Doing science is fun

DOING SCIENCE IS FUN V.G. KULKARNI R.M. BHAGWAT V.G. GAMBHIR Doing Science Is Fun V.G. Kulkami, R.M. Bhagwat, V.G. Ga...

Author: V. G. Kulkarni | Pradeep Banerjee | R. M. Bhagwat | V. G. Gambhir | A. D. Ghaisas | Neeru Sharma

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DOING SCIENCE IS FUN V.G. KULKARNI R.M. BHAGWAT V.G. GAMBHIR

Doing Science Is Fun

V.G. Kulkami, R.M. Bhagwat, V.G. Gambhir Homi Bhabha Centre for Science Education Tata Institute of Fundamental Research Bombay

Publications & Information Directorate (CSIR) Dr K. S. Krishnan Marg New Delhi 110 012 India

Doing Science Is Fun V.G. Kulkarni, R.M. B h a g w a t V.G. Gambhir © Publications & Information Directorate First Edition : November 1993 Second Edition : September 1995 ISBN : 91-7236-082-57

Volume Editor

: Dr(Ms) B.S. Mahajaa Dr Sukanya Datta Ms Sudha Kannan

Cover Design Illustrations

: A.D. Ghaisas : A.D. Ghaisas, Pradeep Banerjee, Neeru Sharma, P.R. Mehta, Sushila Vohra, Neeru Vijan and Malkhan Singh

Production

: Radhe Sham, Gopal Porel, Sudhir Mamgain, Bachitter Singh, Maran Pandian and Vinod Sharma

For sale in India only.

Designed, Printed and Published by Publications & Information Directorate (CSIR) Dr. K.S. Krishnan Marg, New Delhi 110012 India

Foreword Experiments constitute a crucial component of learning science. Our rich store of knowledge in science and technology has been made possible by the brilliant experimental investigations conducted by scientists in the past. In fact, the only way to satisfy one's curiosity about nature is to investigate systematically into the how and why of natural phenomena. Unfortunately, developing skills to investigate and to explore is largely ignored in our science curricula at all levels. Even the scanty laboratory programmes that have survived in schools and colleges are so heavily dominated by the need to obtain the correct value of some physical entity that the spirit of exploring is simply lost. I am, therefore, delighted to write this foreword to "Doing Science is Fun", produced by scientists at the Homi Bhabha Centre for Science Education. The experiments suggested in this book cut across artificial barriers like physics, chemistry or biology, and deal with real situations encountered in daily life. That is why these experiments deal with curiosities arising out of common everyday observations. The aim is not to convey information alone, but to help young minds to explore on their own. I am happy to note that these experiments can be conducted using materials and implements readily available even in rural areas. I am sure that students will enjoy conducting investigations along lines suggested in this book. They will also develop a deep insight into science besides acquiring precious skills to explore nature. Chairman Atomic Energy Commission

The Authors V.G. Kulkarni (1932-) Began his research career in 1953 as a physicist at the Tata Institute of Fundamental Research (TIFR). His interest in science education led in 1974 to the establishment of the Homi Bhabha Centre for Science Education at the TIFR. As the Founder Director he has nurtured this institution to a status of considerable reputqtion. His interests include, education of the underprivileged, role of language in science education and use of mass media. Was awarded the G.D. Parikh Memorial Award in 1985 for his contributions to the education of the underprivileged. R.M. Bhagwat(1931 -) Holds Masters degrees in mathematics and education. Joined the Homi Bhabha Centre in 1975 with considerable experience and reputation as a successful teacher. Clear and lucid writing and a deep understanding of children's difficulties are his fortes.

V.G. Gambhir (1948 -) Holds a degree in geology and a Masters degree in education. Joined the Homi Bhabha Centre in 1976 after a brief but successful career as a teacher. His interests in science cut across disciplines. A skilled experimenter he is equally at ease in animal dissections, rock collections, photography, printing technology and model building. Has a wonderful rapport with children.

Preface The experiments and activities described in the book cover a wide range of topics in science and technology. Many of these deal with experiences which a typical Indian child encounters in daily life. However, the prescribed conventional textbooks constrained to present the formal structure of science, can hardly discuss such experiences. It is, therefore, necessary to bring out supplementary material to cater to the natural inquisitiveness of young minds. Conventional experiments for children are short and crisp, to be finished in a few minutes. What is described in this book is not a set of experiments, but an experimental investigation which may involve a series of experiments. This choice is deliberate. We hope that in a systematic investigation, where the choice of the next step depends on the outcome of previous steps, children will develop aspects like planning, strategy, resourcefulness, and patience. We have also made an effort to present the method of science through activities. Careful observations leading to a guess, further activities to test the validity of the guess, the power of reasoning to establish causal relationships, and an ability to isolate factors and test them one by one, are all emphasized at appropriate places. Social aspects of science have also been highlighted when relevant. High technology is always dazzling, especially to a young mind. The experiences provided in this book are expected to reveal the connections between basic principles of science and the processes used in laboratories and industry. At the same time, the reader is encouraged to look for and understand the differences between simple and pure experiences in a well designed home experiment and the complex technology.

Obviously, a book of this type can hardly aim at completeness. It can never be an exhaustive repertoire of all experiments. It can only be a sample. Many more books on similar lines would be needed to satisfy the truly enormous thirst of children. We have enjoyed performing these activities with children. We have also enjoyed the task of writing this book. It is hoped that children will enjoy conducting these investigations, ask questions, and demand more information.

Acknowledgements This book is based on field experiences gathered by research scientists working at the Homi Bhabha Centre for Science Education, (HBCSE). Their field-work enabled us to understand the way a child thinks about natural phenomena, to appreciate the typical questions that occur to a young mind and the intellectual equipment and material resources available to children. Also, some of these experiments have been tried out in rural areas in action research programmes undertaken by the HBCSE. We are grateful to all our colleagues for making this fund of information available to us. We are grateful to Dr (Ms) Jayashree Ramadas for reading the manuscript critically and for her valuable suggestions. It is a pleasure to thank Dr (Ms) B.S. Mahajan for patiently editing the book, incorporating several changes that kept coming up till the end. Thanks are also due to Mr V.N. Purohit for typing the manuscript. The authors feel extremely obliged to Dr G.P. Phondke who took a personal interest in the publications of HBCSE and prompted us to try innovative formats for this book. The final editing and formatting of the manuscript was done by the ever willing staff of the Publication and Information Directorate. Without the patient and meticulous work of Dr Sukanya Datta and Ms Sudha Kannan, the book would not have come out in this format. V.G. Kulkarni would like to thank his wife, Vijaya, whose silent support has contributed in no small measure to this endeavour. VGK RMB VGG

Contents Endpoint

...

1

Matchmaker

...

8

Natural gates

...

15

No noise is good noise

... 24

Short circuit

... 31

Your own spiderman

... 33

Sound's fun

... 45

Safety first

... 51

Not by hands alone

... 57

Floods and flows

... 63

A silver lining

... 74

Strings of music

... 83

Universal currency

... 88

Friend or foe

... 95

A lightning flash in your room

...101

Silence is golden

...107

Ready reference

...112

ENDPOINT

"Wash behind your ears", admonishes mum ever so often. Wash your hands, wash your hanky, wash your cup...the iist of things to wash seems endless and each time you have to use a different soap. There are many kinds of soaps in the market. There is an entire range of pleasantly perfumed and coloured toilet soaps. There are soaps for washing, for shaving and even liquid soaps. These soaps differ in their colour, smell, smoothness or texture and of course, in their price. A really good toilet soap can cost many times more than a cake of washing soap. But why should the prices vary so much? Your hands can immediately tell you why, Just lather a washing soap and use it for bathing. Your skin will protest and show its irritation by feeling dry and rough. So what is it that a toilet soap has, that makes it softer on delicate skin? How does a toilet soap differ from a washing soaps? Let's find out. For this we will need two kinds of soap, one should be a bathing soap and

2

DOING SCIENCE IS FUN

The two cubes of soap should be of the same size

the other an inexpensive washing brand, two dishes, a strainer, a dropper, a knife, a lemon, and some turmeric powder. Let's cut a small piece of about 3 gms from each cake of soap or else we could cut pieces of equal size (say 1 cm long, 1 cm broad and 1 /2 cm thick) from each of

ENDPOINT

3

the two cakes and assume that their weights are the same. Now in two different clean dishes let us dissolve the pieces separately. To each dish we will add equal amounts of water, 5-7 teaspoons should be enough. This means that we have two soap solutions with about the same quantity of soap in them, We have to be very careful at this stage and label the dishes, so that we know which dish contains bathing soap and which one the washing soap, Now we will add a pinch of turmeric powder to each of the solution and see what happens! The colour changes, doesn't it? Have you paid attention to the delicate tints of the changed colour in both solutions? If not, do it again, Which solution shows a deeper tint? For the next bit of the experiment, we will need lemon juice so let's squeeze the lemon and filter the juice through the strainer so that we get clear juice, This is the juice that we will add drop by drop to the solution of bathing soap, There are many interesting details to observe in this experiment. If you have been following the colour changes keenly you must have noticed that as soon as you added a drop of lemon juice, the soap solution changed colour locally. The solution around the drop changed from red to yellow but the rest of the solution remained red. When you stirred the solution, the yellow colour readily disappeared. But as you went on adding the lemon juice, drop by drop, it took longer and longer for the yellow colour to disappear. Finally, the entire solution turned yellow. It is a really colourful experiment isn't it? So let, us do it again, this time with the dish containing washing soap solution. What is your guess? Will it require the same number of drops or a few more to reach the yellow colour stage? Well, let's do it and find out.

