Light Physics PDF

LIGHT T he world is largely known through comb and close all its openings the senses. The sense of sight except one in the middle. You can is one of the most important use a strip of black paper for senses. Through it we see mountains,...

LIGHT T he world is largely known through comb and close all its openings the senses. The sense of sight except one in the middle. You can is one of the most important use a strip of black paper for senses. Through it we see mountains, this purpose. Hold the comb rivers, trees, plants, chairs, people and perpendicular to the sheet of paper. so many other things around us. We also Throw light from a torch through see clouds, rainbows and birds flying the opening of the comb from one in the sky. At night we see the moon side (Fig. 13.1). With slight and the stars. You are able to see the adjustment of the torch and the words and sentences printed on this page. How is seeing made possible? comb you will see a ray of light along the paper on the other side of the 13.1 What makes Things comb. Keep the comb and the torch steady. Place a strip of plane mirror Visible in the path of the light ray Have you ever thought how we see (Fig. 13.1). What do you observe? various objects? You may say that eyes see the objects. But, can you see an object in the dark? It means that eyes alone cannot see any object. It is only when light from an object enters our eyes that we see the object. The light may have been emitted by the object, or may have been reflected by it. You learnt in Class VII that a polished or a shiny surface can act as a mirror. A mirror changes the direction of light that falls on it. Can you tell in which direction the light falling on a surface Fig. 13.1 : Arrangement for showing reflection will be reflected? Let us find out. After striking the mirror, the ray of 13.2 Laws of Reflection light is reflected in another direction. Activity 13.1 The light ray, which strikes any surface, is called the incident ray. The ray that Fix a white sheet of paper on a comes back from the surface after drawing board or a table. Take a reflection is known as the reflected ray. 2024-25  A ray of light is an idealisation. In Table 13.1 : Angles of Incidence reality, we have a narrow beam of and Reflection light which is made up of several S. Angle of Angle of rays. For simplicity, we use the term No. Incidence (∠∠i) ∠r) Reflection (∠ ray for a narrow beam of light. 1. Draw lines showing the position of 2. the plane mirror, the incident ray and 3. the reflected ray on the paper with the help of your friends. Remove the mirror 4. and the comb. Draw a line making an 5. angle of 90º to the line representing the mirror at the point where the incident Do you see any relation between the ray strikes the mirror. This line is known angle of incidence and the angle of as the normal to the reflecting surface reflection. Are they approximately equal? at that point (Fig. 13.2). The angle If the experiment is carried out carefully, it is seen that the angle of incidence is always equal to the angle of reflection. This is one of the laws of reflection. Let us perform another activity on reflection. Fig. 13.2 : Drawing the normal What would happen if I between the normal and incident ray is threw the light on the called the angle of incidence (∠i). The mirror along the normal. angle between the normal and the reflected ray is known as the angle of Activity 13.2 reflection (∠r) (Fig. 13.3). Measure the angle of incidence and the angle of Perform Activity 13.1 again. This reflection. Repeat the activity several time use a sheet of stiff paper or a times by changing the angle of chart paper. Let the sheet project a incidence. Enter the data in Table 13.1. little beyond the edge of the Table (Fig. 13.4). Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure Reflected Incident that the reflected ray extends to the ray ray projected portion of the paper. Bend Normal that part of the projected portion on which the reflected ray falls. Can you still see the reflected ray? Bring Fig. 13.3 : Angle of incidence and angle of the paper back to the original reflection 166 SCIENCE 2024-25  position. Can you see the reflected with the Sun as the source of light ray again? What do you infer? instead of a torch. You, too, can use the Sun as the source of light. These activities can also be performed by making use of the Ray Streak Apparatus (available in the kit prepared by NCERT). Boojho remembered that in Class VII, he had studied some features of the image of an object formed by a plane mirror. Paheli asked him to recall (a) those features: (i) Was the image erect or upside down? (ii) Was it of the same size as the object? (iii) Did the image appear at the same distance behind the mirror as the object was in front of it? (iv) Could it be obtained on a screen? Let us understand a little more about (b) the formation of an image by a plane Fig. 13.4 (a), (b) : Incident ray, reflected ray and the normal at the point mirror in the following way: of incidence lie in the same plane Activity 13.3 When the whole sheet of paper is A source of light O is placed in front spread on the table, it represents one of a plane mirror PQ. Two rays OA plane. The incident ray, the normal at and OC are incident on it (Fig. 13.5). the point of incidence and the reflected Can you find out the direction of ray are all in this plane. When you bend the reflected rays? the paper you create a plane different Draw normals to the surface of from the plane in which the incident the mirror PQ, at the points A and ray and the normal lie. Then you do not C. Then draw the reflected rays at see the reflected ray. What does it the points A and C. How would you indicate? It indicates that the incident draw these rays? Call the reflected ray, the normal at the point of rays AB and CD, respectively. incidence and the reflected ray all lie Extend them further. Do they meet? in the same plane. This is another law Extend them backwards. Do they of reflection. meet now? If they meet, mark this Paheli and Boojho performed the point as I. For a viewer’s eye at E above activities outside the classroom (Fig. 13.5), do the reflected rays LIGHT 167 2024-25  Fig. 13.6 : Parallel rays incident on an irregular surface Fig. 13.5 : Image formation in a plane mirror appear to come from the point I. Since the reflected rays do not actually meet at I, but only appear to do so, we say that a virtual image Fig. 13.7 : Rays reflected from irregular of the point O is formed at I. As you surface have learnt already in Class VII, such an image cannot be obtained When all the parallel rays reflected on a screen. from a rough or irregular surface are You may recall that in an image not parallel, the reflection is known as formed by a mirror the left of the object diffused or irregular reflection. appears on the right and the right Remember that the diffused reflection appears on the left. This is known as is not due to the failure of the laws of lateral inversion. reflection. It is caused by the irregularities in the reflecting surface, 13.3 Regular and Diffused like that of a cardboard. On the other hand, reflection from Reflection a s m o o t h s u r face like that of a Activity 13.4 mirror is called regular reflection (Fig. 13.8). Images are formed by Imagine that parallel rays are regular reflection. incident on an irregular surface as shown in Fig. 13.6. Remember that the laws of reflection are valid at each point of the surface. Use these laws to construct reflected rays at various points. Are they parallel to one another? You will find that these rays are reflected in different directions. (Fig. 13.7) Fig. 13.8 : Regular reflection 168 SCIENCE 2024-25  Do We See all Objects due to Reflected Light? Nearly everything you see around is seen due to reflected light. Moon, for example, receives light from the Sun and reflects it. That’s how we see the moon. The objects