what do vegetables andfruits do? Let us try to find that out.

Activity-4

Testing the effect of electriccurrent on potato

Take a potato. Cut into two halvesand take one half of it. Construct testerwith LED bulb, insert two copper wires ofthe tester into the potato leaving somedistance (around 1 cm) between them. asshown in figure 4.

Fig. 4

Does the LED glow?

Leave the inserted wires for 20-30minutes.

What do you observe on the surface ofthe potato?

A greenish blue spot is seen on thepotato around the wire connected to thepositive terminal of the battery. But no suchspot is seen around the other wireconnected to the negative terminal. Thisgreenish spot is due to chemical change inthe potato.

What could be the cause behind thischange?

Will other vegetables also show suchan effect?

Try it out with vegetables like carrot,beetroot, cucumber, raddish, brinjal, sweet-potato, etc.

Think and discuss

If a battery is packed in a box and ifonly two wires from two terminals aregiven out, how can we decide thepositive and negative terminal of thebattery?

Electrolytic cellIn all the above activities, the battery

we have used is made up of dry cells. Inlower classes you have studied about drycell.

Can you produce electric current inanother way?

Do you know, how was the first cellmade?

SCERT TELA

NGANA


How the first cell was made?People in

Europe began

experimenting with

electricity around

400 years ago. They

generated electricity

in different ways and conducted various

experiments. However, they faced one

major problem which prevented them

from understanding electricity in depth.

They did not have a stable and permanent

source of electricity. This may sound

like a minor problem today, but it took

scientists nearly 200 years to find a

solution.

That solution came in the year 1780. And

it came almost by chance. A biologist

named Luigi Galvani from Bologna, Italy,

once saw a frog’s leg hung from a cop-

per hook twitching violently when it

touched another metal. It seemed as if

the frog’s leg had suddenly come to life.

Galvani did many more experiments

with the legs of dead frogs. He finally

came to the conclusion that frog’s legs

twitched every time electricity flowed

through them. Galvani thought he had

discovered living or biological electricity.

He presented his theory to the world,

saying that all living beings contained

electricity and it was this electricity that

was their main source of life.

Galvani’s experiments took thewhole of Europe by storm. Manyscientists began performing similarexperiments with various species ofanimals. Among them was AlessandroVolta of Italy. He, too, performedexperiments with frog’s legs. However,he discovered that if a frog’s leg hungfrom an iron hook is touched withanother iron rod, it does not twitch. Voltawas a bit puzzled..

If the reaction in a frog’s leg is dueto the electricity in its body, why are twodifferent metals required to make ittwitch, he wondered? After a lot ofthinking he arrived at the conclusion thatelectricity does flow through the frog’sleg when two different metals touch it.However, this electricity is not containedin the leg of the frog but is generated bysome other process.

Volta repeated his experiment usingdifferent liquids instead of frog’s legs.He found that it did not require ananimal’s body to generate electricity. Itis possible to generate electricity if twodifferent metals are placed in someliquids.

These experiments showed the wayto a steady source of electricity. Voltamade his first cell in 1800 using zinc andcopper plates dipped in sulphuric acid.His discovery made him famous in therealm of science. The cell he made iscalled a Volta cell in his honour. The wordvoltage is also derived from his name.

Do you know?

SCERT TELA

NGANA


two terminals. Touch the wire from oneterminal to the zinc plate and the wire fromthe other terminal to the copper wire. Didthe LED light up? If not, change theconnections vice-versa. Did the LED glow?

Repeat the above activity using lemonjuice, tamarind juice and tomato juice oneby one instead of sulphuric acid to makeyour cells.

What other liquids can be used to makethe cell?

Will detergent solution be useful?

Find it out yourself.

How does the above cell function?

After a few seconds of immersion ofzinc and copper wires into dilute sulphuricacid, zinc slowly begins to dissolve in thesulphuric acid. We can see bubbles formingon the copper rod.

The current is passed from copper rodto zinc rod. These rods are known aselectrodes and dilute sulphuric acid isknown as electrolyte.

Here the chemical energy is convertedinto electric energy by “electrolysismethod”.

Can you compare this cell with dryCell?

Which is good one? Why?

Think and discuss

What is electrolysis?

Discuss with your teacher or collectthe information about electrolysismethod form your school library books.

Let us make a cell with the same metalsand chemicals used by Volta.

Activity-5

Make your own cellCollect two injection bottles. Cut two

5 cm-long bits of thick copper wire. Usesandpaper to scrape about 1 cm of thecoating from both ends of the wires.

Break open a exhausted dry cell andremove its outer metal covering (made ofzinc). Cut two 2 mm-wide and 5 cm-longstrips from this zinc plate. Insert the copperwires and zinc strips into the rubber capsof the injection bottles as shown in figure5.Ensure that the copper wire and zinc stripsdo not touch each other.

Now take a wire and connect the copperwire of one bottle with the zinc plate ofthe other bottle. Fill both bottles withdilute Sulphuric acid carefully and fix thecaps in which the copper wires and zincstrips are inserted. Your cell is ready.

How do you test it?

Take an LED. Attach two wires to its

Copper

Fig. 5dil.sulphuric acid

Zinc Copper Zinc

LED

Conducting wires

SCERT TELA

NGANA


take a thick copper wire and hammer it toflatten it instead of the above mentionedcopper plate.)

Procedure:

Dissolve crystals of copper sulphate inpure water to prepare concentrated solution(deep blue in colour). Pour the solution ina beaker and add a few drops of dilutesulphuric acid to it. (Acid helps inincreasing the conductivity of electrolyte.)

Tie one end of a connecting copperwire to the iron object (key) to be coatedwith copper. Connect its other end to thenegative terminal of a battery. Suspend thetied iron object into the copper sulphatesolution. Suspend the copper plate intocopper sulphate from positive end of thebattery through a switch as shown in fig-6.

Care should be taken that the key andplate do not touch each other and are a littleaway from one another. Put the switch onfor about 10 minutes. Switch off the circuitand take the iron key out.

Observations :

Does the iron key get coated with ashiny, brown colour?

ElectroplatingCan you list some

objects around you thatkeep shining? Forexample, the clip of yourwriting pad or rim of anewly bought bicycle.

However, if theseobjects are scratched deliberately oraccidently, their shine diminishes.Scratching of such objects removes somecoating from their surface and we can seea relatively dull surface below the coating.Safety pins, when they are new, shinebrightly. However, with repeated use, theylose the brightness of shining. Repeatedhandling makes the coating of the pins wearoff and the non-shiny metal beneath isexposed.

In the above examples, the materialunderneath contains a coating of anothermetal. How is this achieved? How is theshining metal coated on the dull object?

Well, let us try doing it ourselves.

Aim: Coating an iron key with copper byelectroplating method.

Required material: Copper plate of size2 cm x 5 cm, crystals of copper sulphate(blue vitriol), a key made by iron, glassbeaker, water, sulphuric acid, Battery andsome connecting copper wires. (You may

Lab ActivityFig. 6: Electro plating

Copper plate

Coppersulphate

Iron key

SCERT TELA

NGANA


What is the colour due to?

What will happen if you interchange thebattery terminals?

Why does copper get deposited on theiron key?

When electric current is passed throughthe copper sulphate solution, in which thecopper sulphate is present in the form ofcopper and sulphate ions, the free copperions are drawn to the electrode connectedto the negative terminal of the battery anddeposited on it.

Thus one metal is coated with anothermaterial. This process is known aselectroplating.

If the key is to be coated with zinc oraluminium instead of copper, what changesdo we need to make in the aboveexpeirment?

In electroplating, an inferior metal(metals which are effected by theatmospheric humidity, carbon dioxide, etc.)is coated with a superior metal (metalswhich are not effected by the humidity oroxygen). The following are therequirements for fine coating.

(a) The object to be electroplated shouldbe free from greasy matter.

(b) The surface of the article should berough so that the metal deposited stickspermanently.

(c) The concentration of the electrolyteshould be so adjusted as to get smoothcoating.

(d) Current must be the same throughout.

Electroplating – usesElectroplating is widely used in

industry for coating metal objects with athin layer of different metals.

For example, metals like iron which areeasily corroded by atmospheric air,moisture and oxygen are coated withdeposits of nickel or chromium which aremost resistant to such corrosion by electroplating method. Machinery parts are oftenchromium plated to protect them fromcorrosion and at the same time to give themgood polish. (see Figure 7)

Sometimes, electroplating is done witha view to repairing worn out parts ofmachinery. In such cases the suitable metalis deposited on the affected parts of themachinery by electro plating method.

Electroplating is also used inornamentation and decoration. For exampleseveral articles made of copper or itsalloys, such as table wares, decorationpieces are coated with silver or gold. (seeFigure 8)

Fig. 7

SCERT TELA

NGANA


Fig. 8In general the processed food items

are preserved in tin coated iron cans. Tinis less reactive to the food than iron. Sothe cans are made by electroplating tin oniron. (see Figure 9)

Fig. 9


Key words

Good conductors, Poor conductors, Electrodes, Electrolyte, Electrolysis,

Electroplating

1. Some liquids also conduct electricity as some solids do.

2. Some liquids are good conductors of electricity and some are poor conductors.

3. Distilled water does not allow the current to pass through it.

4. Most Liquids that conduct electricity are solutions of acids, bases and salts.

5. Electrolyte is a solution of a substance through which electric current can pass.

6. Electroplating is possible through electrolysis.

Refelections on concepts1. Give examples for good solid conductors and liquid conductors. (AS

1)

2. Give examples for poor solid conductors and liquid conductors. (AS1)

3. Give two examples for electrolyte. (AS1)

4. Draw the diagram of Electrolytic cell and explain. (AS5)

When iron is coated with zinc metal,iron becomes more resistive to corrosionand formation of rust. So, zinc coated ironis used for bridges and in automobiles.

What we have learnt

SCERT TELA

NGANA


Application of concepts1. Which energy is cause for glowing of bulb in electrolytic cell? (AS

1)

2. What do you add to distilled water for making it to conduct electricity? (AS1)

3. Kavya observed that a discharged dry cell which kept in sun light by her father forfew hours got ability to glow LED. She got many doubts and questions to raise. Canyou guess those questions or doubts? (AS

2)

4. Make a battery from four lemons and test it with a LED in the circuit. (AS5)

5. Collect the information and make list of good conductors and bad conductors. Howdo you use this information in your daily life works? (AS

1)

Higher Order Thinking Questions1. If the key is to be coated with aluminium instead of copper,what changes do we

need to make in the experiment of coating on iron key with copper? (AS3)

2. Is plastic coated by the process of electroplating?Why? (AS2)


1. Which of the following is also used in ornamentation and decoration [ ]

a) Electro typing b) Electro plating c) Electro printing d) Galvanizing

2. Pure water is [ ]

a) Electric conductor b) Semi conductor c) Insulator d) Resistor

3. The material which do not allow electric current to pass through it isknown as [ ]

a) Electric conductor b) Insulator c) Electric resistance d) semi conductor

4. Electro plating is possible through [ ]

a) Electrolysis b) Chemical process c) Dissolving d) Filtration

5. One of the following is not an electrolyte [ ]a) Sulphuric acid b) Lemon juice c) Tamarind juice d) Detergent solution

Suggested Experiments1. Conduct an experiment for coating on iron key with copper by electroplating

method and prepare a note.2. Conduct an experiment for testing the electric conductivity of liquids.3. Conduct an experiment for testing the electric conductivity of electrolytes.

Suggested Projects1. Collect the information from various sources on the applications of electroplating

in daily life and prepare a note on that.2. In many of the activities in this chapter, we have used a tester made up of LED.

Can we avoid LED and use something else as a tester? Collect the information andmake a model.

SCERT TELA

NGANA


SOME NATURALPHENOMENA

Chapter

10

In the earlier classes you have learntabout winds, storms and cyclones.You have learnt that cyclones can cause alot of damage to life and property. You alsolearnt that we can protect ourselves fromthese destructive phenomena to someextent.

In the present chapter, we shall discusstwo other natural phenomena that causedestruction. These are lightning andearthquake. We shall also discuss whatsteps we can take to reduce the destructioncaused by these phenomena.

