NAVODAYA VIDYALAYA SAMITI

• CLASS- VIII
• SUBJECT – SCIENCE
• CHAPTER- 15 SOME NATURAL PHENOMENA

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• CREATED BY : P.K.MEENA
• (T.G.T, SCIENCE)
• ( JNV,GODDA,JH.)

Some natural destructive phenomena

etc.

Some natural destructive phenomena are cyclones, lightning, earthquakes

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 phenomena include volcanic eruptions, weather, and decay. Most natural phenomena, such as rain, are relatively harmless so far as humans are concerned.

• There are various types of natural phenomena, which include (and are not limited to), Meteorlogical phenomena, weather, including hurricanes, thunderstorms and tornadoes, and geological phenomena , including volcanic activity and earthquakes.

Lightning

• 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 clouds.

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.

c) Charges acquired by different objects

• A glass rod rubbed with silk gets positively charged.
• A plastic rod rubbed with fur gets negatively charged.

Frictional order

• Wool
• Flannel
• Fur
• Sealing wax
• Glass
• Paper
• Silk
• Hand
• Rubber
• Amber
• Ebonite
• Plastic

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 2balloons. 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 you observe?

Answer : A charged balloon repelled a charged balloon.

• 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 charged refill.

• It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive.

The other kinds 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.

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 paper clip.
• 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 other?

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 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 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 electrical current.

•

Transfer of charge

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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 other.

Electroscope

• 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 ourselves.

Lightning conductor

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

1. If you are outdoors :-
1. Open vehicles like motorbikes, trucks, open cars are not safe.
2. Open fields, parks, elevated places, tall trees are not safe.
3. Carrying an umbrella is not safe.
4. If in a forest take shelter under short trees.
5. 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.

2. If inside the house :-
2. Lightning can strike electric wires, telephone cords and metal pipes so contact with these objects should be avoided.
3. It is safer to use mobile and cordless phones than wired phones.
4. Electrical appliances like TV, computers etc should be unplugged.
5. Bathing should be avoided to avoid contact with running water.

Earthquakes

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