Beginning the colourful experiment

Now that the colourful experiments are over, there may be many questions in your mind. Why did turmeric become red when it was added to the soap solution? How could the addition of lemon

ENDPOINT

5

Drop by drop

juice turn the turmeric to yellow again? Is there any relation between the depth of the red tint of turmeric in the soap solution and the quantity of lemon juice required to change the tint to yellow? Can you recall a similar observation (changed colour of a turmeric stain) from your experience? Think about

6


it. It's a common everyday affair. Questions such as these also had the scientists scratching their heads till they finally arrived at the answer. Turmeric is yellow. However,the yellow colour of turmeric changes to red if it is added to lime water, slaked lime, washing soda solution, or to caustic soda solution. Caustic soda, washing soda, lime water are some of the substances called alkalis. The colour of turmeric changes from yellow to red when it comes in contact with an alkali. If the alkaline nature of the solution is changed later and no more alkali is left in it, the turmeric regains its original yellow colour. Some other substances, too, change their colour when they are transferred from an acidic solution to an alkaline one. Such substances can, therefore, be used to indicate the nature of the solution, These substances are called indicators and are used regularly in the chemistry laboratory. These tell-tale chemicals immediately signal by colour change if the solution is alkaline or acidic. Soap is formed when an alkali and a vegetable oil(or fat) react with each other. Generally, non-edible oils, like castor oil and alkali such as caustic soda or caustic potash, are used in soap- making industry. In the process of soap making, some excess, unreacted alkali may remain in the soap. Removal of this excess alkali in the soap involves further processing. This leads to further expenditure and the soap becomes costly. Now can you guess what happens with inexpensive soap cakes? Since the expensive process of removing excess alkali is not carried out, the alkali remains in the soap. It is the unreacted extra alkali left in the soap that affects our delicate skin. This alkali can cause irritation and roughness of the skin. It is also responsible for the changes in the colour of turmeric. Some alkalis are mild,

ENDPOINT

7

whereas others are strong. Lime water is a mild alkali as compared to caustic soda. Usually caustic soda, a strong alkali, is used to prepare washing soap. You may wonder as to what happened to the alkali in the soap solution on adding lemon juice? Lemon juice contains an acid called citric acid. A neutral salt and water are formed when an acid and an alkali react with each other. This reaction is called neutralization. Both salt and water are neutral. That is, salt and water are neither acidic nor alkaline. For full neutralization of a given quantity of acid, you will need a fixed quantity of alkali. The exact quantities of these chemicals will depend upon the relative strengths of the solutions. While carrying out the neutralization, one must know when to stop adding acid to the alkali. If you add more acid the solution will not be neutral, it will be acidic. Indicators help in indicating the state of neutralization by signalling the end point when the solution is neither acidic nor alkaline. The technique of neutralization is used frequently in laboratories and in many industries. If the volume and strength of one of the solutions is known, the strength of the other can be calculated. To make such accurate measurements, a specially prepared tube with a nozzle and markings to measure volumes is used. Next time you visit a laboratory, try to observe the many apparatus such as burettes, pipettes and indicator bottles, as also the technique of using these, but in the meantime have fun changing the colour red to yellow but don't use up all the soap at home! However, while doing these experiments remember to use soaps only not detergents.

MATCHMAKER ^WtMeet hydrogen peroxide. It is a colourless, odourless liquid and looks like water. Hydrogen peroxide is stored in coloured bottles which have tight caps. It does not evaporate rapidly, nor is it flammable. Yet the bottles are kept tightly corked. Why should this be so? Doctors often use hydrogen peroxide to clean wounds, people use it to bleach hair and it is also used in rockets. You must be wondering about its versatility and special properties that make it indispensable in so many varied fiel.ds. Shall we try to find out something about its properties? Well then, we will need hydrogen peroxide of course. A chemist or a beauty parlour will be able to provide it. We will also need some test tubes or small bottles, and a little bit of manganese dioxide for the first experiment. Let's take some hydrogen peroxide in a clean test tube or bottle. About two or three teaspoons should be

MATCHMAKER

9

Hot or not?

enough. You have noticed that the liquid is clear like water. Now just add a pinch of the black manganese dioxide powder and wait a minute or two. Do you see the tiny bubbles rising in the liquid? The liquid seems to be boiling slowly. But is it really boiling? You know what happens when water boils. Touch the test tube to check if it is hot or not. Also watch for any vapour rising from the test tube! How is the powder in the test tube behav-

10


The apparatus must be correctly set up

ing? Are the particles darting about? Once the bubbling has stopped, look for changes in colour and particle size. Do you think some of the particles have dissolved in hydrogen peroxide? Now let's carry out a few more experiments and see if we can learn more about this useful fluid. For the second experiment, we will need a test tube or a small bottle. It should have a tight fitting cork through which we will fix a rubber tube. We will also need a rubber tube, an injection syringe and lighted agarbatti. First we will fill three-fourth of the test tube with hydrogen peroxide and add about 2 gms of man-

MATCHMAKER

11

ganese dioxide powder to it. Then we will cork the test tube. We will attach the free end of the rubber tube to an injection syringe. As the hydrogen peroxide bubbles vigorously, the plunger of the syringe is slowly pushed back. Let's wait till the plunger is pushed back considerably, say upto the 5 ml mark on the syringe. In the meanwhile, we could note the time taken for the plunger to be pushed back this far. Now one of you take a glowing agarbaffi while your friend releases the rubber tube from the syringe and then pushes the plunger slowly, releasing the gas towards the agarbaffi. Does this make a difference to the intensity of the glowing tip? Of course, the agarbaffi is now glowing more brightly. Now can you tell which gas causes a glowing splinter to glow more brightly? Yes! It is oxygen. But, where did it come from? To get the answer let's have a variation of the second experiment. This time we will also require a potato in addition to the items we have already used. The potato has to be cut into two small cubes each weighing about 2 gms. A cube 1 cm x 1 cm x 1 cm will approximately be 2 gms by weight. This time we will repeat the experiment but with one difference. We will add the potato cubes to hydrogen peroxide instead of the manganese dioxide. And then in another test tube we will add potato slices cut from the other cube Have you taken note of the time taken to push back the plunger to the 5 ml mark on the syringe? Also take a look at the pieces of potato. Are they changed in any way? Many questions will crowd your mind but to some you have already noted the answer. With which of the two materials, manganese dioxide or potato was the reaction faster? Were there any

12


Potato cubes or slices, with which is the reaction faster?

changes noticed in manganese dioxide or in potato? Slices of potato worked better than the cube even though the weight was the same. Can you guess why? The experiments have hinted at the nature of hydrogen peroxide and the results become clear when we learn that hydrogen peroxide is a compound of hydrogen and oxygen. Water, too, is a compound of hydrogen and oxygen. Chemically, water is H2O (that is, hydrogen oxide). Hydrogen peroxide is H2O2. It is not

MATCHMAKER

13

as stable as water. It slowly releases oxygen leaving behind water. The breaking up of hydrogen peroxide is faster when it is warmed or exposed to light. That is why it is kept in coloured bottles with a tight cap. Why is hydrogen peroxide used in treating wounds? You have seen that it gives off oxygen which is useful in disinfecting the wound. Hydrogen peroxide is also used in a rocket as a source of oxygen. Breaking or splitting of hydrogen peroxide into oxygen and water is a process of decomposition. Hydrogen peroxide decomposes faster when it is in contact with manganese dioxide. At the end of the reaction, hydrogen peroxide changes into water and oxygen. Substances called catalysts affect the rate of reaction. In the experiments we carried out, manganese dioxide and potato (actually an enzyme present in potato) acted as catalysts. Can we show that the rate of the reaction really changes when a piece of potato is added to hydrogen peroxide? Let's take some hydrogen peroxide in a test tube and dip into it a piece of potato tied to a string. The liquid bubbles as soon as the potato is dipped into it. Repeated lowering and raising the piece of potato alters the rate of bubbling. You can also speed up the bubbling by heating hydrogen peroxide. However, you will need 'energy' to heat hydrogen peroxide. The catalyst, in this case potato, has saved this energy. A substance which functions as a catalyst in one reaction may not act in a similar manner in other reactions. So far, scientists have found quite a few catalysts for specific chemical reactions. Catalysts are used in industrial processes manufacture of urea, sulphuric acid and refrigerants. Even the simple process of converting an edible oil into a ghee-

14


Dip, dip, dip

like substance needs a catalyst. These processes will be uneconomical without the use of catalysts. The area of contact between the catalyst and the reactants is an important factor in deciding the rate of a chemical reaction. The effect is enhanced if the area of contact is increased. Therefore, to increase the surface area, catalysts are generally used in the form of a fine powder or thin layers spread on some supporting substahc^jke asbestos. ' The reactants and the products do not form stable compounds with aaatalyst. Therefore, the catalyst can, in principle; bfe usefd repeatedly." Sometimes, however, other. substances''Involved in the reaction affect the

MATCHMAKER

15

action of the catalyst adversely. In such situations, the catalyst becomes useless, it is called 'poisoning of the catalyst'. While using the catalysts in industry, considerable care needs to be taken, especially to avoid catalyst poisoning. There are some other catalysts that slow down chemical reactions. They are called negative catalysts. A few drops of ammonium hydroxide added to hydrogen peroxide can slow down the decomposition of hydrogen peroxide. We may wish to speed up some reactions and slow down many others. Therefore, we need both the positive and negative catalysts. Can you think of some instances of each type?

NATURAL GATES

The doors that man has designed are simple. They are opened when we wish to enter or leave a room. While open, they allow other people to enter or leave at will. They will also allow animals and dust to enter. They cannot distinguish between the desirable elements and the undesirable ones. Nature, on the other hand, has sensitive doors. These doors are selective. They may allow certain chemicals to enter but not leave. With other chemicals they may exercise the restriction in reverse. This means that the doors will allow a chemical to leave but not allow it to re-enter. Still other chemicals may be strictly forbidden to enter into the cell. Sounds fun, doesn't it? But how do these doors work? Can we see them? Of course, these incredibly efficient doors are too tiny, microscopic really, to be seen with the unaided eye. But we can study the way they work. To do so, let's raid the kitchen for a couple of raw potatoes, some sugar,

NATURAL GATES

17

Let's raid the kitchen

water, a knife, two dishes and, if available, any water soluble colouring matter (potassium permanganate will do fine or even some colours left over from Holi). Let us take the potatoes and scoop them out so that they resemble a cup. You may also have to cut a proper base so that the cups will sit well on a dish. Then we will dissolve 5 spoonfuls of sugar in a cup of water and pour this carefully into the potato-cup so that it is half-full with the concentrated sugar syrup. After that let's fill the dish with water taking care that it does not overflow into the potato-cup. Experiments become easy to observe if we add a pinch of colour to the plain water in the dish.

18


Making the potato-cup

Now the experiment is set up and we are eager to see if the doors work, But we must give the invisible doors some time to operate smoothly and silently. So we will wait for about fifteen minutes or so, before we check if the doors are on their job. In the meanwhile we can make waiting easy by writing down our guesses about what might happen. Then we could compare our notes with what we will see in fifteen minutes time. Won't it be fun if we were right? We could even try and find out how the doors worked. For some extra fun while we are waiting, why don't we reverse the experiment we have just set up? Let us put plain water in the potato-cup this time and sugar solution in the dish. This will be our second experiment.

