Science Class Eight  


Some Natural Phenomena

Lightning

We always see sparks on a electric pole when wires become loose. This phenomenon is quite common when a wind is blowing and shaking the wires. We also often see sparks when a plug is loose in its socket. Lightning is also an electric spark, but on a huge scale.

Lightning is an atmospheric discharge of electricity usually accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. Lightning is caused when two clouds collide or slide against one another. This involves same process which happens when you rub a comb on your dry hair and the hair gets an electric charge.

As amount of charge generated during collision of clouds is huge so we witness it as flashing lights in the sky. This flashing light is called lightning. Once a cloud gets electrically charged it releases the charge to earth. In other words the earth receives the charge and neutralizing the cloud in the process.

Lightning Safety:

During lightning and thunderstorm no open place is safe.

• Hearing thunder is an alert to rush to a safer place.

• After hearing the last thunder, wait for some time before coming out of the safe place.

Do’s and Don’ts during a Thunderstorm:

Outside: Open vehicles, like motorbikes, tractors, construction machinery, open cars are not safe. Open fields, tall trees, shelters in parks, elevated places do not protect us from lightning strokes. Carrying umbrella is not a good idea at all during thunderstorms. If in a forest, we should take shelter under shorter trees. If no shelter is available and we are in an open field, stay far away from all trees. We should stay away from poles or other metal objects. We should not lie on the ground. Instead, we should squat low on the ground.

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. 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: 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 is buried deep in the ground. The rod provides easy route for the transfer of electric charge to the ground. The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.

You can notice trident shaped iron rod on top of temples, TV towers and cell phone towers. These are lightning conductors.

Earthquake

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 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 to life and property. The earthquakes can cause floods, landslides and tsunamis.

Causes of Earthquake

The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a tectonic plate. These plates are in continual motion. When they brush past one another, or a plate goes under another due to collision, they cause 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, or a meteor hits the earth, or an underground nuclear explosion is carried out. However, most earthquakes are caused by the movement of 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. In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North East, Rann of Kutch, Rajasthan and the Indo – Gangetic Plane. Some areas of South India also fall in the danger zone

Power of Earthquakes: The power of an earthquake is expressed in terms of a magnitude on a scale called Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale. Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5.

Protection against Earthquakes

It is, important that we take necessary precautions to protect ourselves all the time. People living in seismic zones, where the earthquakes are more likely to occur, have to be specially prepared. First of all, the buildings in these zones should be designed so that they can withstand major tremors. Modern building technology can make it possible. It is advisable to make the structure simple so that it is ‘Quake Safe’.

• Consult qualified architects and structural engineers. In highly seismic areas, the use of mud or timber is better than the heavy construction material. Keep roofs as light as possible. In case the structure falls, the damage will not be heavy.

• It is better if the 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.

Summary

• 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.

• 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.

• 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 earth’s plates. These boundaries are known as fault zones.

• Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property.

• We should take necessary precautions to protect ourselves from earthquakes.

 
 

 
                 Aesop's Fables - Story for kids
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