class 8 ch 17

Chapter 17. STARS AND THE SOLAR SYSTEM | Science | CBSE Class VIII (8th) | Solved Exercises

Things to remember....

1. The phases of the moon occur because we can see only that part of the moon which reflects the light of the Sun towards us.
2. Stars are celestial bodies that emit light of their own. Our sun is also a star.
3. It is convenient to express distances of stars in light years.
4. Stars appear to move from east to west.
5. The pole star appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth.
6. Constellations are groups of stars that appear to form recognizable shapes.
7. The solar system consists of eight planets and host of asteroids, comets and meteors.
   
8. A body revolving around another body is called a satellite.
9. Moon is the natural satellite of the Earth.
10. Some planets also have natural satellites.
11. Venus is the brightest planet in the night sky.
12. Jupiter is the largest planet of the solar system.
13. The artificial satellites revolve around the Earth. They are much closer than the moon.
14. Artificial satellites are used for weather forecasting, long distance communication and remote sensing.

Choose the correct answer in Questions 1-3 :
Question 1. Which of the following is NOT a member of the solar system?

1. (a) An asteroid
2. (b) A satellite
3. (c) A constellation
4. (d) A comet

Answer : (c) A constellation

Question 2. Which of the following is NOT a planet of the sun?

1. (a) Sirius
2. (b) Mercury
3. (c) Saturn
4. (d) Earth

Answer : (a) Sirius

Question 3. Phases of the moon occur because

1. (a) we can see only that part of the moon which reflects light towards us.
2. (b) our distance from the moon keeps changing.
3. (c) the shadow of the Earth covers only a part of moon’s surface.
4. (d) the thickness of the moon’s atmosphere is not constant.

Answer : (a) we can see only that part of the moon which reflects light towards us.

Question 4. Fill in the blanks:

1. (a) The planet which is farthest from the Sun is ____________ .
2. (b) The planet which appears reddish in colour is ____________ .
3. (c) A group of stars that appear to form a pattern in the sky is known as a ____________ .
4. (d) A celestial body that revolves around a planet is known as __________.
5. (e) Shooting stars are actually not ____________.
6. (f) Asteroids are found between the orbits of _________ and _________ .

Answer : 

1. (a) The planet which is farthest from the Sun is Neptune .
2. (b) The planet which appears reddish in colour is Mars .
3. (c) A group of stars that appear to form a pattern in the sky is known as a Constellations .
4. (d) A celestial body that revolves around a planet is known assatellite.
5. (e) Shooting stars are actually not stars.
6. (f) Asteroids are found between the orbits of Mars and Jupiter .

Question 5. Mark the following statements as true (T) or false (F):

1. (a) Pole Star is a member of the solar system. [ ]
2. (b) Mercury is the smallest planet of the solar system. [ ]
3. (c) Uranus is the farthest planet in the solar system. [ ]
4. (d) INSAT is an artificial satellite. [ ]
5. (e) There are nine planets in the solar system. [ ]
6. (f) Constellation Orion can be seen only with a telescope. [ ]

Answer : 

1. (a) Pole Star is a member of the solar system. [ false (F)]
2. (b) Mercury is the smallest planet of the solar system. [true (T) ]
3. (c) Uranus is the farthest planet in the solar system. [false (F) ]
4. (d) INSAT is an artificial satellite. [ true (T)]
5. (e) There are nine planets in the solar system. [false (F) ]
6. (f) Constellation Orion can be seen only with a telescope. [false (F) ]

Question 6. Match items in column A with one or more items in column B:

A	B
(i) Inner planets (ii) Outer planets (iii) Constellation (iv) Satellite of the Earth	(a) Saturn (b) Pole Star (c) Great Bear (d) Moon (e) Earth (f) Orion (g) Mars

Answer : 

A	B
(i) Inner planets	(e) Earth (g) Mars
(ii) Outer planets	(a) Saturn
(iii) Constellation	(c) Great Bear (f) Orion
(iv) Satellite of the Earth	(d) Moon

Question 7. In which part of the sky can you find Venus if it is visible as an evening star? 
Answer : Venus, if it is visible as an evening star, can be found in the western part of the sky only, just 1-3 hours after sunset

Question 8. Name the largest planet of the solar system. 
Answer : Jupiter is the largest planet of the solar system.

