NCERT Textbook - Some Natural Phenomena Class 8 Notes | EduRev

Science Class 8

Created by: Divey Sethi

Class 8 : NCERT Textbook - Some Natural Phenomena Class 8 Notes | EduRev

 Page 1


SOME NATURAL PHENOMENA
SOME NATURAL PHENOMENA
In Class VII you read about winds,
storms and cyclones. You learnt that
cyclones can cause a lot of damage to
human life and property. You also
learnt that we can protect ourselves from
these destructive phenomena to some
extent. In this chapter we shall discuss
two other destructive natural
phenomena. These are lightning and
earthquakes. We shall also discuss what
steps we can take to minimise
destruction caused by these
phenomena.
15.1Lightning
You might have seen sparks on a
electric pole when wires become loose.
This phenomenon is quite common
when a wind is blowing and shacking
the wires. You might also have seen
sparks 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 was
visiting them. Now, of course, we
understand that lightning is caused by
the accumulation of charges in the
clouds. We need not be afraid of
lightning, but we have to take
precautions to protect ourselves from
the deadly sparks.
The Sparks that the Greeks Knew
About
The ancient Greeks knew as early as
600 B.C. that when amber (amber is a
kind of resin) was rubbed with fur, it
attracted light objects such as hair. You
might have seen that when you take
off woollen or polyester clothes, your
hair stands on ends. If you take off
these clothes in the dark, you see even
a spark and hear crackling sound. In
1752 Benjamin Franklin, an American
scientist, showed that lightning and the
spark from your clothes are essentially
the same phenomena. However, this
realisation took 2000 years.
Scientific discoveries are a
result of hardwork by many
people. It can sometime takes
a long time.
I wonder why they took so
many years to realise the
similarity.
We shall now study some properties
of electric charges. We shall also see how
they are related to the lightning in the
sky.
Page 2


SOME NATURAL PHENOMENA
SOME NATURAL PHENOMENA
In Class VII you read about winds,
storms and cyclones. You learnt that
cyclones can cause a lot of damage to
human life and property. You also
learnt that we can protect ourselves from
these destructive phenomena to some
extent. In this chapter we shall discuss
two other destructive natural
phenomena. These are lightning and
earthquakes. We shall also discuss what
steps we can take to minimise
destruction caused by these
phenomena.
15.1Lightning
You might have seen sparks on a
electric pole when wires become loose.
This phenomenon is quite common
when a wind is blowing and shacking
the wires. You might also have seen
sparks 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 was
visiting them. Now, of course, we
understand that lightning is caused by
the accumulation of charges in the
clouds. We need not be afraid of
lightning, but we have to take
precautions to protect ourselves from
the deadly sparks.
The Sparks that the Greeks Knew
About
The ancient Greeks knew as early as
600 B.C. that when amber (amber is a
kind of resin) was rubbed with fur, it
attracted light objects such as hair. You
might have seen that when you take
off woollen or polyester clothes, your
hair stands on ends. If you take off
these clothes in the dark, you see even
a spark and hear crackling sound. In
1752 Benjamin Franklin, an American
scientist, showed that lightning and the
spark from your clothes are essentially
the same phenomena. However, this
realisation took 2000 years.
Scientific discoveries are a
result of hardwork by many
people. It can sometime takes
a long time.
I wonder why they took so
many years to realise the
similarity.
We shall now study some properties
of electric charges. We shall also see how
they are related to the lightning in the
sky.
Let us perform some activities to
understand the nature of electric
charges. But recall first what you might
have played as a game. When you rub
a plastic scale on your dry hair, the
scale can attract very small pieces of
paper.
15.2 Charging by rubbing
Activity 15.1
Take a used ballpen refill and rub
it vigorously with a piece of
polythene. Bring it close to small
pieces of paper. Take care not to
touch the rubbed end of the refill
with your hand or with a metallic
object. Repeat the activity with small
pieces of dry leaf, husk and mustard
seeds. Record your observations.
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.
Let’s try to charge some other
objects  that are familiar to you.
Activity 15.2
Collect the objects and the
materials listed in Table 15.1. Try
to charge each by rubbing with the
materials mentioned in the Table.
Record your findings.
You can add more items to the
Table.
Table 15.1
Objects rubbed Materials Attracts/does not Charged
used for rubbing attract pieces /not charged
of paper
Refill Polythene,
woollen cloth
Balloon Polythene, woollen
cloth, dry hair
Eraser Wool
Steel spoon Polythene,
woollen cloth
SOME NATURAL PHENOMENA 187
Page 3