NATURAL GATES

19

Invisible gates at work

For this experiment too, we will allow fifteen minutes for the gates to operate. The results are interesting because although apparently it seems the flow of water in the two experiments are opposite to each other, that is not the case. If we look again, we see that in both, the movement of water takes place towards the sugar solution. How did this happen? How did water know which way to go? And why did not sugar particles (molecules) also pass in or out of the potato-cup? It would seem that water has been selectively allowed to pass out but not sugar. Scientists say that the potato-cup is made up of cells and each cell has its boundary wall. These walls allow

20


only water to pass through them but do not allow sugar particles to pass through. Such a screen that is selective about the molecules, it allows to pass through is called a semipermeable membrane. Parchment paper and the membranous sac or the white thin membrane which covers the inside of an egg are also semi permeable in nature. The name semipermeable membrane itself indicates the nature of the membrane. If we make a solution, say, of water and sugar (as we have just done), sugar which dissolves is called the solute and the water or medium in which it dissolves is called a solvent. Now, a semipermeable membrane will allow a solvent free passage through it but will prevent or resist the passage of the solute. It has become clear that some of the water in the dish has entered the potato-cup in the first experiment. In the second experiment it is equally certain that water from the potato-cup has entered the dish. This process of selective passage of a solvent in preference to the solute through a semipermeable membrane is termed osmosis. But for how long will the process continue? Will it go on for ever? The answer is No, and we can see it ourselves if we carry on the experiments for a sufficiently long time. Why does this happen? Why does the movement of solvent stop? Scientists tell us that nothing is perfect, not even the sensitive doors of nature. The semipermeable membrane also allows a few solute molecules to pass through. The entire process continues till concentration of solutions on both sides becomes equal. At that point there is equal number of solute and solvent molecules on either side and osmosis stops.

NATURAL GATES

21

Making a semipermeable membrane

In all the cases, The tiny pores in the walls of the potato worked as efficient doors. They allowed water to move in or out depending on the location of the concentrated sugar solution. Shall we try to get some other semipermeable membrane? It's easy. All we need to do is to drain the contents of an egg through a small hole at one end. Then we will immerse the egg in dilute hydrochloric acid which is available in any chemist's shop and which is often used at home to clean sinks. In about 10 minutes, the shell will be dissolved and the egg sac left behind. This sac can be used as a semipermeable membrane. Now that we have a semipermeable membrane, let us try another experiment. For this we will need a thistle funnel, which is a little like the funnel used at home to

22


pour kerosene or oil but with a longer stem. The school science laboratory will surely have it. We will also need concentrated sugar solution and a beaker filled with water. Let's fill the thistle funnel partly with concentrated sugar solution and then close the mouth of the funnel tightly, either with a piece of parchment paper or with the egg sac. Invert the funnel and place it in a beaker filled with water. Also mark the level of the sugar solution in the stem of the funnel and closely observe over a period of time the difference in the level of water in the stem. Yes, of course, the level of water in the stem rises with time. Why don't we try out further experiments and see what happens when we take some dry grapes (kismis) and put them in water for some time? Or put some fresh grapes in a concentrated solution of common salt and note the results. Does repeating the potato-cup experiment with concentrated solution of common salt instead of sugar yield results similar to those we got during the experiments we did using sugar solutions? Why is salt added to the cucumber, onion, and tomato salad only just before serving? What do you think will happen if we add the salt beforehand and keep for sometime prior to eating? We have read that it is harmful to use fertilisers like ammonium phosphate and urea indiscriminately. When too much salt is put around the roots of a plant, water from the cells of the roots comes out into the soil. Fertilisers are added only at specific stages in the life of a plant. Now we understand why gardeners and farmers are advised to use the right concentration of fertilisers and that too not frequently. We know that plants and animals cannot live without water. Plants and animals require many substances

NATURAL GATES

23

called nutrients for their growth. They can absorb nutrients only when these are dissolved in water. That is why animals and plants depend on water for their survival and growth. The nutrients necessary for plants are present in the soil in the form of salts. When we water a plant the salts in the soil get dissolved in water. The roots of plants have these interesting doors which allow food and water to come in. These roots have minute root hairs made of cells. The walls of these cells act as a semipermeable membrane. Does everything pass through this way? No, obviously not. Otherwise the cells would also lose some important molecules to the soil. Each membrane is a special one. It decides what will go across and in which direction. Thus, cell walls ensure that water and nutrients go into the cells, but useful materials in the cell do not get out.

NO NOISE IS GOOD NOISE

the last ball is about to be bowled and that will decide the fate of the match. The commentator is about to announce the events of the next few moments. Someone outside the house starts his motorcycle, precisely at that moment. The loud noise of the motorcycle and also the crackling disturbance over the radio! The voice of the commentator is lost - irritating, isn't it? But why blame the motorcycle alone? When someone rings the doorbell, switches the tube-light or puts the mixie on, there is a simultaneous disturbance on our television and radio sets. Why do you think this happens? Let us try to deliberately cause such a disturbance and check out when exactly the noise is created in the radio. All we need for this is a dry cell from the flashlight and also a wire. Switch the radio on, first. Then, hold one end of the wire at the base of the cell, and with the other end, scratch the upper end of the cell. We must do this


25

Causing noise in the radio

near the radio. The voices on the radio are no longer clear, but are disturbed by a simultaneous crackling sound, right? The noise disappears the moment we stop rubbing the wire. The scratching of the wire on the cell leads to making or breaking of a circuit which causes noise in the radio. Similar is the case with the doorbell. The spring of the buzzer in the doorbell repeatedly makes and breaks contacts. This causes the noise in the radio. But, why should the electrical disturbance elsewhere affect the radio at all? The radio functions as a receiver of radio waves. If there is any hindrance or interference in the waves reaching the receiver, then we cannot hear the trans-

26


mission clearly. Electrical disturbances obviously send out unwanted waves, and the radio receives them, in addition to those sent out by the radio station. We have noticed that when we are attempting to tune in a particular station, we hear a continuous background noise. Where does this noise come from? Where are these noise-producing radio waves generated? Actually, electromagnetic radiation consists of several different types of waves carrying different amounts of energy across space. One such wave is the radio wave. The speciality of radio waves is that they can be used to send messages between distant points, without any wires to carry them. The first to succeed in this attempt was a young Italian engineer, Guglielmo Marconi. After some preliminary experiments, on December 12, 1901 he sent the first telegraphic code across the Atlantic Ocean from England to Newfoundland. From this humble beginning came such developments as television radio broadcasting and RADAR. In the case of radio wave transmission, electromagnetic waves produced accidentally or unintentionally, as in the case of the doorbell, interfere with the' main transmission and produce hoise. Noise of this kind can be eliminated if we prevent the disturbing waves from interacting with the radio waves coming from the radio station. In other words, we must shield the radio from the disturbing waves. A simple solution to the problem is covering the radio with a metal can. Let us tune in the radio to its loudest, and cover it with a can. Now, let's try ringing the bell. There is no disturbance, is there? Similarly, let's try covering the source of the disturbance - the electric bell. We will also need a metal can and an insulated electric wire. Let's make a small hole

27


Enjoy the music

on one side of the can and pass an insulated electric wire through it. Let us join this wire to the bell. We can now keep the bell in the can and the can on the ground. We now cover the can with the lid and ring the bell. Does it create a disturbance in the radio? Remove the lid and check again. What do you think happens when the radio and the bell are covered with the metal can? The electromagnetic waves generated by the bell are received by the can. These waves produce a current which travels to the ground via the can instead of affecting the radio set. Thus a grounded metal cover protects the radio from the noise. The metal acts as a

28


Does this work as well?

shield and hence this method is called 'shielding' the apparatus from radio disturbance. It is really not possible to cover our radio sets with this kind of a shielding apparatus. It will make it very difficult for us to listen to the radio. Generally, certain devices installed in the radio set help decrease the noise. These are in the form of circuits, and they suppress or filter out the unwanted waves and allow only waves of desired frequency to pass through them. Cars too have radio sets. Can you guess how radio noise is prevented in cars? Cars have a long wire-like antenna fitted outside the car,, which receives the broadcast. The body of the car acts as a shield and


29

prevents the disturbing waves from entering the car. Moreover the spark-plug in the cars is also completely covered with a metal casing. Now you can guess why we cannot hear the radio clearly when we are in a moving train or bus. The study of radio-noise had an unexpected, but extremely fruitful result. In 1932, Karl Jansky, an engineer in Bell Telephone Company, USA, was trying to find out how much noise he would receive from the atmosphere in his short-wave radio receiver. He had built an aerial which could locate the direction of the source of radionoise. He was trying to listen to the crackling noises from distant thunderstorms. In addition to the expected noises, he received a continuous noise from an unknown source in the sky. Jansky found that the radio noise was coming from the centre of the Milky Way. This was the first detection of radio waves coming from a source in outer space. After several years of this discovery, Grote Reber, a German scientist constructed a radio-telescope to receive radio waves from stellar objects. The telescope consisted of a large metal disc, shaped like a parabolic mirror (like a mirror of a car lamp), with a sensitive radio receiver at its focus. This telescope could receive faint signals from galaxies and stars and could fix the direction of the source of radio waves. Thus, the basic technique of Radio-Astronomy was established. This field is now one of the most important disciplines in the study of the universe. You might be surprised to know that apart from visible light, stars emit many other radiations like radio waves and X-rays. All these are, of course, electromagnetic radiations. There are some sources which emit mostly radio waves. These would never be visible to the naked eye and can be seen only by radio-telescopes.

30


Radio telescope

The biggest and the best radio-telescope is now in operation at a site near Pune in Maharashtra. Of course, the most familiar uses of radio waves are in the fields of communication broadcasting, telecasting, and radio telegraphy and telephony between ships and across continents or oceans.

