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Class VIII Science, Chapter 15 , SOME NATURAL PHENOMENA, Topic:-, Spark & Nature Of Electric Charges, , Prepared by :-Vijay Kumar Sethi
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Electric Spark, Sparks (चिंगारी) on a electric pole when wires become loose., This phenomenon is quite common when wind is blowing and shaking the wires., Sparks can also be seen when a plug is loose in its socket., Lightning is also an electric spark, but on a huge scale., 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 (ईश्वरीय प्रकोप) was visiting them., Now, we understand that lightning is caused by the accumulation of charges in the clouds. , We need not be afraid of lightning.
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Examples of Sparks, The ancient Greeks knew that when amber (amber is a kind of resin) was rubbed with fur, it attracted light objects such as hair., when you take off woollen or polyester clothes, your hair stands on end. , If you take off these clothes in the dark, you even see a spark and hear a crackling sound.(कर्कश ध्वनि) , In 1752 Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomena., Some trees produce sticky substance , called resin
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When you rub a comb on your dry hair, the comb can attract very small pieces of paper., Activities To Understand The Nature Of Electric Charges, Take a used ballpen refill and rub it vigorously with a piece of polythene. , Bring it close to small pieces of paper. The small pieces of paper attracts towards refill., When a plastic refill is rubbed with polythene, it acquires a small electric charge. , Similarly, when a plastic comb is rubbed with dry hair, it acquires a small charge. , These objects are called charged objects. , In the process of charging the refill and the plastic comb, polythene and hair also get charged.
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Activity, Through this activity, it can be concluded that certain objects acquire charge on being rubbed with certain materials. An object possessing charge will attract bits of paper.
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Types of Charges and Their Interaction, Activity, Inflate two balloons., Hang them in such a way that they do not touch each other (Fig.)., Rub both the balloons with a woollen cloth and release them., Observation:- Balloons repel each other
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Activity, Now take two used pen refills. , Rub one refill with polythene. , Place it carefully in a glass tumbler using the tumbler as a stand(Fig), Rub the other refill also with polythene. , Bring it close to the charged refill., Observation:- refills repel each other., In these activities we have brought close together the charged objects that were made of the same material.
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Activity, Rub a refill and place it gently in a glass tumbler (Fig.), Bring an inflated charged balloon near the refill., Observation:- there is attraction between refill and balloon, Let’s summarise the observations:, A charged balloon repelled a charged balloon., A charged refill repelled a charged refill., But a charged balloon attracted a charged refill.
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Conclusion:, It indicates that the charge on the balloon is of a different kind from the charge on the refill., We can say that there are two kinds of charges., The charges of the same kind repel each other, while charges of different kind attract each other., It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive.
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The other kind of charge is said to be negative., When a charged glass rod is brought near a charged plastic straw rubbed with polythene there is attraction between the two., So we can say that the plastic straw or rod rubbed with polythene carries a negative charge., The electrical charges generated by rubbing are static. , They do not move by themselves., When charges move, they constitute an electric current. , 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.
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Transfer of Charge, Activity, Take an empty jam bottle. , Take a piece of cardboard slightly bigger in size than the mouth of the bottle., Pierce a hole in it so that a metal paper clip can be inserted. , Open out the paper clip as shown in Fig., Cut two strips of aluminium foil and Hang them on the paper clip as shown in fig.. , Insert the paper clip in the cardboard lid so that it is perpendicular to it. , Charge a refill and touch it with the end of the paper clip., Observation:- foil strips repel each other. , Now, touch the end of the paper clip with hand. Foil strips come in original position.
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The aluminium foil strips receive the same charge from the charged refill through the paper clip (metals are good conductors of electricity). , The strips carrying similar charges repel each other and they become wide open. , A device that can be used to test whether an object is carrying charge or not is known as electroscope., Thus, electrical charge can be transferred from a charged object to another through a metal conductor., On touching with hands, foil strips lose charge to the earth through your body. We say that the foil strips are discharged come in original position, The process of transferring of charge from a charged object to the earth is called earthing., Continue… Transfer of Charge, Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current.
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Lightning, Lightning can be explained in terms of the charges produced by rubbing., During thunderstorm, the air currents move upward while the water droplets move downward. , These vigorous movements cause separation of charges., The positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. , There is accumulation of positive charges near the ground also.
