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Chapter – 15 Some Natural Phenomena
Some objects can be charged by rubbing with other objects.
There are two kindsof charges — positive charge and negative charge.
Likecharges repel andunlike charges attract each other.
Theelectrical charges produced by rubbing arecalled static charges. Whencharges move, theyconstitute an electric current. Anelectroscope may beused to detect whether a bodyis charged ornot. Theprocess of transfer of charge froma charged object to the earthis called earthing. The process of electric discharge between clouds and the earth or between different clouds causes lightning. The formation of clouds involves friction between water droplets in the atmosphere. The friction charges the particles in the atmosphere. The negative charges accumulate at the bottom of the cloud and the positive charges at the top. As the accumulation of the charges increases, the cloud will induce positive charges on the ground nearby. Asthe amount ofcharge increases, thenegative charges onthe cloud tendto make apath towards theground, and it results in a narrow streak of electrical discharge, which we call lightning. Lightning strike could destroy life and property. Protective measures are of utmost importance during lightning strikes. Taking shelter ininteriors(house or otherclosed places) and vehicles(closed e.g.cars) are themost preferred measures. Lightning conductors can protect buildings from theeffects of lightning. A natural phenomenon that cannot be predicted is an earthquake. The earth consists of threemajor layers, called the crust, the mantle andthe core. The core is further divided into the inner core and theouter core. Themantle consists ofsemi-solid material above which the crust floats. The crust consists of oceans andcontinents. The crust is divided into several parts, called tectonic plates. The regions where one tectonic plate slides against another are referred toas fault zones, andthese are theregions where an earthquake is likely tooccur. Hence, thesezones are referred to as seismic zones. The placein the interior of the earth where an earthquake occurs is the focus, andthe region onthe surface ofthe earth thatis the closest to focus islikely to experience the largest damage. This region is called the epicenter of the earthquake. The instrument that measures the severity of an earthquake is a seismograph. It basically consists of a drum that rolls and a pendulum with a stylus that traces the waves of an earthquake on asheet like agraph paper. Theenergy released atthe focus ofan earthquake, propagates outwardly in form of waves knownas seismic waves. Destructive energy of an earthquake is measured onthe Richter scale. It is a logarithmic scale, ranging from1 to 10 for indicating the intensity ofan earthquake. Theearthquake measuring 7 ormore on Richter scale can cause severe damage tolife and property.
Protective measures for earthquake:
1.If you areat home: Take shelter under a table and stay there till shaking stops. Stayaway from talland heavy objects that may fallon you. If you are in bed, do not getup. Protect yourhead with a pillow. 2. If you areoutdoors: Find a clear spot, away from buildings, trees and 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 notcome out tillthe tremors stop. Moreover, it is advisable to make thestructure of buildings simple so thatthey are ‘Quake Safe’.
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