LightningYou might have seen

sparks on an electricpole when wires becomeloose or when windblows and shakes thewires. You might alsohave seen sparks when aplug is loosely put in its socket and theswitch is put on. (Do not try this if youhave never seen such a spark.)

Lightning is also an electric spark, buton a huge scale. In ancient times peopledid not understand the cause of thesesparks.

They were, therefore, afraid oflightning and thought that the wrath of godsor evil spirits was striking them. Now, of

course, to some extent we understandreasons for lightning.

We have to take some precautions toprotect ourselves from the deadly sparksof lightning.

The Sparks that the GreeksKnew About !

The ancient Greeks knew, as early as in600 B.C. that when amber (amber is a kindof resin) was rubbed with fur, it attractedlight objects such as hair.

You might have seen that when you takeoff woollen or polyester clothes, especiallyin dry season the hair on your skin standson end.

If you take off these clothes in the dark,you even see a spark and hear cracklingsound.

Why does hair get attracted towardsclothes?

In 1752, Benjamin Franklin, anAmerican scientist, showed thatlightning and the spark from yourclothes are essentially the samephenomena.

People before Franklin knew about thephenomena of lightning and hair gettingattracted to clothes but did not have an idea

SCERT TELA

NGANA

132 Some Natural Phenomena

Fig-1

Take an inflated balloon and rub itagainst your clothes. Bring the balloonclose to small pieces of paper.

Take a drinking-straw and rub it againsta smooth wall or against your clothes, thenbring it near pieces of paper.


Are they able to attract bits of paperafter being rubbed?

Repeat the activity by rubbing each oneof the above mentioned objects (refill,comb, drinking straw, balloon) and usesmall pieces of dry leaf, husk, etc as testingmaterials. Record your observations intable-1.

What can we infer from the aboveactivity?

Do the objects like refill or combattract pieces of paper only afterrubbing?

Do all objects show this property?

Can we rub a comb against our palm andmake it attract the paper bits? Try it andsee.

that these are related. However, to realisethat these two are the same phenomena butat different scales, it took about 2000years.

Scientific discoveries are a result ofhard work by many people. It cansometimes take a long time.

We shall now study some properties ofelectric charges. We shall also see howthey are related to the lightning in the sky.

Let us perform some activities tounderstand the nature of electric charges.Let us recall what you might have playedas a game.

When you rub a plastic scale on yourdry hair, the scale can attract very smallpieces of paper.

Why is the plastic scale not able toattract pieces of paper before it getsrubbed by dry hair?

Charging by rubbing

Activity-1Effect of rubbing

Take a used ball-pen refill and bring itnear small pieces of paper. The refill shouldbe close enough but not touch the piecesof paper.

Check what happens to the paper-pieces. Now, rub the refill vigorously witha piece of polythene.

Bring it close to small pieces of paper.Note your observation. Take care that therubbed end is not touched by your hand orwith a metallic object.

Now, take a comb and move it throughdry hair a few times. Take the comb nearsmall pieces of paper and check whathappens.

SCERT TELA

NGANA


Let us try to find out whether all objectsshow this property. We will also try to findout whether attracting paper-pieces is onlydue to rubbing or rubbing with specificmaterials.

When a plastic refill is rubbed withpolythene, it acquires a small electriccharge. Similarly, when a plastic comb isrubbed with dry hair, it acquires a smallcharge.

These objects are called chargedobjects. In the presence of a charged refillor plastic comb, bits of paper and hair alsoget charged. Let’s try to charge some otherobjects that are familiar to you.

Aim: To find effects of charged bodieswhich have been rubbed by differentmaterials.

Material required: A ball pen refill, aballoon, a comb, an eraser, a steel spoon,polythene sheet, plain paper, woollen cloth,etc.

Procedure :

Rub the above objects against materialslisted in table-2. In each case, bring therubbed object near small pieces of paperand note whether they attract pieces ofpaper or not. Record your observations intable-2 by writing 'yes' or 'no'.

Object Material used for testing Effect before rubbing Effect after rubbing

Bits of Paper Paper bits Paper bits getremain stationary attracted to the refill

Pieces of dry leaves

Husk

Bits of Paper


Husk

Bits of Paper


Husk

Bits of Paper

Pieces of dry leavesHusk

Refill

Comb

Balloon

DrinkingStraw

Table-1

Lab Activity

SCERT TELA

NGANA


What do you conclude from the abovetable?

Some objects like refill, comb, etcwhen rubbed with some specific materialsare able to attract light objects like bits ofpaper. But some objects like steel spoondo not attract pieces of paper even afterrubbing.

Why do not some materials attractpieces of paper even after rubbing?

Let us try to explore this phenomenonfurther.

Object Material used forrubbing

Whether paper piecesare attracted?

Refill

Inflated Ballon

Comb

Eraser

Steel spoon

Plain Paper

Polythene sheet yes

Woollen cloth

Dry hair

Polythene sheet

Woollen cloth

Plain Paper

Polythene sheet

Woollen cloth

Dry hair

Polythene sheet

Woollen cloth

Plain Paper

Polythene sheet

Woollen cloth

Table-2

Types of charges and theirinteraction

Activity-2

Understanding types of charges

Fig-2(a) Fig-2(b)

Inflate two balloons and hang them insuch a way that they do not touch each other.Rub both the balloons with a woollen clothand release them.

SCERT TELA

NGANA


Let us summarise our observationscarefully.

1. A balloon rubbed with woollen clothrepelled another balloon of the sametype.

2. A refill rubbed with polythene repelledanother refill rubbed with similarmaterial.

3. A ballon rubbed with woollen clothattracted by a refill rubbed withpolythene sheet.

What can we conclude from theseobservations?

Does the repulsion between chargedballoons indicate that they possessimilar charge?

Does the attraction between a chargedballoon and a charged refill indicate thatthey posses different charges?

Does this activity remind you some ofexperiments that you have done in ‘Playingwith Magnets’ chapter of class VI?

We know that magnets attract objectsmade up of magnetic materials like Iron,Nickel, Cobalt, etc.

We also know that unlike poles ofmagnets (North-South or South-North)attract each other and like poles (North-North or South-South) repel each other.

Can we say that something similar ishappening in above activities?

Does it indicate that the charge on theballoon is of a different kind from thecharge on the refill!

Can we say that there exist two kindsof charges?

(Make sure that your hand doesn’ttouch the balloon while rubbing withwoollen cloth).


Take a refill and rub it with a polythenesheet. Keep it gently in a plastic tumbler.Take another refill and also rub it with thesame polythene sheet.

Bring the second refill near the firstone in the tumbler. Take care that you donot touch either of the rubbed portions ofthe refills with your hand. Is there any effect on the first refill in

the tumbler?Do they attract each other or repel eachother?

Bring two ballons rubbed with samesilk cloth together what happend ?

Now, take a rubbed balloon near therubbed refill in the tumbler and check theaction (fig-3).

Do they attract each other or repel eachother?In the first two parts of the above

activity, two objects that were made of thesame material have brought near to eachother after being rubbed with someappropriate material.

In the third part, objects made of differentmaterials were brought near to each otherafter being rubbed with different material.

Fig-3

SCERT TELA

NGANA


Fig-4: Testing the presence of charge

Bring a glass rod which is rubbed witha silk cloth near the suspended ball. Whathappens? Does it get attracted towards theglass rod or move away from it.

Now touch the silver foil on thethermocol ball with charged glass rod.Remove the glass rod from the ball andagain rub it with silk cloth and bring it closeto the suspended ball.


Does it get attracted towards the glassrod or move away from it?

You may notice in the first instance thatthe thermocol ball is attracted towards theglass rod and in second instance it movesaway from the glass rod.

What could be the reason for thischange in movement of the ball?

If a charged body is brought near anuncharged body it induces an oppositecharge on it and it gets attracted by thebody.

In the above experiment when a chargedbody (glass rod rubbed with silk cloth) isbrought near an uncharged body(thermocol) it induces an opposite chargein it and hence it get attracted by the glassrod.

In second case (above experiment) wehave charged the thermocol ball bytouching it with a charged glass rod. Hence

Can we also say that the charges of thesame kind repel each other, whilecharges of different kinds attract eachother?It is a convention to call the charge

acquired by a glass rod when it is rubbedwith silk cloth positive and the chargeaquired by the silk cloth is negative.

It is observed that when a charged glassrod is brought near to a charged plasticstraw which is rubbed with polythene sheet,there is attraction between the two. What do you think about the kind of

charge on the plastic straw?You may guess that the plastic straw

would carry a negative charge.Is your guess correct or wrong?Discuss with your teacher.The electrical charges generated by

rubbing are static. They do not move bythemselves.

When charges move, they constitute anelectric current. You studied about thecurrent in a circuit which makes a bulbglow, or the current that makes a wire getheated in class VI & VII.

The electric current in a circuitrepresents motion of charges.

Activity-3

To find the presence of chargeon a body

Make a small ball of thermocol.Collect thin silver foil used to decoratesweets. Wrap this thin silver foil to coverthe thermocol ball and suspend it from astand with the help of a thread as shown inthe figure 4.

SCERT TELA

NGANA


Do the foil strips behave in the sameway in all cases?

Can this apparatus be used to detect thepresence of charge on a body or not?

Can you explain why the strips repeleach other?

The strips of aluminium foil receive thesame charge from the charged refill throughthe paper clip. The strips carrying similarcharges repel each other and hence theymove apart.

This device can be used to test whetheran object is carrying charge or not. Thisdevice is known as electroscope. (Earlierdays gold foil used in electroscope). In theabove activity you can observe thatelectrical charge can be transferred from acharged object to another through a metalconductor.

Touch the end of the paper clip gentlywith hand and you will find a change in thefoil strips. They move closer and comeback to their original state.

Repeat charging of foil strips andtouching the paper clip. Every time you willfind that the foil strips collapse as soon asyou touch the paperclip with hand.

Why does it happen?

The reason is that the foil strips losecharge to the earth through your body. Wesay that the foil strips are discharged.

The process of transferring of chargefrom a charged object to the earth is calledearthing.

Earthing is provided in buildings toprotect us from electrical shocks due toany leakage of electrical charge.

when we brought the glass rod near the ball,as both of them have similar charge the ballgets repelled by the glass rod.

From the above experiment we canconclude that attraction is not a sure testto know the presence of a charge on a body.

Activity-4

Demonstrating transfer of charge Take an empty jam bottle. Take a piece

of cardboard slightly bigger in size than themouth of the bottle.

Pierce a hole in it so that a metal paperclip could be inserted. Open out paper clipas shown in the fig-5

Fig-5

Cut two strips of aluminium foil about4 cm×1 cm each and hang them on the paperclip.

Insert the paper clip having the stripsof aluminium foil in to the cardboard lidso that it is perpendicular to it as shown inthe fig-5.

Charge a refill and make it touch theend of the paper clip. Observe what happens.

Is there any effect on the strips ofaluminium foil?Do they repel each other or attract eachother?Now bring other charged bodies andmake them touch the end of the paperclip.

SCERT TELA

NGANA


The Story of Lightning

Fig-6

It is now possible to explain lightningin terms of the charges produced byrubbing. You have learnt in Class VII thatduring the development of a thunderstormthere will be fast movement of air currents.

The clouds moving in air acquire acharge on their surface due to the frictionwith particles of air. As the surface area ofa cloud is very large, the amount of chargeaccumulated on its surface is very high.

When a charged cloud comes close toanother cloud it induces an opposite chargeon the later and the accumulated chargetries to move from one cloud to anothercloud.

But the air present between them beinga poor conductor of electricity resists theflow of charge between them.

When the magnitude of theaccumulated charge becomes very large,the air, which is normally a poor conductorof electricity, is no longer able to resisttheir flow.

Hence discharge takes place betweennegative and positive charges whichproduce streaks of bright light and sound.We see streaks as lightning. The process iscalled an electric discharge.

The process of electric discharge canoccur between two or more clouds, orbetween clouds and the earth. Today weneed not get frightened by lightning like theancient people did. Now we understand thephenomenon.