SHORT-CIRCUIT

So many of us have heard our grandparents speak of their childhood. A childhood in a remote village where the summer nights were spent without the luxury of even a fan. The comfortable life we now live is mainly because of electricity, one of man's greatest discoveries in the modern world. Elctricity is indeed responsible for thousands of inventions and appliances that make life in the twentieth century so comfortable. Most of us probably think of electricity as a recent discovery. But men experimented with it many centuries ago. The word 'electricity' comes from the Greek word electron which means 'amber'. Thales, a Greek philosopher who lived in about 600 B.C. noticed a strange effect when he rubbed a piece of amPer with a woollen cloth. The amber, as a result of the friction, became 'electrified'. The electric current which flows through wires and reaches our homes is in fact, the movement of electrons,

SHORT-CIRCUIT

32

Electricity is of tremendous use to man. But we all know that it must be used extremely carefully. The consequences of one careless step can be disastrous. We hear of fires in peak summer. 'Must have been a short-circuit', we hear people say. What is a short-circuit? How does it cause so much damage? Let us see how a short-circuit occurs. All we need are some electrical material we often have lying around at home. To be precise, we will need two batteries of the kind which we use in our torch lights, a bulb with the holder, a switch, some copper wire, a dozen crocodile clips and a thermocol sheet. We will use the sheet as the base. Let us place the two batteries on one side and the

Lighting up the bulb

33 SHORT-CIRCUIT

bulb with its holder on the other. Let us then linkup the wires with crocodile clips at both ends. We then connect up all these. When we press the switch , the bulb glows and when we switch it off, the bulb does not light up. Thus for the bulb to glow, the electric current must flow round and round continuously in the circuit. This is a closed circuit. When the current path is broken at any point, as when the switch is off, we have an open circuit. When we switch off the circuit, no current is allowed to flow. What if we create some obstruction to the flow of electric current in the circuit? A pencil refill made of graphite will do. Let us connect it in the circuit. All we have to do is link up the graphite refill in the circuit with the help of wires and crocodile clips. Let us now put the switch on. Do you think the bulb will glow more brightly, less brightly or not glow at all? Supposing we connect a wire across the circuit? Do you think the bulb would still glow? We realize now that the amount of current through the circuit is not decided by the battery alone. It is the components in the circuit which determine how much current they will draw from a given battery. Some materials allow larger current to flow through them. But other materials do not. In other words, certain materials offer more resistance to current than other. A substance that has extremely high resistance will not allow electricity to flow through. It is called an insulator. Electric current takes the path of least resistance. Thus, when we connected the graphite refill in the circuit, the current could have gone directly through the wire or through the high resistance of the graphite. It took the usual wire route and hence there was no change in the gJowing of the bulb. This wire had extremely low resistance and the current took that path,

34


bypassing the bulb altogether. In other words, an easy short-cut was available and the current took it. In such a short-cut, the current meets little resistance. Hence large amounts of current can flow. This leads to considerable heating. If a contact at any point along the circuit is loose or becomes loose due to heating, a spark can occur. In our experiment, we used a power source which was very feeble. At home or in a factory, however, considerable power is used, Serious damage can occur if two points with little resistance get connected accidentally. A minor spark can lead to a major fire. Wires are often wrapped in rubber, which is an insulator. What will happen if the rubber on two wires fixed side by side wears off and the two wires touch each other? In such cases a large current will flow, and an inferno can result. Many precautions and safety measures are taken to protect the circuits against damage due to overloading or a short-circuit. All wires used in an electric circuit are coated with a layer of insulating material. In addition they are covered by a rubber or plastic layer. As a result ordinarily the wires do not come in direct contact. The most important safety device used for protection of electric circuits is the fuse, The fuse is a piece of wire made of a material with a very low melting point. When a high current flows through the circuit due to overloading or a short-circuit, the fuse wire gets heated and melts. As a result, the circuit is broken and current stops flowing. What is earthing of an electrical gadget? It is yet another safety measure always used in electrical appliances to prevent us from getting a shock. Let us prepare a similar circuit as we did the first time, Then let us connect the bulb holder to the base of a

35 SHORT-CIRCUIT

Earthing electrical gadgets

metal can. We must take care that the wires of the circuit do not touch the can anywhere. What must we do for this? Yes, we should use an insulator such as an adhesive tape between the wires and the can. Now let's switch the circuit on. The lamp glows! What if the wires touch the metal can? Will the lamp glow? Why not? The current is in fact flowing through the can. What will happen if we provide an easy path for the current from the can to the earth? We can easily do that by attaching two crocodile clips to a long flexible wire. One clip can be attached to the can and the other to a nearby pipe.

36


Now we can guess why electrical gadgets have a three-pin plug. The third pin is connected to the ground. Normally, the body of the gadget does not get any current, but should the wires inside it ever accidentally touch such a gadget, it can give us a shock. Thus when we touch it, the current has two options, to flow through our body to the earth, or to flow through the wire connected to the earth. The resistance of our body being much higher than that of the wire, the current chooses to flow through the wire. The earthing of instruments thus saves us from shock. Can you recollect, the 'tester' an electrician uses when we complain about the refrigerator or iron giving a shock? How about trying to make one?

YOUR OWN SPIDERMAN

Just mentioning the word 'spider' often makes many of us look for the cobweb duster. But have you ever stopped to admire the delicate web the spider weaves in the garden? Transparent, lightweight yet strong, sticky enough to trap flies and other prey, yet allowing the spider to swing merrily like a trapeze artist in the circus. The web is made up of threads spun by the spider itself. The thread is strong enough to hold the spider's weight and this factor is, of course, vitally important from the view point of safety. In fact, the same factor operates when commandos carry out risky rescue operations. They airlift stranded people using a wire or rope that is lowered from the helicopter. The wire or rope used in such rescue operations is always manufactured to meet precise specifications so that we know how much weight it can support. It is also rigorously tested before use so that it does not suddenly snap in mid air.


38

When does it snap?

But how c$in the strength of thread and rope or even wire be studffed? It's rather fun and simple as well, All we need Jor that, are equal lengths of strings of various types, ohail on the wall, a hook and some weights. We will tie the hook to one end of the string which should be about 50 cm long and the other end we will tie to a strong nail on the wall. But we must be careful that when we tie the hook to the string, the thread should remain taut. Placing a meter scale behind the thread will allow us to know the length of the thread or string. Let's then attach a small weight to the hook on the thread. This will stretch the thread and by reading its length against the meter scale, we can see that the thread is elongated. If the addition of the weight does not cause the string to break, -we can go on adding extra weights till at last the thread snaps as soon as we

YOUR OWN SPIDERMAN

39

add more weight, If we record the weights needed to break the different types of threads, we can easily estimate the materials able to withstand extra weight and those that cannot. In fact, an index of this can also be prepared from the way the string elongates, Let's note the length of the string each time a new weight is attached and the thread comes to a steady position after elongation. We can get an idea about the elasticity of the string if we record the length of the string just before it breaks. If we prepare a systematic table like the one shown, the results will be obvious at a glance. Material

Original Length

Length at Breaking

Increase in Length

Breaking Point Weight

Cotton

Silk

Wool

This table will readily reveal which thread is the strongest. But what makes it strong? Is it the thickness that determines the strength of the thread? To confirm our guess, we can repeat this experiment using threads of the same material but with different thicknesses. Does thickness really make a difference to the weight the thread can support before snapping? When we observe a cotton thread closely, we realize that it is not really a single strand of cotton. The thread is made up of a number of strands or yarns spun into one thread. What do we find if we compare the number of

40


strands in two different threads of the same material? There are many different kinds of cotton threads. A thin thread of fine cotton may even be stronger than a thickerthread of coarse cotton. Interesting, isn't it? If you obtain and test the strengths of a single strand of each kind of thread, you might be surprised at your findings. Apart from the common choice of cotton, silk and wool, we could also try threads obtained from plants like hemp. We could also jute, etc. Metal wires would also be suitable for our experiments. Iron, brass, copper and aluminium wires are easily available. We could also use man-made fibres like rayon and nylon and, of course, there is the spider's fragile looking thread! Scientists have found that the thread of a spider's web is not weak. It is, in fact, stronger than a steel strand of the same thickness. To compare different types of threads for their strength, we should find the thickness of the thread and the weight that is necessary to break the thread. From these two, we can obtain a single number by dividing the break-weight by the thickness of the thread. This will give us the comparison of the strength of different threads, as if all of them are of the same thickness. But with the material and equipment that is available to us, it will be difficult to test the strength of very thin threads. This can be done only in technical laboratories. But that does not mean we cannot measure the thickness of threads at all. To measure the thickness of threads, we can use different methods, depending on the material of the thread, and the measuring apparatus that is available to us. The best method is to use a special kind of microscope which is used in the laboratory for this purpose. The thread is held fast on a slide under the lens of the

YOUR OWN SPIDERMAN

41

microscope. The thickness is measured with the help of the scale attached to the microscope. Another method is to use a micrometer screw. Perhaps it is available in the school laboratory. It is used in engineering workshops. To measure the thickness of your thread, take a rod, or a pencil, and wind two or three turns of the thread around the rod, taking care that the threads do not overlap one another. Now the diameter of the rod with the threads is measured. Then the diameter of the rod only is measured. The difference gives us double the thickness of the thread. From this we can get the thickness of a single thread. If we do not have any instruments like a microscope or a micrometer screw, we may use a graph paper formeasuring the thickness of a thread. Let's take a few threads (say four) together and stretch them between our fingers and hold them on the graph paper while taking care that no gap is left between the two adjacent strands. Now the thickness of the four threads is easily measured. Dividing this by the number of threads taken, will give us the thickness of each thread. For example, if the thickness of the four threads together is equal to 2 mm,, then each thread is 0.5 mm thick, This is usually the thickness of thread used in sewing. But what if we do not have graph paper? Never mind, we can measure the thickness of a thread even if we do not have a graph paper. Let's wind the thread on a thin pencil (so that we will get many turns), such that there is no gap between turns. With an ordinary ruler, we can measure the total thickness of (say) ten turns. Now we can estimate the thickness of one turn easily. The threads that we use are seldom single stranded. They are formed by spinning several threads together. How does this affect the strength of the thread? Let's find out.


42

Measuring thickn'ess

YOUR OWN SPIDERMAN

43

For this, we have to take two single strands of nylon. We will test their strengths by attaching a weight to the first strand and then testing the strength of this thread. Is it different from the strength of two single threads taken together? A striking example of how a rope of twisted strands may be made quite strong merely by twisting together the many individual strands can be seen here. Ordinary grass is too flimsy to make strong ropes. But thick ropes of grass are strong enough to tie an elephant and were used not long ago for this purpose. "United we stand divided we fall' — the old saying seems to have a scientific application! Talking about strength reminds us of the heavy cranes that lift loads, The maximum load which can be handled by a crane is marked on it. But on what does it depend? The steel cables of a crane consist of a number of wires twisted together. Why are these wires twisted together to form a single cable? Why are not the same number of wires used separately? Strings in the form of threads, ropes, wires, or cables have been used from ancient times for a number of purposes. We have seen pictures of workers carrying heavy stones to build a pyramid. From their experience (or perhaps by experiment also), they must have judged (or measured) the strength of the ropes, Strong ropes have many uses. Steel cables are used to tow ships, These are carefully tested to ensure that they are strong enough to withstand the pull of the ship.lt is not enough to simply ensure the strength of the towing cable. The hooks to which the cables are attached must also be strong enough to withstand the pull. There is a considerable difference in the thickness of the rope used to tie the ships to the capstan in a harbour, and a steel cable used on a crane. Why do you think there is so much difference in their sizes?

