Chapter SOME NATURAL PHENOMENA from CBSE science textbook 8th class
Published on: Mar 3, 2016
Transcripts - Natural phenomena
Some natural destructive phenomena
Some natural destructive phenomena are cyclones, lightning, earthquakes etc.
Lightning is streaks of bright light and sound caused by electric discharge
between clouds and earth or between different clouds.
Earthquake is a sudden shaking or trembling of the earth caused by a
disturbance deep inside the earth’s crust.
Lightning and earthquakes cause a lot of damage to human life and property.
A natural phenomenon is a
non-artificial event in the
physical sense, and
therefore not produced by
humans, although it may
affect humans (e.g.
bacteria, aging, natural
disasters, death.) Common
examples of natural
weather, and decay. Most
natural phenomena, such as
rain, are relatively harmless
so far as humans are
There are various types
of natural phenomena,
which include (and are
not limited to),
geological phenomena ,
You may have seen sparks
on an electric pole when
wires become loose. This
phenomenon is quite
common when wind is
blowing and shaking the
wires. You may have also
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 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
woolen or polyester clothes, your hair stands on end. If you take
off these clothes in the dark, you even see a spark and hear a
crackling sound. In 1752 Benjamin Franklin, an American
scientist, showed that lightning and the spark from your clothes
are essentially the same phenomena. However, it took 2000
years for this realization to occur.
Charging by rubbing (friction)
Some objects can be charged by rubbing with other objects. When objects
are rubbed, they attract light objects like pieces of paper. This process is called
charging by friction.
Eg :- A plastic refill, scale or comb when rubbed with dry hair, paper or
polythene gets charge and attracts pieces of paper.
The charges acquired by rubbing are static charges. They do not move. When
charges move, it is electric current.
Electric current is the motion of charges through a conductor.
Types of charges and their interaction
• There are two types of charges. They are positive charge (+) and
negative charge (-).
• Two charged bodies may attract or repel each other.
• Like (similar) charges repel each other and unlike (dissimilar)
charges attract each other.
a) Like charges repel each other
Activity :- Inflate two balloons and hang them in such a way
that they do not touch each other. Rub both the balloons with a
piece of woolen cloth and release them. They repel each other.
Activity :- Rub a plastic refill with polythene and place it in a
glass tumbler. Rub another plastic refill with polythene and bring
it near the charged refill. They repel each other.
b) Unlike charges attract each other
• Activity :- Charge a plastic refill by rubbing with polythene and
keep it in a glass tumbler. Charge an inflated balloon by rubbing
with a piece of woolen cloth and bring it near the charged plastic
refill. They attract each other.
• A glass rod rubbed with silk gets positively charged.
• A plastic rod rubbed with fur gets negatively charged.
c) Charges acquired by different objects
When any of these materials are rubbed together, the one in the higher order
gets positive charge(+) and the one in the lower order gets negative charge (-)
A glass rod rubbed with silk gets positive charge (+)
An ebonite rod rubbed with fur gets negative charge (-)
• (a) Inflate 2 balloons.
Hang them in such a
way that they do not
touch each other. Rub
both the balloons with
a woolen cloth and
release them. What do
Answer : A charged
balloon repelled a charged
• Now let us repeat this activity with
the used pen refills. Rub one refill
with polythene. Place it carefully in a
glass using the tumbler as a stand
• Rub the other 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?
Answer :A charged refill repelled a
• It is a convention to call the charge acquired
by a glass rod when it is rubbed with silk as
The other kinds of charge is said to
• It is observed that when a charged glass rod is
brought near a charged plastic straw rubbed
with polythene there is attraction between
Transfer Of Charge
• 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 can be
inserted. Open out the
• Cut two strips of
aluminum foil about 4cm
x 1cm each. Hang them
on the paper clip.
• Insert the paper clip in the cardboard lid so that it
is perpendicular to it. Charge a refill and touch it
with 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?
Now, touch other charged bodies with the end of
the paper clip. Do foil strips behave in the same
way in all cases? Why the foil strips repel each
Answer : The aluminum 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
• 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
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
paper clip 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
transferring of charge from charged object to the
earth is called earthing.