Scientists are trying hard to improveour understanding. However, lightningstrike could destroy life and property. It is,therefore, necessary to take measures toprotect ourselves.

Lightning Safety1. Which is the safe place during a thunder

storm?

A house or a building of low height

If you are travelling in a bus or in a caryou are safe inside provided that doorsand windows are closed.

If you are in a forest, taking shelterunder shorter trees than a taller tree isa good idea during the thunder storm.

2. Which is not a safe place during athunder storm?

Travelling in an open place.

Standing under tall trees in open fieldsor in a garden / park.

Staying in multi storied building whichdo not have lightning conductors.

Standing near electric poles ortelephone poles.

Speaking on landline telephones.

Using electric appliances like T.V andComputers.

Lightning ConductorsLightning Conductor is a device used

to protect buildings from the effect oflightning. A metallic rod, taller than the

SCERT TELA

NGANA


building, is installed in the walls of thebuilding during its construction.

One end of the rod is kept out in the airand the other is buried deep in the ground asshown in fig-7. The rod provides an easy routefor the transfer of electric charge to the ground.

The projected end of the metal rod isat a height more than the height of thebuilding. Hence it receives the charge firstduring lightning because it is closer to thecloud than the building.

As it is a good conductor of electricity,it allows all the charge to flow through itthereby causing no damage to the building.

Fig-7

The metal columns used duringconstruction and water pipes in thebuildings also protect us to some extent.But do not touch them during athunderstorm.

Earthquakes Natural phenomena like lightning,

floods, cyclones etc., can cause largescale destruction of human life andproperty. Fortunately, these phenomena canbe predicted to some extent.

The meterological department can warnabout a thunderstorm developing in somearea. If a thunderstorm occurs there isalways a possibility of lightning andcyclones accompanying it.

So, we get time to take measures toprotect ourselves from the damage causedby these phenomena.

There is, however, one naturalphenomenon which we are not yet able topredict. It is an earthquake. It can causedamage to human life and property on a hugescale.

A major earthquake occurred in Indiaon 8th October 2005 in Uri and Tangdhartowns of North Kashmir. Before that amajor earthquake occurred on 26th January2001 in Bhuj District of Gujarat.

Activity-5

Collecting information about thedamages caused by earth quakes

Ask your parents about the hugedamages to life and property caused bythese earthquakes. Collect a few picturesshowing the damage caused by theseearthquakes from newspapers andmagazines of those days.

Prepare a short report on the sufferingof the people during the earthquakes.

What is an earthquake?

What happens when it occurs?

What can we do to minimise its effects?

These are some of the questions whichwe shall discuss here.

What is an Earthquake?An earthquake is a

sudden shaking ortrembling of the earthlasting for a very shortperiod of time. It iscaused by a disturbancedeep inside the earth’s crust.

SCERT TELA

NGANA


Collect accounts of the devastationcaused by the tsunami in India from yourparents, or other elders in the family or inthe neighbourhood.

What Causes an Earthquake?

What could cause a disturbance insidethe earth?

In ancient times, people did not knowthe true cause of earthquakes. Their ideaswere, therefore, expressed in mythical/folkstories. Similar myths were prevalent inother parts of the world.

Now we know that the tremors arecaused due to the disturbance at deep downinside portion of uppermost layer of theearth. This uppermost layer of the earth iscalled crust.

The outer most layer of the earth is notin one piece. It is fragmented. Eachfragment is called a plate. These plates arein continuous motion. [See fig-9(a), 9(b)]

When they brush past on one another,or a plate goes under another plate due tocollision, they cause disturbance in theearth’s crust.

Crust

Mantle

Inner Core

Outer CoreFig-8

Some mythical/folk stories told that theearth is balanced on the horn of a bull andwhen the bull shifts it to the other horn, anearthquake takes place.

How could it be true?

Earthquakes occur all the time, all overthe earth. They are not even noticed. Majorearthquakes are much less frequent. Theycan cause immense damage to buildings,bridges, dams and people.

There can be a great loss to life andproperty. The earthquakes can cause floods,landslides and tsunamis.

A major tsunami occurred in the IndianOcean on 26th December 2004. All thecoastal areas around the ocean sufferedhuge losses.

Activity-6

Locating the tsunami affectedareas in the map

Take an outline map of the world.Locate the eastern coast and Andaman andNicobar Islands in India. Mark othercountries around the Indian Ocean whichcould have suffered damage.

Fig-9(a)

Fig-9(b)SCERT TELA

NGANA


It is this disturbance that shows up asan earthquake on the surface of the earth.However, most earthquakes are caused bythe movement of earth’s plates.

Since earthquakes are caused by themovement of plates, the places atboundaries of the plates are considered asweak zones where earthquakes are morelikely to occur.

The weak zones are also known asseismic or fault zones. In India, the areasmost threatened are Kashmir, Western andCentral Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo– Gangetic Plane. Some areas of SouthIndia also fall in the danger zone as shownin figure 10.

Can we predict when and where the nextearthquake takes place?How do we know the occurrence of anearthquake?Seismologists use two main devices to

measure an earthquake, a seismograph anda seismoscope. The seismograph is aninstrument that measures seismic wavescaused by an earthquake.

The seismoscope is an instrument thatmeasures the occurrence or the time ofoccurrence of an earthquake. Unlike othermeasuring devices, the seismoscope is asimple device which can be used withoutany technological background.

How do we measure the intensity of theearthquake?

The power of an earthquake isexpressed in terms of a magnitude onRichter scale. The destructive earthquakeshave magnitudes higher than 7 on theRichter scale.

Both Bhuj and Kashmir earthquakes hadmagnitudes greater than 7.5.

Although, we know for sure whatcauses an earthquake, it is not yet possibleto predict when and where the nextearthquake might occur. Tremors on theearth can also be caused when a volcanoerupts, or a meteor hits the earth, or anunderground nuclear explosion takes place.

Fig-11

The tremors produce waves on thesurface of the earth (see figure 11). Theseare called seismic waves. The waves arerecorded by an instrument called theseismograph.

Fig-10

SCERT TELA

NGANA


Richter magnitude Earthquake effects

less than 3.5 Generally not felt, but recorded.

3.5-5.4 Often felt, but rarely causes damage.

5.5-6.0 Atmost slight damage to well-designed buildings. Can causemajor damage to poorly constructed buildings over smallregions.

6.1-6.9 Can be destructive in areas upto about 100 kilometres acrosswhere people live.

7.0-7.9 Major earthquake. Can cause serious damage over larger areas.

8 or greater Great earthquake. Can cause serious damage in areas severalhundred kilometres across.

The instrument is simply a vibratingrod, or a pendulum, which starts vibratingwhen tremors occur. A pen (stylus) isattached to the vibrating system.

The pen (stylus) records the seismicwaves on a graph paper which moves underit. By studying these waves, scientists canconstruct a complete map of the earthquake.

They can also estimate its power tocause destruction.

Like many other scales, Richter scaleis not linear. This means that an earthquakeof magnitude 6 does not have one and halftimes the destructive energy of anearthquake of magnitude 4.

In fact, an increase of 2 in magnitudemeans 1000 times more destructive energy.For example, an earthquake of magnitude6 has thousand times more destructiveenergy than an earthquake of magnitude 4.

There is another method of measuringthe intensity of an earthquake using themoment magnitude scale which is based onthe amount of displacement that occurredalong a fault zone rather than themeasurement of ground motion at a givenpoint.

The moment magnitude measuresenergy released by the earthquake moreaccurately than the Richter scale. It is theonly magnitude scale that adequatelymeasures the size of large earth quakes.

Fig-12: seismograph

Table - 3 : Richter scale reading and affects of earthquake

SCERT TELA

NGANA


Protection against earthquakes

We know from the earlier pages thatearthquakes cannot be predicted. We havealso seen that they can be highly destructive.It is, therefore, important that we takenecessary precautions to protect ourselvesall the time.

People living in seismic zones, wherethe earthquakes are more likely to occur,have to be specially prepared. First of all,the buildings in these zones should bedesigned so that they can withstand majortremors.

Modern building technology can makeit possible. It is advisable to make thestructure simple so that it is ‘Quake Safe’.

Consult qualified architects andstructural engineers.In highly seismic areas, the use of mudor timber is better than heavyconstruction material. Keep roofs aslight as possible. In case the structurefalls, the damage will not be heavy.

It is better if the cupboards and shelvesare fixed to the walls, so that they do

not fall easily.

Be careful where you hang wall clocks,photo-frames, water heaters etc., sothat in the event of an earthquake, theydo not fall on people

Since some buildings may catch firedue to an earthquake, it is necessary

that all buildings, especiallytall buildings, have firefighting equipment inworking condition.

The Central BuildingResearch Institute,Roorkee, has developed tomake quake proof houses.

In the event that anearthquake does strike, takethe following steps toprotect yourself:

Inside the homeTake shelter under a table and stay theretill shaking stops.

Stay away from tall and heavy objectsthat may fall on you.

Outside the homeFind a clear spot, away from buildings,trees and over head power lines. Dropto the ground.

Earth quakes in Telangana

Do you know the places in Telanganawhere earthquakes have occurred andits intensity?

According to seismic hazard map inTelangana, the city of Hyderabad lies in zoneII. Discuss about zones with your teacher.

Fig-13

SCERT TELA

NGANA


Key words

Crust, discharge, earth’s plates, earthquake, electroscope, lightning, lightningconductor, negative charge, positive charge, richter scale, seismograph,Seismoscope, thunder, thunderstorm, transfer of charge, tsunami, tremor

Some objects can be charged by rubbing with other objects.

There are two kinds of charges — positive charge and negative charge

Like charges repel and unlike charges attract each other.

The electrical charges produced by rubbing are called static charges.

When charges move, they constitute an electric current.

An electroscope may be used to detect whether a body is charged or not.

Attraction is not a sure test to know the presence of charge on a body.

The process of transfer of charge from a charged object to the earth is calledearthing.

The process of electric discharge between clouds and the earth or betweendifferent clouds causes lightning.

Lightning strike could destroy life and property.

Lightning conductors can protect buildings from the effects of lightning.

An earthquake is a sudden shaking or trembling of the earth.

Earthquake is caused due to the disturbance deep inside the earth’s crust.

It is not possible to predict the occurrence of an earthquake.

Earthquakes tend to occur at the boundaries of earth’s plates. These boundariesare known as fault zones.

Destructive energy of an earthquake is measured on the Richter scale. Theearthquake measuring 7 or more on Richter scale can cause severe damage tolife and property.

We should take necessary precautions to protect ourselves from earthquakes.


SCERT TELA

NGANA



1. Describe with the help of a diagram an instrument which can be used to detect a

charged body. (AS1)

2. Which places are not safe during a thunderstorm?(AS1)

3. List three states in India where earthquakes are more likely to occur.(AS1)

4. Explain how do you determine the intensity of earthquake.(AS1)

5. Suppose you are outside your home and an earthquake occurs. What precaution

would you take to protect yourself?(AS7)


1. Sometimes, a crackling sound is heard while taking off sweater during winter.

Explain.(AS1)

2. Give two examples of your daily life about effects which are caused by transfer

of charges. (AS1)

3. Inflate two balloons and rub both of them with a cloth first and then with

different material. Will they attract each other in both cases?(AS3)

4. Explain why a charged balloon is repelled by another charged balloon whereas

an uncharged balloon is attracted by a charged balloon?(AS1)

5. How do you appreciate the efforts of scientists in developing an instrument to

measure the intensity and detect the source of earthquake?(AS6)


1. The weather department has predicted that a thunderstorm is likely to occur on a

certain day. Suppose you have to go out on that day. Would you carry an umbrella?