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When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. , Negative and positive charges meet, producing streaks of bright light and sound. , We see streaks (लंबी व पतली लकीर या धारियाँ) as lightning . , The process is called an electric discharge., Continue…, Lightning, The process of electric discharge can occur between two or more clouds, or between clouds and the earth. , Lightning strike can destroy life and property., It is, therefore, necessary to take measures to protect ourselves.
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Lightning Safety, During lightning and thunderstorm no open place is safe., Hearing thunder is an alert to rush to a safer place., 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, you are safe inside with windows and doors of the vehicle shut.
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Do’s and Don’ts during a Thunderstorm, Outside, Open vehicles, like motorbikes, tractors, construction machinery, open cars are not safe. , Open fields, tall trees, shelters in parks, elevated (ऊंचा, high) places do not protect us from lightning strokes., Do not take an umbrella during thunderstorms., If in a forest, take shelter under shorter trees., If no shelter is available and you are in an open field, stay far away from all trees. , Stay away from poles or other metal objects., Do not lie on the ground. , Instead, squat (उकडूँ बैठना) low on the ground. , Place your hands on your knees with your head between the hands . This position will make you the smallest target to be struck.
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Continue…, Do’s and Don’ts during a Thunderstorm, Inside the house, Lightning can strike telephone cords, electrical wires and metal pipes (lightning is an electrical discharge). During a thunderstorm contact with these should be avoided. , It is safer to use mobile phones and cordless phones. , Do not call up a person who is receiving your phone through a wired phone., Bathing should be avoided during thunderstorms to avoid contact with running water., Electrical appliances like computers, TVs, etc., should be unplugged. , Electrical lights can remain on. They do not cause any harm.
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Lightning Conductors, Lightning Conductor is a device used to protect buildings from the effect of lightning. , A metallic rod, taller than the building, is installed in the walls of the building during its construction. , One end of the rod is kept out in the air and the other is buried deep in the ground (Fig.). , The rod provides easy route for the transfer of electric charge to the ground., The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.
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Earthquakes, Thunderstorm (with lightning) and cyclones can be predicted to some extent. , So, we get time to take measures to protect ourselves from the damage caused by these phenomena., There is one natural phenomenon which we are not yet able to predict accurately. , It is an earthquake. , It can cause damage to human life and property on a huge scale., A major earthquake occurred in India on 8 October 2005 in Uri and Tangdhar towns of North Kashmir . , Before that a major earthquake occurred on 26 January 2001 in Bhuj district of Gujarat.
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Kashmir Earthquake, What is an earthquake? , What happens when it occurs? , What can we do to minimise its effects?
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What is an Earthquake?, An earthquake is a sudden shaking or trembling (vibration, काँपना) of the earth which lasts for a very short time. , It is caused by a disturbance deep inside the earth’s crust. , Major earthquake can cause immense(अत्यधिक) damage to buildings, bridges, dams and people. , There can be a great loss to life and property. , Earthquakes can cause floods, landslides and tsunamis. , A major tsunami occurred in the Indian Ocean on 26 December 2004. , All the coastal areas around the ocean suffered huge losses.
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What Causes an Earthquake?, In ancient times, people did not know the true cause of earthquakes. , Their ideas were expressed in mythical (कल्पित)stories. , Like earth is balanced on the horn of a bull and when the bull shifts it to the other horn, an earthquake takes place., But actually tremors (भूकंप के झटके) are caused by the disturbance deep down inside the uppermost layer of the earth called the crust.
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The outermost layer of the earth is not in one piece., It is fragmented. Each fragment is called a plate (Fig.)., These plates are in continual (लगातार) motion., When they brush past (हल्के से छूना) one another or a plate goes under another due to collision (Fig.), they cause disturbance in the earth’s crust., It is this disturbance that shows up (arrive पहुँचना) as an earthquake on the surface of the earth., Continue… What Causes an Earthquake?
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Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion is carried out. , However, most earthquakes are caused by the movement of earth’s plates., Since earthquakes are caused by the 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 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 the danger zone, Continue… What Causes an Earthquake?, A meteor is a smaller body in space that collides with the Earth. It's also another word for a shooting star.
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The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale., Really destructive earthquakes have magnitudes higher than 7 on the Richter scale., Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5., 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
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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., Seismograph
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By studying these waves, scientists can construct a complete map of the earthquake., They can also estimate its power to cause destruction, Map Of The Earthquake, An increase of 2 in magnitude on Richter scale means 1000 times more destructive energy., Therefore, an earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4.