Question 9. What is a constellation? Name any two constellations.
Answer : Constellations are groups of stars that appear to form recognisable shapes.Constellations were devised by ancient people to easily recognise stars in the sky. The constellations were named as per theirs shapes resembling familiar objects of that time. We can easily identify some constellations in the night sky if know how a particular constellation looks like and where to look for it. Two constellations are Ursa Major and Orion

Question 10. Draw sketches to show the relative positions of prominent stars in
(a) Ursa Major and (b) Orion

Answer : 

(a) Ursa Major

 

Orion

Question 11. Name two objects other than planets which are members of the solar system. 

Answer : (1.) Asteroids   (2.) Comets

Question 12. Explain how you can locate the Pole Star with the help of Ursa Major. 
Answer : Ursa Major is one of the prominent constellations, which we can see during summer time in the early part of the night. To locate the pole star, with the help of Ursa Major, We will first, look at the two stars at the end of Ursa Major, then will the Imagine a straight line passing through these stars as shown in the fig. Now looking in the direction of this imaginary line towards the north direction, we can see the Pole Star,which is not too bright and located at a distance about five times the distance between the two inline stars of Ursa Major. The Pole Star is not visible from the southern hemisphere

Question 13. Do all the stars in the sky move? Explain.
Answer : Yes, in real, all the stars in the sky move. As we know, all the celestial bodies comprising our solar system, Stars, Constellations and galaxies, are part of big Universe, which is ever expanding. Every celestial object with in Universe, in order to over come the gravitational pull of other celestial object around, has to move in some relative motion. The pole star, which is also moving, but appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth. From earth, all others stars appear to move from east to west as earth makes rotation from west to east around its axis.


Question 14. Why is the distance between stars expressed in light years? 
What do you understand by the statement that a star is eight light years away from the Earth? 

Answer : The stars are located at far greater distance from each other.It is convenient to express distances of stars in light years. For example, The Sun is nearly 150,000,000 kilometres away from the Earth. Alpha Centauri, the next nearest star , is at a distance of about 40,000,000,000,000 km from the Earth. This distance is not convenient to express in normal units of length. To measure such large distances, we make use of another unit known as light year, which is the distance traveled by light, in one year with a speed of 300,000 km per second . So using this unit, the distance of Alpha Centauri is about 4.3 light years, which is very convenient to express.
A star is eight light years away from the Earth implies that, for light, which travels with speed of 300,000 km per second, it takes 8 years to reach Earth. In other way, the distance between star and earth is around 9460800000000 km (365 X 24 X 60 X 60 X 3,00, 000 Km).

Question 15. The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate? 

Answer : 

Let the radius of the Earth	= r
∴ The radius of Jupiter	=11r
Volumes of earth	= 4/3√π(r)3
volumes of Jupiter	=4/3√π(11r)3
Ratio of the volumes of Jupiter and the Earth	4/3√π(11r)3 = 4/3√π(r)3
	=1331 : 1
∴  Jupiter can accommodate 1331 numbers of Earths

Question 16. Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it. 

Answer :  The Sketch is incorrect. The Correct one is as shown below : -

class 8 ch 16

Chapter 16. LIGHT | Science | CBSE Class VIII (8th) | Solved Exercises

Question 1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.

Answer : We can not see objects in the dark room without the presence of light or a lighted object it self . We may see an object only when, the light from an object enters our eyes. The light may have been emitted by the object directly (luminous objects.), or may have been reflected by it ( illuminated objects). Similarly, we can not see outside objects, sitting inside the darkroom, if there is no opening or passage for any incidental light from out side. How ever, out side the darkroom, in open, we can easily see the objects, due to either reflected light from the surface of illuminated objects or direct light from luminous objects such as Sun, candle, torch etc. Nearly everything we see around is seen due to reflected light.