Explain.(AS7)

2. Is there any alternative method to find the intensity of earthquake?(AS2)

3. We know that the clouds have charges. Can we produce current through these

charges?(AS2)

4. How do you appreciate the efforts of the scientists to develop a Lightning conductor

to protect buildings from the effect of lightning.(AS6)


SCERT TELA

NGANA



1. Which of the following cannot be charged easily by friction [ ]

a) a plastic scale b) a copper rod c) a wollen cloth d) piece of wood

2. When a glass rod is rubbed with a piece of silk cloth [ ]

a) Rod and the cloth acquire positive charge

b) Road becomes positively charged while the cloth has a negative charge

c) Rod and the cloth both acquire negative charge

d) Rod becomes negatively charged while the cloth has a positive charge

3. The magnitude of destructive earthquakes on the Richter scale is greater than:

a) 3.0 b) 4.0 c) 7.0 d) 2.0 [ ]

4. The instrument is used to estimate the intensity of earth quake is

a) Sesimograph b) Sesimoscope [ ]

c) Gold leaf electroscope d) Lightning conductors

5. It can protects buildings from the effects of lightning [ ]

a) Sesimograph b) Sesimoscope

c) Gold leaf electroscope d) Lightning conductors


1. Conduct an experiment to find the effects of charged bodies which have been rubbed

by different materials.

2. Conduct an experiment to find out the presence of charge on a body.

Suggested Project Works

1. Which country in the world is frequently effected by earth quakes? Collect the

information and photographs on the recent earthquakes.

2. Find out if there is an organisation in your area which provides relief to those suffering

from natural disaster. Enquire about the type of help they render to the victims of

earthquakes. Prepare a brief report on the problems of the earthquake victims.

3. Collect accounts of the devastation caused by tsunami in India from your

parents,friends and neighbourhood and write a report.SCERT TELA

NGANA


Chapter

11STARS AND

THE SOLAR SYSTEM

Observing the night sky is afascinating experience for everyone. Youmight have watched the clear blue sky sometimes and also have observed sunrise andsunset several times. What have youobserved in the sky? What do you knowabout celestial objects and their movement?

Elderly people seem to know a lot ofthings. Some of them can tell the time ofthe day simply by looking at the shadowsof some objects. How do they make suchguesses?

Read the following questions and checkwhat you know about the sky and our earth.

What are the celestial objects that wecan see in the sky?Are the stars moving?Do you see the same stars at night andearly in the morning?Do you see the same stars duringsummer and winter nights?What is the shape of the moon? Whydoes it change? Why doesn’t the sunchange its shape daily like the moon?Where exactly is the sun situated in thesky at noon?Why does the shadow of a tree changefrom morning to evening?It is definitely interesting to know

answers to the above questions, but it willbe more interesting to understand how our

ancestors came to an understanding aboutall these in the olden days and what theyobserved and how they observed the sky andcelestial bodies without using instrumentslike telescopes.

One thing we have to remember thatpeople made these observations from theearth and not from any other point in thesky. We shall now perform some activitieswhich will help us understand the abovequestions with some insights.

Activity-1Observing the changes in thelength of shadow

This experimentshould be performed ona day when the sky isclear, preferablybetween nine in themorning and four in theevening.

Pick a spot in the open ground whereyou can be sure to have sunlight throughoutthe day. Also there should not be any treesor buildings nearby which can cast a shadowon this spot during the period of theexperiment.

SCERT TELA

NGANA

148 Stars and Solar System

· Look at your table and figure out thetime of the day of shortest shadow.

· When did you observe the longestshadow in your activity?

· How does the length of the shadowchange with time? Illustrate youranswer with the help of some diagrams.Draw the diagrams of the stick and itsshadow for 5 different times, that is, at9am, 11am, 12noon, 2pm, and 4pm.

· If you continue your activity fromsunrise to sunset, at what times do youthink the shadow would be the longest?

· Where is the sun situated in the sky atnoon? Where does the shadow of stickfall at that time? Think about how yourown shadow will be at that time.

· Do you think that your shadow will bethe same on all the days at noon?

· In which direction does the shortestshadow of the stick fall in your activity?The shortest shadow cast by a vertical

object on the ground always falls in thenorth–south direction. You can use this factto locate directions. The time when theshortest shadow occurs is called the localnoon time at that place.

Think and Discuss

Look at the nails or pegs you havefixed on the ground to keep track of theshadow of the stick throughout the day.From their positions, can you tell howthe position of the sun changes in the skyfrom sunrise to sunset?

The spot should be as flat as possible.You may find such a spot on your schoolplayground.

Take a stick which is a little over ameter long and fix it vertically in theground. Ensure that exactly one meter ofthe stick remains above the surface ofground. You could even build a fencearound your stick as shown in figure-1 tokeep people away from it.

Fig-1: Changes in lengths of shadowMake your first observation at nine in

the morning. Make a mark with a nail orpeg at the point where the tip of the shadowfalls on ground. Measure the length of theshadow.

Then, make similar observations forevery half an hour throughout the day tillfour in the evening.

Use a clock to fix the time for makingyour observations. Enter the measurementsof the length of the shadow and the time ofmeasurement in a table making twocolumns, one for time and another forlength of shadow.

(Since you will be making observationsover the next two weeks at least, you shouldensure that the pegs and stick are notdisturbed.)

SCERT TELA

NGANA


Continue your observationsObserve on the next day whether the

shadow of the stick falls at the same spotsat the same times throughout the day.

Can you use your stick as a clock (sundial) to tell the time? If your answer is ‘yes’,explain how this is possible.

Two weeks later, once again check tosee whether the stick’s shadow falls at thesame spots at the same times during the day.

If the shadow does not fall on the samespot, what could be the possible reason?

You observed in activity 1 that theposition of the sun in the sky changes duringthe day. If you continue your experimentfor a full year, you will find that the positionof the sun changes from day to day as well.

That is, the position of the sun at 10.amtoday will be different from its position twoweeks later at the same time. If you choosea particular time every week and mark theposition of the sun with a peg at that time,you can build a calendar for the full year.You could use this calendar for thefollowing year to figure out dates.

In the olden days, people used tocalculate the time by observing the shadowsof different objects.

During a period of two weeks you hadmade an observation that the length of theshadow at a particular time is changing dayby day. Did it become longer or shorter?

By observing the direction of shadows,can you guess the arrival of summer orwinter?

Does the sun rise at the same spotthroughout the year? Let us do an activityto understand this.

Activity-2

Understanding the North – Southmovement of the sun.

Fix a spot near your home from whereyou can observe the sunrise. You may haveto go to the terrace of a RCC building orgo to an open field for the purpose. Choosea tree, electric pole or some otherstationary object as a reference point. Overthe next 10 to 15 days, note the spot atwhich the sun rises daily, keeping in mindyour reference point. Make a daily sketchof the rising sun as well as your referencepoint in your note book during this period.(See figure-2).

Fig-2 : Observing the position of sun

Does the spot of sunrise change? If itdoes, in which direction does it seem tomove?

When the sun looks like travellingtowards south of the sky, it is called thedakshinayanam. When it looks liketravelling towards north of the sky it is calledthe uttarayanam. (Ask your parents aboutUttarayanam and Dakshinayanam)

Was the sun appear travelling towardssouth or north during the time you madeyour observations?Do you think that is the reason for the

SCERT TELA

NGANA


Latitude in DegreeSl.No. Districts North (rounded to

whole number)1. Mahabubnagar 162. Ranga Reddy, Hyderabad,

Khammam, Nalgonda 173. Medak, Nizamabad, Karimnagar, Warangal 184. Adilabad 19

A list of latitudes of districts of Telangana is given below in table 1.

change in the length of the shadow ofthe stick day by day in activity 1?Assuming that you did not have anycalendar and knowledge of months andseasons, can you use movement of thesun to predict the arrival of winter orsummer?

Think and Discuss

Why does the sun appears to traveltowards north or south? Try to find theanswer by reading your social studieschapter “Earth movements andseasons” along with this lesson.

Collect the information : Are thetimings of rising and setting of the sunsame every day? Collect the informationfrom news papers for at least a period ofa fortnight. Think why the lengths of dayand night are different every day. Try toget answers through internet or fromother books or from teachers.

Activity 1 can be used to make asundial (a clock based on shadows of anobject due to sunlight). But the length ofthe shadow of our stick is changing day today because of the north – south movementof the sun which is a problem in making asundial.

People in olden days overcame thisproblem and made sundials also.

The Jantarmantar monument inJaipur, Rajasthan state is built by theRajput king sawai Jai Singh-II. It featuresthe world’s largest stone sundial and is aUNESCO’s world heritage site.

How can we make our own sundial?

Activity -3

Make your own sun-dialFirst of all, you will need to cut a right-

angled triangle ABC from a sheet ofcardboard. Angle C of the triangle shouldequal to the latitude of your city or townand angle A should be 90 degrees, as shownin figure 3.

Do you know?

Table -1

SCERT TELA

NGANA


Fix your cardboard triangle verticallyin the middle of a rectangular woodenboard. Glue strips of paper along both edgesof BC and the wooden board to make thetriangle stand erect.

Place your board with the triangle onlevel ground in an open space which getssunlight throughout the day. Base BC of thetriangle should be placed in the north-southdirection, with B pointing to the north.

At nine in the morning, draw a linealong the shadow of side AC on the woodenboard. Write the time alongside the line.Draw lines of the shadow of side AC at one-hour intervals (use a clock to check thetime) through the day till sunset and markthe time for each line. Your sun-dial isready.

You can tell the time by looking at theshadow on the sun-dial. But remember thatbase BC of the triangle in the sun-dial mustalways be in the north-south direction if youwish to read the time correctly.

(You can find out the north-southdirection at a place by using a stick like inActivity 1, but don’t use magneticcompass.)

Now we will try to know a few thingsabout the moon.

Fig-3 Sun dial

A

B C SN

Fig-4:Drawing the phases of the Moon

· Have you ever observed the movementof moon in the sky?

· Does the moon appear at same point ata particular time every day?

· Is the shape of the moon same on everyday?To understand these issues, let us do

some activities. (You can do these activitieson your own at home.)

Activity-4Observing phases of the moon

1. Note the date of the day after newmoon day (Amavasya), when the moon firstappears in the sky.

Also note the time at night when themoon sets (goes down in the western sky).In the same way every day locate the moonin the sky at the time of sunset orimmediately after sunset.

Record the date and time of the moonset and draw a picture of the moon as yousee it on that day in your note book as shownin figure 4.

SCERT TELA

NGANA


Continue making observations for asmany nights as possible.

2. Observe the moon a few daysbefore full moon day (Pournami) to a fewdays after it. Locate the position of moonin the sky at the time of sunset beforePournami and note the time and positionof moon in the sky at that time.

After Pournami, note the time at whichthe moon rises (comes in the eastern sky)and also note the date. Draw pictures of theshape of the moon on each of these days.

What do you understand from theseobservations?· Can you calculate the number of hours

between one moon rise and the nextmoon rise or the number of hoursbetween one moon setting and the nextmoon setting, with help of theseobservations?

· How many hours lapse between onesunrise to the next, or one sunset to thenext?

· Is the time period same for sun andmoon to appear at selected positionafter completing a cycle in the skyevery day?

· Does the moon appear at the same pointevery day during the time of the sunset?

· What is the shape of the moon? Is itsame every day?You might have observed that the shape

of the moon changes night after night.These changes in its appearance are calledthe phases of the moon. Can you guess whythe shape of the moon changes?

You may have noticed that the timeperiod taken by sun to complete a cycle inthe sky and come to selected position isalmost same every day and it is about 24hours i.e. 1 day. Whereas moon takes about50 minutes more than a day to completethe cycle and which results in the phasesof the moon.

Let us perform two activities tounderstand why the phases of moon occur.

Activity-5A Moon-shaped lemon

Choose a day one week after the newmoon day when the moon is visible in thesky before the sunset.

Take a yellow lemon or a whitewashedclay ball and pivot it on a long needle or ona spoke of bicycle. Hold it up towards themoon as shown in figure 5.

Fig-5 : Observing the shape formed bysun light on a lemon

Ensure that you are standing in thesunshine when you do this activity.

SCERT TELA

NGANA


Fig-7 : Phases of moon

sun rays

Fig-6

Observe the shape formed by thesunlight on the surface of the lemon.

Is there some similarity between theshape formed and the shape of the moon?

Activity-6

Why does the shape of the moonchange?