• Earthing is proving in buildings to protect us
from electrical shocks due to any leakage of
When a charged body is brought in contact with an uncharged body charges
pass from the charged body to the uncharged body.
This method of charging a body is called charging by contact or conduction.
If a body is charged by contact it gets the same charge as the charged body.
We can observe the transfer of charge by using an electroscope. If a charged
plastic rod is brought in contact with an electroscope, the charges are
transferred to the metal strips and they repel each other.
This is because both the metal strips get the same charge and they repel each
Transfer of charge
An electroscope is a device used to test whether an object is having electric
charge or not.
A simple electroscope can be made by using an
empty bottle, a piece of cardboard, a paper clip and
two small strips of aluminium foil. Pass one end of
the paper clip through the cardboard and hang the
two aluminiun strips from the other end.
If a charged body is touched to the end of the
paper clip, the aluminium strips repel each other.
The Story Of Lightning
• During the development of a
thunderstorm, the air current 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
become 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. The
process is called an electric discharge.
• The process of electric discharge can occur
between two or more clouds, or between
clouds and earth. Today we need not get
frightened by lightning like our ancestors did.
Now we understand the basic phenomenon.
Scientists are trying hard to improve this
understanding. However, lightning strike could
destroy life and property. It is, therefore,
necessary to take measures to protect
A lightning conductor is a device used to protect buildings from
damage due to lightning.
A lightning conductor has a metallic rod. The upper end is pointed
and the lower end is joined to a metal plate buried inside the
ground. When lightning strikes, the electric charges pass through it
to the earth without causing any damage to the building.
Protection from lightning and thunderstorm
a) If you are outdoors :-
i) Open vehicles like motorbikes, trucks, open cars are not safe.
ii) Open fields, parks, elevated places, tall trees are not safe.
iii) Carrying an umbrella is not safe.
iv) If in a forest take shelter under short trees.
v) If in an open field stay away from trees, poles, metallic objects. Do
not lie down. Squat on the ground with hands on the knees and
the head between the hands.
b) If inside the house :-
i) Lightning can strike electric wires, telephone cords and metal
pipes so contact with these objects should be avoided.
ii) It is safer to use mobile and cordless phones than wired phones.
iii) Electrical appliances like TV, computers etc should be unplugged.
iv) Bathing should be avoided to avoid contact with running water.
SAFE POSITION DURING LIGHTNING
if shelter is not available
An earthquake is a sudden shaking or trembling of the earth caused by
a disturbance deep inside the earth’s crust.
Earthquakes cause damage to buildings, dams, bridges. It can cause
landslides, floods and tsunamis. It can also cause damage to life and
What cause an earthquake ?
The outermost layer of the earth called crust is not one piece. It is fragmented
into many plates. These plates are moving continuously. When these plates
brush past each other or when one plate goes under another plate due to
collision, they cause disturbance and causes earthquakes.
The boundaries of the plates are weak zones called siesmic zones or fault
zones. Earthquakes often occur in the seismic zones.
MOVEMENTS OF EARTH’S PLATES
The seismic zones in India are Kashmir, Himalayas, Indo Gangetic plane, North
eastern region, Kutch, Rajasthan, Andaman And Nicobar Islands some regions of
South India etc.
The power of an earthquake is measured on a scale called Richter scale. Very
destructive earthquakes have magnitudes more than 7.5 on the Richter scale.
Earthquakes produce waves on the surface of the earth called seismic waves.
Seismic waves are recorded by an instrument called seismograph.
Seismic zones of India
Protection from earthquakes
i) The buildings in seismic zones should be designed such that they
can withstand tremors due to earthquakes.
ii) Materials like mud, wood and light roof should be used instead of
iii) Heavy objects like cupboards, water heaters etc. should not be fixed
on the walls.
iv) All buildings should have fire fighting equipments.
Precautions during an earthquake :-
a) If you are at home :-
i) Take shelter under a table.
ii) If you are in bed do not get up. Protect your head with a pillow.
iii) Stay away from tall and heavy objects which may fall on you.
b) If you are outdoors :-
i) Find an open area away from buildings, trees and power lines and
lie on the ground.
ii) If you are in a car or bus ask the driver to slow don and don’t get
• out till the tremors stop.