which shine in the light of other objects are called illuminated objects. Can you name some other such objects? There are other objects, which give their own light, such as the Sun, fire, flame of a candle and an electric lamp. Their light falls on our eyes. That is how we see them. The objects which emit their own light are known as luminous objects. Paheli recalls having constructed a periscope as an Extended Activity in I have a question. Can the Class VI. The periscope makes use of reflected rays be further reflected two plane mirrors. Can you explain if incident on another mirror? how reflection from the two mirrors enables you to see objects which are Let us find out. not visible directly? Periscopes are 13.4 Reflected Light Can be used in submarines, tanks and also Reflected Again by soldiers in bunkers to see things outside. Recall the last time you visited a hair dresser. She/he makes you sit in front of 13.5 Multiple Images a mirror. After your hair cut is complete, You are aware that a plane mirror forms she/he holds a mirror behind you to show you how the hair has been cut only a single image of an object. What (Fig. 13.9). Do you know how you could happens if two plane mirrors are used see the hair at the back of your head? in combination? Let us see. Fig. 13.9 : Mirror at the hair dresser shop LIGHT 169 2024-25 Activity 13.5 Can you now explain how you can see the back of your head at the hair Take two plane mirrors. Set them dresser’s shop? at right angles to each other with This idea of number of images formed their edges touching (Fig. 13.10). To by mirrors placed at an angle to one hinge them you can use adhesive another is used in a kaleidoscope to tape. Place a coin in between the make numerous beautiful patterns. You mirrors. How many images of the can also make a kaleidoscope yourself. coin do you see (Fig. 13.10)? Kaleidoscope Activity 13.6 To make a kaleidoscope, get three rectangular mirror strips each about 15 cm long and 4 cm wide. Join them together to form a prism as shown in Fig. 13.12(a). Fix this arrangement of mirrors in a circular Fig. 13.10 : Images in plane mirror at right cardboard tube or tube of a thick angle to each other chart paper. Make sure that the tube Now hinge the mirrors using the is slightly longer than the mirror adhesive tape at different angles, say strips. Close one end of the tube by 45°, 60°, 120°, 180° etc. Place some a cardboard disc having a hole in object (say a candle) in between the centre, through which you can them. Note down the number of see [Fig. 13.12(b)]. To make the disc images of the object in each case. durable, paste a piece of transparent Finally, set the two mirrors plastic sheet under the cardboard parallel to each other. Find out how many images of a candle placed between them are formed (Fig. 13.11). (a) mirrors (b) (c) Fig. 13.11 : Image in plane mirror parallel to each other Fig. 13.12 : Making a kaleidoscope 170 SCIENCE 2024-25  disc. At the other end, touching the 13.6 Sunlight — White or mirrors, fix a circular plane glass Coloured plate [Fig. 13.12(c)]. Place on this In Class VII, you learnt that the sunlight glass plate several small pieces of is referred to as white light. You also coloured glass (broken pieces of learnt that it consists of seven colours. coloured bangles). Close this end of Here is another activity (Activity 13.7) the tube by a ground glass plate. showing that sunlight consists of several Allow enough space for the colour colours. pieces to move around. 13.7 What is inside Our Eyes? Your kaleidoscope is ready. When you peep through the hole, you will be We see things only when light coming able to see a variety of patterns in the from them enters our eyes. Eye is one tube. An interesting feature of a of our most important sense organs. It kaleidoscope is that you will never see is, therefore, important to understand the same pattern again. Designers of its structure and working. wallpapers and fabrics and artists often The eye has a roughly spherical use kaleidoscopes to get ideas for new shape. The outer coat of the eye is patterns. To make your toy attractive, white. It is tough so that it can protect you can wrap the kaleidoscope in a the interior of the eye from accidents. coloured paper. Its transparent front part is called Activity 13.7 Get a plane mirror of a suitable size. prism. As you learnt in Class VII, Place it in a bowl (katori) as shown this breaks up the light into its in Fig. 13.13. Fill the bowl with colours, Splitting of light into its water. Put this arrangement near a colours is known as dispersion of window in such a way that direct light. Rainbow is a natural sunlight falls on the mirror. Adjust phenomenon showing dispersion. the position of the bowl so that the reflected light from the mirror falls on a wall. If the wall is not white, fix a sheet of white paper on it. Reflected light will be seen to have many colours. How can you explain this? The mirror and water form a Fig. 13.13 : Dispersion of light LIGHT 171 2024-25 cornea (Fig. 13.14). Behind the cornea, Class VII. The lens focuses light on the we find a dark muscular structure back of the eye, on a layer called retina called iris. In the iris, there is a small (Fig. 13.14). The retina contains several opening called the pupil. The size of nerve cells. Sensations felt by the nerve the pupil is controlled by the iris. The cells are then transmitted to the brain iris is that part of eye which gives it its through the optic nerve. There are two distinctive colour. When we say that a kinds of cells– person has green eyes, we refer actually (i) cones, which are sensitive to bright to the colour of the iris. The iris controls light and the amount of light entering into the (ii) rods, which are sensitive to dim light. eye. Let us see how. Cones sense colour. At the junction of the optic nerve and the retina, there Ciliary are no sensory cells, so no vision is muscle possible at that spot. This is called the Iris blind spot. Its existence can be Optic Nerve demonstrated as follows: Lens Cornea Activity 13.9 Make a round mark and a cross on Retina a sheet of paper with the spot to the Fig. 13.14 : Human eye right of the cross (Fig. 13.15). The distance between two marks may be Caution : For this activity, never use 6-8 cm. Hold the sheet of paper at a laser torch. an arm’s length from the eye. Close Activity 13.8 your left eye. Look continuously at the cross. Move the sheet slowly Look into your friend’s eye. Observe towards you, keeping your eye on the size of the pupil. Throw light on the cross. What do you find? Does her eye with a torch. Observe the the round mark disappear at some pupil now. Switch off the torch, and point? Now close your right eye. observe her pupil once again. Do Look at the round mark now and you notice any change in the size repeat the activity. Does the cross of the pupil? In which case was the disappear? The disappearance of the pupil larger? Why do you think it cross or the round mark shows that was so? there is a point on the retina which In which case do you need to cannot send messages to the brain allow more light in the eye, when when light falls on it. the light is dim or bright? Behind the pupil of the eye is a lens which is thicker in the centre. What kind of lens is thicker at the centre? Fig. 13.15 : Demonstration of blind spot Recall what you learnt about lenses in 172 SCIENCE 2024-25  The impression of an image does not Nature has provided eyes with vanish immediately from the retina. It eyelids to prevent any object from persists there for about 1/16th of a entering the eye. Eyelids also shut out second. So, if still images of a moving light when not required. object are flashed on the eye at a rate Eye is such a wonderful instrument faster than 16 per second, then the eye that it (normal) can clearly see distant perceives this object as moving. objects as well as objects nearby. The minimum