(Do this activity around 4p.m.)Wrap a ball tightly with a white

handkerchief or with a piece of white cloth.Assume this is the moon. Hold this ball infront of your eyes in bright sunshine asshown in figure 6 and turn around yourselfslowly. Observe how the shape of theilluminated part of the ball changes.· Does sunlight fall on half the ball at all

times while you turn around?

· Is the shape of the illuminated part onthe ball viewed by you same in allpositions during your rotation?

· Why does this happen?

To understand the reason better, lookat figure7 carefully.

SCERT TELA

NGANA


The large circle in the middle of figure7is the earth and the smaller circles aroundit represent the moon in differentpositions. You can also see the phases ofthe moon on different days in the figure.The sunrays falling on the moon illuminatehalf its surface in all the positions.However, we cannot see the entireilluminated surface from the earth in all thepositions. In some cases we see the entireilluminated surface while in others we seeonly part of it. In one particular position,we cannot see the illuminated surface at all.

The shape of the moon we see is theshape of the illuminated portion visible tous.

In figure 7, the day of the new moon iscalled day 0 or day 28 (position 1). In thisposition, the illuminated surface is notvisible from earth, so the moon cannot beseen from earth.

Four days later, when the moon is inposition 2, a small part of its illuminatedsurface is visible from earth. On day-7, themoon is in position 3, so more of itsilluminated part is visible from earth.

After fourteen days (at position 5) theentire illuminated surface of the moon isvisible from earth. This is the day of thefull moon.

Subsequently, the moon appearssmaller with each day as it passes throughpositions 6 (day-18), 7(day-21) and 8(day-25). After 28 days, the moon is once againin position 1.

Try to duplicate position 1 with the ball.For this, you will have to hold the ball

towards the sun (between your eyesight andthe sun).

In this position, which half of the ballis illuminated?Although half the surface of the moon

is illuminated everyday, we cannot see themoon on new moon day since theilluminated surface is on the side oppositeto the point of observation on the earth. Ona full moon day, the situation is reversed.The illuminated half of the moon faces thepoint of observation, so we see a full moon.

From the above explanation, you mayhave understood that the sun and moonmust be on the same side of the earth on anew moon day and they are on oppositesides of the earth on a full moon day.

During the phases in between, we seedifferent shapes of the moon.

Hold the ball in different positions anddraw pictures of the shapes of the visibleilluminated portion in each case.

The different shapes of the moon in itsdifferent phases, as seen from the earth, areshown in figure 7. Compare the drawingsyou have made with those in figure 7.

Can you now state as to in whichdirection the moon will rise on a fullmoon day?While we observe moon in clear sky

on a full moon day, we think about the spotsthose are visible on the moon. In olden daysalso people were curious about those spotson the moon. They did not know the natureof the surface of moon as we know it today.This led to creation of a lot of stories andmyths about the spots on the moon.

SCERT TELA

NGANA


Do you know any such stories?Today we have many satellites that

observe the surface of the moon. Humanbeings landed on the moon way back in1969. We have better information aboutmoon’s surface than our ancestors.

The surface of the moonWhen astronauts landed on the moon,

they found that the moon’s surface is dustyand barren. There are many craters ofdifferent sizes. It also has a large numberof steep and high mountains. Some of theseare as high as the highest mountains on theearth. But the moon has no atmosphere likethat on the earth.

Will we be able to hear any sound if wewere on the moon? Why?Can any life exist on the moon? Why?

Do you know?Our country launched Chandrayan-1

(Satellite to Moon) on 22nd October2008 to know about the moon. The objectives of Chandrayan-1 are:

1. To check the possibility of finding water on the moon

2. Finding out the elements of matter onmoon

3. To search for Helium-34. To make 3-dimensional atlas of the moon

5. To study about the evolution of thesolar systemNow India is one of the six countries

which have sent satellites to the moon.Search the findings of Chandrayan-1 oninternet or in news papers.

Think and Discuss

Scientists are planning to build settlementson moon and are trying to make arrangementsto live there. You know that there is no air onmoon. How will it be possible to live on themoon then?

We feel pleasant during the night offull moon. But sometimes on full moonday the moon loses its brightness forsome time, it appears covered – partiallyor fully. This is called Lunar eclipse.Why does the moon get covered? Likethe moon, the sun also gets covered

partially or fully on someof the new moon days.This is called Solareclipse. Let us try tounderstand thesephenomena.

Solar EclipseA solar eclipse occurs when the

shadow of the moon falls on the earth. Itoccurs only on new moon day.Types of solar eclipse1. Total Solar eclipse: It occurs when the

moon completely covers the Sun, asseen from earth.

2. Partial Solar eclipse: It can beobserved when only the partially shadedouter region of the shadow cast bymoon (Lunar penumbra) touches theearth.

3. Annular eclipse: It occurs when themoon appears smaller than the sun as itpasses centrally across the solar diskand a bright ring or annulus of sunlightremains visible during eclipse.

SCERT TELA

NGANA


4. Hybrid eclipses: These are a rare formof Solar eclipse, which changes froman annular to a total solar eclipse alongits path.

Lunar eclipseA lunar eclipse occurs when the

shadow of the earth falls on the moon. Itoccurs only on full moon day.

Types of lunar eclipse1. Total Lunar eclipse: It occurs when

the earth’s shadow(Umbra) obscures allof the moon’s visible surface.

2. Partial Lunar eclipse: It can beobserved only when part of the moon’svisible surface is obscured by theearth’s shadow.

3. Penumbral Lunar eclipse: It happenswhen the moon travels through thepartially shaded outer region of theshadow cast by the earth(earth’spenumbra).Why does a lunar eclipse occur onlyon a full moon day?

According to figure-7, in whichposition can the shadow of the earth fallon the moon?Can this position occur only on oneparticular day?Can you now explain why a solar eclipseoccurs only on a new moon day?However, why is it that a solar eclipse

does not occur on every new moon day anda lunar eclipse does not occur on every fullmoon day? Let us try and understand thereason.

A total solar eclipse occurred on theafternoon of February 16, 1980 (it wasseen in Mahaboobnagar, Nalgonda andKhammam districts and also in some areasof Krishna district in Andhra Pradesh.)Because the sun was covered during theeclipse, it looked like night during the daytime.

Figure 8 contains a sketch of the timeexposure photograph of this eclipse. Thatmeans the exposures of the different stagesof the eclipse were made at 10-minuteintervals on the same frame.

Fig-8

SCERT TELA

NGANA


The sketch from left to right shows themoon slowly covering the sun and thenmoving away. The uncovered portion of thesun appears white and the black circlesrepresent the moon in the sketch.

You can easily guess the position of themoon at each stage of the eclipse. Can youdraw lines tracing the paths of the sun andmoon in the sketch?

The white portions of the discs infigure-8 represents the sun and the blackportions represents the moon. Each ofthese discs depict the position of the sunand moon at various stages of the eclipse.

Make 2 discs, one white and the otherblack, as the size of sun and moon as infigure-8.

We shall now find the centers of thesun and moon at each stage. To do this, takethe white disc you have made and place itexactly on the white portion of any of thestages in the diagram.

Pierce a hole through the center ofyour disc with a pin to mark the spot at thecenter of the sun’s position at that stage inthe diagram. Remove the white disc andmark the spot with a pencil.

In this way, mark the sun’s center atevery stage of the eclipse in your diagram.Join these spots with a line. This linedepicts the path of the sun.

To find the moon’s path, repeat theexercise, but this time use the black discand mark the centers of the black portionsat each stage of the eclipse. Join thesespots with a line and you will get the pathof the moon during the eclipse.

Do the sun and moon follow parallelpaths or do their paths cross each otherduring the course of the eclipse?

There is another aspect to note infigure-8. The sun and moon reached thepoint of intersection of their paths atexactly the same time during the eclipseon February 16, 1980.

If this had not happened, would a totalsolar eclipse still have occurred?Can you now tell why a solar eclipsedoes not occur on every new moon day?What would be the difference in the

position of the sun and moon on new moondays when no eclipse takes place and whenthere is an eclipse? Use the sketch to tryand figure out your answer.

Some other fascinating celestialobjects in the sky are stars. They usuallyappear in groups and members of thesegroups when viewed together formdifferent shapes. People used to assignsome shapes of animals and human beingto those small groups. Those groups arecalled constellations. A group of starswhich contains millions of stars are calledgalaxy. Millions of galaxies together makeour universe.

Let us know something about starsWhen you look at the night sky, do the

stars appear to be moving? If you wish tostudy the movement of stars across the skyand to trace their paths you must observethe pole star, the seven stars of theSaptarishi (Great Bear) constellation andthe six stars of the Sharmistha (Cassiopeia)constellation.

SCERT TELA

NGANA


You can easilyrecognize the greatbear with itsrectangular head inthe northern sky(figure 9a).

Fig–9(a): Great bear constellation(saptarishi) - position of pole star

In winter, this constellation rise a fewhours before sunrise (wecan see this fromanywhere in Telangana). Inthis season, you can alsosee Cassiopeia in the samepart of the sky, its six starsforming the letter ‘M’

Fig-9(b): Cassiopeiaconstellation (Sharmistha) - position ofpole star

You can locate the pole star ( Polaris )with the help of these two constellations.If you are able to spot only the great bear,look at the two stars that form the outerside of its rectangular head. Extend animaginary line from these two stars (asshown in figure 9a). The pole star will belocated on that extended line with adistance of about 5 times the distancebetween these two stars.

If only Cassiopeia is visible, the polestar will be located on a line extended fromthe middle star of the ‘M’ (see figure 9b).

Once you have located the Great Bear,Cassiopeia and pole star in the night sky,do the following activity.

Activity -7

Observing the movement ofconstellations (stars)

Take a 20cm x 20cm square sheet ofpaper and make a 1cm diameter hole in itscenter. Mark a cross ( X ) on one side ofthe sheet of paper as shown in figure 10.

Hold the sheet in front of your eyeswith the ‘x’ mark at the bottom and lookfor the pole star through the hole. Once youhave located the pole star, check in whichdirection the Great Bear or Cassiopeia lie.

Write ‘G’ for Great Bear and ‘C’ forCassiopeia on the paper in the directionsin which you see each of the constellations.Mark the timing at which you made yourobservation in both cases.

Pole star

Pole star

Fig-10

SCERT TELA

NGANA


Choose a nearby tree or house as areference point. Draw a picture of yourreference point on the paper sheet taken,clearly indicating its location.

Repeat your observations at one-hourintervals. Ensure that you are standing onthe same spot each time you look at thestars. Write G, C in the direction of theposition of the great bear and Cassiopeiaduring each observation and note the timeof the observation next to the letters G andC.

Using the tree or house you havechosen as your reference point, checkwhether the position of the pole star haschanged or not. If it has changed, note thechanged position.

Repeat this activity as many times aspossible, the minimum being four times.But ensure that the ‘x’ mark on your sheetof paper remains at the bottom during allyour observations.

You could also use other known starsor constellations close to the pole star toperform this activity.

Study the picture you have drawn andanswer the following questions.

Do the positions of the stars changewith time?Does the position of the pole star alsochange with time?Does the shape of the great bear andCassiopeia change with time or doesthe position of the entire constellationsin the sky change?What kind of path do theseconstellations trace in the sky?From your observations, you would

have realized that the stars do not remainin the same spot in the sky but revolvearound the pole star. The pole star, however,remains fixed at one place. It takes the stars24 hours to complete a revolution aroundthe pole star. We can observe only half thisrevolution during the course of a night.

If all stars move, why doesn’t the polestar move? Let us try to understand it bydoing the following activity.

Activity -8

Why does the pole star appearsfixed at one point?

Take an umbrella and open it. Makeabout 10 – 15 stars out of white paper having5 cm length, 2.5 cm breadth. Paste one starat the position of the central rod of theumbrella and others at different places onthe cloth near the end of each spoke (figure11).

Fig-11

Now rotate the umbrella by holding itscentral rod in your hand. Observe the starson the umbrella. Is there any star whichdoes not appear moving? Where is this starlocated? Is it located where the rod of theumbrella holds the cloth of the umbrella?