44


More recently, fibres are being used to strengthen other materials. Plastics, metals, and other materials become extremely strong when they are impregnated with fibres of various materials, like nylon, rayon, glass, carbon, etc. By this method, it is possible to obtain light yet strong materials, so necessary in building air-crafts and space-crafts where both strength and low weight are necessary. Glass fibres are extensively used in various industries.

sound's fun

Isn't it irritating when the chatter in the next room or the shrieks of the neighbourhood children disturb us just when we are enjoying an interesting novel? How nice it would be, if we could read in peace. Or, even have secret talks with our friends without an inquisitive ear getting to know about it. There must be some way to stop the sound from outside coming in or the sounds of our whispers going out. Do you know that scientists have studied sound and ways to adjust sound levels to our convenience? But what exactly is sound? Say a tree crashes to the ground. As it topples, it produces disturbances in the air. These disturbances give rise to waves. These waves are,in fact, vibrations or movements back and forth, of the air. They travel out in all directions. Some of these strike our eardrums and the message is signalled to and recorded by the brain as sound. If we could redirect these sound waves occurring outside our room in another direction or let them be

40


SOUND'S FUN

47

absorbed en route so that they never reach us, we could have a sound-proof room! Similarly, if we direct all the waves in one direction (without letting them scatter), to a destination of our choice, we would again have a sound-proof system. Remember when you made phones out of tin cans and a twine, for secret talks with your friend? When we speak into the tin can, and our friend has the other tin against his ear, the sound waves are carried through the twine to our friend's ears. There is an easy way to study the behaviour of sound. All we need are a mirror, two hollow cylindrical tubes about 30 cm long, and a clock that ticks. Let us place the mirror vertically on a table. It would be easier to place it against the wall. We will now partition the mirror into two halves by placing a cardboard in the middle. Then we will place two-tubes on either side of the cardboard making the same angle with the cardboard. This is easy. All we have to do is to place one tube at an angle and look through the mirror on the other side of the cardboard. We will see an image of this tube in the mirror. We can keep the second tube in line with this image. Now, place the clock at the mouth of one tube and your ear at the mouth of the other. Can you hear the steady ticking of the clock? Do you think the ticking would be as clearly audible if the tubes were not kept at the same angle to the mirror? Try shifting the tubes and then listen to the ticking. Let us now replace the mirror with an uneven surface. The pressure waves emanating from the ticking clock are channelled in the tube, and hit the surface of the mirror placed on the other end. They are then reflected back. When the tubes are kept at the same angle, we hear the loudest ticking, right? The smooth, polished

48


Tick-tock

surface of the mirror directly reflects the sound waves into the other tube. The uneven surface, on the other hand, reflects the sound in all directipns and only a small portion of it comes through the tube. This is exactly what happens, when we talk or sing in a room. The sound that we produce hits the walls of the room and is bounced back in all directions.

SOUND'S FUN

49

Can you recall the booming sound you hear when you speak in a room devoid of any furniture? We don't hear it in a room full of furniture, do we? Curtains and upholestry absorb part of the sound wave9. Now, let us do another experiment which will give us a clue as to how we can soundproof our room. Let us place the mirror and tubes as before. Now cover the mirror with a piece of felt, thick cloth or a woollen blanket with lots of folds. We can easily guess that the ticking sound is hardly audible. Why is that so? The folds of the woollen blanket absorb the sound waves and don't reflect them back as the mirror does. How about trying to make our rooms sound-proof? Let us line the walls with soft porous material, cover the walls, doors and windows with thick curtains, Even tiny cracks can allow sound to pass through . So let us make sure to cover the entire edge of the door with a lining of cloth or paper and last, but not the least, take care to close the windows and doors. We have a soundproof room ready! Can you name a few places where a lot of care needs to be taken regarding damping of sound? A broadcasting studio is one of them. The studio needs to be totally sound-proof in order to allow good quality sound recording. The walls of studios are covered with boards with a number of holes. Sometimes an additional layer of felt is fitted behind the boards. A gap of air between the two layers helps still more in absorbing sound, In certain broadcasting studios,the walls and ceilings are provided with curved surfaces. The floors are left flat. In these studios, sound is so perfectly diffused that microphones can be set almost anywhere!

50


Audiotoriums must have good acoustics

Next time you visit an auditorium try to find out the precautions taken to allow the audience to hear the clear, crisp notes from the stage.

SAFETY FIRST

Nature is beautiful. But nature can also be very frightening. Have you ever been caught in a thunderstorm while travelling in a car or a bus? Wasn't the experience frightening? The bright flashes of lightning, the heavy rumble of thunder and the dark clouds must have made everyone uneasy. But what did they do? Did they sit quietly in the bus or did they run out into the open to shelter under trees? Yes, of course everyone sat nice and dry in the car while the storm lashed everything and the lightning lit up the sky. Lightning is usually not dangerous but it does sometimes strike tall buildings or trees with devastating results. Even running or walking in the open is likely to attract lightning while sheltering under a tree is equally dangerous. But it is relatively safer inside a closed car. This is because the metallic body of the car safely carries the charge to the ground. In fact, the car acts as a shield and protects the passengers.

52


Safety first

SAFETY FIRST

53

But what happens to an aeroplane that encounters a thunderstorm? When lightning strikes a plane, the charge spreads over the metal body of the plane and passes to the surrounding air through pointed conductors. These conductors are specially provided on the wings and the rudder of the plane, and are pointed in shape so that the charges pass out easily and so the passengers are not harmed. Very tall buildings also have special lightning conductors on the roofs to safely conduct lightning to the ground such that the building and its inhabitants remain secure. Many places, including laboratories also need special shielding from electrostatic charges. Some electrical instruments may show wrong measurements or readings when affected by external electrical charges. Such instruments are shielded by grounded metal enclosures or wire-cages. It is necessary to shield electronic equipment, electrical instrument and telephone cables from electrical and magnetic disturbances which may be present all around us. These instruments are shielded in wellgrounded enclosures of metals, usually made of copper or aluminum. Telephone cables are enclosed in metal sheaths. The sheath is grounded in many places to obtain effective electrical shielding. Although a direct lightning hit can be dangerous, we can perform a few simple experiments to learn more about the principles that protect us from it. The experiments are not dangerous and are easy to perform. For the first experiment we will need a small round metal pot, an insulating base like, a wooden block, a thick sheet of plastic or a glass piece, two pieces of string or thread and a clamp system. Let's put the metal pot on the insulating base and attach the two strings from the clamp rod in such a way that the first string hangs

54


Duplicating the effects of a lightning strike

free outside the pot while the second string dangles inside it. Now we will duplicate the effects of a lightning strike but on a very low level by charging the pot with a plastic strip or a glass rod. Immediately the string outside the pot is attracted towards it and moves till it touches the pot. The string inside the pot does not move at all. This shows that all the charge given to the pot spreads over the outer surface of the pot. The inside of the pot is completely free from any charge. This experiment shows that electrostatic charge always stays on the outer surface of a conductor, That is why you are safe from lightning if you stay inside the car and do not touch any metal parts, The charge from the lightning spreads over the metal body of the car and jumps to the ground from

SAFETY FIRST 65

Magnetic shield

its lower parts. Some of the charge may also pass through the tyres, especially when they are wet. The second experiment that we will perform will give us more information about magnetic shielding. Let's make a ring from a tin can, of course, a tin can is usually an iron can plated with tin. Or else we can make a ring by bending a piece of soft iron. Let's make a nice wide ring so that we have more space to play in. Once the ring is made we will place a magnet inside the ring and sprinkle iron filings around the magnet. So now both the magnet and the filings are inside the ring, we may even tap the arrangement gently until the filings arrange themselves. What will happen if we bring another magnet near the first one? Interestingly the iron filings do not move as long as the other magnet is outside the ring. We can even repeat the experiment using a magnetic needle and the results are the same.

56


The iron ring acts as a magnetic shield. The needle will only move if we lift the ring. What happens when we put the ring back? What happens when we put the ring around the second magnet? Let's try out all the combinations! What is the ultimate result? From our observations we see that the iron ring also protects the outside region from the field of the magnet inside the ring. Thus a proper shield is needed to provide protection from both electrical and magnetic energies.

NOT BY HANDS ALONE

In the early morning while hurrying to go to school we switch on the electric iron to press our uniforms. A tiny light on the iron starts glowing. A little later, the light stops glowing and we know that the iron in hot enough for us to go ahead with the ironing. Soon after we start ironing, the light begins to glow again. How does the light go on and off automatically? The light on the iron is actually an indicator. Actually, glowing light is an indication that the iron is getting heated up. The light going off is an automatic switching off of the heating system within the iron. It ensures that the iron does not get overheated. The automatic switching on or off of a machine is called automation. Automation has become part of so many aspects of our daily lives. We generally do not notice it. Elevators, central-heating and air-conditioning, refrigerator, the direct-dial telephone system — all involve automation.

58


Automation is a part of life

NOT BY HANDS ALONE

59

M - Magnet S - Switch S'-Strip B - Battery

Simple automation at work

In large scale production of many goods, from canned food to cars, automation is used widely. But, how exactly does simple automation work? Wouldn't you like to find out? This time we need a U-shaped piece, and also a tiny straight strip of soft iron, some insulated wires, a battery and a wooden board. Let us wind insulated wires around the arms of the U-shaped piece of soft iron. But, we must remember that the direction of the winding on one arm should be opposite to that on the other arm. Do you know that soft iron becomes a magnet, if you connect the two ends of the wires wound around it to a battery? Indeed, we can pick up nails and pins and other light iron objects with this magnet. This is what is called an electromagnet. The moment we break the connection, the iron loses its magnetism and the nails fall off. We will now make an electric circuit on the wooden board. First, we will fix up the electromagnet along with

60


T, >> •n

\

M

1

Green Red

M~ Magnet S - Switch S' - Strip

B - Batten

Red to Green in seconds

the battery to the wooden board. Then we link up the circuit with a switch. Opposite to the U ends of the electromagnet, we place the small movable strip of soft iron at a distance of 2 to 3 mm. We now fix two metal screws between the strip and the magnet. We link up the metal screws in another circuit with a battery and small bulb. Now begins the experiment. As soon as the switch of the first circuit is switched on, the magnet is activated. It pulls the soft iron strip. The strip moves forward and touches the two screws. The second circuit is now complete and the bulb lights up. Just pause and think how the traffic signal changes from red to green and vice versa at periodic intervals. Do you think we can build up a circuit to do this at home?