distance at which the eye can Activity 13.10 see objects distinctly varies with age. The most comfortable distance at which one Get a square piece of cardboard of can read with a normal eye is about size 6-8 cm. Make two holes as 25 cm. shown in Fig. 13.16. Thread a Some persons can see objects close string through the two holes. Draw/ to them clearly but cannot see distant paste a cage on one side of the objects so clearly. On the other hand, cardboard and a bird on the other some persons cannot see objects nearby side. Twist the string and make the clearly but they can see distant objects card twirl rapidly. Do you see the quite well. With suitable corrective bird in the cage? lenses, these defects of the eye can be corrected. Sometimes, particularly in old age, eyesight becomes foggy. It is due to the Reverse side Front side of eye lens becoming cloudy. When it of cardboard cardboard happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe. It is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer. 13.8 Care of the Eyes It is necessary that you take proper care Fig. 13.16 : Bird in cage of your eyes. If there is any problem you should go to an eye specialist. Have The movies that we see are actually a regular checkup— a number of separate pictures in l If advised, use suitable spectacles. proper sequence. They are made to move across the eye usually at the rate l Too little or too much light is bad of 24 pictures per second (faster for eyes. Insufficient light causes than 16 per second). So, we see a eyestrain and headaches. Too much moving picture. light, like that of the Sun, a powerful LIGHT 173 2024-25  Did you know? Animals have eyes shaped in different ways. Eyes of a crab are quite small but they enable the crab to look all around. So, the crab can sense even if the enemy approaches from behind. Butterflies have large eyes that seem to be made up of thousands of little eyes (Fig. 13.17). They can see not only in the front and the sides but the Eyes back as well. A night bird (owl) can see very well in the night but not during the day. On the other hand, day light birds (kite, eagle) can see well during the day but not in the night. The owl has a large cornea and a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more Fig. 13.17 : Eyes of a butterfly cones and fewer rods. lamp or a laser torch can injure the vegetables (such as spinach) and cod retina. liver oil are rich in vitamin A. Eggs, l Do not look at the Sun or a powerful milk, curd, cheese, butter and fruits such as papaya and mango are also rich light directly. in vitamin A. l Never rub your eyes. If particles of dust go into your eyes, wash your 13.9 Visually Impaired eyes with clean water. If there is no Persons Can Read and improvement go to a doctor. Write l Always read at the normal distance for vision. Do not read by bringing Some persons, including children, can the book too close to your eyes or be visually impaired. They have very keeping it too far. limited vision to see things. Some You learnt about balanced diet in persons cannot see at all since birth. Class VI. If food is deficient in some Some persons may lose their eyesight components, eyes may also suffer. Lack because of a disease or an injury. Such of vitamin A in foodstuff is responsible persons try to identify things by for many eye troubles. Most common touching and listening to voices more amongst them is night blindness. carefully. They develop their other One should, therefore, include in the senses more sharply. However, diet components which have vitamin A. additional resources can enable them Raw carrots, broccoli and green to develop their capabilities further. 174 SCIENCE 2024-25  Non-optical and optical aids for visually impaired Non-optical aids include visual aids, tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids. Visual aids, can magnify words, can provide suitable intensity of light and material at proper distances. Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing. Auditory aids include cassettes, tape recorders, talking books and other such devices. Electronic aids, such as talking calculators and computers, are also available for performing many computational tasks. Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination. Nowadays, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text. Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids. While the lens combinations are used to rectify visual limitations, telescopic aids are available to view chalkboard and class demonstrations. 13.10 What is the Braille Braille system has 63 dot patterns or System? characters. Each character represents a letter, a combination of The most popular resource for visually letters, a common word or a challenged persons is Braille. grammatical sign. Dots are arranged in cells of two vertical rows of three Louis Braille, himself a visually dots each. challenged person, developed a Patterns of dots to represent some system for visually challenged persons English letters and some common and published it in 1821. words are shown below. Louis Braille Fig. 13.18 : Example of dot patterns used in Braille System The present system was adopted in These patterns when embossed on 1932. There is Braille code for common Braille sheets help visually languages, mathematics and scientific challenged persons to recognise words notation. Many Indian languages can by touching. To make them easier to be read using the Braille system. touch, the dots are raised slightly. LIGHT 175 2024-25  Visually impaired people learn the character has to be memorised. Braille Braille system by beginning with texts can be produced by hand or by letters, then special characters and machine. Typewriter-like devices and letter combinations. Methods depend printing machines have now been upon recognition by touching. Each developed. Some visually impaired Indians have great achievements to their credit. Diwakar, a child prodigy has given amazing performances as a singer. Ravindra Jain, born completely visually impaired, obtained his Sangeet Prabhakar degree from Allahabad. He had shown his excellence as a lyricist, singer and music composer. Lal Advani, himself visually impaired, established an Helen A. Keller Association for special education and rehabilitation of disabled in India. Besides this, he represented India on Braille problems in UNESCO. Helen A. Keller, an American author and lecturer, is perhaps the most well- known and inspiring visually challenged person. She lost her sight when she was only 18 months old. But because of her resolve and courage she could complete her graduation from a university. She wrote a number of books including The Story of my Life (1903). 176 SCIENCE 2024-25 KEYWORDS WHAT YOU HAVE LEARNT ANGLE OF Ü Light is reflected from all surfaces. INCIDENCE Ü Regular reflection takes place when light is ANGLE OF incident on smooth, polished and regular REFLECTION surfaces. BLIND SPOT Ü Diffused or irregular reflection takes place from rough surfaces. BRAILLE Ü Two laws of reflection are CONES (i) The angle of incidence is equal to the angle CORNEA of reflection. (ii) Incident ray, reflected ray and the normal DIFFUSED OR drawn at the point of incidence to the IRREGULAR reflecting surface, lie in the same plane. REFLECTION Ü Image formed in a plane mirror undergoes INCIDENT RAYS lateral inversion. IRIS Ü Two mirrors inclined to each other give multiple KALEIDOSCOPE images. Ü Beautiful patterns are formed in a LATERAL INVERSION kaleidoscope because of multiple reflections. LAWS OF Ü Sunlight, called white light, consists of seven REFLECTION colours. PUPIL Ü Splitting of light into its constituent colours is REFLECTED RAYS known as dispersion. Ü Parts of the eye are cornea, iris, pupil, lens, REFLECTION retina and optic nerve. REGULAR Ü A normal eye can see nearby and distant REFLECTION objects clearly. RETINA Ü Visually impaired persons can read and write RODS using the Braille system. Ü Visually impaired persons develop their other senses more sharply to improve their interac- tion with their environment. LIGHT 177 2024-25  Exercises 1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain. 2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection? 