On similar lines, if there were a starlocated where the axis of rotation of the

SCERT TELA

NGANA


earth meets the sky, couldthis star also be stationary?

The pole star is situatedin the direction of the earth’saxis and that is why it does notappear to move even thoughall stars appears that they aremoving because of therotation of the earth.(figure 12).

Fig-13: Saptha rushi Sharmista Orion Leo ( Simha Rashi )

Some of the Galaxies in our Universe

Some of the constellations visible from Telangana

Fig-14

polaris

Towards

the sunEarth’sOrbital Plane

Fig-12:Direction of Pole Star

SCERT TELA

NGANA


Among millions of galaxies, our sun isa star in Milky Way galaxy. Our earth isrevolving around the sun. And moon isrevolving around the earth. Do you knowthat not only the earth but also some othercelestial bodies are revolving around thesun? Let us know something about thosecelestial bodies.

The solar systemThe sun and the

celestial bodies whichrevolve around it formthe solar system. Itconsists of large

number of bodies such as planets, comets,asteroids and meteors. The gravitationalattraction between the sun and these objectskeeps them revolving around it.

The earth revolves around the sun. It isa member of the solar system. It is a planet.There are seven other planets that revolvearound the sun. The eight planets in theirorder of distance from the sun are:Mercury, Venus, Earth, Mars, Jupiter,Saturn, Uranus and Neptune.

Figure -15 shows a schematic view ofthe solar system.( Not to the Scale ).

Let us learn aboutsome members of thesolar system.

The SunThe Sun is the

nearest star to us.

Fig-15It is continuously emitting huge

amounts of heat and light. The sun is thesource of almost all energy on the earth. Infact, the sun is the main source of heat andlight for all planets of our solar system.

SCERT TELA

NGANA


The planetsThe planets look like stars, but they do

not have light of their own. They merelyreflect the sunlight that falls on them.

A planet has a definite path in which itrevolves around the sun. This path is calledan orbit. The time taken by a planet tocomplete one revolution is called itsperiod of revolution. The period ofrevolution increases as the distance of theplanet from the sun increases.

Besides revolving around the sun, aplanet also rotates on its own axis like atop. The time taken by a planet to completeone rotation is called its period ofrotation.

Some planets are known to havemoons/satellites revolving round them. Anycelestial body revolving around anothercelestial body is called its satellite.

The earth revolves around the sun. Doesit make earth a satellite of the sun?

The earth and other seven celestialbodies can be said to be as satellite of thesun, though generally we call them as planetsof the sun. We use the term satellite for thebodies revolving around planets. Moon is asatellite of the earth. There are many man-made satellites revolving round the earth.These are called artificial satellites.

Mercury (Budhudu)The planet mercury

is nearest to the sun. Itis the smallest planetof our solar system.

Because mercury is very close to thesun, it is very difficult to observe it, as mostof the time it is hidden in the glare of thesun. However, it can be observed just beforesunrise or just after sunset, near thehorizon.

So it is visible only at places wheretrees or buildings do not obstruct the viewof the horizon. Mercury has no satellite ofits own.

Venus (Sukrudu)Venus is earth’s

nearest planetaryneighbour. It is thebrightest planetin the night sky.Sometimes, Venusappears in theeastern sky before sunrise.

Sometimes, it appears in the westernsky just after sunset. Therefore, it is oftencalled a morning or an evening star,although it is not a star. Try to locate Venusin the night sky during early winter.

Venus has no satellite of its own.Rotation of Venus on its axis is somewhatunusual. It rotates from east to west whilethe earth rotates from west to east.

Does the sun rise in the east on Venus?

If you get a chance, try to observe Venusthrough a telescope. You will observe thatVenus shows phases just like the moon.SCERT T

ELANGANA


The Earth (Bhoomi)The earth is the

only planet in the solarsystem on which life isknown to exist.

Some speciale n v i r o n m e n t a lconditions are responsible for theexistence and continuation of life on theearth. These include just the right distancefrom the sun so that it has the righttemperature range, the presence of waterand suitable atmosphere and a blanket ofozone. We must take special care to protectour environment so that life on earth is notdisturbed.

From space, the earth appears blue-green due to the reflection of light fromwater and landmass on its surface. The earthhas only one moon revolving around it.

Mars (Kujudu / Angarakudu)The first

planet outside theorbit of the earthis mars. It appearsslightly reddishand therefore, itis also called the red planet. Mars has twosmall natural satellites.

Mars science laboratory:National Aeronautic andSpace Administration (NASA) in America started amission called ‘Mars Science Laboratory’ on 26th

November 2011, to know more about Mars. A rovernamed ‘Curiosity’ landed on Mars on 6th August 2012. Itis analyzing the elements in the rocks of the surface ofthe Mars. It found few indications of water on Mars. Itis searching whether the favorable conditions for lifeexist on Mars.

Jupiter (Brihaspati)Jupiter is the

largest planet ofthe solar system. Itis so large thatabout 1300 earthscan be placed insidethis giant planet.However, the massof Jupiter is about 318 times that of ourearth. It rotates very rapidly on its axis.Jupiter has a large number of satellites.

It also has faint rings around, appearsquite bright in the sky. If you observe it withthe help of a telescope, you can also seefour of its large moons.

Saturn (Shani)B e y o n d

Jupiter is Saturnwhich appearsyellowish incolour. Whatmakes it unique in

SCERT TELA

NGANA


Mercury 0.38 5.79 88 days 0

Venus 0.95 10.8 225 days 0

Earth 1 15 365 days 1

Mars 0.53 22.8 687 days 2

Jupiter 11.20 77.8 12 years 69

Saturn 9.45 142.7 29.5 years 62

Uranus 4.00 286.9 84 years 27

Neptune 3.88 449.7 165 years 14

Name of thePlanet

ComparativeDiameter with

Diameter ofEarth(Approximately)

Distance from theSun in Crore km.

(Approximately)

Period ofrevolution

(Approximately)

No. of satelites(detected so far )

the solar system is that it has rings. Theserings are not visible to the naked eye. Youcan observe them with a small telescope.Saturn also has a large number of satellites.

Uranus and Neptune

These are the outermost planets of thesolar system. They can be seen only withthe help of large telescopes. Like Venus,Uranus also rotates from east to west. Themost remarkable feature of Uranus is thatit has highly tilted rotational axis (figure16). As a result, in its orbital motion itappears to roll on its side.

Fig. 16The first four planets, mercury, venus,

earth and mars are much nearer the sun thanthe other four planets. They are called theinner planets. The inner planets have veryfew moons.

The planets outside the orbit of Mars,namely Jupiter, Saturn, Uranus and Neptuneare much farther away from the sun thanthe inner planets. They have a ring systemaround them. The outer planets have a largenumber of moons.

Table-2 : Comparison between planets

We took Earth’s diameter (12756 Km) as 1 unit. With this information find the diametersof other planets using the comparison given in table-2.

Uranus Neptune

SCERT TELA

NGANA


AsteroidsThere is a large gap in between the

orbits of mars and Jupiter (figure-17). Thisgap is occupied by a large number of smallobjects that revolve around the sun. Theseare called asteroids. Asteroids can only beseen through large telescopes.

Fig. 17

CometsComets are also members of our solar

system. They revolve around the sun inhighly elliptical orbits. However, theirperiod of revolution round the sun isusually very long. A comet appearsgenerally as a bright head with a long tail.The length of the tail grows in size as itapproaches the sun. The tail of a comet isalways directed away from the sun (figure18).

Fig. 18

Think and Discuss

The diameter of the sun is 13,92,000 Km.The diameter of the earth is 12,756Km.The diameter of the moon is 3,474 Km.The distance from the sun to earth is15,00,00,000 Km.The distance from the earth to moon is3,84,399 Km.

Take the scale as 1 lakh km = 1 cm,and imagine how the arrangement of sun,earth and moon is in our universe. Canyou make this arrangement on yourschool-ground?

Do you know? Till 25th August 2006 we used to say

there are nine planets in our solar system.The ninth planet at that time was ‘Pluto’.In the 26th general assembly ofInternational Astronomical Union it wasdecided that ‘Pluto’ is no more a planet.The decision was taken since it wasobserved that Pluto does not follow therule of “cleared the neighborhood.” Thatmeans, sometimes it is entering into theorbit of Neptune.

Some other members of thesolar system

There are some other bodies whichrevolve around the sun. They are alsomembers of the solar system. Let us knowabout some of them.

SCERT TELA

NGANA


Many comets are known to appearperiodically. One such comet is Halley’scomet, which appears after every 76 years.It was last seen in 1986. Can you tell whenHalley’s comet will be visible again?

Meteors and MeteoritesAt night, when the sky is clear and the

moon is not visible, you may sometimessee bright streaks of light in the sky (fig.19).

Fig. 19These are commonly known as

shooting stars, although they are not stars.They are called meteors. A meteor is usuallya small object that occasionally enters theearth’s atmosphere. It has a very highspeed. The friction due to the atmosphereheats it up. It glows and evaporates quickly.That is why the bright steak lasts for a veryshort time. Some meteors are large and sothey can reach the earth before theyevaporate completely. The body thatreaches the earth is called a meteorite(figure-20).

Fig. 20Meteorites help scientists in

investigating the nature of the materialfrom which solar system was formed.

Artificial satellitesYou must have heard that there are a

number of artificial satellites which areorbiting the earth. Artificial satellites areman-made. They are launched from theearth. They revolve around the earth muchcloser than earth’s natural satellite, themoon.

India has built and launched severalartificial satellites. Aryabhatta was the firstIndian artificial satellite (figure-21).

S o m eother Indiansatellites areINSAT, IRS,Kalpana-1,E D U S AT,etc.

Artificial satellites have many practicalapplications. They are used for forecastingweather, transmitting television and radio

Fig. 21SCERT TELA

NGANA


signals. They are also used for telecommunication, remote sensing(collecting information from a distance) inaviation and military use.

This information about solar system isknown to us for the past 2 to 3 hundredyears, after the telescopes were made. Buthow did the people of olden days know somuch about the celestial bodies and alsoabout earth?

Let us know about some of theinteresting constructions of theirknowledge.How did people come to anunderstanding that earth isspherical?

In olden days people felt that earth isflat because it looks flat. However they hada doubt, if it is flat how does the water inoceans remain there, why does it not spellout of the earth? To get clarity theyassumed a fencing around the flat earth.After that1) They assumed the shape of earth asround by observing the shadow of earth inlunar eclipse. In every eclipse they foundthe shape of earth is in round even thoughthere is a chance of getting linear, ellipticalshadows by a circular object.2) Some sailors, who started their journeyin ocean, reached the same place aftertravelling large distance in one directiononly.3) Observing ships approaching the portalso helped to change their opinion aboutthe shape of the earth, that is, usually theysee smoke of the ship first and then the topof the ship after that the whole ship.4) Observations about the movement ofstars and different stars visible fromdifferent places on the earth also helped tothink about the shape of the earth.

Through all such observations made byso many people at so many places on theearth they came to an understanding thatearth is spherical. And then it is get clarifiedin 1969 when man landed on the moon andobserved the earth’s shape from the moon.

How did people come to anunderstanding that earth rotateson its own axis?

People from olden days thought thatearth is located in the center of the universewith sun, moon and stars moving aroundit. They also thought that sun, moon andstars are located on transparent concentricspheres sorrounding the earth, becausethey are not falling down.

The three spheres are rotating on theiraxis from east to west that is why sun,moon and stars appear to revolve from eastto west around the earth. They also assumedthat the sphere on which sun is locatedrotates east to west and oscillates fromsouth to north that is why uttarayanam anddakshinayanam are happening.

Because of the uneven movement ofsome stars (actually they are planets) whichthey observed, it became very difficult toexplain the model of universe whichrequired so many transparent spheresaround the earth. Nicholas Copernicussuggested that the sun is at the center ofthe universe and all other celestial objectsare revolving around the sun. Then how doday and nights occur. It was assumed thatearth rotates on its axis. This model couldexplain the occurence of day and night.

In this way people came to anunderstanding that earth rotates on its axis.