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Protection against Earthquakes, The buildings in seismic zones should be so designed that they can withstand major tremors., It is advisable to make the structure simple so that it is ‘Quake Safe’., In highly seismic areas, the use of mud or timber is better than using heavy construction material. , Keep roofs as light as possible. In case the structure falls, the damage will not be heavy., It is better if the cupboards and shelves are fixed to the walls, so that they do not fall easily., Be careful where you hang wall clocks, photo-frames, water heaters etc., so that in the event of an earthquake, they do not fall on people.
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Continue…, Protection against Earthquakes, Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have fire fighting equipment in working order. , The Central Building Research Institute, Roorkee, has developed know how to make quake-resistant houses.
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In the event that an earthquake does strike, take the following steps to protect yourself:-, If you are at home, Take shelter under a table and stay there till the shaking stops., Stay away from tall and heavy objects that may fall on you., If you are in bed, do not get up. Protect your head with a pillow., If you are outdoors, Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground., If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.
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Class VIII Science, Chapter 15 , SOME NATURAL PHENOMENA, Topic:-, Answers of Exercise Questions, , Prepared by :-Vijay Kumar Sethi
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Q1 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., Answer. (b) A copper rod., Q2 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., Answer. (b) The rod becomes positively charged, while the cloth has a negative charge., Only non-conducting materials can be easily charged by friction. Copper is a highly conducting materials. Therefore, a copper rod cannot be charged easily by friction.
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Q3 Write T against true and F against false in the following statements., (a) Like charges attract each other (T/F) Answer:-F, (b) A charged glass rod attract a charged plastic straw (T/F) Answer:-T, (c) Lightning conductor cannot protect a building from lightning (T/F) Answer:- F, (d) Earthquakes can be predicted in advance (T/F) Answer:- F, , Q4 Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain?, Answer:- When a sweater is taken off, the woollen sweater gets charged because of the friction, between the sweater and the body. Hence, one can hear a crackling sound during the given, process.
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Q5 Explain why a charged body loses its charge if we touch it with our hand., Answer. Human body is a conductor of electricity. When a charged body is touched with hand, our body conducts its charges to the earth. Hence, by this way charged body loses its charge. This phenomenon is known as electric discharge., , Q6 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?, Answer. The destructive energy of an earthquake is measured by the Richter scale. , Yes, it would be recorded by seismograph because this scale has the readings from 1 to 10. , No it is not likely to cause much damage as earthquakes of magnitude higher than 5 is considered destructive in nature.
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Q7 Suggest three measures to protect ourselves from lightning., Answer:-, (i) Stay in a completely closed place. If you are moving in a car, then remain there until the lightning is over. Close the windows of the car immediately., (ii) Do not touch any electrical wires, telephone cables, metal pipes, etc., (iii) Do not bath in running water. This may cause an electric shock., Q8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon? Answer: A charged balloon is repelled by another charged balloon because both carry same type of charges. When a charged body is brought near an uncharged body, the uncharged body acquires charges which is opposite with respect to charged body. , So an uncharged balloon is attracted by another charged balloon as they have opposite charges. We know that same charges repel and opposite charges attract each other.
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Q9 Describe with the help of a diagram an instrument which can be used to detect a charged body., Answer. An electroscope can be used to detect whether a body is charged or not. The following, figure shows a simple electroscope., It consists of a metal rod or wire. , At one end of the rod, two strips of aluminium foil are fixed. , Another end is inserted in a disc or cardboard. , The end which has aluminium foil is hanged in a glass container (fig.), When the outer end of metal rod is touched with a charged body, the aluminium strips move away from each other because the aluminium foil strips receive the same charge from the charged body so repels each other, When the Aluminium foil strips repel each other proves that the body is charged because repulsion is the sure test to detect that body is charged or not through an electroscope.
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Q10 List three states in India where earthquakes are more likely to strike., Answer:- The three states in India where earthquakes are more likely to strike are Jammu and Kashmir, Gujrat, Rajasthan and Assam., Q11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself? Answer: We will take the following precautions to protect ourselves: (i) Move to open space away from buildings, trees and overhead power lines. Drop to the ground. (ii) If we are in a car or a bus, will not come out. Drive slowly to a clear spot and remain in it till the earthquake stops.
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Q12. 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. Answer: No, I would not carry an umbrella at all. Because during thunderstorms, which are, accompanied with lightning, electric discharge from the clouds can travel through the metallic rod of the umbrella. This may give an electric shock to me., , , , THE END
