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Page No 86:

Question 1:

An electroscope is a device which is used to find if an object is
(a) charged         (c) free of cracks
(b) magnetic      (d) hot

Answer:

Electroscope is a device used to detect the presence of charge on an object.
Hence, the correct answer is option (a).

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Question 2:

Electric current is to be passed from one body to another. For this purpose the two bodies must be joined by
(a) cotton thread        (c) copper wire
(b) plastic string        (d) rubber band

Answer:

Copper wire can conduct electricity through it, so bodies must be joined by copper wire to pass electricity from one body to other.
Hence, the correct answer is option (c).

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Question 3:

The movement of the earth’s plates causes
(a) cyclones           (c) earthquakes
(b) lightning           (d) thunderstorms

Answer:

Earthquakes are caused due to the movement of earth's plates.
Hence, the correct answer is option (c).

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Question 4:

Two charged objects are brought close to each other. Choose the most appropriate statement from the following options:
(a) they may attract
(b) they may repel
(c) they may attract or repel depending on the type of charges they carry
(d) there will be no effect

Answer:

Similar charges ( two positive or two negative charges) repel each other while dissimilar charges ( positive and negative charges) attract each other. So, if two charged objects are brought close to each other, they may attract or repel depending on the type of charges they carry.
Hence, the correct answer is option (c).

Page No 86:

Question 5:

Which of the following is not likely to cause Tsunami?
(a) A major nuclear explosion under sea
(b) Earthquake
(c) Volcanic eruption
(d) Lightning

Answer:

Tsunami is not caused by lightning.
Hence, the correct answer is option (d).

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Question 6:

The earth’s plate responsible for causing earthquakes is
(a) the crust of the earth
(b) the mantle of the earth
(c) the inner core of the earth
(d) the outer core of the earth

Answer:

Earth's crust, called the lithosphere, consists of tectonic plates. The sudden disturbance in the earth's tectonic plates cause earthquake.
Hence, the correct answer is option (a).

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Question 7:

Consider the list of terms given below:
(i) Seismic Zone          (iii) Mantle
(ii) Fault Zone              (iv) Inner Core
The boundaries of the earth’s plate are known as
(a) (i) & (ii)       (c) (iii) & (iv)
(b) (i) & (iii)      (d) (ii), (iii) & (iv)

Answer:

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. 
Hence, the correct answer is option (a).



Page No 87:

Question 8:

The outermost layer of earth is called
(a) mantle            (c) crust
(b) outer core      (d) inner core

Answer:

The outermost layer of earth is called crust.
Hence, the correct answer is option (c).

Page No 87:

Question 9:

Major earthquakes are less likely to occur in
(a) North East India         (c) Rann of Kutch
(b) Rajasthan                    (d) Orissa

Answer:

Earthquake prone areas in India are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo–Gangetic Plane. Major earthquakes are less likely to occur in Orissa.
Hence, the correct answer is option (d).
 

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Question 10:

Consider the list of terms given below
(i) Tsunami       (iii) Floods
(ii) Landslide    (iv) Lightning
Earthquakes can cause
(a) (i), (ii) & (iii)         (c) (ii), (iii) & (iv)
(b) (ii) & (iv)               (d) (iii) & (iv)

Answer:

Earthquakes can cause tsunami, floods and landslide. Lightning is caused due to electric discharge occur between two or more clouds, or between clouds and the earth.
Hence, the correct answer is option (a).

Page No 87:

Question 11:

State whether the following are True or False.
(a) Earthquakes occur all the time all over the world.
(b) The plates of the outermost layer of the earth are always in continuous motion.
(c) Tremors on the earth can also be caused by the eruption of a volcano.
(d) The process of electric discharge cannot occur between clouds and the earth.
(e) Bathing outdoors should be avoided during thunderstorm.

Answer:

(a) True
An earthquake is a sudden shaking or trembling of the earth which lasts for a very short time.  It is due to a disturbance deep inside the earth’s crust. Earthquakes occur all the time, all over the earth. Small intensity earthquakes are not even noticed.

(b) True
The outermost layer of the earth is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another or a plate goes under another due to collision, they cause disturbance in the earth’s crust.
(c) True
 The tremors are caused by the disturbance deep down inside the uppermost layer of the earth called the crust. 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.

(d) False
The process of electric discharge can occur between two or more clouds, or between clouds and the earth.

(e)True
Bathing in open should be avoided during thunderstorms to avoid contact with running water. Otherwise lightning strike may occur and harm the person. 

 

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Question 12:

Is it possible to predict the occurrence of an earthquake?

Answer:

No, the occurrence of an earthquake is not predictable.  It is not yet possible to predict when and where the next earthquake might occur.

Page No 87:

Question 13:

If a charged plastic straw is brought near another uncharged plastic straw, what will happen?

Answer:

If a charged plastic straw is brought near another uncharged plastic straw, then charge of opposite nature is induced on the uncharged plastic straw. So, both will attract each other if brought closer.

