Thursday, 18 April 2013

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ch15 sci class8

Select the correct option in Questions 1 and 2.
Question 1. Which of the following cannot be charged easily by friction?

  1. (a) A plastic scale
  2. (b) A copper rod
  3. (c) An inflated balloon
  4. (d) A woolen cloth.
Answer : (b) A copper rod

Question 2. When a glass rod is rubbed with a piece of silk cloth the rod
  1. (a) and the cloth both acquire positive charge.
  2. (b) becomes positively charged while the cloth has a negative charge.
  3. (c) and the cloth both acquire negative charge.
  4. (d) becomes negatively charged while the cloth has a positive charge.
Answer : (b) becomes positively charged while the cloth has a negative charge. 

Question 3. Write T against true and F against false in the following statements:
  1. (a) Like charges attract each other (T/F)
  2. (b) A charged glass rod attract a charged plastic straw (T/F)
  3. (c) Lightning conductor cannot protect a building from lightning (T/F)
  4. (d) Earthquakes can be predicted in advance (T/F)
Answer : 
  1. (a) Like charges attract each other (F)
  2. (b) A charged glass rod attract a charged plastic straw (T)
  3. (c) Lightning conductor cannot protect a building from lightning (F)
  4. (d) Earthquakes can be predicted in advance (F)
Question 4. Sometime, a crackling sound is heard while taking off sweater during winters. Explain.

Answer : We know th at some objects can be charged by rubbing with other objects.The electrical charges produced by rubbing are called static charges. A woolen sweater while taking off comes in contact with the surface of inner clothes like shirts etc which are either made of cotton or synthetic material. Due to rubbing , or friction between the surfaces of cloth materials, where inner cloth lose some charge in the form of lose electrons, and the sweeter acquires excess of static charge. This excess of static charge, accumulated in sweater, when it comes near or in contact with body, gets discharged to either back or to ground through body, producing electric sparks with crackling sound. 

Question 5. Explain why a charged body loses its charge if we touch it with our hand.

Answer : When we touch a charged body, with our hand, the excess of accumulated charge or static charges on it, gets transfer to ground through our body. Thus the charged body loses its charge, and becomes neutral 


Question 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?

Answer : Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 3 on Richter scale, may be recorded by a seismograph and it is not likely to cause much damage as it will result in  mild tremors of very low intensity on the earth surface. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property.


Question 7. Suggest three measures to protect ourselves from lightning.

Answer : The three measures to protect ourselves from lightning, are

  1. We should avoid driving or sitting in Open vehicles, like motorbikes, tractors, construction machinery, open car etc as they being made up of metal, may induce lightening strike, hence are not safe. Similarly Open fields, tall trees, shelters in parks, elevated places do not protect us from lightning strokes.
  2. We should secure, high rise building with lightening conductor. At the same time, We should avoid carrying umbrella during thunderstorms as lightening may strike its pointed end of metal pipe
  3. In a forest, we should take shelter under shorter trees to an extent as possible. In an open field, we should stay away from trees, metal poles or structures and squat low on the ground, Placing hands on knees with head between the hands
  4. In an open field, we should stay away from trees, metal poles or structures and squat low on the ground, placing hands on knees with head between the hands
Question 8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon? 

Answer : We know that like charges repel and unlike charges attract each other. Therefor a charged balloon is repelled by another charged balloon, whereas an uncharged balloon, in the presence of a charged balloon gets induced with opposite charge, is attracted by another charged balloon 


Question 9. Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer : The instrument which is used to detect a charged body, is called electroscope. Such instrument can be made very easily. As shown in the figure, the mouth of empty jam bottle is covered with a piece of cardboard slightly bigger in size than the mouth of the bottle.
A stretched paper clip, is inserted in the middle of cardboard as shown. Two small strips of aluminium foil about 4 cm × 1 cm each are Hanged by the lower end of paper clip inside the bottle. When a charged refill is brought in contact with the upper end of paper clip and as metals are good conductors of electricity, the aluminium foil strips receive the same charge from the charged refill through the metal body of paper clip . The aluminium foil strips carrying similar charges repel each other and they become wide open, thus showing the presence of a charge in refill body.


 Question 10. List three states in India where earthquakes are more likely to strike.

Answer : 
  1. Jammu & Kashmir
  2. Gujrat
  3. Rajsthan


Question 11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Answer : The precautions, which we would take to protect our self are : 
  1. We would find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
  2. If we are in a car or a bus, we would not come out. If moving in vehicle, would ask the driver to drive slowly to a clear spot and wait there inside the vehicle till the tremors stop.
Question 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.

