CBSE Class 8 Science Chapter 15 Some Natural Phenomena Revision Notes
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Lightning is an electric discharge in the form of a spark or flash originating in a charged cloud. Thunder clouds are charged. The negative charge centre is located in the lower part of the cloud. Lightning is nothing but huge static electric sparks that jump from one cloud to another cloud or from one cloud to some object on the earth. Let us now have a quick look at static electricity and learn a little bit about the instrument used to measure electrical charges.
Charging by rubbing
Static electricity is the result of an imbalance between negative and positive charges in an object. These charges can build up on the surface of an object until they find a way to be released or discharged. One way to discharge them is through a circuit. The rubbing of certain materials against one another can transfer negative charges or electrons.
▪ If we rub our shoe on the carpet, our body collects extra electrons. The electrons cling to our bodies until they can be released.
▪ If we blow up a balloon and tie a knot in the end, and then rub the inflated balloon on our hair, it will create a static charge on the balloon. The balloon will then stick to another surface such as a wall.
▪ Cut tissue paper into small pieces and lay them on the desk or table. If we rub a comb with our hairs and hold it above the pieces of tissue paper, they will jump up and attach themselves to the charged comb.
Earthquake is the vibrations of the earth’s crust that follows a sudden release of energy in the earth’s surface. This energy may be generated by a sudden dislocation of segments of the crust or by volcanic eruptions. The earth’s outer surface is made of pieces called tectonic plates. These plates constantly move away or towards each other. When stresses caused by plate movements are released, an earthquake occurs.
Richter scale is an instrument that helps in measuring the strength of earthquakes. The tremors produce waves on the surface of the earth. These waves are called seismic waves and are recorded by an instrument called a seismograph. Whenever an earthquake occurs the pendulum in it starts
vibrating and the pen attached to it records the waves on a paper.
CBSE Class 8 Science Chapter 15 Some Natural Phenomena Revision Notes PDF