3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case. (a) Polished wooden table (b) Chalk powder (c) Cardboard surface (d) Marble floor with water spread over it (e) Mirror (f) Piece of paper 4. State the laws of reflection. 5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane. 6. Fill in the blanks in the following. (a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image. (b) If you touch your ____________ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ____________. (c) The size of the pupil becomes ____________ when you see in dim light. (d) Night birds have ____________ cones than rods in their eyes. Choose the correct option in Questions 7 – 8 7. Angle of incidence is equal to the angle of reflection. (a) Always (b) Sometimes (c) Under special conditions (d) Never 8. Image formed by a plane mirror is (a) virtual, behind the mirror and enlarged. (b) virtual, behind the mirror and of the same size as the object. (c) real at the surface of the mirror and enlarged. (d) real, behind the mirror and of the same size as the object. 9. Describe the construction of a kaleidoscope. 10. Draw a labelled sketch of the human eye. 178 SCIENCE 2024-25  11. Gurmit wanted to perform Activity 13.8 using a laser torch. Her teacher EXERCISES advised her not to do so. Can you explain the basis of the teacher’s advise? 12. Explain how you can take care of your eyes. 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray? 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm? 15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19. Draw the reflected ray from the second mirror. Fig. 13.19 16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 13.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R? Fig. 13.20 17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 13.21). (b) Can Paheli at B see this image? (c) Can Boojho at C see this image? (d) When Paheli moves from B to C, where does the image of A move? Fig. 13.21 LIGHT 179 2024-25  Extended Learning — Activities and Project 1. Make your own mirror. Take a glass strip or glass slab. Clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In which case do you see yourself better and why? 2. Make friends with some visually impaired students. Enquire from them how they read and write. Also find out how they are able to recognise objects, hurdles and currency notes. 3. Meet an eye specialist. Get your eye sight checked and discuss how to take care of your eyes. 4. Survey your neighbourhood. Find out how many children below the age of 12 years use spectacles. Find out from their parents what, in their view, could be the reason for the weak eyesight of their children. Did You Know? Eyes can be donated by any person as an invaluable gift to visually impaired persons suffering from corneal blindness, The person may be (a) a male or female. (b) of any age. (c) of any social status. (d) using spectacles. (e) suffering from any normal disease but not AIDS, Hepatitis B or C, rabies, leukemia, lymphoma, tetanus, cholera, encephalitis. The eyes have to be donated within 4-6 hours after death at any place, home or hospital. A person who wants to donate the eyes may pledge eyes during his/her lifetime to any registered eye bank. He/she should also inform his/her relatives about this pledge so that they can take necessary action after his/her death. You can also donate a Braille kit. 180 SCIENCE 2024-25 Notes 2024-25 Notes 2024-25  SOME NATURAL PHENOMENA In this chapter we shall discuss two kind of resin) was rubbed with fur, it destructive natural phenomena. These attracted light objects such as hair. You are lightning and earthquakes. We shall might have seen that when you take also discuss what steps we can take to off woollen or polyester clothes, your hair stands on end. If you take off these minimise destruction caused by clothes in the dark, you even see a these phenomena. spark and hear a crackling sound. In 1752 Benjamin Franklin, an American 12.1 Lightning scientist, showed that lightning and the You might have seen sparks on a spark from your clothes are essentially electric pole when wires become loose. the same phenomena. However, it took This phenomenon is quite common 2000 years for this realisation to occur. when wind is blowing and shaking the wires. You might also have seen sparks when a plug is loose in its socket. I wonder why they took so Lightning is also an electric spark, but many years to realise the on a huge scale. similarity. In ancient times, people did not understand the cause of these sparks. They were, therefore, afraid of lightning and thought that the wrath of gods was Scientific discoveries are a visiting them. Now, of course, we result of hardwork by many understand that lightning is caused by people. It can sometimes take the accumulation of charges in the a long time. clouds. We need not be afraid of lightning, but we have to take We shall now study some properties precautions to protect ourselves from of electric charges. We shall also see how the deadly sparks. they are related to the lightning in the sky. The Sparks that the Greeks Knew Let us perform some activities to About understand the nature of electric The ancient Greeks knew as early as charges. But recall first what you might 600 B.C. that when amber (amber is a have played as a game. When you rub 2024-25 a plastic scale on your dry hair, the small pieces of dry leaf, husk and scale can attract very small pieces of mustard seeds. Record your paper. observations. 12.2 Charging by Rubbing When a plastic refill is rubbed with Activity 12.1 polythene, it acquires a small electric Take a used ballpen refill and rub charge. Similarly, when a plastic comb it vigorously with a piece of is rubbed with dry hair, it acquires a polythene. Bring it close to small small charge. These objects are called pieces of paper. Take care not to charged objects. In the process of touch the rubbed end of the refill charging the refill and the plastic comb, with your hand or with a metallic polythene and hair also get charged. object. Repeat the activity with Let’s try to charge some other objects that are familiar to you. Activity 12.2 Collect the objects and the materials listed in Table 12.1. Try to charge each by rubbing with the materials mentioned in the Table. Record your findings. You can add more items to the Table. Table 12.1 Objects Rubbed Materials Attracts/does not Charged/ Used for Rubbing Attract Pieces Not Charged of Paper Refill Polythene, woollen cloth Balloon Polythene, woollen cloth, dry hair Eraser Wool Steel spoon Polythene, woollen cloth SOME NATURAL PHENOMENA 151 2024-25 12.3 Types of Charges and Rub the other refill also with Their Interaction polythene. Bring it close to the We will select some objects fr om charged refill. Be careful not to Table 12.1 for the next activity. touch the charged end with your hand. Is there any effect on the refill Activity 12.3 in the tumbler? Do the two attract each other, or repel each other? (a) Inflate two balloons. Hang them In this activity we have brought in such a way that they do not touch close together the charged objects each other (Fig. 15.1). Rub both the that were made of the same balloons with a woollen cloth and material. What happens if two release them. What do you observe? charged objects made of different materials are brought close to each other? Let’s find out. (b) Rub a refill and place it gently in a glass tumbler as before (Fig. 12.3). Bring an inflated charged balloon near the refill and observe. Fig. 12.1 : Like charges repel each other Now let us repeat this activity with the used pen refills. Rub one refill with