Answer : At the event of thunderstorm prediction, while going out, We should avoid carrying an umbrella, as lightening has great affinity to strike at any available metal object in the open. An umbrella with pointed metal tube and wire frame structure inside, offers and ideal situation for a lightening strike in a thunderstorm. 


Things to remember....

  1. Some objects can be charged by rubbing with other objects.
  2. There are two kinds of charges — positive charge and negative charge
  3. Like charges repel and unlike charges attract each other.
  4. The electrical charges produced by rubbing are called static charges.
  5. When charges move, they constitute an electric current.
  6. An electroscope may be used to detect whether a body is charged or not.
  7. The process of transfer of charge from a charged object to the earth is called earthing.
  8. The process of electric discharge between clouds and the earth or between different clouds causes lightning.
  9. Lightning strike could destroy life and property.
  10. Lightning conductors can protect buildings from the effects of lightning.
  11. An earthquake is a sudden shaking or trembling of the earth.
  12. Earthquake is caused by a disturbance deep inside the earth’s crust.
  13. It is not possible to predict the occurrence of an earthquake.
  14. Earthquakes tend to occur at the boundaries of earth’s plates. These boundaries are known as fault zones.
  15. 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.
  16. We should take necessary precautions to protect ourselves from earthquakes.

ch14 sci class8


Things to remember....

  1. Some liquids are good conductors of electricity and some are poor conductors.
  2. Most liquids that conduct electricity are solutions of acids, bases and salts.
  3. The passage of an electric current through a conducting liquid causes chemical reactions.
  4. The resulting effects are called chemical effects of currents.
  5. The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.
Question 1. Fill in the blanks 
  1. (a) Most liquids that conduct electricity are solutions of ,______________ and ______________.
  2. (b) The passage of an electric current through a solution causes ______________ effects.
  3. (c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the __________ terminal of the battery.
  4. (d) The process of depositing a layer of any desired metal on another material by means of electricity is called _________ .
Answer : 
  1. (a) Most liquids that conduct electricity are solutions of , acids, basesand salts.
  2. (b) The passage of an electric current through a solution causeschemical reactions effects.
  3. (c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the Negative terminal of the battery.
  4. (d) The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating .

Question 2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Answer : When the free ends of a tester are dipped into a solution, and the magnetic needle shows deflection, It implies that the solution is good conductor of electricity and it is allowing electric current to pass through its passage between the dipped free ends of tester thus making the circuit complete. The electric current passing through conducting wire of tester, due to magnetic effect of electric current, produces magnetic field around it, which causes the deflection of magnetic needle.




Question 3. Name three liquids, which when tested in the manner shown in Figure, may cause the magnetic needle to deflect.
 

Answer : 
  1. Lemon juice
  2. Vinegar
  3. Common Salt Solution


Question 4. The bulb does not glow in the setup shown in Figure. List the possible reasons. Explain your answer.



Answer : 
  1. The Liquid is either bad or poor conductor of Electricity
  2. The connections are loose
  3. The bulb is fused
  4. The cells being old or used up, are not producing electric current


Question 5. A tester is used to check the conduction of electricity through two liquids, labeled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that 
  1. (i) liquid A is a better conductor than liquid B.
  2. (ii) liquid B is a better conductor than liquid A.
  3. (iii) both liquids are equally conducting.
  4. (iv) conducting properties of liquid cannot be compared in this manner.

Answer : (i) liquid A is a better conductor than liquid B.

Question 6. Does pure water conduct electricity? If not, what can we do to make it conducting? 

Answer : The pure water does not conduct electricity as it is poor conductor of electricity. We can make pure water conducting by adding either some common salt, or few drops of acidic solution such as lemon juice or dilute hydrochloric acid 

Question 7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Answer : In case of fire, the firemen shut off the main electrical supply for the area, before they use the water hoses for the following reasons :

  1. (i) The melting of electric wire insulation, resulting from the heat, may cause short circuit. It may result in escalating the fire in the area
  2. (ii)The using of water for putting off the fire will be dangerous, as melting of electric wire insulation, together with increased conductivity of wet surface, will lead to the leakage of electric current in the area, resulting in electric shocks to struggling inhabitants and fire men rescuing them.
Question 8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Answer : We know that pure water is a poor conductor of electric current. The presence of some impurities in the form of some salt or acid, turns it to a good conductor. The seawater and the drinking water both contains salt impurities dissolved in them. The salt concentration in seawater is much more as compare to the normal drinking water. The presence of excess of salt in seawater makes it as better conductor of electricity than drinking water. Thus the child testing the the drinking water and the seawater, finds compass needle deflecting more in the case of seawater, due to excess flow of current in the tester.