Question 2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer : 

	Regular reflection	Diffused reflection
(1.)	When all the parallel rays reflected from a plane surface are parallel, the reflection is known as regular reflection	When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection
(2.)	Reflection from a smooth surface like that of a mirror is an example of regular reflection Images are formed by regular reflection.	Reflection from a rough surface, like that of a cardboard, is an example of irregular reflection. It is caused by the irregularities in the reflecting surface .
(3.)	Images are formed by regular reflection.	Images are not formed by irregular reflection.

Question 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.

1. (a) Polished wooden table
2. (b) Chalk powder
3. (c) Cardboard surface
4. (d) Marble floor with water spread over it
5. (e) Mirror
6. (f) Piece of paper

Answer : 

1. (a) Polished wooden table -----> regular reflection
2. (b) Chalk powder -----> diffused reflection
3. (c) Cardboard surface -----> diffused reflection
4. (d) Marble floor with water spread over it -----> regular reflection
5. (e) Mirror -----> regular reflection
6. (f) Piece of paper -----> diffused reflection

In case of objects with smooth, polished and shiny surfaces, like Polished wooden table, Marble floor with water spread over it and Mirror, when a incident beam of light strikes on their surfaces, regular reflection takes place. Whereas, in case of objects with rough surfaces, such as Chalk powder, Cardboard surface, Piece of paper etc, when a incident beam of light strikes on their surfaces, irregular reflection takes place. It is caused by the irregularities in the reflecting surface

Question 4. State the laws of reflection.

Answer : There are two laws of reflection as given below :

1. (i) The angle of incidence is equal to the angle of reflection.
2. (ii) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane

Question 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.

Answer : When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane.

When we bend the paper we create a plane different from the plane in which the incident ray and the normal lie.

 Then we do not see the reflected ray. What does it indicate? It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. This is second law of reflection.

Similarly, in activity below, as the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane, we can clearly see the glare of led torch

Where as, in activity below, as the incident ray, the normal at the point of incidence and the reflected ray all do not lie in the same plane, therefor,  the glare of led torch is not as powerful.

Question 6. Fill in the blanks in the following :

Answer : 

1. (a) A person 1 m in front of a plane mirror seems to be 2 m from his image.
2. (b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with left hand
3. (c) The size of the pupil becomes large when you see in dim light.
4. (d) Night birds have  small number of cones than rods in their eyes.

Choose the correct option in Questions 7 – 8 
Question 7. Angle of incidence is equal to the angle of reflection 

1. (a) Always
2. (b) Sometimes
3. (c) Under special conditions
4. (d) Never

Answer : (a) Always

Question 8. Image formed by a plane mirror is 

1. (a) virtual, behind the mirror and enlarged
2. (b) virtual, behind the mirror and of the same size as the object
3. (c) real at the surface of the mirror and enlarged
4. (d) real, behind the mirror and of the same size as the object.

Answer : (b) virtual, behind the mirror and of the same size as the object

Question 9. Describe the construction of a kaleidoscope.

Answer : A kaleidoscope, is made up of three rectangular mirror strips about 15 cm long and 4 cm wide each. This mirrors, as shown in the figure, are Joined together to form a prism. This assembly, may be housed inside a a circular cardboard tube or tube of a thick chart paper.

The length of tube is slightly longer than the mirror strips. One end of the tube is covered by a cardboard disc having a hole in the center, through which we can see . Other end of the cardboard tube is covered by a circular plane glass plate touching the edge of mirror strips. Now several small pieces of coloured glass (broken pieces of coloured bangles) are put inside . Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.
Your kaleidoscope is ready. When we peep through the hole, we will be able to see a variety of patterns in the tube. Interesting feature of a kaleidoscope is that we will never see the same pattern again.
Question 10. Draw a labeled sketch of the human eye.

Answer : 

Question 11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teachers advise?

Answer : The intensity of laser light is extremely high. If we throw beam from laser torch directly on the eye, the eyes may be injured or blinded temporarily or partially due to sudden contraction of pupil and damage of  retina

Question 12. Explain how you can take care of your eyes.