SOME NATURAL PHENOMENA
SOME NATURAL PHENOMENA
In Class VII you read about winds,
storms and cyclones. You learnt that
cyclones can cause a lot of damage to
human life and property. You also
learnt that we can protect ourselves from
these destructive phenomena to some
extent. In this chapter we shall discuss
two other destructive natural
phenomena. These are lightning and
earthquakes. We shall also discuss what
steps we can take to minimise
destruction caused by these
phenomena.
15.1Lightning
You might have seen sparks on a
electric pole when wires become loose.
This phenomenon is quite common
when a wind is blowing and shacking
the wires. You might also have seen
sparks 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 was
visiting them. Now, of course, we
understand that lightning is caused by
the accumulation of charges in the
clouds. We need not be afraid of
lightning, but we have to take
precautions to protect ourselves from
the deadly sparks.
The Sparks that the Greeks Knew
About
The ancient Greeks knew as early as
600 B.C. that when amber (amber is a
kind of resin) was rubbed with fur, it
attracted light objects such as hair. You
might have seen that when you take
off woollen or polyester clothes, your
hair stands on ends. If you take off
these clothes in the dark, you see even
a spark and hear crackling sound. In
1752 Benjamin Franklin, an American
scientist, showed that lightning and the
spark from your clothes are essentially
the same phenomena. However, this
realisation took 2000 years.
Scientific discoveries are a
result of hardwork by many
people. It can sometime takes
a long time.
I wonder why they took so
many years to realise the
similarity.
We shall now study some properties
of electric charges. We shall also see how
they are related to the lightning in the
sky.
Let us perform some activities to
understand the nature of electric
charges. But recall first what you might
have played as a game. When you rub
a plastic scale on your dry hair, the
scale can attract very small pieces of
paper.
15.2 Charging by rubbing
Activity 15.1
Take a used ballpen refill and rub
it vigorously with a piece of
polythene. Bring it close to small
pieces of paper. Take care not to
touch the rubbed end of the refill
with your hand or with a metallic
object. Repeat the activity with small
pieces of dry leaf, husk and mustard
seeds. Record your observations.
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.
Let’s try to charge some other
objects  that are familiar to you.
Activity 15.2
Collect the objects and the
materials listed in Table 15.1. Try
to charge each by rubbing with the
materials mentioned in the Table.
Record your findings.
You can add more items to the
Table.
Table 15.1
Objects rubbed Materials Attracts/does not Charged
used for rubbing attract pieces /not charged
of paper
Refill Polythene,
woollen cloth
Balloon Polythene, woollen
cloth, dry hair
Eraser Wool
Steel spoon Polythene,
woollen cloth
SOME NATURAL PHENOMENA 187 SCIENCE 188
15.3 Types of Charges and
their Interaction
We will select some objects from Table
15.1 for the next activity.
Activity 15.3
a) Inflate two balloons. Hang them
in such a way that they do not touch
each other (Fig. 15.1). Rub both the
balloons with a woollen cloth and
release them. What do you observe?
refill also with polythene. Bring it
close to the charged refill. Be careful
not to touch the charged end with
your hand. Is there any effect on
the refill in the tumbler? Do the two
attract each other, or repel each
other?
In this activity we have brought close
together the charged objects that
were made of the same material.
What happens if two charged objects
made of different materials are
brought close to each other? Let’s
find out.
b) Rub a refill and place it gently in
a glass tumbler as before (Fig. 15.3).
Bring an inflated charged balloon
near the refill and observe.
Fig. 15.1 : Like charges repel each other
Fig. 15.2 : Interaction between like charges
Now let us repeat this activity with
the used pen refills. Rub one refill
with polythene. Place it carefully in
a glass tumbler using the tumbler
as a stand (Fig. 15.2). Rub the other
Fig. 15.3 : Unlike charges attract each other
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.
Does it indicate that the charge on
the balloon is of a different kind from
the charge on the refill? Can we say
then, that there are two kinds of
Page 4