NOT BY HANDS ALONE

61

Make your own Thermostat

We will use the same circuit except now we will use two bulbs, a green bulb and a red one. They will be linked up to the second circuit in such a way that when the electromagnet is not active, the soft strip will close the circuit of the green lamp. Whien the magnet is activated, it pulls the strip which breaks the circuit of the green lamp and completes that of the red lamp. Automatic processes to control temperature levels in electronic devices are called thermostats. Electric irons, refrigerators, geysers, all have thermostats. Thermostats consist of a thin strip of two different metals welded together to form a bimetallic strip. The metals used are usually copper and steel. When heated, the two metals expand to different lengths. The strip bends on the side of the metal which expands less. Thus a bimetallic strip of copper and steel will bend on the side of steel. Why not try this out in our circuit? Two thin strips of copper and iron will be required. They have to be

62


welded together, We could get it done at a welding workshop. If we fix it to a wooden stand and gently heat it using a candle flame, we can see how the strip bends as temperature increases. This bimetallic strip can work as a switch in our circuit. As temperature increases, it will bend and touch the wire when a certain temperature is reached. This will complete the circuit. As the temperature decreases, the strip will bend backwards and the circuit will open again. Don't you think this will be very useful in an alarm system in case there is a fire? Where else do you think such automatic mechanisms will be of use? There is really no magic in automation, is there?

FLOODS AND FLOWS

The kitchen is such a nice place. Tasty things like jams, murambas, gulab jamuns and jaleebes are made there. But the cook is always so busy and does not like to be disturbed, He takes a drop of the syrupy solution and tests it between his fingers. He tries to judge the stickiness of the solution before deciding if the fruits or the fried jamuns should be added. The painter who comes to paint the house also does something similar. He opens the lid of the tin, stirs the contents with a brush and watches as the paint drips off it. Then he adds a thinner to the paint, stirs it and closely studies the flow of the paint down the brush, He frowns and may add a little thinner once again, This time he is happy. The consistency is just right so he paints a strip on the wall and observes the movement of the paint as it flows down. The cook and the painter — what were they looking for? They were both examining a basic property of

64

••


Judging viscosity is an everyday affair

FLOODS AND FLOWS

65

Drop by drop

liquids, All liquids flow, but not at the same speed, Some, like water, flow readily, Others, like honey, flow ever so slowly, This property of flowing is called fluidity, Liquids that do not flow well, or flow slowly and sluggishly, are called viscous. Viscosity is the technical term for this condition. It is the opposite of fluidity. Viscous liquids offer considerable resistance to flow and so their rate of movement is slow. Viscosity is an important property of liquid; several other properties of liquids are related to the rate of flow. In fact the difference in the rate of flow can provide some fun. For this we will need a ring stand, a funnel, a vessel, some water and other liquids like castor oil, honey and gum. We wiil also need a stop-watch, or else a digital watch, or a watch with a seconds hand.

66


Setting up this experiment is easy. Just fixing a funnel to the ring stand is ail that you have to do. See that the funnel is large. Also don't forget to place the beaker just under the funnel or else you will get wet and it won't be fun. Ask your friend to block the outlet of the funnel while you fill the funnel with water. Now ask another friend to give a signal. As soon as the signal is heard, your friend has to remove the finger from the outlet stem and let the water flow while everyone else times the event. Water flows very fast, and so maybe you and your friends will have some difficulty in timing it. But never mind, this time we will do it with gum or honey and we will have enough time to measure the flow accurately. This is because honey, gum and castor oil flow slowly. In fact, this easy experiment can be repeated with other liquids with which we are familiar. We can try it with milk, cooking oil and kerosene. Comparing the time taken by each of these liquids to empty the funnel gives us an idea about their viscosity. The viscosity of a liquid is high if it takes a long time to empty the funnel. Thus we can assert with confidence that cooking oil is more viscous than water. In fact, we can even assign numbers to denote the viscosity of each liquid. We can maintain an index with the most fluid liquid at the top and the most viscous liquid at the end. To do so, let's take the viscosity of water to be 1. We can then find the ratio of time taken by a liquid to flow out completely to the time taken by water. This number gives us the viscosity of the liquid as compared to water. There are other ways, too, by which we can measure the difference in viscosity between two or more liquids. For this we will need a tall, narrow, glass tumbler or bottle. We will also need a small heavy steel ball like a ball bearing in the wheels of a bicycle.

FLOODS AND FLOWS

67

Reaching the bottom

We will at first fill the tumbler with water and then gently release the ball so that it sinks. Timing the event may be difficult as the ball sinks almost instantly, but if we repeat the experiment a few times we will surely be able to time it correctly. Now we will fill the tumbler with a different liquid. Maybe we could fill it with cooking oil this time. We can then record how long it takes for the ball to sink this time. But to carry out the experiment successfully we have to take care about certain things. The ball should be

68


heavy enough to sink completely even in very viscous and sticky liquids like gum. That is why we used a ball bearing. The liquid should be transparent or else we will not be able to see the ball move through the liquid. Also, when dropping the ball in the liquid, we have to be careful about releasing it slowly without splashing the liquid. And it would also help if we could position the tumbler in a way that light falls on it from one side. This would help us to see the ball properly, especially in semi-transparent liquids. For another experiment of a similar nature, let's fill a bottle half-way with some liquid and then close the mouth of the bottle tightly. At first we will hold the bottle in a vertical position with its mouth up and then we will quickly turn it to the horizontal position. As we do so the liquid in the bottle also gets repositioned. With water, as usual, this change is very quick. But what happens when we use gum or honey? To make the experiment more interesting, we can fill the bottle almost completely with a liquid before corking it. Now, when the bottle is turned upside down very quickly, we see the movement of the air bubble, This air bubble always stays at top when the bottle is erect. When the bottle is turned upside down, the bubble moves accordingly. This movement of the bubble can be timed. However, as expected it differs from liquid to liquid. Can you now guess why the air bubble always moves to the top? Let us go back to the painter and the cook who encouraged us to launch our experiments. Do you know that when we use oil-bound paints to colour doors and windows, the viscosity of the paint has to be checked extremely carefully. This is because if the paint is too thick, it would be difficult to spread the paint evenly over the surface, On the other hand, it it is too thin, it will not give a good coat. Therefore, we use a thinner, usually

FLOODS AND FLOWS

69

Cars use many kinds of oils

turpentine, to make the paint thin enough to flow properly and give a good coat of paint. In other words, we adjust the viscosity of the paint by using a thinner. The manufacturers of paints test the quality of different paints by measuring their viscosities. For this purpose, special standard cup, with a hole is used. The time taken for the cup to become empty is taken as the measure of viscosity of a paint. Cars and some other machines need to use different oils. Viscosity of these oils is one of the critical properties deciding their quality and specific use. Also crude oil and several other products obtained by refining the crude oil have to be transported over long

70


distances. They are usually pumped through pipes. The rate of flow, as well as the power required to pump the liquid flow are dependent on the viscosity of the liquid. Walking down the road is an everyday affair for most of us and it is a pleasure to walk down a newly surfaced road. Tar is used for surfacing roads. It is a thick black sticky substance. But its viscosity can be easily changed. Have you ever looked closely at what is done to the tar prior to using it to surface the road? We can also do something similar. We can also experimentally change the viscosity of a substance. Let's take castor oil in a small bottle so that it is half full. Now let's close the mouth of the bottle tightly. If now we turn the bottle sideways, it will take some time for the oil to adjust to the new horizontal position. This time span is sufficiently long for us to measure. The second time around, let's repeat the experiment but with a difference. Let's hold the bottle in hot water for some time before carrying out the experiment. Doesn't the castor oil flow more readily now? Viscosity depends upon temperature and liquids flow more readily when they are heated. But fluidity is not the property of liquids alone. There are many solids which flow, although the rate of flow may be very very sluggish indeed and the process may have to be carried out under pressure. These substances are called 'plastic substances'. The dough made with flour and water, and plasticine are two common examples. It sounds unreal doesn't it?So why don't we check it out? Let's take some plasticine or dough and make a thick rectangular block from it. Let's also measure its dimensions which means that we now know its length, breadth and height. Now we will place a flat weight on the plasticine block. A big fat book will suffice as the weight. We will measure the dimensions of the block every hour.

FLOODS AND FLOWS

Plasticity can be easily demonstratea

71

72


From the observation, we get on idea about how slowly the flow has taken place. Which flows faster — the plasticine or the dough? We can also do something similar with sealing wax. It is hard at room temperature and does not easily show any plasticity. But when we warm it a little and press a coin on the heated sealing wax, the impression of the coin is left behind on it. This shows that the viscosity of sealing wax changes as we warm it. You can all guess now, why it is called sealing wax. The fact that many materials become plastic at high temperatures is taken advantage of by many manufacturers. Plastic toys, utensils, boxes, etc., are manufactured by taking advantage of the plasticity of the material concerned at high temperatures. Coloured sealing wax is used to colour wooden toys. The wax is held firmly against the toy which rotates with great speed. The heat of the friction is enough to melt the wax which then sticks to the object. Even in nature, examples of various viscous materials abound. When a volcano erupts, it throws out molten lava which flows for some distance. The distance covered by the lava-flow depends on the viscosity of the lava. Some lavas are more viscous than others and hence do not flow over long distances. Rocks under the earth's crust also provide an interesting example of plasticity. You know that as you go deep into the earth, the interior of the earth becomes increasingly hot. The deep layers of rocks under the crust become plastic due to the heat and flow under the pressure of the upper layers. In fact, the continental land masses are-virtually floating on the plastic layers of rocks under the crust.

FLOODS AND FLOWS

73

Now we know that liquids and some solids flow under pressure. What about gases? Of course, gases also flow. They flow many times faster than the liquids. This property is something that we have to be careful about in the kitchen — especially if using LPG for fuel.

