Thursday , April 9 2020

NCERT 8th Class (CBSE) Science: Some Natural Phenomena

Question: How does a lightning conductor work?

Answer: A lightning conductor consists of a metal rod ending in spikes at the top. The lower end of the rod is attached to a copper plate buried deep in the earth. When lightning strike the building, spikes receive charge and transfer to ground through the metal rod.

Question: List four precautions you will take to save yourself from a lightning strike during a thunderstorm.

Answer: Four precautions to be taken to save myself from a lightning strike during a thunderstorm are:

  1. Install lightning conductor on building.
  2. If you are in car or bus shut the windows.
  3. Do not take shelter under a tree.
  4. Stay away from trees and poles.
  5. If you are in an open place squat low on the ground.

Question: What is the Richter scale? Why do we say that it is not a linear scale?

Answer: An instrument used to measure the magnitude or intensity of an earthquake is called Richter scale. The number indicating the magnitude or intensity on Richter scale ranges between 0 and 9. The Richter scale is not a linear scale because on this scale, the vibrations of an earthquake with a magnitude of 2 are 10 times greater in amplitude than those of an earthquake with a magnitude of 1.

Question:  List three effects of an earthquake.

Answer: The effects of earthquakes include deformation of ground surface, damage and destruction of human-made structures, towns and cities, loss of life, violent devastating fires, landslides, floods, etc.

Question: Draw a labelled diagram of a gold-leaf electroscope. Describe how you will use it to detect a charge, measure a charge, and find the nature of the charge.

Answer: The gold-leaf electroscope consists of a metal disc to which a metal rod is attached. The metal rod passes into a bell jar through a tightly-fitted rubber stopper. At the bottom of the metal rod are mounted two thin strips (or leaves) of metal. These can be made of copper, brass or gold. The bell jar rests on a wooden base. Tin foil is attached to the sides of the bell jar.

The gold-leaf electroscope is used:

  1. To detect and measure charge. The amount of divergence of the leaves is a measure of the amount of charge on the body.
  2. To test whether a body has a positive or negative charge, at first we charge the electroscope with a known charge.
  3. If the divergence of the leaves increases, the body has a similar charge.
  4. If the divergence of the leaves decreases the body has opposite charge, i.e. it is positively charged.

Gold-leaf electroscope


Question: Explain how lightning occurs between two clouds during a storm.

Answer: When two clouds with unlike charges come near each other, there can be a very heavy discharge between them. Huge amount of energy is released in the form of heat, light and sound during such a discharge. This is the lightning we see and the thunder we hear during a storm.

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