SOME NATURAL PHENOMENA
SOME NATURAL PHENOMENA
In Class VII you read about winds,
storms and cyclones. You learnt that
cyclones can cause a lot of damage to
human life and property. You also
learnt that we can protect ourselves from
these destructive phenomena to some
extent. In this chapter we shall discuss
two other destructive natural
phenomena. These are lightning and
earthquakes. We shall also discuss what
steps we can take to minimise
destruction caused by these
phenomena.
15.1Lightning
You might have seen sparks on a
electric pole when wires become loose.
This phenomenon is quite common
when a wind is blowing and shacking
the wires. You might also have seen
sparks 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 was
visiting them. Now, of course, we
understand that lightning is caused by
the accumulation of charges in the
clouds. We need not be afraid of
lightning, but we have to take
precautions to protect ourselves from
the deadly sparks.
The Sparks that the Greeks Knew
About
The ancient Greeks knew as early as
600 B.C. that when amber (amber is a
kind of resin) was rubbed with fur, it
attracted light objects such as hair. You
might have seen that when you take
off woollen or polyester clothes, your
hair stands on ends. If you take off
these clothes in the dark, you see even
a spark and hear crackling sound. In
1752 Benjamin Franklin, an American
scientist, showed that lightning and the
spark from your clothes are essentially
the same phenomena. However, this
realisation took 2000 years.
Scientific discoveries are a
result of hardwork by many
people. It can sometime takes
a long time.
I wonder why they took so
many years to realise the
similarity.
We shall now study some properties
of electric charges. We shall also see how
they are related to the lightning in the
sky.
Let us perform some activities to
understand the nature of electric
charges. But recall first what you might
have played as a game. When you rub
a plastic scale on your dry hair, the
scale can attract very small pieces of
paper.
15.2 Charging by rubbing
Activity 15.1
Take a used ballpen refill and rub
it vigorously with a piece of
polythene. Bring it close to small
pieces of paper. Take care not to
touch the rubbed end of the refill
with your hand or with a metallic
object. Repeat the activity with small
pieces of dry leaf, husk and mustard
seeds. Record your observations.
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.
Let’s try to charge some other
objects  that are familiar to you.
Activity 15.2
Collect the objects and the
materials listed in Table 15.1. Try
to charge each by rubbing with the
materials mentioned in the Table.
Record your findings.
You can add more items to the
Table.
Table 15.1
Objects rubbed Materials Attracts/does not Charged
used for rubbing attract pieces /not charged
of paper
Refill Polythene,
woollen cloth
Balloon Polythene, woollen
cloth, dry hair
Eraser Wool
Steel spoon Polythene,
woollen cloth
SOME NATURAL PHENOMENA 187 SCIENCE 188
15.3 Types of Charges and
their Interaction
We will select some objects from Table
15.1 for the next activity.
Activity 15.3
a) Inflate two balloons. Hang them
in such a way that they do not touch
each other (Fig. 15.1). Rub both the
balloons with a woollen cloth and
release them. What do you observe?
refill also with polythene. Bring it
close to the charged refill. Be careful
not to touch the charged end with
your hand. Is there any effect on
the refill in the tumbler? Do the two
attract each other, or repel each
other?
In this activity we have brought close
together the charged objects that
were made of the same material.
What happens if two charged objects
made of different materials are
brought close to each other? Let’s
find out.
b) Rub a refill and place it gently in
a glass tumbler as before (Fig. 15.3).
Bring an inflated charged balloon
near the refill and observe.
Fig. 15.1 : Like charges repel each other
Fig. 15.2 : Interaction between like charges
Now let us repeat this activity with
the used pen refills. Rub one refill
with polythene. Place it carefully in
a glass tumbler using the tumbler
as a stand (Fig. 15.2). Rub the other
Fig. 15.3 : Unlike charges attract each other
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.
Does it indicate that the charge on
the balloon is of a different kind from
the charge on the refill? Can we say
then, that there are two kinds of
SOME NATURAL PHENOMENA 189
charges? Can we also say that 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.  The
other kind of charge is said to be
negative.
It is observed that when a charged
glass rod is brought near a charged
plastic straw rubbed with polythene
there is attraction between the two.
What do you think would be the kind
of charge on the plastic straw? Your
guess, that the plastic straw would carry
a negative charge is correct.
The electrical charges generated by
rubbing are static. They do not move by
themselves. When charges move, they
constitute an electric current. You have
been reading about electric current since
Class VI.  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.
15.4Transfer of Charge
Activity 15.4
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 could be inserted. Open
out paper clip as shown in Fig. 15.4.
Cut two strips of aluminium foil
about 4 cm × 1 cm each. Hang them
on the paper clip as shown. Insert
the paper clip in the cardboard lid
so that it is perpendicular to it (Fig.
15.4). Charge a refill and touch it
Fig 15.4 : A simple electroscope
with the end of the paper clip.
Observe what happens. Is there any
effect on the foil strips? Do they
repel each other or attract each
other? Touch now, other charged
bodies with the end of the paper clip.
Do foil strips behave in the same
way in all cases? Can this apparatus
be used to detect whether a body is
charged or not? Can you explain
why the foil strips repel each other?
The aluminium foil strips receive the
same charge from the charged refill
through the paper clip (remember that
metals are good conductors of
electricity). The strips carrying similar
charges repel each other and they
become wide open. Such a device can
be used to test whether an object is
carrying charge or not. This device is
known as electroscope.
Thus, we find that electrical charge
can be transferred from a charged object
to another through a metal conductor.
Touch the end of the paper clip gently
with hand and you will find a change in
Page 5


