CBSE Class 8 Science, Some Natural Phenomena

•   Lightning: Lightning is an electric spark, but on a huge scale. Lightning is caused by the accumulation of charges in the cloy s.

•   Some objects can be charged by rubbing with other objets.

•   The electrical charge produced by rubbing is called static charge.

•   There are two ]rinds of charges-positive charge and negative charge.

      It i a car vention to call the charge acquired by a glass rod when it is rubbed with silk as positive.

      The other kind of charge is said to be negative.

•   Charges of the same kind repel each other-, while charges of different kind attract each other.

•   When charges move, they constitute an electric current.

•   Electroscope; It is a device that can be used to test whether an object is carrying charge or not.

      Electroscope consists of closely placed two metallic (aluminium) foils or strips.

      When both the strips are charged with similar charges, they repel each other and become wide on.

•   Electrical charge can be transferred from a charged object to another through a metal conductor.

•   Discharged: When a body loses charge to the earth or any other body, it is said to he disch.acrged.

•   Earthing: The process oftransfemng of charge from a charged object to the earth is known as earthing.

•   The story of Lightning:

      — Lightning occurs due to r'birtg.

      — During a thunderstorm, the air currents move upwards while the water droplets move downward.

      This vigorous movements cause separation of charges. This leads to the positive charges collect near the upper edges of the clouds and the negative charge to accumulate near lower edges of the clouds.

      — There is an accumulation of positive charges near the ground also.

      — Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning. The process is known as electric discharge. ne process of electric discharge between different clouds or between clouds and the earth causes lightning.

Fig. 15.L Accumulation ofcharges leading to lightning.

•   Lightning strike could destroy life and property. So it is necessary to take measures to protect ourselves.

•   Lightning conductors: It is a device used to protect buildings from the effects of lightning.

      Lightning conductor consists of

      — a metallic rod, taller than the building.

      — a portion of the metal rod remains in air at the top of the building.

      — a major portion of the metal rod/wire installed in the walls during its construction.

      — the other end of the rod is connected to a copper plate placed deep into the soil.

      — the rod provides easy route for the transfer of electric charge to the ground without harming the building.

Fig 15.2. Lightning Conductor.

•   Earthquakes: Are earthquake is a sudden shaking or trembling of the earth which lasts for a very short time. Earthquakes can cause immense damage to buildings, dams, etc. They may also cause floods, land slides, tsunamis and loss of life.

      It is caused by a disturbance deep inside the earth's crust. The outer layer of the earth is fragmented.

      Each fragment is called a plate.

      Earth's plates are in continualen these plates brush ptone another or undergo collision, causes earthquakes.

Fig. 15.3 Earth plates.

•   Seismic or fault zones: Thie boundaries of the earth's plate are the weak zones where earthquakes are more likely to occur. These weak zones are called as seismic or fault zones.

•   Seismic or fault zones of India: In India, the most of the threatened areas are:

      — Kashmir

      — Western and central Himalalyes

      — The whole of North-East

      — Rann of Kutch and the Indo-Gangetic Plane.

      — Some areas of south India.

•   The power of an earthquake is expressed in terms of a magnitude on a scale, known as the Richter Seale.

      Destructive earthquakes have magnitudes higher than 7 on Richter Scale.

Fig. 15.4. Movements of Indian earth's plate.

•   The tremors caused by the earthquake produce waves on the surface of the earth.

      These waves are callled seismic waves.

•   Seismograph: It is an instrument to record seismic waves.

•   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. Actually, an increase of 2 in magnitude means 1000 times more destructive energy.

•   Protection Against Earthquakes Earthquakes cannot be predicted. So, the buildings in fault zones/seismic zones should be designed so that they can withstand major tremors.

•   Take the following steps to protect yourselves in the event of the earthquake:

      — If you are at home

      — Hide under a table till the shaking stops.

      — Do not stand or sit near tall and heavy objects.

      — If you are in bed, do not get up. Protect your head with a pillow,

      — If you are at outdoor

      — Drop down away from buildings, trees and overhead power lines.

      — If you are in a bus, a car or any other covered vehicle, do not come out till the tremors stop. Ask the driver to drive slowly to a safe place.