Class 8 Science Some Natural Phenomena Exam Notes

Class 8 Science Some Natural Phenomena Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Covers the following topics:

1.Charging by Rubbing

Class 8 Science: Some Natural Phenomena Exam Notes

CHARGING BY RUBBING

The ancient Greeks knew as early as 600 B.C. that when amber (a kind of resin) was rubbed with fur, it attracted light objects such as hair. If you take off woollen clothes in the dark, you may 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 phenomenon.

When a plastic comb is rubbed with dry hair, it acquires a small charge. Such objects are called charged objects. In the process of charging the plastic comb, the hair also gets charged. The method of charging by friction is called frictional charging.

TYPES OF CHARGES AND THEIR INTERACTION

There are two kinds of charges. Charges of the same kind repel each other, while charges of different kinds attract each other.

It is a convention to call the charge acquired by a glass rod when it is rubbed with silk positive. The other kind of charge is said to be negative.

TRANSFER OF CHARGE AND EARTHING

Take an empty jam bottle. Take a piece of cardboard slightly bigger than the mouth of the bottle. Pierce a hole in it so that a metal paper clip can be inserted. Cut two strips of aluminium foil. Hang them on the paper clip as shown. Insert the paper clip in the cardboard lid so that it is perpendicular to it. Charge a refill and touch it with the end of the paper clip. Observe that the foil strips repel each other.

A simple electroscope.

This device is known as an 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 your hand and you will find a change in the foil strips. They come back to their original state. 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 charge from a charged object to the earth is called earthing.

RAIN

Water from the water bodies on the surface of the earth is converted into water vapour by evaporation, goes into the atmosphere and rises up in the air. The water droplets float in the air in the form of clouds. These water droplets collide against each other and stick together to form bigger water droplets. When these water droplets become too heavy to float, they come down as rain.

LIGHTNING

During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges. 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 the 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 the earth, and at that moment the air near the lightning path reaches up to 30,000°C.

LIGHTNING SAFETY

Finding a safe place: A house or a building is a safe place. If you are travelling by car or by bus, you are safe inside with the windows and doors of the vehicle shut.

If outside the home:

• Open vehicles, like motorbikes, tractors, construction machinery and open cars, are not safe. Open fields, tall trees, shelters in parks and elevated places do not protect us from lightning strokes. Carrying an umbrella is not a good idea at all during thunderstorms.
• If you are in a forest, take shelter under shorter trees. If no shelter is available and you are in an open field, stay far away from all trees. Stay away from poles or other metal objects. Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between your hands. This position will make you the smallest target to be struck.

If inside the house:

• Lightning can strike telephone cords, electrical wires and metal pipes. During a thunderstorm, contact with these should be avoided.
• It is safer to use mobile phones and cordless phones. However, it is not wise to call up a person who is receiving your call on a wired phone.
• Bathing should be avoided during thunderstorms to avoid contact with running water.
• Electrical appliances like computers, TVs, etc., should be unplugged. Electrical lights can remain on. They do not cause any harm.

LIGHTNING CONDUCTORS

A lightning conductor is a device used to protect buildings from the effect of lightning. A metallic rod, taller than the building, is installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other end is buried deep in the ground. The rod provides an easy route for the transfer of electric charge to the ground.

Lightning conductor.

The metal columns used during construction, and the electrical wires and water pipes in the buildings, also protect us to an extent. But do not touch them during a thunderstorm.

EARTHQUAKES

If a thunderstorm occurs there is always a possibility of lightning and cyclones accompanying it. So we get time to take measures to protect ourselves from the damage caused by these phenomena. There is, however, one natural phenomenon which we are not yet able to predict. It is an earthquake. It can cause damage to human life and property on a huge scale.

An earthquake is a sudden shaking or trembling of the earth lasting for a very short time. It is caused by a disturbance deep inside the earth's crust.

Earthquakes occur all the time, all over the earth. They are often not even noticed. Major earthquakes are much less frequent. They can cause immense damage to buildings, bridges, dams and people. There can be a great loss of life and property.

Earthquakes can cause floods, landslides and tsunamis. A major tsunami occurred in the Indian Ocean on 26th December 2004. All the coastal areas around the ocean suffered huge losses.

CAUSES OF EARTHQUAKES

The tremors are caused by a disturbance deep down inside the uppermost layer of the earth, called the crust.

Structure of the earth.

The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another, or when a plate goes under another due to collision, they cause a disturbance in the earth's crust. It is this disturbance that shows up as an earthquake on the surface of the earth.

Tremors on the earth can also be caused when a volcano erupts, a meteor hits the earth, or an underground nuclear explosion is carried out. However, most earthquakes are caused by the movement of the earth's plates.

Since earthquakes are caused by the movement of plates, 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. The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale.

PROTECTION AGAINST EARTHQUAKES

(a) Before earthquakes

• Consult qualified architects and structural engineers.
• In highly seismic areas, the use of mud or timber is better than heavy construction material. Keep roofs as light as possible. In case the structure falls, the damage will not be heavy.
• It is better if cupboards and shelves are fixed to the walls, so that they do not fall easily.
• Be careful where you hang wall clocks, photo frames, water heaters, etc., so that in the event of an earthquake they do not fall on people.
• Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have fire-fighting equipment in working order.

(b) At the time of earthquakes

In the event that an earthquake does strike, take the following steps to protect yourself.

If you are at home:

• Take shelter under a table and stay there till the shaking stops.
• Stay away from tall and heavy objects that may fall on you.
• If you are in bed, do not get up. Protect your head with a pillow.

If you are outdoors:

• Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
• If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.

SEISMOGRAPH

The tremors produce waves on the surface of the earth. These are called seismic waves. The waves are recorded by an instrument called the seismograph.

The instrument is simply a vibrating rod, or a pendulum, which starts vibrating when tremors occur. A pen is attached to the vibrating system. The pen records the seismic waves on a paper which moves under it. By studying these waves, scientists can construct a complete map of the earthquake. They can also estimate its power to cause destruction.

Like many other scales in science (decibel is another example), the Richter scale is not linear. This means that an earthquake of magnitude 6 does not have one and a half times the destructive energy of an earthquake of magnitude 4. In fact, an increase of 2 in magnitude means 1000 times more destructive energy. For example, an earthquake of magnitude 6 has a thousand times more destructive energy than an earthquake of magnitude 4.

IMPORTANT POINTS TO REMEMBER

• Some objects can be charged by rubbing with other objects.
• There are two kinds of charges — positive charge and negative charge.
• Like charges repel and unlike charges attract each other.
• The electrical charges produced by rubbing are called static charges.
• When charges move, they constitute an electric current.
• An electroscope may be used to detect whether a body is charged or not.
• The process of transfer of charge from a charged object to the earth is called earthing.
• The process of electric discharge between clouds and the earth, or between different clouds, causes lightning.
• A lightning strike can destroy life and property.
• Lightning conductors can protect buildings from the effects of lightning.
• An earthquake is a sudden shaking or trembling of the earth.
• An earthquake is caused by a disturbance deep inside the earth's crust.
• It is not possible to predict the occurrence of an earthquake.
• Earthquakes tend to occur at the boundaries of the earth's plates. These boundaries are known as fault zones.
• The destructive energy of an earthquake is measured on the Richter scale. An earthquake measuring 7 or more on the Richter scale can cause severe damage to life and property.
• We should take necessary precautions to protect ourselves from earthquakes.

3.Transfer of charge & Earthing

4.Rain

5.Lightning

6.Lightning safety

7.Lightning conductors

8.Earthquakes

9.Causes of Earthquakes