SOME NATURAL PHENOMENA
SOME NATURAL PHENOMENA
In Class VII you read about winds,
storms and cyclones. You learnt that
cyclones can cause a lot of damage to
human life and property. You also
learnt that we can protect ourselves from
these destructive phenomena to some
extent. In this chapter we shall discuss
two other destructive natural
phenomena. These are lightning and
earthquakes. We shall also discuss what
steps we can take to minimise
destruction caused by these
phenomena.
15.1Lightning
You might have seen sparks on a
electric pole when wires become loose.
This phenomenon is quite common
when a wind is blowing and shacking
the wires. You might also have seen
sparks 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 was
visiting them. Now, of course, we
understand that lightning is caused by
the accumulation of charges in the
clouds. We need not be afraid of
lightning, but we have to take
precautions to protect ourselves from
the deadly sparks.
The Sparks that the Greeks Knew
About
The ancient Greeks knew as early as
600 B.C. that when amber (amber is a
kind of resin) was rubbed with fur, it
attracted light objects such as hair. You
might have seen that when you take
off woollen or polyester clothes, your
hair stands on ends. If you take off
these clothes in the dark, you see even
a spark and hear crackling sound. In
1752 Benjamin Franklin, an American
scientist, showed that lightning and the
spark from your clothes are essentially
the same phenomena. However, this
realisation took 2000 years.
Scientific discoveries are a
result of hardwork by many
people. It can sometime takes
a long time.
I wonder why they took so
many years to realise the
similarity.
We shall now study some properties
of electric charges. We shall also see how
they are related to the lightning in the
sky.
Let us perform some activities to
understand the nature of electric
charges. But recall first what you might
have played as a game. When you rub
a plastic scale on your dry hair, the
scale can attract very small pieces of
paper.
15.2 Charging by rubbing
Activity 15.1
Take a used ballpen refill and rub
it vigorously with a piece of
polythene. Bring it close to small
pieces of paper. Take care not to
touch the rubbed end of the refill
with your hand or with a metallic
object. Repeat the activity with small
pieces of dry leaf, husk and mustard
seeds. Record your observations.
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.
Let’s try to charge some other
objects  that are familiar to you.
Activity 15.2
Collect the objects and the
materials listed in Table 15.1. Try
to charge each by rubbing with the
materials mentioned in the Table.
Record your findings.
You can add more items to the
Table.
Table 15.1
Objects rubbed Materials Attracts/does not Charged
used for rubbing attract pieces /not charged
of paper
Refill Polythene,
woollen cloth
Balloon Polythene, woollen
cloth, dry hair
Eraser Wool
Steel spoon Polythene,
woollen cloth
SOME NATURAL PHENOMENA 187 SCIENCE 188
15.3 Types of Charges and
their Interaction
We will select some objects from Table
15.1 for the next activity.
Activity 15.3
a) Inflate two balloons. Hang them
in such a way that they do not touch
each other (Fig. 15.1). Rub both the
balloons with a woollen cloth and
release them. What do you observe?
refill also with polythene. Bring it
close to the charged refill. Be careful
not to touch the charged end with
your hand. Is there any effect on
the refill in the tumbler? Do the two
attract each other, or repel each
other?
In this activity we have brought close
together the charged objects that
were made of the same material.
What happens if two charged objects
made of different materials are
brought close to each other? Let’s
find out.
b) Rub a refill and place it gently in
a glass tumbler as before (Fig. 15.3).
