Chapter 12 Some Natural Phenomena - PDF

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 w...

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