A SILVER LINING

cloud has a silver lining. What they mean is that no matter how dark and unhappy the situation, there is always a ray of hope. But literally speaking too, clouds sometimes seem to be fringed with a silver streak. Often on a rainy day, dark clouds gather on the horizon. As the sun sets behind them, many clouds acquire a bright silvery edge. Can we demosntrate this at home? Can we pretend we hold the clouds in our hands? Why, of course we can! For this we will need a lump of cotton wool, a lamp and a few friends. Hold the cotton between the lighted lamp and your friend and then ask him what he sees. You can even reverse positions and take a look for yourself too. The cotton lump appears like a dark cloud with bright edges. At the same time, to the person holding the lamp on the other side, it appears white. We know that when light falls on an object, it can either be reflected, ab-

75

A SILVER LINING

Creating the silver lining at home

sorbed or can even go through the object. Most of the light falling on a mirror is reflected, while that falling on a transparent piece of glass passes through it. Let's observe the cotton wool closely. Most of it is white and opaque. But the lump of cotton does not have any sharp edges like those of a disc of wood or metal. Light that falls on the main mass of the cotton

76


gets reflected backk. To the person who receives this reflected light, the lump appears bright and white. But not all the light falling on the cotton is reflected. A small part of the light falling on the cotton is also absorbed, while light falling on the very edges of the cotton has another fate. It simply goes through the openings in the edge to reach the viewer's eye. Thus the edges of the lump of cotton appear to be bright. They almost look like the brightly edged clouds in the sky. What do you think would happen if we took a bit of wet cotton instead of dry cotton? The margins of wet cotton are not fluffy. They are firm. So now can you guess if the wet cotton shows a bright edge or not? Lets try out our experiment with wet cotton again, shall we? Rain clouds have water droplets and dust particles. These make the clouds opaque. But there are also clouds of ice crystals which look white and appear thin and transparent. You must have seen these white ciuuas;i5 "< ' U " Th& Sum or • • » ' TEM WAITS 54*7601•^^M^OAM

I am very tired, I don't have the 'energy' to do any more work. We often say such things, don't we? The word energy, generally suggests motion, vitality and strength. The limitless amount of radiant energy from the sun is captured by green plants. This provides food for all. We are told that food with 'high energy content' should be part of our daily diet. The food we eat gives us energy for our day to day activities. Man has learnt to use the energy available in nature. Petroleum, 'the high energy fuel' helps drive vehicles. Electricity, another form of energy, plays a major role in our day to day activities. Strolling down the roadside, you see so many stones. Do you think this stone has energy? Let us find out. Let us drop a stone from a height on to a mud ground. What do we see? We hear a typical 'thud' and the stone leaves an impression on the ground. Let us be a little more systematic in our experiment. Let us go out into the park, Let us dig a pit, say one metre

THE UNIVERSAL CURRENCY

89

Let's dig a pit

long, one metre wide and about two centimetres deep into the ground. We will then fill up this pit with fine sand. Let us hunt around for a big stone, it must weigh at least about one kg. Hold the stone over the centre of the pit, ask one of your friends to measure the height of the stone from the ground. Now release the stone. The stone will drop to the ground with a thud, and as it hits the ground, sand will be thrown out, all around the stone.

90


The Quebec Crater Lakes (Canada)

Carefully, without disturbing the sand, let us lift the stone. What do we see? Isn't there a deep depression, where the stone fell? Now, let us measure the diameter and depth of this depression. Do you think this will differ, depending on the height from which the stone is released? How about trying it out? If you cannot go out into the park and play in the sand you can modify the experiment and carry it out at home. For this you will need some flour (atta, maida or even besan will be fine). Just pile the flour into a nice, smooth rectangle. This is the substitute for the pit you would have dug in the sand. Now drop a marble or a similar object on it and measure the depression created. Such depression on land caused by the impact of a falling object, is called a crater. Remember reading


91

about the craters on the surface of the moon? Craters on the moon are really big. Some such craters are even seen on the earth's surface. These occur when huge meteorites hit the earth's surface. If you don't feel like going out into the sun, would you like to try out some similar experiment in your room? Let us take a sheet of paper and fill an ink dropper with ink. Just allow a drop of ink to fall on the centre of the sheet. As the ink drop hits the sheet of paper, it will splash around and create a star-like figure. Measure the diameter of the star. Try dropping the ink from different heights. What difference do you find in the diameters? What does all this have to do with energy, you may ask. Indeed, even the piece of stone has energy, as does the drop of ink. The falling stone had the energy to displace sand from the pit. Stone falling from greater heights moves more sand. Now we know, how a meteorite creates such a huge crater.

Experimenting with ink

92


An object at a height acquires energy because of the earth's pull or gravity. The object released from a higher position falls towards the earth and as the height increases, so does the speed with which it hits the ground and hence acquired more energy. The ancient scientist Gottfried W. von Leibnitz described this energy as vis viva meaning living force. The word 'energy' first entered the technical vocabulary of science in 1807. Energy due to motion is called 'kinetic energy' Kinetic simply means 'due to motion'. Would you like to study some more forms of energy? Can you try and obtain a convex lens from your school laboratory? Try holding this convex lens in the sun and focussing the rays on a piece of paper. We must be very careful doing this. The paper will catch fire. We are capturing and concentrating so much of the sun's heat energy at one place, that (the paper) gets heated enough to catch fire. Would you like to have some more fun? Let us make a hollow tube about 5 cm wide with a cardboard. Let us cover one end of the tube with a piece of thin paper. Then, we fix a small piece of mirror to the centre of the paper. Using a torch we flash a beam of light on the mirror and get a reflection on a wall or a screen. Now, comes the fun part. Ask one of your friends to speak into the other end of the tube. What do you see? The reflected light dances on the walls, doesn't it? Sound too, is a form of energy. This energy is transferred to the paper which moves, The movement or vibrations of this paper, cause the reflection to dance around. Energy thus manifests itself in many forms. There are at least five major forms of energy, heat, light, chemical, electrical and nuclear energy. Energy can be converted from one form to another. The water of lakes and oceans evaporates due to the heat produced by the radiant energy of the sun. Water vapour collects as


93

Energy manifests itself in many ways

clouds, then falls as rain. Flowing downhill, it turns the generators of power houses. The current generated may flow through a wire to light a bulb, heat houses or charge a chemical storage battery. Striking a match or lighting a candle, are all examples of the conversion of one form of energy into another. Look around you. Can you explain the energy conver-

94


sions taking place around you? Whenever you see energy being used, try to trace its source. While, in principle, energy in any form can be converted into another form, in practice, we convert it into a form which suits our needs. For example, the wind that blows in our fields can do useful work for us when it is made to drive a windmill. However, when we change the form of energy, all of the energy is not converted into the desired form. For example, when we strike a match, we convert the chemical energy into usable heat. The energy spent in producing the sound and in heating the side of the match box are examples of wasted energy. The energy stored in petrol and coal, was made by nature over millions of years. Nature will not make this energy for us in a hurry. We must make an effort to prevent wastage of energy.

FRIEND OR FOE

Have you ever wondered why a carrom coin stops just before the pocket even when it is aimed to reach it? Or why a cricket ball stops after travelling some distance? Scientists say that the ground surface opposes the moving ball and the board opposes the motion of the coin. Such opposing force is called friction. Friction tends to stop two surfaces moving over each other. It is greater for some surfaces than for others. When we slide two glass strips over each other, don't they slide smoothly? The same is not the case when we rub together blotting or sand paper. What is the difference between the two surfaces? Now you can guess why the cricket ball moves faster and over a longer distance on a smoother surface. Yes, you guessed it right. A carrom coin, too, moves more smoothly when the board is sprinkled with powder. Friction is usually greater between two rough surfaces than between two smooth ones.

96


Demonstrating friction

We could check this out using different surfaces. We need a flat board and a coin. Besides, we shall also need sheets of glass, polythene, wood and a cotton cloth. Let us place the coin on the board, and raise the board carefully from one side. As we raise the board, at one point the coin starts sliding down the board. The coin starts sliding down only when a particular angle is reached. Let us measure this angle. We can repeat the process after fixing a polythene sheet, glass, wood or cotton on the board. In each case, the coin starts sliding down at a different angle. The object starts sliding only when the downward pull along the surface overcomes the force of friction between the coin and the surface of the board.

FRIEND OR FOE

97

Hold fast

Now, a large number of questions are answered. Can we play tennis with greasy hands? What would happen if the knobs of a guitar or violin do not hold fast in the desired position? Can you now explain why we can hold objects firmly in our hands? Have you ever wondered as to why fishermen always make two to three turns with their ropes around a hook and then release the sails of their boats? A rope can hold large weights with a few turns around a rod. The friction of the rope with the rod contributes a large part of the force required to hold the weight. This is the trick used to sail boats. The rope holding the sail is just passed around a hook, a couple of times.

98


Overcoming friction

Friction stops the cricket bail from crossing the boundary, spoils the game of carrom, and forces us to pedal our bicycles. No wonder, we are constantly making efforts to reduce friction one way or the other. Ball bearings and roller bearings are used everywhere in factories, vehicles and in machinery for smooth motion. The discovery that a rolling object has to overcome less friction is one of the breakthroughs achieved by pre-historic man. Roller bearings in the form of logs were used in ancient times to move heavy stones for building monuments like big temples and pyramids. A bicycle ride is perfect for studying friction. Ride your bicycle and pedal hard till you reach good speed. Then stop pedalling and see how far the bicycle will take you. We can do the same on various roads: on a good

99

FRIEND OR FOE

Testing the power of friction

surface, a hard stony road, a dust tract. What difference will we find? Let us try another experiment. Let us keep the bicycle upside down, and rotate the rear wheel using a pedal. Let us then hold a ball of crumpled paper hard against the rim of the wheel. Quite obviously the speed of the wheel is reduced until it stops. The friction of the paper

100


with the rim reduces the speed. The ball of paper on the other hand is worn out by friction. The brakes of the bicycle are made of hard rubber and this rubber does not wear out easily when it rubs against the rim of the wheel. But prolonged use of the bicycle does wear out the rubber pads. Ancient men used to start fires by rubbing dry sticks of wood together. Nowadays we make a flame by striking a match. Does that mean, heat is produced due to friction? Quite right. The ancient method of making fire was to produce enough heat to produce a flame. Similarly, rubbing the match on a rough surface gives enough heat to set fire to the chemicals at the end of the match. We rub our hands together on cold winter evenings, and the hands begin to get comfortably warm. Sometimes, the heat generated by friction can be inconvenient. When a spacecraft re-enters the earth's atmosphere there is friction between the air and the surface of the spacecraft. This produces a lot of heat. A special heat shield has to be fitted around the craft to protect the astronauts. When a motor car engine is running there are many moving metal surfaces, These would cause a lot of friction if they are rubbed together. So, oil is used in the engine. The oil forms a film between the metal surfaces so that they do not rub together. This prevents the engine from getting too hot. Do you know that the bone joints in our body have some arrangement to protect them from rubbing against surrounding parts and to prevent the wearing caused by this? Be it, our friend or foe, next time we take the first step to walk, we must remember that if there were no friction •• ' between the soles of our shoes and the ground, we would" soon as we tried to walk.

