LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

EARTHQUAKES:

"An earthquake is a sudden shaking or trembling of the earth which lasts for a very short period."

"Earthquakes can cause immense damage to houses, other buildings, bridges, dams and people etc. A lot of people get killed when they get buried under the debris of collapsed houses and other buildings during an earthquake. Earthquakes can also cause floods, landslides and tsunamis."

Earthquake is a destructive natural phenomenon which can not be predicted in advance. 

STRUCTURE OF EARTH:

The inside of earth is made up of three main layers:  Core, Mantle, Crust. 

Core: The innermost part of the earth is called its core. The core of earth is made up mostly of iron. The core of earth is extremely hot. Most of the earth's core called outer core is liquid (molten iron) whereas the inner part of the core called inner core is under such high pressure that it is solid (solid iron).  

Mantle: The central region of earth (between the core and crust) is called mantle. Mantle is mostly made of dense, solid rocks. Some of the mantle is, however, a mixture of solid rocks and hot molten rocks (liquid rocks) like the lava from a volcano. Heat coming from the core of earth warms the mantle. This heating sets up huge convection currents in the mantle. The giant convection currents occuring in the mantle can make the mantle move very slowly.

Crust:

The outermost layer of earth is called crust. The crust of earth is made of comparatively lighter rocks than that of mantle. The crust of earth is thicker where there is land. The crust of Earth is not in one piece. It is divided into many plates and those are called continental plates and oceanic plates. The plates of earth on which continents exist are called continental plates, whereas those plates of earth on which oceans exist are called oceanic plates. The solid plates which make up the earth's crust are floating on the partially molten rocks of mantle beneath. Due to convection currents taking place in the mantle, the plates of earth's crust are moving around very, very slowly. The reason the earthquakes occur is that the earth's crust is made of a number of plates which are able to move. 

Why do Earthquakes occur:

The Earthquakes occur when the moving plates of the earth's crust:

(i) slide past one another, and

(ii) collide with one another. 

When the entangled rocks of the crust plates break open suddenly with a big jolt, the Earthquake occurs releasing a tremendous amount of energy. 

A tremendous amount of energy is released when two plates collide with eache other. This large amount of energy produces a wave throughout the earth, which is known as shock wave. It is also known as Seismic Waves. 

(iii) Tremors on the earth can also be caused due to the strike of large mateorites to the earth and it can be due to the  explosion of nuclear devices. 

SEISMIC ZONES ( FAULT ZONES )

The weak zones of earth 's crust which are prone to earthquake are called seismic zone.

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

Lightning:

Lightning is an electric Spark which is caused by the accumulation of electric charges in the clouds. 

Electric charges:

Electric charges are the property of matter. When Amber is rubbed with silk cloth, both the Amber and the silk cloth become charged. Similarly when a rubber balloon is rubbed with a woolen cloth The rubber balloon gets electric charges due to friction. 

Uncharged and charged objects:

An object having no electric charge on it is called an uncharged object. An object having electric charge on it is called a charged object. 

If we take a glass rod and bring it near some tiny pieces of paper it will not have any effect on them, but if the glass rod is first rubbed with a piece of silk cloth and then brought near the tiny pieces of paper then the glass rod attracts the tiny pieces of paper towards itself. It means that when the glass rod is rubbed with silk cloth then it gets electric charge. 

The process of giving electric charge to an object is called charging the object. 

Charging an object by rubbing:

The charging of an object by rubbing it with another object is called charging by friction. The electric charges generated by rubbing are static electric charges. These electric charges remain bound on the surface of the charged object. 

Friction charges both the objects which are rubbed together. 

When two objects are rubbed together then both the objects get charged by friction. 

When a glass rod is rubbed with silk cloth, glass rod becomes positively charged and the silk cloth becomes negatively charged. 

Two types of electric charges and their interaction:

There are two types of electric charges positive charges and negative charges. 

A positive charge repels another positive charge but attracts a negative charge. Similarly a negative charge repels another negative charge, but attracts a positive charge. 

Q. When a charged glass rod rubbed with silk cloth is brought near a charged plastic straw rubbed with polythene there is attraction between the two. What is the nature of charge on the plastic straw?

Ans: when a glass rod is rubbed with silk cloth, glass rod becomes positively charged, if it attracts a plastic straw, then plastic straw must be negatively charged. 

Electroscope:

The electric charge on an object can be detected by using an instrument called electroscope. 

The electroscope is a device for detecting electric charge on an object. 

Working of an electroscope:

Charge a glass rod by rubbing its own and with a piece of silk cloth. Touch the charged glass rod with the top and of metal clip. We will see that the two aluminium leaves move away from each other. We say that the aluminium leaves diverge or open up. 