Page No 87:

Question 14:

The aluminium strips in an electroscope as shown in the given figure are replaced by plastic strips and a charged body is brought in contact with the metal clip. What will happen?

Answer:

Aluminium is good conductor of electricity while the plastic is not. So, if aluminium strips are replaced by plastic strips, the divergence of the strips will not take place and hence we cannot predict whether the object is charged or not.



Page No 88:

Question 15:

Plastic straws A and B are rubbed with dry cotton cloth. What will happen if they are brought near each other?

Answer:

After rubbing the straws with cotton cloth, similar charge is induced on both the straws. So when these charged straws are brought near each other, they repel each other, as similar charges repel.

Page No 88:

Question 16:

During the construction of a building the lightning conductor was left hanging in the air by mistake. Would the lightning conductor be still effective? Explain.

Answer:

No, it will not be effective. As the lightning conductor was not connected properly to the earth, therefore, the charge will not pass from the lightning conductor to the earth.

Page No 88:

Question 17:

If air and cloud were good conductors of electricity, do you think lightning could occur? Explain.

Answer:

No, lightning will not occur. The charge separation does not take place in conductors. If air and cloud were good conductors of electricity, then charge accumulation on clouds will not occur, and hence lightning will not take place.

Page No 88:

Question 18:

Identify the lightning conductor and the copper plate in the given figure

Answer:

A is lightning conductor and B is copper plate.



Page No 89:

Question 19:

If the materials used for constructing a building were good conductors, do you think lightning will strike the building. Will the lightning conductor be still required to be installed in the building?

Answer:

The lightning will not strike the building because all the lightning falling on the building will pass to the ground through the conducting materials used in the building materials. So, there is no need for installing any lightning conductor. 
(But it is always advisable to install a lightning conductor in a high rise building to protect it from lightning)

Page No 89:

Question 20:

You might have observed on a dry day that when you touch the screen of a television or computer monitor (with picture tube), you get a slight shock. Why does it happen?

Answer:

Electric charge gets accumulated on the screen of television or  monitor of the computer. On touching the screen, the charge discharges through our body. Thus, we get a slight shock.

Page No 89:

Question 21:

Explain how does lightning conductor protects a building from getting struck by lightning.

Answer:

The lightning conductor does not allow the charge to accumulate on a building. It conducts the charge to the earth following the path of least resistance and passing harmlessly through the rod and its cables, protecting the building from being struck by lightning.

Page No 89:

Question 22:

In an electroscope if a negatively charged body is brought in contact with the metal clip, the strips of the electroscope diverge. If now another charged object carrying equal amount of positive charge is brought in contact with the clip, what will happen?

Answer:

When a positively charged object is brought in contact with the clip of an electroscope, the negative charge given earlier will get neutralised. Hence, the strips will collapse. 

Page No 89:

Question 23:

The strips of an electroscope diverge when a charged body is brought in contact with the metal clip. Now the clip is touched gently by our hand. What will happen to the strips? Explain.

Answer:

The human body also conducts electricity. The charged electroscope strips will get discharged by touching it with our hands. Due to which, the strips will come back to the original state after discharge.

Page No 89:

Question 24:

Explain how lightning takes place?

Answer:

During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These sudden movements cause accumulation of positive charges near the upper edges of the clouds and the negative charges near the lower edges. There is an accumulation of positive charges near the ground too. 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 in the form of lightning. 

Page No 89:

Question 25:

Mention three precautions that you will take to protect yourself if earthquake strikes when you are inside the house.

Answer:

We should take the following precautions to protect ourselves if earthquake strikes and we are inside our home:
(i) Take shelter under a table
(ii) Stay away from tall and heavy objects that may fall on us.
(iii) Protect your head with a pillow 

Page No 89:

Question 26:

Explain why it is safer to use a wireless telephone instead of a landline telephone during lightning.

Answer:

Lightning is an electrical discharge. Conducting wire, helps to conduct the charge through itself. During lightning, electric charge at the edges of the cloud may discharge through landline telephone conducting wires, and this discharging process may be dangerous. So, it is safer to use a wireless telephone instead of a landline telephone during lightning. It is safer to use mobile phones and cordless phones. However, it is not wise to call up a person who is receiving your phone through a wired phone.

Page No 89:

Question 27:

What precautions would you take if lightning occurs while you are outside the house?

Answer:

Following  precautions should be taken if lightning occurs while we are outside the house
(i) Do not take shelter in open vehicles, like motorbikes, tractors, construction machinery, open cars, open fields, tall trees or at elevated places.
(ii) Do not carry an umbrella during thunderstorms. 
(iii) Stay away from tall trees.
(iv) Stay away from poles or other metallic objects.

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Question 28:

If the metal clip used in the electroscope is replaced by an ebonite rod and a charged body is brought in contact with it, will there be any effect on the aluminium strips? Explain.

Answer:

An ebonite rod is an insulator. So, the charged body will not transfer any type of charge to the ebonite rod. Thus, there will be no charge on aluminum strips and hence there will be no repulsion.



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