A LIGHTNING FLASH IN YOUR ROOM

A brilliant flash of lightning followed by the deafening roll of thunder, is indeed one of nature's most awe-inspiring displays. Ancient Greeks believed that thunderbolts were actually hurled by Zeus, the father of gods. Today, even though we hold lightning in awe, and may be even a bit of fear, we know there is nothing supernatural about it. It is simply a rapid discharge of electric charges which have accumulated on the thunder clouds. This landmark discovery was made by the famous American scientist, Benjamin Franklin in 1752.

But what exactly is an electric charge? Remember the crackle we hear when our nylon clothes rub against our woollens in winter? The crackle, too, is because of an electrical charge, except that it is on a much smaller scale compared to the one in thunder cloyd^

102


Commonplace example of electric charge

Let us try rubbing a handkerchief on a polythene plastic sheet. We will realize the presence of a charge, the moment we take the rubbed polythene sheet near tiny bits of paper. The paper pieces are instantly attracted to the sheet and cling to it atleast for a short period of time. Do you think we will feel the presence of the charge when we don't rub the plastic sheet and the kerchief? Let's try that too. Scientists pondered over what actually brought about this kind of an attraction, and why it happened only when two substances were rubbed against each other. The answer to this came when the structure of the atom, the fundamental unit of ail matter, was unraveled. Typically on atom has a nucleus, or centre, containing positively charged particles — protons — and neutral or uncharged particles — neutrons. Negatively charged particles known as electrons revolve around


103

Let's puzzle our friends

the nucleus. Ordinarily, there are as many protons as there are electrons in an atom. But when an atom either gains or loses electrons, a charge is acquired. The atom becomes negatively charged when electrons are gained; it gets positively charged if electrons are lost. Now can you guess what happens when we rub the polythene sheet with the handkerchief? The friction between the two surfaces removes some of the electrons from one material to the other. One surface acquires more negative charge on it. The other surface loses electrons and gets positively charged. Thus these surfaces become 'electrically charged' when rubbed together. An electrically charged object attracts other light objects. That explains why the tiny bits of paper are attracted to the plastic sheet, doesn't it? Wouldn't it be nice if we could puzzle our friends with some tricks for which we have an explanation and they don't? Let us tie a thin cotton or silk thread to a support

104


from where it hangs down, Supposing we take one of our plastic scales, rub it on a piece of nylon cloth and bring it near the free end of the hanging thread, what do you think will happen? The thread is attracted to the plastic, isn't it? Not only does the thread move, it also becomes taut. The latter happens when the charge on the plastic is more. We could try it out with other materials also. Isn't it an easy way to test whether an object is electrically charged or not? Do you think if we touched the charged portion of the plastic scale with our fingers, it would still attract the thread? Let's try it. An electric charge on one object can be easily transferred to other objects just by contact. If a charged rod touches another object, the charges on the rod are transferred to that object. Would you like to try creating a lightning-like spark in your room? Let us take a glass or plastic tumber. We cover the lower half of the tumbler both from inside and outside with a thin metal foil. We then fix a copper wire to the outside of the tumbler and allow it to touch the ground. We will take a long key chain and pass it through a small hole in a piece of cardboard. We only need to take care that when we place the cardboard on the tumbler, the chain should touch the bottom of the tumbler. The upper end of the chain with the key ring must remain above the piece of cardboard. Now comes the most exciting part. We will switch off the light and charge the key ring just the way we have done before, After we feel that enough charges have accumulated, we touch the ring with a finger. In that darkness, we will actually see a spark. Indeed, a bright spark will jump from the ring to the finger, along with a crackling sound. Don't worry, it is all perfectly harmless.


105

Lightning flashes made to order

The lightning flash we see in the sky is a very large amount of charge (compared to the ones we just created) which are discharged through the air. A thunder cloud is formed when the water droplets are pulled up vigorously by extremely fast winds. A turbulent cloud gets charged due to friction. The large number of negative charges on the base of the cloud induces positive charges on the earth below. The charges leak when the system builds more charges than it can hold, We cannot see the electrons themselves. What we see in air is the glow by the passage of these charged particles. Scientists call this accumulation of charges, static electricity. Static electricity has been exploited for several uses. Understanding static electricity and its role in causing lightning has helped us to make a device that protects tall structures from being hit by lightning. Invented by Benjamin Franklin, it is a rod made of copper

106


Lightning conducters protect tall buildings

with pointed ends. It is fixed near the top of the high structure. This rod is connected by a cable to another rod buried in the ground. Lightning is safely conducted into the ground when it strikes the rod on tall structures. Have you ever noticed that these lightning rods are prominently found on wooden and non-metallic structures, while metallic structures do not have these rods? This is because the metallic frame itself provides a path for the lightning to reach the ground. Did you know that even a photocopier we make use of static charges? These charges allow the carbon powder to cling to the paper on which the copy is to be made. Can you think of any other uses of static electricity?

SILENCE IS GOLDEN

Have you ever wondered why a balloon makes so much of noise when it bursts? When a car or a motorcycle starts, then too, it makes a lot of noise. A gun makes an explosive noise when a bullet is fired. What do you think creates this noise? The answer to all these questions lies jn the speed of air. When a balloon bursts, air within it, which was maintained under a great deal of pressure, is instantly released with great speed. Similarly noise is created when exhaust gases coming out of the car engine hit the air with great speed. The sharp report of a gun is due to the exhaust gases that follow the bullet with great speed. As the gases come out of the muzzle of the gun, a loud noise is created. Noise is not always bad. We make noise to chase away wild animals, Here we are talking about noise that hurts us.

108


Does the paper strip move?

If if is the speed of air that causes such a lot of noise, there must be some way to reduce the speed of air, if only to ensure silence. Let us see how this can be done. Let us find (or make) a hollow cardboard tube, about 5cm wide and 30cm long. Now, hold a tiny strip of paper near one end of the tube and blow air through the other end. The strip will obviously move depending on the speed of air. We could try this out with air blowing at varying speeds. How about calling some friends over? Each of them would blow air at a different speed. in the second step, let us try and get two plastic funnels, Let us join their broad rims using an adhesive tape. Now, we will hold the same.strip of paper at one end and blow through the other, There will be a definite

SILENCE IS GOLDEN

109

Drilling holes into the pipe needs care

difference in the movement of the strip. Will the strip move faster or will it not move at all? Yes, the strip hardly moves. The bulge in the middle has reduced the speed of air considerably. When the air enters the bulge, it expands and its speed is reduced. Can you try and get a plastic tube about a metre long from somewhere? Even an old pipe, lying around the house will do. We will drill some holes into this tube. Tiny ones, not more than half a centimetre wide. If you can find a screw, just heat it up and pierce the tube with it, Let us place this tube in a cardboard box, about 30 cm long.

110


- How far does the paper strip move?

instead of blowing air through the tube, let us use a table fan as the air source. If this air is channelized through a big cardboard funnel, fitted to the tube, the speed will be sufficiently high. As before, let us hold a strip of paper at the other end, and then switch on the fan. What do we see? Despite such a fast speed of air, the strip is not really blowing away, is it? The air, in fact, expands as it enters the tube. This reduces the speed of the air before it comes out at the other end of the tube. Aptly called the silencer, a device quite similar to what we just constructed, is attached to cars, to stop them from making too much noise. The car silencer is made up of a wide cylinder around a narrow perforated tube. A perforated metal plate is fixed in front of the tube. The exhaust gases which come out of the narrow tube are obstructed by the plate. The gases bounce back and expand in the cylinder. Finally, they can move

SILENCE IS GOLDEN

111

A car silencer

out only through the perforations in the metal plate, and are no longer at a fast speed. Silencers are sometimes used in guns as well. This silencer is also a long tube fitted with a number of thin plates. Each plate has a hole at the centre through which the bullet passes. The exhaust gases expand in the space between the thin plates. At each step the speed of the exhaust gases is reduced, Ultimately, when the gases come out of the silencer tube, their speed is not sufficient to create the loud report. Aircraft models are tested in tunnels where air is blown with tremendous speed. This can create deafening noise and disturb the surroundings. This will need large silencers to minimize this noise. Where else, do you think silencers can be used?

Ready Reference Acid 1-7 Alkali 1-7 Automation

57-62

Catalyst 8-14 Circuit 31-36 Decomposition 8-14 Earthing 31-36 Endpoint 1-7 Elastic limit 37-44 Elasticity 37-44 Electricity 31-36, 101-106 Electromagnet

51-56

Electromagnetic shielding 51-56 Electron 31-36, 101,106 Energy 88-94 Fluidity Friction

63-73 95-100

Indicator

1-7

Insulator

31-36

Light

74-82

Lightning conductors

51-56

Material science 74-82 Music 83-87 Neutalization 1-7 Neutron 101-106 Noise 24-30,45-50,107-111 Osmosis 15-23 Plasticity 63-73,101-106 Radio telescope 24-30 Reflection 74-82 Resistance 31-36 Semipermeable membrane 15-23 Short circuit 31-36 Silencer 107-111 Sound 24-30 Soundproof room 45-50 Sound waves 45-50 Static electricity 101 -106 Tensile strength 37-44 Thermostat 57-62 Viscosity 63-73

"The experiments suggested in this book cut across artificial barriers like physics, chemistry or biology, and deal with real situations.encountered in daily life. That is why these experiments deal with curiosities arising out of common everyday observations. The aim is not to convey information alone, but to help young minds to explore on their own. Moreover, these experiments can be conducted using materials and implements readily available even in rural areas".

ISBN : 81-7236-082-7

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