Bring an inflated charged balloon
near the refill and observe.
Fig. 15.1 : Like charges repel each other
Fig. 15.2 : Interaction between like charges
Now let us repeat this activity with
the used pen refills. Rub one refill
with polythene. Place it carefully in
a glass tumbler using the tumbler
as a stand (Fig. 15.2). Rub the other
Fig. 15.3 : Unlike charges attract each other
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.
Does it indicate that the charge on
the balloon is of a different kind from
the charge on the refill? Can we say
then, that there are two kinds of
SOME NATURAL PHENOMENA 189
charges? Can we also say that 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.  The
other kind of charge is said to be
negative.
It is observed that when a charged
glass rod is brought near a charged
plastic straw rubbed with polythene
there is attraction between the two.
What do you think would be the kind
of charge on the plastic straw? Your
guess, that the plastic straw would carry
a negative charge is correct.
The electrical charges generated by
rubbing are static. They do not move by
themselves. When charges move, they
constitute an electric current. You have
been reading about electric current since
Class VI.  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.
15.4Transfer of Charge
Activity 15.4
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 could be inserted. Open
out paper clip as shown in Fig. 15.4.
Cut two strips of aluminium foil
about 4 cm × 1 cm each. Hang them
on the paper clip as shown. Insert
the paper clip in the cardboard lid
so that it is perpendicular to it (Fig.
15.4). Charge a refill and touch it
Fig 15.4 : A simple electroscope
with the end of the paper clip.
Observe what happens. Is there any
effect on the foil strips? Do they
repel each other or attract each
other? Touch now, other charged
bodies with the end of the paper clip.
Do foil strips behave in the same
way in all cases? Can this apparatus
be used to detect whether a body is
charged or not? Can you explain
why the foil strips repel each other?
The aluminium foil strips receive the
same charge from the charged refill
through the paper clip (remember that
metals are good conductors of
electricity). The strips carrying similar
charges repel each other and they
become wide open. Such a device can
be used to test whether an object is
carrying charge or not. This device is
known as electroscope.
Thus, we find that electrical charge
can be transferred from a charged object
to another through a metal conductor.
Touch the end of the paper clip gently
with hand and you will find a change in
SCIENCE 190
the foil strips. They come back to their
original state. Repeat charging of foil
strips and touching the paper clip. Every
time you will find that the foil strips
collapse as soon as you touch the
paperclip with hand. Why does it
happen? The reason is that the foil strips
lose charge to the earth through your
body. We say that the foil strips are
discharged. The process of transfering
of charge from a charged object to the
earth is called earthing.
Earthing is provided in buildings to
protect us from electrical shocks due
to any leakage of electrical current.
15.5 The Story of Lightning
It is now possible to explain lightning
in terms of the charges produced by
rubbing.
You learnt in Class VII that during
the development of a thunderstorm, the
air currents move upward while the
water droplets move downward. These
vigorous movements cause separation
of charges. By a process, not yet
completely understood, 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. 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
as lightning (Fig. 15.5). The process is
called an electric discharge.
Fig 15.5 : Accumulation of charges leading to lightning.
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