NCERT Solution
Some Natural Phenomena Chap-12 for Class 8
Exercises
Question 1
Which of the following cannot be charged easily by friction?
1. A plastic scale
2. A copper rod
3. An inflated balloon
4. A woollen cloth.
Answer
A copper rod.
Reason — When an
object is charged by friction, charges develop on the two materials involved in
friction. Copper being a good conductor, allows the charges to move easily and
redistributes the excess charges quickly and doesn't retain a significant
charge through friction. Therefore, it's difficult to charge a copper rod
through friction.
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REPRODUCTION IN ANIMALS NIMALS
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Question 2
When a glass rod is rubbed with a piece of silk cloth, the
rod
1. and the cloth both acquire a positive charge.
2. becomes positively charged while the cloth has a negative
charge.
3. and the cloth both acquire a negative charge.
4. becomes negatively charged while the cloth has a positive
charge.
Answer
becomes positively charged while the cloth has a negative
charge.
Reason — When two
objects are rubbed against each other, electrons transfer from one material to
the other. This transfer of electrons leads to one material becoming positively
charged (losing electrons) and the other becoming negatively charged (gaining
electrons). It is a convention to call the charge acquired by a glass rod when
it is rubbed with silk as positive. The other kind of charge is said to be
negative.
Question 3
Write T against true and F against false in the following statements.
(a) Like charges attract each other. (T/F)
(b) A charged glass rod attracts a charged plastic straw.
(T/F)
(c) Lightning conductors cannot protect a building from
lightning. (T/F)
(d) Earthquakes can be predicted in advance. (T/F)
Answer
(a) Like charges attract each other. — False
Reason — Like charges repel each
other.
(b) A charged glass rod attracts a charged plastic straw. — True
(c) Lightning conductors cannot protect a building from
lightning. — False
Reason — Lightning
conductors protect a building from lightning.
(d) Earthquakes can be predicted in advance. — False
Reason — Earthquakes cannot be
predicted in advance.
Question 4
Sometimes, a crackling sound is heard while taking off a
sweater during winter. Explain.
Answer
The crackling sound heard when removing a sweater in winter
is a result of the friction between the woollen fabric and the body or the
shirt worn underneath. This friction generates opposite electric charges, and
the discharge of these charges produces the audible 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, the charges get conducted to
the earth through our body and the conductor loses its charge. This process of
transferring of charge from a charged object to the earth is called earthing.
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
The destructive energy of an earthquake is measured on
Richter scale. This scale has the readings from 1 to 10. An earthquake
measuring 3 on Richter scale would be recorded by a seismograph. However,
earthquakes of this magnitude are not likely to cause significant damage.
Question 7
Suggest three measures to protect ourselves from lightning.
Answer
Three measures to protect ourselves from lightning are:
1. During lightning, no open place is safe. We should be inside
a house or a building. If travelling by car or bus, we should shut the doors
and windows of the vehicle.
2. Electric wire, telephone lines, and metal surfaces should
never be touched during lightning.
3. Incase no shelter is available, we should not lie on the
ground. Instead, we should squat low on the ground with hands on the knees and
head between the hands. This position makes us the smallest target to be
struck.
Question 8
Explain why a charged balloon is repelled by another charged
balloon, whereas an uncharged balloon is attracted by another charged balloon.
Answer
The repulsion between two charged balloons occurs because
both balloons carry the same type of charge, resulting in like charges
repelling each other. On the other hand, when a charged balloon is brought
close to an uncharged balloon, due to induction, the uncharged balloon acquires
opposite charge. Due to this they are attracted to each other.
Question 9
Describe with the help of a diagram an instrument which can
be used to detect a charged body.
Answer
The instrument used to detect a charged body is known
as electroscope.
It consists of a metal rod on which two leaves of aluminium
foil are fixed to one end and a metal disc at the other end. The leaves are
kept inside a conical flask, and it is corked to isolate them from the
atmospheric air.
When a charged body comes in contact with the metal disc, the aluminium leaves move away from each other because some charges get transferred to aluminium leaves through the metal rod. This process is called charging by conduction. The charges on the leaves and the charged body are of the same nature, and thus, the leaves of aluminium repel each other. If the body is not charged then they would attract each other.
Question 10
List three states in India where earthquakes are more likely
to strike.
Answer
Three earthquake prone states in India are:
1. Kashmir
2. Gujarat
3. Assam
Question 11
Suppose you are outside your home and an earthquake strikes.
What precautions would you take to protect yourself?
Answer
Precautions that I will take to protect myself in case of an
earthquake while I am outside are:
1. I will find a clear spot, away from buildings, trees and
overhead power lines and then I will drop to the ground.
2. If I am in a car or a bus, I will not come out. I will ssk
the driver to drive slowly to a clear spot. I will stay 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
I will not carry an umbrella during a predicted thunderstorm.
A thunderstorm is accompanied by lightning. The lightning might travel along
the metallic rod of the umbrella and cause an electric shock to the person
holding it, making it an unsafe choice in such weather conditions.
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