SCERT TELA

NGANA



Key words

Celestial bodies, Local noon, Sundial, Dakshinayanam, Uttarayanam, Phasesof the moon, Constellation, Galaxy, Pole star, Solar system, Planets, Satellites,Artificial satellites, Asteroids, Comets, Meteors, Meteorite.

The shortest shadow cast by a vertical object on the ground always falls in North,South direction.The shortest shadow of on object occurs at local noon.The time duration for appearance of sun and moon after completion of a cycle isdifferent.Changes in appearance of moon are called phases of the moon.On the new moon day, sun and moon are on the same side of the earth.On the full moon day, sun and moon are on either sides of the earth.Moon has no atmosphere like we have on the earth.The polestar is situated in the direction of the earth’s axis and hence it appears asnot moving.There are eight planets in our solar system.Among eight planets of solar system earth is the only planet which supports life.Large number of objects that revolve around the sun between the orbits of Mars andJupiter are asteroids.The length of the tail of the comet grows in size as it approaches the sun.A meteor is a small object that occasionally enters the earth’s atmosphere.A body that reaches the earth is called a meteorite.Aryabatta was the first Indian artificial satellite.Forecasting weather, transmitting Television and Radio signals, Telecommunication,remote sensing are the practical applications of artificial satellites.

Reflections on concepts1. Why does pole star seem to be stationary? (AS1)2. How many planets are there in our solar system? What are they?

(AS1)3. Among all 8 planets what is the special thing about earth? (AS1)4. How do people come to an understanding that earth is spherical? (AS1)5. How do people come to an understanding that earth rotates on its own axis? (AS1)

What we have learnt

SCERT TELA

NGANA


Application of concepts1. What factors to be taken into consideration to view the pole star at your place?

(AS1)

2. How can you find north – south direction at your place? (AS1)

3. Is it possible to see the pole star for the people who live in the southern hemisphereof the earth? Why? (AS1)

4. Draw the different phases of moon. Arrange them in a order from pournami toamavasya. (AS5)

5. What are the questions that arise in your mind when you look at night sky? (AS2)

6. What are the planets you have seen in the sky? When do you observe those planets?(AS3)

7. What is the use of artificial satellites in our daily life? (AS7)

8. How do day and night occur? (AS1)

Higher Order Thinking Questions1. Even though we do not have clock, we can know the time by observing some shad-

ows in day time. Think and discuss with your friends how can we know the time atnight. (AS2)

2. We launched many artificial satellites around our earth for different purposes. Whatdo you think about the impact of artificial satellites and their radiation on bio diver-sity? (AS7)

3. How do you appreciate the construction of knowledge about the Universe by ourancestors?(AS6)

4. Among eight planets of our solar system, earth is the only planet supporting life.Explain how we should protect our earth and its environment. (AS7)


1. When the moon completely covers the sun as seen from the earth is known as…

a) Partial solar eclipse b) Total solar eclipse [ ]

c) Annular eclipse d) Hybrid eclipse

2. The planet which is near to earth is [ ]

a) Mercury b) Venus c) Jupiter d) Saturn

SCERT TELA

NGANA


3. The brightest planet among all the planets [ ]

a) Mercury b) Venus c) Jupiter d) Saturn

4. Moon is [ ]

a) the natural satellite to earth b) an artificial satellite to earth

c) a comet d) an asteroid

5. The first Indian artificial satellite [ ]

a) INSAT b) Kalpana - I c) Aryabhatta d) EDUSAT

Suggested Experimetns1. Conduct an experiment to find out the local noon time of your village/Town.2. Conduct an experiment to make a sundial.Suggested Project Works1. Collect the information what the Chandrayaan-I brought the information from the

Moon through news papers,magazines.2. Collect information about cosmic dust (wastage) from news papers, internet and

make a poster on your school panel board about the consequences of cosmic dust.3. What is the duration of a day and night today? Collect the information about dura-

tion of day and night for the past 7 days from the news papers, analyze it and saywhether summer or winter is going to come.

SCERT TELA

NGANA


GRAPHS OF MOTIONChapter

12In class 7, We studied about motion,

types of motion and relation between speed,distance and time.

Can we describe motion by usinggraphs?

Let us try with some situations.

Have you travelled in a bus or trainor bullock cart or auto? Try to recall a tripyou made and answer the followingquestions.

− Where did you begin your journey fromand where did you go?

− What was the distance between thesetwo places?

− How long did it take for you to makethe trip?

− What was the average distance yourvehicle (bus or train or bullock cartor auto) travelled in one hour?

The distance travelled by an object in aunit of time (hour, minute, second etc.) iscalled the average speed of the object. Theequation to calculate the average speed is

If we measure the distance inkilometers and the time in hours, the unitof speed will be kilometer per hour orKMPH. We can use other units of distanceand time to measure the speed.

• Can we guess the unit of speed if thedistance covered is measured incentimeters and the time in seconds?

• What will the unit of speed be if thedistance covered is measured in metersand the time in minutes?

• Raja travelled 15 kilometers in 3 hours.Calculate his average speed and writethe correct unit of speed.

Note: It is important to mention the unitwhen we denote any quantity likedistance, time, speed, weight etc.Otherwise it becomes meaningless. Soalways remember to write the unit afterthe quantity that you denote.

There are many different ways inwhich you can describe a journey. In thischapter we shall learn how to use graphs todescribe and represent motion. We shallalso see in what other ways graphs ofmotion can be useful to us.

Activity-1

Swathi walked from her home to herschool. The details of her journey are givenin table - 1.

Average speed = Total distance travailed

Total time to cover the distance

Total distance travelledTotal time taken to cover the

distanceSCERT TELA

NGANA

172 GRAPHS OF MOTION

This data tells us the distance Swathiwalked in consecutive two - minutesegments of her journey but it doesn't tellus how far she walked at any given time ofher journey, for example say after 10-minutes. So, we can not find the distancefrom her home to her school just bylooking at a table. To get this information,we must present the data in the table in adifferent way. We must show the totalelapsed time and the total distance coveredas in table - 2.

Let us now use this data to make a graphof Swathi's journey, showing the time takenand the distance covered.

You have learnt how to make graphs inmathematics. For this graph we shall showthe time on X-axis, and distance coveredon Y-axis. Draw the X and Y axes on yourgraph paper and choose a scale for eachaxis. Record the scale in the upper righthand corner of your graph paper.

To represent the data given in table - 2as a graph, mark the first point on the graphwhich corresponds to a time of twominutes on the X-axis and a distance of 120meters on the Y-axis. In the same way, plotthe remaining five points on graph paper.Connect these points with the help of aruler to get a straight line as shown in fig.

This graph shows swathi's journeyfrom her home to school.

Think and Discuss

· Why should we take time on X-axisand distance covered on Y-axis

A graph is not a mapYou must remember that the graph

you have drawn and the other graphs youwill draw in this chapter are graphs that are

TIME(in minutes)

0-2 (first two minutes)

2-4 (second two minutes)

4-6 (third two minutes)

6-8 (fourth two minutes)

8-10 (fifth two minutes)

10-12 (sixth two minutes)

DistanceTravelled

(in meters)

120

120

120

120

120

120

Table - 1

Total elapsed time(in minutes)

24681012

Total distancecovered

(in meters)

120240360480600720

Table - 2X - axis (time in minutes)

GRAPH - 1

Y -

axis

(dis

tanc

e tra

velle

d in

met

ers)

Scale :On X - axis 1 cm = 1 minuteOn Y - axis 1 cm = 60 meters

SCERT TELA

NGANA


· How would the graph of time anddistance look for an object travellingwith uniform motion?

If an object is travelling withuniform motion, the distance it covers in aunit of time is its speed.· What was Swathi's speed for each two

minute segment of her journey.· calculate the average speed of Swathi's

complete journey?· Is the speed for each two minutes

segment the same as her averagespeed for the entire journey?

The speed of an object travellingwith uniform motion does not change. insuch cases the speed and average speed arethe same.

Activity-2Graphs of objects moving atdifferent uniform speeds

Anish and Hitesh raced from theirhome to school. Hitesh ran at uniformspeed, so did Anish. But their uniformspeeds were different. Graph - 2 showstheir motions.

plotted time elapsed against the distancecovered. They are not maps showing theroute of journey. Never make a mistake ofthinking that a graph shows route of thejourney.

Figure -1 isa map showing theroad from Swathi'shouse to school.Swathi walks toschool along thisroad. Compare thegraph that showsSwathi's journey(graph -1) and themap. (Figure - 1)· Can you

estimate howlong Swathitakes to reachher school bylooking at themap?

· Can we guess how many turns are therealong the road from Swathi's home toher school, or where the road crossesthe river, by looking at the graph?

It is evident that the information youget from a route map can not be obtainedfrom a graph. Similarly, informationabout the speed at which Swathi walkedcan be obtained only from the graph, notfrom the map.

· Did Swathi cover an equal distance inevery two minutes interval of herjourney?

If an object covers an equal distancein equal time intervals it is said to be in"uniform motion". X - axis (time in minutes)

GRAPH - 2

Y -

axis

(dis

tanc

e tra

velle

d in

met

ers)


Hite

sh

Anish

Swathi'shouse

Roa

dR

iver

Fig. 1

SCERT TELA

NGANA


· Can you tell who ran faster just bylooking at the graph?

· How much time did Hitesh take to runfrom home to school? Calculate hisaverage speed.

· What was the average speed of Anish?Relation between speed and theslope of a graph

If we have two graphs of uniformspeed, we just by looking at the two linescan tell which speed is greater. We shouldlook at the angle that the two graph linesmake with the X-axis. We can estimate theslope of the graph lines from these angles.The greater the angle, the greater the slopefor graph line.· Take another look on graph - 3. Which

graph line has the greater slope -Hitesh's or Anish's?

· Is his speed faster too?Any graph of uniform motion will

be a straight line. The faster the speed ofuniform motion the greater will be theslope of the graph line. That is, the anglethat graph line makes with the X-axis willbe larger. But remeber you can make suchvisual comparisons only between graphswhich have the same scale. You can notcompare graphs with different scales justby looking at them.

Activity-3Graphs of stationary objects.

Bhoomika went to a journey and thedata of her trip is given in table - 3.

· Can you say by looking at the tablewhether Bhoomika rested some whereduring her journey?

· After walking how many minutes didBhoomika take rest? For how manyminutes did she take rest.

· How would you show the duration ofher rest in a graph?

To understand how this is to bedone, let us draw a graph of Bhoomika'sjourney. But before we do this we mustrearrange the figures in table - 3 and writethem in the way we did for Swathi's graph.

X - axis (time in minutes)GRAPH - 3

Y -

axis

(dist

ance

trav

elle

d in

met

ers)


Hite

sh

Anish

Lesser the angle lesser the slope

Greater the angle Greater the slope

Time(in minutes)

0-2

2-4

4-6

6-8

8-10

10-12

12-14

14-16

DistanceTravelled

(in meters)

60

60

60

0

0

0

60

60

Table - 3

SCERT TELA

NGANA


· Copy table -4 in your exercise book andfill the blanks.

· Use the data from table-4 to draw agraph of Bhoomika's journey.

· Look at the graph and estimate thedistance covered by Bhoomika after 8minutes.

· How far did she travel after 12 minutes?

From the 8th to 12th minutes of herjourney, the time increased but the totaldistance covered remained same.

When any object stops at a place,the time continues to increase but thedistance covered does not change duringits journey. Then the graph line remainsparallel to the X-axis. This shows that theobject is at rest.

Activity-4

The graph of Sana's journey is givenin graph 4. Look at the graph and answerthe following questions.

· What is the scale for X-axis?· What is the scale for Y-axis?· What was Sana's average speed for

section AB of her journey?· What was her average speed for section

CD of her journey?· Calculate Sana's average speed for her

entire journey.· After covering what distance did Sana

take rest and for how long did she takerest?

· Which section of the graph has agreater slope AB or CD?

Activity-5Graphs of non uniform motion

We have learnt about graphs ofuniform motion in the activities that wehave done so far. We shall now look atgraphs of motions which are not uniform.You might have seen a train leaving orarriving at a station.· Is the motion of the train uniform when

it leaves the station?