When we touch the top end of metal clip with the positively charged glass rod, then some of its positive charge is transferred to the top and of metal clip. Since the metal clip is a good conductor of electricity, it conducts the positive electric charge to the two aluminium leaves held on its other end. In this way the two aluminium leaves get charged with the same kind of electric charge. And therefore they repel each other. 

Electric discharge: production of sparks

The passage of electric current in air due to movement of electric charges is called electric discharge. During electric discharge the positive and negative electric charges cancel out each other and and electric Spark and the crackling sound are produced. In nature, electric discharge within a cloud during thunderstorm produces use electric sparks known as lightning along with a loud sound called thunder. And electric discharge between a thunder cloud and the earth also produces lightning followed by thunder. 

LIGHTNING:

The bright flash of light which we see in the clouds is called lightning. 

A cloud is a visible mass of condensed water vapour floating in the atmosphere high above the ground. A heavy dark rain cloud is also called storm cloud. When a storm cloud develops in the sky strong winds move upwards through the cloud and make the water drops present in the cloud to rub against one another. This rubbing together of water drop produces extremely large electric charges in the cloud due to friction. The small water drops acquire a positive charge and being lighter move to the upper part of the cloud with rising wind. On the other hand the larger water drops acquire a negative charge and being here come down in the lower part of the cloud. In this way the top of the cloud becomes positively charged where is the bottom of the cloud becomes negatively charged. 

When the amount of opposite electric charges on the top and bottom of a storm cloud becomes extremely large then electric charges start flowing with high speed through the air between them. When the positive and negative charges of a cloud meet they produce and intense Spark of electricity between the cloud in the sky. This electric Spark as a flash of lightning in the sky. The electric sparks of lightning heat the nearby air in the sky to very high temperatures due to this heat the air in the sky expands repeatedly and produces a loud sound which we call thunder. 

Lightning is usually occurs within a cloud in the sky. It is called sheet lightning. Lightning also occurs between a cloud and the earth or tall all objects of the earth. It is then called fork lightning.

Storm clouds carry electric charges. Now if a storm cloud having negative charges at its bottom passes over a tall building it induces positive charges on the roof of the building. When the electric charges on the bottom of the cloud become extremely large, then this tremendous electric charges present on the bottom of the charged cloud suddenly starts to flow to the roof of the building and we see a flash of lightning coming towards the building.

Lightning strikes are more frequent in the hilly areas because in such areas clouds are comparatively closer to the ground than in the plains. 

When we take off woolen or synthetic clothes like polyester or nylon class sometimes we hear a crackling sound and if it is a dark or as during night we can even see tiny sparks. 

Dangers of lightning:

When lightning strikes the earth it can cause a lot of destruction by damaging property building, trees and killing people. The damage caused to buildings and other structures by lightning can be prevented by installing lightning conductors on them. 

Lightning conductor:

Lightning conductor is a device used to protect a building from the effects of lightning. 

The tall buildings and other tall structures are protected from lightning strikes by using a device calling lightning conductor. A lightning conductor is made of a thick strip of metal made of copper. The top end of lightning conductor is pointed like a sharp spike and it is fixed above the highest point of the building.

 From the top of a building the thick metal strip runs along the outer wall of the building to the ground. The lower end of metal strip is joined to a metal plate and buried deep in the ground near the base of the building. 

If lightning strikes it will hit the top of the lightning conductor rather than the building. The electric energy of lightning passes through the metal strip and gets discharged safely into the ground through the buried metal plate. Since no electric energy produced by lightning passes through the building no damage is caused to it.

Measures to protect ourselves from lightning:

We can take the following measures to protect ourselves from lightning strikes during a thunderstorm. 

1. No open space is safe during lightning and thunderstorms. A house is a safe place during a lightning. 

2. Open vehicles like motorbikes, scooters, tractors and construction machinery are not safe during lightning and thunderstorm. 

3. When in open space, a person should never stand under a tree to take shelter during a thunderstorm because there is danger of lightning striking the tree and burning it up. 

4. When in open space, a person should not lie on the ground during thunderstorm and lightning. A person should squat low on the ground during lightning. 

5. We should avoid raising an umbrella over our head during lightning. 

6. The TV antennas and dish antennas fixed on the tall buildings are especially prone to lightning strikes. We should, therefore, switch off our TV sets during frequent lightning otherwise TV sets may get burnt. 

7. Lightning can strike metal pipes fixed in buildings. So, during a thunderstorm when lightning is taking place, we should avoid touching the metal pipes fixed in a house or building.