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15. Some Natural Phenomenon

This chapter talks about few natural phenomena like storms, cyclones, earthquakes etc that cause a lot of damage to human life & property

Lightning is an electric spark, on a huge scale, caused by the accumulation of charges in the clouds.

Ancient Greeks knew as early as 600 B.C. that Amber, a kind of resin, when rubbed with fur, attracted light objects like hair. However, in 1752, Benjamin Franklin, an American Scientist showed that lightning & the sparks & the crackling sound that we see in polyester & woolen clothes are essentially the same phenomena.

15.1     Charging by Rubbing

Charged Objects are the objects that acquire a small charge by rubbing it with other objects. Example:

  • When a plastic refill id rubbed with polythene, it acquires a small charge.
  • When a plastic comb is rubbed with dry hair, it acquires a small charge.

15.1.1       Types of charges

Charges are of two kinds – Positive & Negative

  • The charge acquired by a glass rod when it is rubbed with silk is positive.
  • The plastic when rubbed with polythene, acquires a Negative charge.

15.1.2             Interaction of charges

  • Charges of the same kind repel each other, while charges of different kind attract each other.
  • The electrical charges generated by rubbing are static as they do not move by themselves.
  • When charges move, they constitute an electric current. Eg. The current in a circuit which makes a bulb glow is actually a motion of charges.

15.1.3             Transfer of Charge

Electroscope: A device that is used to test whether an object is carrying charge or not is called as Electroscope.

The electrical charge can be transferred from a charged object to another a metal conductor.

15.1.3.1                Earthing

The process of transferring of charge from a charged object to the earth is called Earthing. Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current.

15.2     The story of Lightning

  • During thunderstorm, the air current 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 & the negative charges accumulate near the lower edges.
  • There is an accumulation of positive charges near the ground also.
  • As the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is not able to resist their flow any more.
  • Negative & positive charges meet.
  • Streaks of bright light & sound are seen which we see as lightning.

This whole process is called as Electric Discharge.

15.2.1             Lightning Safety

We understand the basic phenomenon of lightning. Lightning strike could destroy life & property. So, it is necessary to take measures to protect ourselves.

  • During lightning & thunderstorm, no open place is safe, so on hearing thunder, rush to a safer place.
  • After hearing the last thunder, wait for some time before coming out of the safe place.

15.2.2             Safe Place During Lightning

A house or a building is a safe place. If traveling by car or by a bus, it is safe inside with windows & doors of the vehicle shut.

15.2.3             Do’s & Don’ts during a thunderstorm

15.2.3.1                Outside the house

Don’ts

  • Open vehicles like motorbikes, open cars are not safe.
  • Open fields, tall trees, shelters in parks etc do not protect us from lightning strokes.
  • Do not carry an umbrella during thunderstorm.
  • Do not lie on the ground. Instead, squat low on the ground.

DOs

  • Take shelter under shorter trees.
  • Place your hands on your knees with your head between the hands. This position will make you the smallest target to the struck.

15.2.3.2                Inside the House

Don’ts

  • Lightning is an electrical discharge, so it can strike telephone cords, electrical wires & metal pipes. So, during a thunderstorm, contact with these should be avoided.
  • Do not call up a person who is receiving the phone through a wired phone.
  • Do not take bath during thunderstorm to avoid contact with running water.
  • Do not plug in electrical appliances like computers, TVs etc.

Note: Electrical lights can remain ON as they do not cause any harm

15.2.4             Lightning Conductors

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

15.3     Earthquakes

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

15.3.1             Eearthquake

Earthquake is caused by a disturbance deep inside the earth’s crust. Earthquakes keep occurring all the time, all over the earth. Small ones are not ever noticed. However, major earthquakes are much less frequent. They cause immense damage to buildings, bridges, dams & people. Earthquakes can also cause floods, tsunamis & landslides.

15.3.2             What causes an Earthquake?

As we know, the tremors are caused by the disturbance deep down inside the uppermost layer of the earth called the crust.

The outermost layer or crust of the earth is not in one piece, but it is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another, or if 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.

DIAGRAM

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.

15.3.3             Seismic Zone

As earthquakes are caused by the movement of earth’s plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. These weak zones are called as Seismic Zone.

In India, the areas which fall in the seismic zone are Kashmir, Western & Central Himalayas, North-East Rann of kutch, Rajasthan & the Indo-Gargetu Place.

15.3.4             Richter Scale

The power of an earthquake is expressed in terms of a magnitude ona scale called the Richter Scale. Very destructive earthquakes have magnitudes higher than 7 on the Richter Scale.

Richter Scale is not linear. This means that an earthquake of magnitude 6 does not have one & half times the destructive energy of an earthquake of magnitude 4. In fact, an increase of 2 is magnitude means 1000 times more destructive energy.

15.3.5             Protection Against Earthquake

Earthquakes cannot be predicted & they cause destruction. Therefore, it is important to take necessary precautions to protect ourselves in time.

  • Buildings in the seismic zones should be designed ‘quake-safe’.
  • Consult qualified architects & structural engineers.
  • In highly seismic areas, the use of mud or timber is better than using heavy construction material.
  • Roof should be as light as possible, so that if the structure falls, the damage will not be heavy.
  • Cupboards & shelves should be fixed on the walls, so that they do not fall easily.
  • Since some buildings may catch fire due to an earthquake, it is necessary that all buildings have fire fighting equipment in working order.

15.3.6             Protection during a an earthquake

When Earthquake strikes, following steps should be taken to protect self:

15.3.6.1                Indoors

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

15.3.6.2                Outdoors

  • Find a clear spot, away from buildings, trees & 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.