Time(in minutes)

2

4

6

8

10

12

14

16

DistanceTravelled

(in meters)

60

120

-

-

-

-

-

-

Table - 4


Y -

axis

(dist

ance

trav

elle

d in

met

ers)

Scale :On X - axis 1 cm =On Y - axis 1 cm =

A

B C

D

E

SCERT TELA

NGANA


· What changes took place in the motionof the train when it comes to a halt at astation.

The motion in which there is anincrease or a decrease in speed is called anon-uniform motion.

Abhilash travelled by a train fromKhammam to Secunderabad. He estimatedthe distance covered for each 30 minutesinterval by counting the telephone polesalong the railway track. He noted thisestimated distances in the form of a graph.Graph 5 shows the motion of the trainstarting from when it left Khammam stationtill it stoped at Secunderabad station.

· Find the distance covered by the trainfor every 30 minutes interval of timeby observing the graph and note thevalues in table 5

· Did the train cover equal distances inequal intervals of time?

· Which section of the graph shows nonuniform motion of the train?

· Which section of the graph showsuniform motion of the train?

· In which section of the graph was thetrain at rest?

· Observe at the sections of non uniformand uniform motion of the train in thegraph. What prime difference do younotice between these two sections?

A curved line in the graph of motionrepresents that the speed is increasingcontinuously. Observe section AB of thegraph. It shows the gradual increase intrain's speed when it leaves Khammamstation.

Activity-6Graph showing the motion of a turtlefalling from the beaks of swans

You might have heard about the storyof the flight of the turtle. In this story twoswans carry their friend turtle by holdingthe two ends of a stick firmly in their beaksand with the turtle gripped on the middleof the stick with its teeth. The swans flewat about hight 180 meters and carry theturtle along.

Time(in minutes)

0-3030-6060-90......................................................................

300-330

Distance covered(in kilometers)

210

32.5......................................................................

250

Table - 5


Y -

axis

(dist

ance

trav

elle

d in

kilo

met

ers)

Scale :On X - axis 1 cm = 30 MinOn Y - axis 1 cm = 25 Kms

AB

C

D E

SCERT TELA

NGANA


Activity-7Anitha riding a bicycle

Let us assume you are riding abicycle. You don't have any problem inpedalling on the plane road with uniformmotion. But when you are pedalling on aslant road against uphill, it gets difficult andyour speed decreases. On other hand, whenyou go along the slope, your speed in-creases and the bicycle moves really fast.

As they were flying above a lake theturtle was overwhelmed by the beautifulscene below. He could not hold hisexcitement and exclaimed "Wow!". Theremaining part of the story of the turtle'sflight of falling down is given in table 6.

Fig. 2

Time(in seconds)

123456

Table - 6

Distance Turtle fell(in meters)

5204580

125180

· Draw a graph of motion of the turtle'sfall.

· What does the graph look like? Whatis the shape of the graph?

· Can you guess whether the motion ofthe turtle is uniform or non-uniformbased on the graph?

· How much time did the turtle take tofall in to the lake from a height of 180meters?

· What was the average speed of the turtleduring its fall?

Graph - 6 shows the motion of bi-cycle due to the pedalling made by Anitha.

Look at the graph and state which ofthe following statments are true.

Fig . 3

X - axis (time)GRAPH - 6

Y -

axis

(dist

ance

trav

elle

d)

A

B

C D

E

SCERT TELA

NGANA


speed of 3 kmph. So the distance hetravelled in these two hours was _____kms.

3. In the next one-and-a-half hours he satand talked with his friend. So the dis-tance he travelled was ________ kms.

4. Finally in the next one-and-a-half hourshe travelled on his friend's bicycle at aspeed of 10 kmph and reached the city. inthis one-and-a-half hours he covered a dis-tance of _________ kms.

Enter this data in table 7

Make table 8 on the basis of this table-7. It should contain the total elapsed timeand the total distance covered.

1. Anitha cycled against the slope forsome time and then cycled down theslope then took rest for a while and thencycled on a plane road.

2. Anitha is pedalling the bicycle on uphillroad continuously.

3. Anitha first went downhill, then on aplane road, then uphill and finallyrested.

4. Anitha first cycled up hill, then stopedand rested for some time because shewas tired, then cycled on a level roadand finally rode downhill.

Activity-8

Ajay set out from his village walk-ing at a speed of 4 kmph. After walking fortwo hours he rested under the shade of atree. An hour later he again began walkingat a speed of 3 kmph. After walking for 2hours he met his friend Rajesh. They satunder a tree talking for one-and-a-halfhours. Then Rajesh took Ajay on his bicycleat a speed of 10 kmph. They cycled for one-and-a-half hours before reaching the city.

Let us proceed step by step to drawa graph of Ajay's journey.

First we shall make a table of thedistance he travelled in different time in-tervals. For example in the first two hourshe walked at a speed of 4 kmph that meanshe cover a distance of 2 x 4 = 8 km. In thesame way the distances he travelled in theremaining time intervals are as follows.1. Ajay rested under a tree for an hour. So

the distance travelled in this time was_______ kms.

2. In the next two hours he walked at a

Time(in hours)

212

1.5 (1 hour 30 min)1.5 (1 hour 30 min)

Table - 7


8________________

Time Elapsed(in hours)

0235

6.5 (6 hours 30 min)8

Table - 8


________________________

SCERT TELA

NGANA


Draw a graph for Ajay's journey withthe data given in the table. Answer thefollowing questions on the basis of thisgraph.· After walking how many kilometers did

Ajay meet Rajesh?· How many hours did Ajay take to reach

the city from his village?· What was average speed of Ajay's for

the first five hours?· What is the distance between the

village of Ajay and the city?· Which section of the graph has the

maximum slope?

Activity-9

Raizee and Jessika decided to visita sweet shop after school. When they wereabout to leave the school, the teacher calledJessika. So Raizee left alone. After a shortwhile Jessika came running and joined withRaizee. Then they went together to thesweet shop and ate sweets there. The entireepisode is shown below in the form of agraph (graph-7). Their journeys are shownby separate graph lines marked withdifferent colours.

Now look at the graph and answerthe following questions.


Y -

axis

(dist

ance

trav

elle

d in

met

ers)

Scale :On X - axis 1 cm = 5 MinOn Y - axis 1 cm = 250 Mtrs

Raizee

Jess

ika

· What was Raizee's speed per minute?· For how long was Jessika detained by

her teacher?· For how long did Jessika run before she

joined with Raizee?· What was Jessika's Average speed per

minute while she ran.· At what distance from the school did

Jessika joined with Raizee?· What distance did the two cover

together?· For how long did they walk together?

Activity-10

A brain teaserWhen school was over, Mounika and

Divya left for their homes. Mounika'shouse lay to the east of the school whileDivya's house lay to the west. The graph oftheir journey to their homes is shown ingraph - 8. Look at the graph and answer thefollowing questions.

· Did Mounika walk with uniform motionthroughout her journey?

· How far is Mounika's home from theschool?


Y -

axis

(dist

ance

trav

elle

d in

met

ers)

Scale :On X - axis 1 cm = 1MinOn Y - axis 1 cm = 200 Mtrs

Mou

nika

Divya

SCERT TELA

NGANA


Key words

What we have learnt

Motion, Speed, Uniform motion,Non-uniform motion, Average speed, Slope.


• How far is Divya's home from theschool?

• How much time did Mounika take toreach her home?

• How much time did Divya take to reachher home?

• Did Divya stop on the way? For howmuch time did she stop?

• Calculate Divya's average speed duringher journey.

• Did Mounika halt anywhere? How muchtime did she stop?

• Calculate Mounika's average speedduring her journey.

Activity-11

The graph of a storyThis is a very old story. You may

have heard it many times before. It is thestory of a race between the rabbit and thetortoise. The two take a bet on who will winthe race. The rabbit takes off swiftly whilethe tortoise begins at a slow and steadypace. The rabbit runs far ahead, then haltsto rest under a tree for a while. He fallsasleep.

The tortoise, meanwhile, continuesto forge ahead steadily. When the rabbitawakes, he runs swiftly to the finishing post.But alas! when he reaches the finishing linehe finds that the tortoise has already wonthe race.• Illustrate the race between the rabbit

and the tortoise in the form of a graph.

1. Look at graph 9.

Y -

axis

(dis

tanc

e tr

avel

led

in k

ilom

eter

s)

Scale :On X - axis 1 cm = 1 HourOn Y - axis 1 cm = 10 Kms

A

B

CD

E

X - axis (time in hours)

GRAPH - 9

•• If an object covers equal distances in equal

intervals of time then the motion is said to beuniform motion.

• Different types of motions can be representedby time-distance graphs.

• Time-Distance graph of uniform motion is astraight line.

• Slop of graph is the angle made by the graphwith X-axis.

• Motion in which the speed increases ordecreases is called non uniform motion.

• Time-Distance graph of a non-uniformmotion is not a straight line rather it is a curve.

• Slope of the time-distance graph line showsthe speed of the object at that time.

• If an object is at rest then the time distancegraph is a line parellel to X axis.

Average speed = Total distance travailed


Total distance travelled


Fig. 4


SCERT TELA

NGANA


· Which section of graph 9 has the greatest slope? What can you say about the speed inthis section? What can you say about the speed in section CD of the graph.

Application of concepts1. A river is 32 kms away from Nikitha's house. There is a hillock on the way. Nikitha left

for the river one morning on her bicycle. She reached the hillock after 2 hours, pedallingat a speed of 5 kmph. Since she could not cycle up the slope, she continued on foot,walking for an hour at a speed of 3 kmph, and reached the top of the hillock. From therethe road was all downhill. She rode her bicycle at a speed of 18 kmph and reached thebottom of the hillock in half-an-hour. She then rested under a tree for half an hour.Refreshed after a rest, she cycled at a speed of 5 kmph and reached the river in 2 hours.Draw a graph of Nikitha's journey from her home to the river.

2. Sunitha and her brother Bharat studyingin the same school. Sunitha walks to theschool while Bharath cycles to theschool. So, Sunitha has to leave an hourbefore Bharath to reach the school ontime. Graph 10 shows the graph linesof their journey from their home to theschool. Look at the graph and answerthe following questions.

A)Which graph line shows Sunitha's journey?B)Whose graph line has a greater slope?C)Whose speed is greater?D)If Sunitha wants to reach school in 3 hours at what speed should she walk?Higher order thinking questions1. Graph 11 shows the journey of Rakesh and Share Khan. Write a story about their journey

on the basis of the graph?

Y -

axis

(dist

ance

trav

elle

d in

kilo

met

ers)

Scale :On X - axis 1 cm = 5 MinOn Y - axis 1 cm = 1 KM

X - axis (time in hours)GRAPH - 10

A B

Y -

axis

(dist

ance

trav

elle

d in

met

ers)

Scale :On X - axis 1 cm = 1 MinOn Y - axis 1 cm = 50 Meters

X - axis (time in minutes) GRAPH - 11

Rake

sh

Sher KhanSCERT T

ELANGANA



1. What does the slope on a distance versus time graph represent? [ ]

a) Displacement b) Speed c) Velocity d) Acceleration

2. Which of the following graph represents constant speed (or) uniform motion[ ]

a) b) c) d)

3. The distance varsus-time graphs of four vehicles 1, 2, 3 and 4 aregiven in the adjacent figure. Which of them has greater speed? [ ]

a) 2 b) 3 c) 1 d) 4

4. The given below distance - time graph represents the motion ofa cart. During in which interval the cart is at rest [ ]

a) AB b) DE c) BC d) CD

5. The graph given below represents the motion of an object.According to graph, as time increases,

the speed of the object [ ]

a) Increases b) decreases c) remains same d) we cannot say

s

t ®

s

t ®

s

t ®

s

t ®

s

t ®

1

23

4

s

t ®

BC D

E

s

t ®

A

SCERT TELA

NGANA

8th phy em.pdf - SCERT Telangana

Documents

Transcript of 8th phy em.pdf - SCERT Telangana