Question 9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Answer : The rain water, mixed with chemical impurities such as acids or salts in the air and ground surface, becomes good conductor of electricity. Therefor it would not be safe for the electrician to carry out electrical repairs outdoors during heavy downpour, as risk of electric shock due to leakage of current, working on a wet surface, will be always there. 

Question 10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons?

Answer : The rainwater is supposed to be as good as distilled water. But now a days due to increased level of air pollution, a lot of impurities or pollutants are present in the air.These impurities may be acidic or may contain some salt. All these impurities in the form of either acid or salt, get dissolved with the rainwater, and make it good conductor of electricity. This is for the reason, why paheli found the compass needle showing deflection on testing with a tester. 

Question 11. Prepare a list of objects around you that are electroplated.

Answer : The list of electroplated objects around us 
  1. (i) The Gold Plated Artificial Jewellery
  2. (ii) The chromium plated Automobiles parts such as Wheel Rim, Engine Parts and Carriage etc
  3. (iii) The chromium  plated kitchen and bathroom fixtures, such as Tapes, Showers, Water Sinks, Racks.
  4. (iv) Tin platted iron cans, used for storing food,
  5. (v) The copper plated metal decorations for home and offices
  6. (vi) The shaving blade with titanium coating on  cutting age.
  7. (vii) The chrome plated Surgical Instrument

Question 12. The process that you saw in Activity 14.7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of battery and why?


Answer : The electrode with thick rod of impure copper should be attached to the positive terminal of battery because in electroplating process, When electric current is passed through the copper sulphate solution,copper sulphate dissociates into copper and sulphate. The free copper gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it. An equal amount of copper gets dissolved from the other electrode with thick rod of impure copper, in the solution. Thus,the loss of copper from the solution is restored .As the process keeps going , the copper gets transferred from positive electrode with thick rod of impure copper to the other electrode made up of thin plate of pure copper.. 

Did You Know?

LEDs (Light Emitting Diodes) are available in many colours such as red, green, yellow, blue, white and are increasingly being used for many applications, for example in traffic signal lights. LEDs are increasingly being used for lighting. A cluster of white LEDs grouped together forms a LED light source. LED light sources consume less electricity and have longer lifetime than light bulbs and fluorescent tubes. But LED light sources are expensive, so CFLs are currently the best choice. However, CFLs contain mercury 




which is toxic. Therefore, used or broken CFLs need to be disposed off safely. Once the technological advances reduce the cost of LEDs, they will become the preferred lighting source.

ch13 sci class8


Things to remember....

  1. Sound is produced by vibrating objects.
  2. In human beings, the vibration of the vocal cords produces sound.
  3. Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.
  4. It sends the signals to the brain. This process is called hearing.
  5. The number of oscillations or vibrations per second is called the frequency of oscillation.
  6. The frequency is expressed in hertz (Hz)
  7. Larger the amplitude of vibration, louder is the sound.
  8. Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.
  9. Unpleasant sounds are called noise.
  10. Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.
  11. Attempts should be made to minimise noise pollution.
  12. Plantation on the roadside and elsewhere can reduce noise pollution.
Question 1. Choose the correct answer :
Sound can travel through:

  1. gases only
  2. solids only
  3. liquids only
  4. solids, liquids and gases.
Answer : d. solids, liquids and gases.