Answer : We can take proper care of our eyes in following ways : 

1. (1) In case of any problem we should go to an eye specialist. Beside, we should have a regular eye checkup
2. (2) In case, after an eye checkup, if advised bye an eye specialist, we should use suitable spectacles.
3. (3) We should avoid bright or dim light. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
4. (4) We should not look at the sun or a powerful light directly.
5. (5) We should not rub our eyes. If particles of dust go into our eyes, we should wash our eyes with clean water. If there is no improvement then we should go to a doctor.
6. (6) We should Wash our eyes frequently with clean water.
7. (7)We should always read at the normal distance for vision. Do not read by bringing our book too close to our eyes or keeping it too far.
8. (6) Our Diet should include food items rich in vitamin A

Question 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer : As per the laws of reflection
The angle of incidence ∠i = angle of reflection ∠r
Here as given ∠i + ∠r = 90°
  ∴ ∠i + ∠i = 90 °
or ∠i = 90/2 = 45 °
Hence angle of incidence of a ray = 45 degree

Question 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Answer : if a candle is placed between two parallel plane mirrors, we will see an infinite numbers of images of candles

Question 15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.

Fig. 16.19

Answer : 

Question 16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Answer : 

 As Boojho is not standing in front of plane mirror, Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, does not lie in the same plane, therefor he can not see him self. He can see the image of objects situated at P, Q as Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane. Similarly, incidental ray from R, will not be reflected to reach point A, hence will not be seen by Boojho.

Question 17.

1. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
2. (b) Can Paheli at B see this image?
3. (c) Can Boojho at C see this image?
4. (d) When Paheli moves from B to C, where does the image of A move?
5.  
   

Answer : 

1. (a) A virtual and lateral image of an object situated at A, will be formed , behind the mirror surface. The image will be exactly at the same distance , as the distance of real object at position A and reflecting surface of the mirror
2. (b)Yes, as incidental ray from object A, is reaching at B as reflected ray, therefor Paheli at B, can see this image
3. (c) Boojho can also see this image, as reflected rays from object at A is reaching at C
4. (d) The point of convergence, of reflected rays reaching at point B and C, is same and at same distance behind the mirror. Therefor, When Paheli moves from B to C, the image of A will not change its position

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CBSE Class VIII (8th) | Science | Chapter 16. LIGHT |  |  Solved Exercises 

Things to remember....

1. Light is reflected from all surfaces.
2. Regular reflection takes place when light is incident on smooth, polished and regular surfaces.
3. Diffused/irregular reflection takes place from rough surfaces.
4. Two laws of reflection are :
   1. The angle of incidence is equal to the angle of reflection.
   2. Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.
5. Image formed in a plane mirror undergoes lateral inversion.
6. Two mirrors inclined to each other give multiple images.
7. Beautiful patterns are formed in a kaleidoscope because of multiple reflections.
8. Sunlight, called white light, consists of seven colours.
9. Splitting of light into its constituent colours is known as dispersion.
10. Important parts of the eye are cornea, iris, pupil, lens, retina and optic nerve.
11. A normal eye can see nearby and distant objects clearly.
12. Visually challenged persons can read and write using Braille system.
13. Visually challenged persons develop their other senses more sharply to improve their interaction with their environment.

class 8 ch 15

Chapter 15. SOME NATURAL PHENOMENA |Science | CBSE Class VIII (8th) | Solved Exercises

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.

class 8 ch 14

Chapter 14. CHEMICAL EFFECTS OF ELECTRIC CURRENT | Science | CBSE Class VIII (8th) | Solved Exercises

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.

CBSE Class VIII (8th)  |  Science |  | Chapter 14  CHEMICAL EFFECTS OF ELECTRIC CURRENT | Solved Exercises

class 8 ch 13

Chapter 13. SOUND |Science | CBSE Class VIII (8th) | Solved Exercises

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 : 

	instruments	Vibrating Part Producing Sound
(a)	Dholak	Stretched membrane
(b)	Sitar	Stretched string
(c)	Flute	Air-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