polythene. Place it carefully in a glass tumbler using the tumbler as a stand (Fig. 12.2). Fig. 12.3 : Unlike charges attract each other Let’s summarise the observations: l A charged balloon repelled a charged balloon. l A charged refill repelled a charged refill. l But a charged balloon attracted a charged refill. Does it indicate that the charge on the balloon is of a different kind from the charge on the refill? Can we say Fig. 12.2 : Interaction between like charges then, that there are two kinds of 152 SCIENCE 2024-25 charges? Can we also say that the it with the end of the paper clip. charges of the same kind repel each Observe what happens. Is there any other, while charges of different kind effect on the foil strips? Do they attract each other? repel each other or attract each It is a convention to call the charge other? Now, touch other charged acquired by a glass rod when it is bodies with the end of the paper clip. rubbed with silk as positive. The Do foil strips behave in the same other kind of charge is said to be way in all cases? Can this apparatus negative. be used to detect whether a body is It is observed that when a charged charged or not? Can you explain glass rod is brought near a charged why the foil strips repel each other? plastic straw rubbed with polythene there is attraction between the two. What do you think would be the kind of charge on the plastic straw? Your guess, that the plastic straw would carry a negative charge is correct. The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current. You have been reading about electric current since Class VI. The current in a circuit which makes a bulb glow, or the current that makes a wire hot, is nothing but a motion of charges. Fig 12.4 : A simple electroscope 12.4 Transfer of Charge The aluminium foil strips receive the Activity 12.4 same charge from the charged refill Take an empty jam bottle. Take a through the paper clip (remember that piece of cardboard slightly bigger in metals are good conductors of size than the mouth of the bottle. electricity). The strips carrying similar Pierce a hole in it so that a metal charges repel each other and they paper clip can be inserted. Open out become wide open. Such a device can the paper clip as shown in Fig. 12.4. be used to test whether an object is Cut two strips of aluminium foil carrying charge or not. This device is about 4 cm × 1 cm each. Hang them known as electroscope. on the paper clip as shown. Insert Thus, we find that electrical charge the paper clip in the cardboard lid can be transferred from a charged object so that it is perpendicular to it to another through a metal conductor. (Fig. 12.4). Charge a refill and touch Touch the end of the paper clip gently with hand and you will find a change in SOME NATURAL PHENOMENA 153 2024-25 the foil strips. They come back to their During the development of a original state. Repeat charging of foil thunderstorm, the air currents move strips and touching the paper clip. Every upward while the water droplets move time you will find that the foil strips downward. These vigorous movements collapse as soon as you touch the cause separation of charges. By a paperclip with hand. Why does it process, not yet completely understood, happen? The reason is that the foil strips the positive charges collect near the lose charge to the earth through your upper edges of the clouds and the body. We say that the foil strips are discharged. The process of transferring negative charges accumulate near the of charge from a charged object to the lower edges. There is accumulation of earth is called earthing. positive charges near the ground also. When the magnitude of the Earthing is provided in buildings to accumulated charges becomes very protect us from electrical shocks due large, the air which is normally a poor to any leakage of electrical current. conductor of electricity, is no longer able to resist their flow. Negative and positive 12.5 The Story of Lightning charges meet, producing streaks of bright light and sound. We see streaks It is now possible to explain lightning as lightning (Fig. 12.5). The process is in terms of the charges produced by rubbing. called an electric discharge. Fig. 12.5 : Accumulation of charges leading to lightning. 154 SCIENCE 2024-25  The process of electric discharge can metal objects. Do not lie on the ground. occur between two or more clouds, or Instead, squat low on the ground. Place between clouds and the earth. Today we your hands on your knees with your need not get frightened by lightning like head between the hands (Fig. 12.6). This our ancestors did. Now we understand position will make you the smallest the basic phenomenon. Scientists are target to be struck. trying hard to improve this understanding. However, lightning strike could destroy life and property. It is, therefore, necessary to take measures to protect ourselves. 12.6 Lightning Safety During lightning and thunderstorm no open place is safe. l Hearing thunder is an alert to rush to a safer place. l After hearing the last thunder, wait for some time before coming out of the safe place. Finding a Safe Place A house or a building is a safe place. If you are travelling by car or by bus, Fig. 12.6 : Safe position during lightning you are safe inside with windows and doors of the vehicle shut. Inside the house Lightning can strike telephone cords, Do’s and Don’ts during a electrical wires and metal pipes (Do you Thunderstorm remember, lightning is an electrical Outside discharge?). During a thunderstorm Open vehicles, like motorbikes, tractors, contact with these should be avoided. It construction machinery, open cars are is safer to use mobile phones and not safe. Open fields, tall trees, shelters cordless phones. However, it is not wise in parks, elevated places do not protect to call up a person who is receiving your us from lightning strokes. phone through a wired phone. Carrying an umbrella is not at all a Bathing should be avoided during good idea during thunderstorms. thunderstorms to avoid contact with If in a forest, take shelter under running water. shorter trees. Electrical appliances like computers, If no shelter is available and you are TVs, etc., should be unplugged. Electrical in an open field, stay far away from all lights can remain on. They do not cause trees. Stay away from poles or other any harm. SOME NATURAL PHENOMENA 155 2024-25 Lightning Conductors 12.7 Earthquakes Lightning Conductor is a device used You just learnt about thunderstorm and to protect buildings from the effect of lightning. These natural phenomena can lightning. A metallic rod, taller than the cause large scale destruction of human building, is installed in the walls of the life and property. Fortunately, these building during its construction. One phenomena can be predicted to some end of the rod is kept out in the air and extent. The weather department can the other is buried deep in the ground warn about a thunderstorm developing (Fig. 12.7). The rod provides easy route in some area. for the transfer of electric charge to the If a thunderstorm occurs there is ground. always a possibility of lightning and The metal columns used during cyclones accompanying it. So, we get construction, electrical wires and water time to take measures to protect pipes in the buildings also protect us to ourselves from the damage caused by an extent. But do not touch them during these phenomena. a thunderstorm. lightning conductor copper plate Fig. 12.7 : Lightning conductor 156 SCIENCE 2024-25  There is, however, one natural caused by these earthquakes from phenomenon which we are not yet able newspapers and magazines of those to predict accurately. It is an days. Prepare a short report on the earthquake. It can cause damage to suffering of the people during and human life and property on a huge scale. after the earthquakes. A major earthquake occurred in India on 8 October 2005 in Uri