CBSE – 8th Standard Science

Some Natural Phenomena

  • Lightning is an electric spark, on a huge scale. It is caused by the accumulation of charges in the clouds.
  • Such a device can be used to test whether an object is carrying charge or not. This device is known as electroscope.

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  • Electric charge can be transferred from a charged object to another through a metal conductor.
  • 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.
  • Metals can be discharged if Negative and positive charges meet each other.
  • During lightning and thunderstorm a house or a building is a safe place wait for some time before coming out of the safe place.
  • Open vehicles, Open fields, tall trees, umbrella Poles, other metal objects are not safe during thunderstorms.
  • Placing hands on knees with head between the hands will make you the smallest target to be struck.
  • Inside the house during a thunderstorm running water, wires are not safe.
  • Lightning Conductor is a device used to protect buildings from lightning.
  • Natural phenomena like thunderstorms, lightning, earthquake can cause large scale destruction of human life and property.

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  • 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.

 

 

 

  • 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.

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  • The power of an earthquake is expressed in terms of a magnitude on a scale called Richter scale.
  • 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
  • In highly seismic areas, the use of light material, mud or timber is safe.
  • Take shelter under a table and stay there till shaking stops. Stay away from tall and heavy objects, Protect head with a pillow.
  • Drop to the ground, Find a clear spot, away from buildings, trees and overhead power lines..
  • Some objects can be charged by rubbing with other objects. The electrical charges produced by rubbing are called static charges.
  • There are two kinds of charges — positive charge and negative charge
  • Like charges repel and unlike charges attract each other.
  • When charges move, they constitute an electric current.
  • The process of electric discharge between clouds and the earth or between different clouds causes lightning.
  • Lightning strike could destroy life and property.
  • Lightning conductors can protect buildings from the effects of lightning.

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  • 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.

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  • 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.
  • You might have seen sparks on an electric pole when wires become loose.
  •  This phenomenon is quite common when a wind is blowing and shacking the wires.
  • You might also have 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.
  • We need not be afraid of lightning, but we have to take precautions to protect ourselves from the deadly sparks.
  • 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 woollen or polyester clothes, your hair stands on ends.
  • If you take off these clothes in the dark, you see even a spark and hear crackling sound. In 1752 Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomena.

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  • However, this realization took 2000 years.
  • The tremors produce waves on the surface of the earth are called seismic waves.
  • The waves are recorded by an instrument called the seismograph

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  •  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), Richter scale is not linear.
  •  This means that an earthquake of magnitude 6 does not have one and 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.

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  • For example, an earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4.

 

  • Earthquakes occur all the time, all over the earth. They are not even noticed.

 

  • Major earthquakes are much less frequent.

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  • 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.
  • A major tsunami occurred in the Indian Ocean on 26th December 2004.
  • All the coastal areas around the ocean suffered huge losses.
  • You learnt that during the development of a thunderstorm, the air currents 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 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 streaks as lightning
  • The process is called an electric discharge.

CBSE 8th Standard Science

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CBSE 8th Standard Science Syllabus and Revision Notes

  • Crop Production & Management
  • Microorganisms – Friends & Foe
  • Synthetic, Fibers & Plastic
  • Materials – Metals & Non-Metals
  • Coal & Petroleum
  • Combustion & Flame
  • Conservation Of Plants And Animals
  • Cell Structure And Functions
  • Reproduction In Animals
  • Reaching The Age Of Adolescence
  • Force And Pressure
  • Friction
  • Sound
  • Chemical Effects Of Electric Current
  • Some Natural Phenomena
  • Light
  • Stars And The Solar System
  • Pollution Of Air And Water