Question 2. Which of the following voices is likely to have minimum frequency?
  1. Baby girl
  2. Baby boy
  3. A man
  4. A woman
Answer : c. A man

Question 3. In the following statements, tick T against those which are true, and F against those which are false: 
  1. (a) Sound cannot travel in vacuum. (T/F)
  2. (b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
  3. (c) If the amplitude of vibration is large, sound is feeble. (T/F)
  4. (d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
  5. (e) The lower the frequency of vibration, the higher is the pitch. (T/F)
  6. (f) Unwanted or unpleasant sound is termed as music. (T/F)
  7. (g) Noise pollution may cause partial hearing impairment. (T/F)
Answer : 
  1. (a) Sound cannot travel in vacuum. (T)
  2. (b) The number of oscillations per second of a vibrating object is called its time period. (F)
  3. (c) If the amplitude of vibration is large, sound is feeble. (F)
  4. (d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
  5. (e) The lower the frequency of vibration, the higher is the pitch. (F)
  6. (f) Unwanted or unpleasant sound is termed as music. (F)
  7. (g) Noise pollution may cause partial hearing impairment. (T)

Question 4. Fill in the blanks with suitable words. 
  1. (a) Time taken by an object to complete one oscillation is called ................... .
  2. (b) Loudness is determined by the .........................of vibration.
  3. (c) The unit of frequency is..........................
  4. (d) Unwanted sound is called ......................... .
  5. ((e) Shrillness of a sound is determined by the ................. of vibration.
Answer : 
  1. (a) Time taken by an object to complete one oscillation is calledTime Period
  2. (b) Loudness is determined by the Amplitude of vibration.
  3. (c) The unit of frequency is Hertz (Hz)
  4. (d) Unwanted sound is called Noise Pollution
  5. ((e) Shrillness of a sound is determined by the Frequency of vibration.
Question 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer : 
We know that the number of oscillations per second is called the frequency of oscillation.
∴The frequency (F) of oscillating pendulum=The number of oscillations/ Time taken in Seconds
 = 40/4 Hz = 10 hz
 = 10 hz
Also, We know, the Time period (T)is the total time taken for making one complete oscillation
∴ Time Period of oscillating pendulum= 1/10 seconds
= .1 second
Question 6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Answer : 

The frequency (F) of Mosquito vibrations =500 Hz
Also, We know, the Time period (T) is the total time taken for making one complete oscillation
∴ Time Period of Mosquito vibration= [1 /frequency (F) ] seconds
 = (1/500) Seconds
 =.002 Seconds
Question 7. Identify the part which vibrates to produce sound in the following instruments: (a) Dholak (b) Sitar (c) Flute

Answer : 

instrumentsVibrating Part Producing Sound
(a)DholakStretched membrane
(b)SitarStretched string
(c)FluteAir-column Variation

Question 8. What is the difference between noise and music? Can music become noise sometimes?

Answer : All excessive and Unpleasant sounds, which may cause discomfort are called noise. The sound of moving vehicles, construction work, Crackers, blaring horns of buses and trucks, loud music are examples of noise, where as the sounds which are pleasant to the ear and one can enjoy them on hearing is termed as Music . Sound produced by a harmonium is a musical sound. The string of a sitar also gives out a musical sound. If a musical sound becomes too loud, it will not be pleasant to hear. it would not remain melodious, rather It will become a noise or a noise pollution 

Question 9. List sources of noise pollution in your surroundings.

Answer : 
List sources of noise pollution in your surroundings. 
  1. (1.) Sounds of vehicles
  2. (2.) Explosions including bursting of crackers,
  3. (3.) Running of machines
  4. (4.) Loudspeakers, Television and transistor radio at high volume
  5. (5.) kitchen appliances such as grinder
  6. (6.) desert coolers
  7. (7.) Air conditioners

Question 10. Explain in what way noise pollution is harmful to humans.

Answer : The noise pollution is harmful to humans in many ways as it may cause : 
  1. (1.) Lack of sleep
  2. (2.) hypertension (high blood pressure)
  3. (3.) Anxiety
  4. (4.) Mood Disorders leading to depression
  5. (5.) Temporary or even permanent impairment of hearing.


Question 11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Answer : I will suggest my parents to buy the house three lanes away from the roadside because :

  1. (1.) There will be less noise pollution caused by the vehicular traffic and crowd
  2. (2.) There will be less air pollution caused by the vehicular traffic
  3. (3.) There will be additional safety from roadside accidents on account of less traffic
  4. (3.) Community based activities or functions are easy to conduct in lanes away from roadside, thus enhancing overall quality living
Question 12. Sketch larynx and explain its function in your own words.

Answer :
In humans, the sound is produced by the voice box or the larynx. It is a hard bump like structure deep inside the throat, located at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air.
When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different
Question 13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?

Answer : The light travels at a speed of 300000000 m/s where as sound travels at a speed of 340 m/s. 
Due to this reason, at the event of a lightening and thunder, taking place in the sky at the same time and at the same distance from us, lightning is seen earlier and thunder is heard later 

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