and Tangdhar towns of North Kashmir What is an earthquake? What (Fig. 12.8). Before that a major happens when it occurs? What can we earthquake occurred on 26 January do to minimise its effects? These are 2001 in Bhuj district of Gujarat. some of the questions which we shall discuss below. Activity 12.5 What is an Earthquake? Ask your parents about the huge An earthquake is a sudden shaking or damage to life and property caused trembling of the earth which lasts for a by these earthquakes. Collect a few very short time. It is caused by a pictures showing the damage disturbance deep inside the earth’s Fig. 12.8 : Kashmir earthquake SOME NATURAL PHENOMENA 157 2024-25 crust. Earthquakes occur all the time, all over the earth. They are not even What could cause a noticed. Major earthquakes are much disturbance inside less frequent. They can cause immense the earth? damage to buildings, bridges, dams and people. There can be a great loss to life Now we know that the tremors are and property. Earthquakes can cause caused by the disturbance deep down floods, landslides and tsunamis. A inside the uppermost layer of the earth major tsunami occurred in the Indian called the crust (Fig. 12.9). Ocean on 26 December 2004. All the coastal areas around the ocean suffered huge losses. crust Activity 12.6 mantle Take an outline map of the world. Locate the eastern coast and outer core Andaman and Nicobar Islands in India. Mark other countries around inner the Indian Ocean which could have core suffered damage. Collect accounts of the devastation caused by the tsunami in India from your parents, Fig. 12.9 : Structure of the earth or other elders in the family or in the neighbourhood. The outermost layer of the earth is not in one piece. It is fragmented. Each What Causes an Earthquake? fragment is called a plate (Fig. 12.10). These plates are in continual motion. When they brush past one another, My grandmother told me that the earth is balanced on the horn of a bull and when the bull shifts it to the other horn, an earthquake takes place. How could it be true? Eurasian In ancient times, people did not know Plate the true cause of earthquakes. Their Arabian ideas were, therefore, expressed in Plate Indan mythical stories such as the one told by Plate Boojho’s grandmother. Similar myths were prevalent in other parts of the world. Fig. 12.10 : Earth plates 158 SCIENCE 2024-25 or a plate goes under another due to carried out. However, most earthquakes collision (Fig. 12.11), they cause are caused by the movement of earth’s disturbance in the earth’s crust. It is plates. this disturbance that shows up as an Since earthquakes are caused by the earthquake on the surface of the earth. movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones. In India, the Colliding areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo–Gangetic Plane. Some areas of South India also fall in Brushing the danger zone (Fig. 12.12). past Fig. 12.11 : Movements of earth’s plates If scientists know so much about earthquakes, can they also predict when and where the next one will strike? Although, we know for sure what causes an earthquake, it is not yet possible to predict when and where the next earthquake might occur. Fig. 12.12 : Movements of Indian earth’s plate I read somewhere that underground explosions The power of an earthquake is could also cause expressed in terms of a magnitude tremors. on a scale called the Richter scale. Really destructive earthquakes have Tremors on the earth can also be magnitudes higher than 7 on the caused when a volcano erupts, or a Richter scale. Both Bhuj and Kashmir meteor hits the earth, or an earthquakes had magnitudes greater underground nuclear explosion is than 7.5. SOME NATURAL PHENOMENA 159 2024-25  The tremors produce waves on the surface of the earth. These are called seismic waves. The waves are recorded by an instrument called the seismograph (Fig. 12.13). The instrument is simply a vibrating rod, or a pendulum, which starts vibrating when tremors occur. A pen is attached to the vibrating system. The pen records the seismic waves on a paper which moves under it. By studying these waves, scientists can construct a complete map of the earthquake, as shown in Fig. 12.14. They can also estimate its power to cause destruction. String Earth’s Focus Surface Seismic waves Support Pendulum Bob Epicentre Pen Rotating Drum Chart Paper Inside the earth A typical seismograph record Fig. 12.13 : A seismograph Fig. 12.14 : Map of the earthquake Like many other scales in science (decibel is another example), Richter scale is not linear. This means that an earthquake of magnitude 6 does not have one and half times the destructive energy of an earthquake of magnitude 4. In fact, an increase of 2 in magnitude means 1000 times more destructive energy. Therefore, an earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4. Protection against Earthquakes living in seismic zones, where the We know from the earlier pages that earthquakes are more likely to occur, earthquakes cannot be predicted. We have to be specially prepared. First of have also seen that they can be highly all, the buildings in these zones should destructive. It is, therefore, important be so designed that they can withstand that we take necessary precautions to major tremors. Modern building protect ourselves all the time. People technology can make it possible. 160 SCIENCE 2024-25  It is advisable to make the structure The Central Building Research simple so that it is ‘Quake Safe’. Institute, Roorkee, has developed knowhow to make quake-resistant l Consult qualified architects and houses. structural engineers. In the event that an earthquake does l In highly seismic areas, the use of strike, take the following steps to protect mud or timber is better than using yourself. heavy construction material. Keep roofs as light as possible. In case 1. If you are at home the structure falls, the damage will l Take shelter under a table and stay not be heavy. there till the shaking stops. l It is better if the cupboards and l Stay away from tall and heavy objects shelves are fixed to the walls, so that that may fall on you. they do not fall easily. l If you are in bed, do not get up. l Be careful where you hang wall Protect your head with a pillow. clocks, photo-frames, water heaters etc., so that in the event of an 2. If you are outdoors earthquake, they do not fall on l Find a clear spot, away from people. buildings, trees and overhead power l Since some buildings may catch fire lines. Drop to the ground. due to an earthquake, it is necessary l If you are in a car or a bus, do not that all buildings, especially tall come out. Ask the driver to drive buildings, have fire fighting slowly to a clear spot. Do not come equipment in working order. out till the tremors stop. SOME NATURAL PHENOMENA 161 2024-25 KEYWORDS WHAT YOU HAVE LEARNT CRUST Ü Some objects can be charged by rubbing with other objects. DISCHARGE Ü There are two kinds of charges — positive EARTH’S PLATES charge and negative charge EARTHQUAKE Ü Like charges repel and unlike charges attract each other. ELECTROSCOPE Ü The electrical charges produced by rubbing LIGHTNING are called static charges. Ü When charges move, they constitute an LIGHTNING electric current. CONDUCTOR Ü An electroscope may be used to detect NEGATIVE CHARGE whether a body is charged or not. Ü The process of transfer of charge from a POSITIVE CHARGE charged object to the earth is called earthing. RICHTER SCALE Ü The process of electric discharge between clouds and the earth or between different SEISMOGRAPH clouds causes lightning. THUNDER Ü Lightning strike could destroy life and property. THUNDERSTORM Ü Lightning conductors can protect buildings from the effects of lightning. TRANSFER OF Ü An earthquake is a sudden shaking or CHARGE trembling of the earth. TSUNAMI Ü Earthquake is caused by a disturbance deep inside the earth’s crust. TREMOR Ü It is not possible to predict the occurrence of an earthquake. Ü Earthquakes tend to occur at the boundaries of earth’s plates. These boundaries are known as fault zones. Ü Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property. Ü We should take necessary precautions to protect ourselves from earthquakes. 162 SCIENCE 2024-25  Exercises Select the correct option in Questions 1 and 2. 1. Which of the following cannot be charged easily by friction? (a) A plastic scale (b) A copper rod (c) An inflated balloon (d) A woollen cloth. 2. When a glass rod is rubbed with a piece of silk cloth the rod (a) and the cloth both acquire positive charge. (b) becomes positively charged while the cloth has a negative charge. (c) and the cloth both acquire negative charge. (d) becomes negatively charged while the cloth has a positive charge. 3. Write T against true and F against false in the following statements. (a) Like charges attract each other (T/F) (b) A charged glass rod attract a charged plastic straw (T/F) (c) Lightning conductor cannot protect a building from lightning (T/F) (d) Earthquakes can be predicted in advance (T/F) 4. Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain. 5. Explain why a charged body loses its charge if we touch it with our hand. 6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage? 7. Suggest three measures to protect ourselves from lightning. 8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon? 9. Describe with the help of a diagram an instrument which can be used to detect a charged body. 10. List three states in India where earthquakes are more likely to strike. 11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself? 12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain. SOME NATURAL PHENOMENA 163 2024-25  Extended Learning — Activities and Projects 1. Open a water tap. Adjust the flow so that it forms a thin stream. Charge a refill. Bring it near the water stream. Observe what happens. Write a short report on the activity. 2. Make your own charge detector. Take a paper strip roughly 10 cm × 3 cm. Give it a shape as shown in Fig. 12.15. Balance it on a needle. Bring a charged body near it. Observe what happens. Write a brief report, explaining its working. Paper 10 cm × 3 cm Fold along the dotted line Cut along the dotted line Needle ThermoCole Fig. 12.15 3. This activity should be performed at night. Go to a room where there is a fluorescent tube light. Charge a balloon. Switch off the tube light so that the room is completely dark. Bring the charged balloon near the tubelight. You should see a faint glow. Move the balloon along the length of the tube and observe how the glow changes. Caution : Do not touch the metal parts of the tube or the wires connecting the tube with the mains. 4. Find out if there is an organisation in your area which provides relief to those suffering from natural disaster. Enquire about the type of help they render to the victims of earthquakes. Prepare a brief report on the problems of the earthquake victims. For more information on these topics, visit: l science.howstuffworks.com/lightning.htm l science.howstuffworks.com/earthquake.htm 164 SCIENCE 2024-25  CHEMICAL EFFECTS OF ELECTRIC CURRENT Y our elders might have cautioned plastic and wood do not conduct you against touching an electrical electricity. However, so far we have used appliance with wet hands. But our tester to test materials which were do you know why it is dangerous to in solid state. But what about liquids? touch an electrical appliance with wet Do liquids also conduct electricity? Let hands? us find out. We have learnt earlier that the materials, which allow electric current to pass through them, are good Paheli and Boojho want to conductors of electricity. On the other remind you that one hand, materials, which do not allow should not experiment electric current to pass through them with the electric supply easily, are poor conductors of electricity. from the mains or a In Class VI, we made a tester to test generator or an inverter. whether a particular material allows the Use only electric cells for electric current to pass through it or not. all the activities suggested here. Do you recall how the tester helped us in deciding that? We found that metals such as copper 11.1 Do Liquids Conduct and aluminium conduct electricity Electricity? whereas materials such as rubber, To test whether a liquid allows electric current to pass through it or not, we can use the same tester (Fig.11.1). Fig.11.1 : A tester 2024-25 However, replace the cell by a battery. of the tester glow? Does lemon juice Also, before using the tester we or vinegar conduct electricity? How should check whether it is working would you classify lemon juice or or not. vinegar— a good conductor or a poor conductor? Activity 11.1 Join the free ends of the tester together for a moment. This completes the circuit of the tester and the bulb should glow. However, if the bulb does not glow, it means that the tester is not working. Can you think of the possible reasons? Is it possible that the connections are loose? Or, the bulb is fused? Or, your cells are used up? Check that all the connections are tight. If they are, then replace the bulb with another bulb. Now test if the tester is working or not. If it is still not Fig. 11.2 : Testing conduction of electricity in working then replace the cells with lemon juice or vinegar fresh cells. When the liquid between the two Now that our tester is working, let ends of the tester allows the electric us use it to test the various liquids. current to pass, the circuit of the tester (Caution: While checking your tester, becomes complete. The current flows in the circuit and the bulb glows. When do not join its free ends for more than a the liquid does not allow the electric few seconds. Otherwise the cells of the current to pass, the circuit of the tester battery will drain very quickly.) is not complete and the bulb does not glow. Activity 11.2 In some situations even though the Collect a few small plastic or rubber liquid is conducting, the bulb may not caps of discarded bottles and clean g l o w. It may have happened in them. Pour one teaspoon of lemon Activity 11.2. What can be the reason? juice or vinegar in one cap. Bring Do you remember why the bulb your tester over this cap and let the glows when the electric current passes ends of the tester dip into lemon juice through it? Due to the heating effect or vinegar as shown in Fig.11.2. Take of current, the filament of the bulb gets care that the ends are not more than heated to a high temperature and it 1 cm apart but at the same time do starts glowing. However, if the current not touch each other. Does the bulb through a circuit is too weak, the filament does not get heated CHEMICAL EFFECTS OF ELECTRIC CURRENT 139 2024-25 sufficiently and it does not glow. And Activity 11.3 why is the current in the circuit weak? Well, though a material may conduct Take the tray from inside a discarded electricity, it may not conduct it as matchbox. Wrap an electric wire a few easily as a metal. As a result, the times around the tray. Place a small circuit of the tester may be complete compass needle inside it. Now and yet the current through it may be connect one free end of the wire to too weak to make the bulb glow. Can the terminal of a battery. Leave the we make another tester which can other end free. Take another piece of detect a weak current? wire and connect it to the other terminal of the battery (Fig. 11.4). We can use another effect of an electric current to make another kind of tester. Do you recall that electric current produces a magnetic effect? What happens to a compass needle kept nearby when current flows in a wire? Even if the current is small, the deflection of the magnetic needle can be seen. Can we make a tester using the magnetic effect of currents? Let us find out in Activity 11.3. You may use an LED (Fig. 11.3) in Fig 11.4 : Another tester place of the electric bulb in the tester of Fig. 11.2. LED glows even when a Join the free ends of two wires weak electric current flows through momentarily. The compass needle it. should show deflection. Your tester There are two wires (called leads) with two free ends of the wire is ready. attached to an LED. One lead is Now repeat Activity 11.2 using this slightly longer than the other. tester. Do you find a deflection in the Remember that while connecting to compass needle the moment you dip a circuit, the longer lead is always the free ends of the tester in lemon connected to the positive terminal of juice? the battery and the shorter lead is Take out the ends of the tester connected to the negative terminal of the battery. from the lemon juice, dip them in water and then wipe them dry. Repeat the activity with other liquids such as tap water, vegetable oil, milk, honey. (Remember to wash and wipe dry the ends of tester after testing each liquid). In each case observe whether the magnetic needle shows deflection or not. Record your Fig. 11.3 : LEDs observations in Table 11.1. 140 SCIENCE 2024-25  Table 11.1 : Good/Poor Conducting Liquids S.No. Material Compass Needle Shows Good Conductor/ Deflection Yes/No Poor Conductor 1. Lemon juice Yes Good Conductor 2. Vinegar 3. Tap Water 4. Vegetable oil 5. Milk 6. Honey 7. 8. 9. 10. From Table 11.1, we find that some We have tested the conduction of liquids are good conductors of electricity electricity through tap water. Let us now and some are poor conductors. test the conduction of electricity through distilled water. When the free ends of the Activity 11.4 tester do not touch each Take about two teaspoonfuls of other, there is an air gap distilled water in a clean and dry between them. Paheli knows plastic or rubber cap of a bottle. (You that air is a poor conductor of may obtain distilled water from your electricity. But she has also read that during lightning, an electric school science lab. You may also get current passes through air. She distilled water from a medical store wonders if air is indeed a poor or a doctor or a nurse). Use the tester conductor under all conditions. to test whether distilled water This makes Boojho ask whether conducts electricity or not. What do other materials classified as you find? Does distilled water poor conductors also allow conduct electricity? Now dissolve a electricity to pass under pinch of common salt in distilled certain conditions. water. Again test. What do you conclude this time? Actually, under certain conditions When salt is dissolved in distilled most materials can conduct. That is why water, we obtain salt solution. This is a it is preferable to classify materials as conductor of electricity. good conductors and poor conductors The water that we get from sources instead of classifying as conductors and such as taps, hand pumps, wells and insulators. ponds is not pure. It may contain CHEMICAL EFFECTS OF ELECTRIC CURRENT 141 2024-25 several salts dissolved in it. Small Most liquids that conduct electricity amounts of mineral salts are naturally are solutions of acids, bases and salts. present in it. This water is thus a good When an electric current flows conductor of electricity. On the other through a conducting solution, does it hand, distilled water is free of salts and produce an effect on the solution? is a poor conductor. 11.2 Chemical Effects of Electric Current Small amounts of mineral salts present naturally in water are In Class VII, we have learnt some effects beneficial for human health. of electric current. Can you list these However, these salts make water effects? What effect does the current a good conductor. So, we should produce when it flows through a never handle electrical conducting solution? Let us find out. appliances with wet hands or while standing on a wet floor. Activity 11.6 We have found that common salt, Take out carbon rods carefully from when dissolved in distilled water, makes two discarded cells. Clean their it a good conductor. What are the other metal caps with sand paper. Wrap substances which, when dissolved in copper wires around the metal caps distilled water, make it conducting? Let us find out. of the carbon rods and join them to a battery (Fig. 11.5). We call these Caution: Do the next activity under the two rods electrodes. (Instead of supervision of your teacher/parent or some elderly person, because the use of acid is involved in it. Activity 11.5 Take three clean plastic or rubber caps of bottles. Pour about two teaspoonfuls of distilled water in each of them. Add a few drops of lemon juice or dilute hydrochloric acid to distilled water in one cap. Now in the second cap containing distilled water, add a few Metal Cap drops of a base such as caustic soda or potassium iodide. Add a little sugar Carbon Carbon to the distilled water in the third cap rod rod and dissolve it. Test which solutions conduct electricity and which do not. Water What results do you obtain? Fig.11.5 : Passing current through water 142 SCIENCE 2024-25  carbon rods, you may take two iron nails about 6 cm long.) Pour a cupful of water in a glass/plastic Boojho decided to test bowl. Add a teaspoonful of salt or a whether some fruits and vegetables also conduct few drops of lemon juice to water to electricity or not. He cut make it more conducting. Now a potato into two halves and immerse the electrodes in this inserted the copper wires of a tester solution. Make sure that the metal into it. Just then his mother called caps of the carbon rods are outside him and he forgot to take out the the water. Wait for 3-4 minutes. wires of the tester inserted into the Observe the electrodes carefully. Do potato. When he came back after you notice any gas bubbles near the half an hour, he noticed that there was a greenish blue spot on the electrodes? Can we call the change potato around one wire whereas taking place in the solution a there was no such spot around the chemical change? Recall the other wire (Fig. 11.6). definition of a chemical change that you learnt in Class VII. In 1800, a British chemist, William Nicholson (1753–1815), had shown that if electrodes were immersed in water, and a current was passed, bubbles of oxygen and hydrogen were produced. Oxygen bubbles formed on the electrode Fig. 11.6 : Testing potato connected to the positive terminal of He was surprised with this the battery and hydrogen bubbles observation and along with Paheli formed on the other electrode. repeated this activity many times. They The passage of an electric current found that it was always the wire through a conducting solution causes connected to the positive terminal, which had a greenish blue spot around chemical reactions. As a result, bubbles it. They felt that this discovery was very of a gas may be formed on the electrodes. useful because it could be used for Deposits of metal may be seen on identifying the positive terminal of a electrodes. Changes of colour of cell or a battery concealed in a box. solutions may occur. The reaction would They decided to report their finding to depend on what solution and electrodes a children’s magazine. are used. These are some of the chemical Remember that Boojho set out to effects of the electric current. test whether potato conducted CHEMICAL EFFECTS OF ELECTRIC CURRENT 143 2024-25  electricity or not. What he found was that current produced a chemical effect in the potato. To him this was very exciting. In fact, this is how science sometimes works. You are looking for something and you discover something else. Many Copper plate important discoveries have been made in this manner. Copper plate Copper sul- 11.3 Electroplating phate solution You know that a brand new bicycle has shiny handlebar and wheel rims. Fig.11.7 : A simple circuit showing However, if these are accidentally electroplating scratched, the shiny coating comes off