Some Natural Phenomena

This chapter deals with two destructive natural phenomena, lightning and earthquakes. You will learn that some objects become electrically charged when rubbed, as a plastic comb does when rubbed on dry hair, and that there are two kinds of charges, positive and negative. Like charges repel and unlike charges attract each other. You will build a simple electroscope to detect charge, learn how charge can be transferred and how earthing removes it, and see that lightning is a huge electric discharge between charged clouds or between a cloud and the Earth. You will learn the safety measures during a thunderstorm and how a lightning conductor protects buildings. An earthquake is a sudden shaking of the Earth caused by the movement of plates in its outermost layer. You will learn about seismic zones in India, the seismograph, the Richter scale, on which an increase of 2 in magnitude means about 1000 times more destructive energy, and ways to protect ourselves and our buildings, recalling events such as the Bhuj earthquake of 2001.

Detailed Worksheet: Some Natural Phenomena Section A - Definitions (10 marks) 1. What are static charges? How can a plastic comb or a balloon be charged? (2 marks) 2. State what happens when (i) two like charges and (ii) two unlike charges are brought close to each other. (2 marks) 3. What is an electroscope? What is earthing? (2 marks) 4. What is lightning? What is a lightning conductor? (2 marks) 5. What is an earthquake? Name the instrument used to record earthquakes and the scale used to measure their magnitude. (2 marks) Section B - Calculations and Applications (15 marks) 6. During a thunderstorm, Riya saw a flash of lightning and heard the thunder 6 seconds later. If sound travels at about 340 m/s in air, how far away was the lightning? Why is the flash seen before the thunder is heard? (3 marks) 7. On the Richter scale, an increase of 2 in magnitude means about 1000 times more destructive energy. How many times more destructive is an earthquake of magnitude 7 than one of magnitude 5? Than one of magnitude 3? (3 marks) 8. A charged balloon is brought near (i) another balloon charged in the same way (ii) a plastic straw charged with the opposite charge (iii) small uncharged bits of paper. State what happens in each case and give a reason. (3 marks) 9. A student counts 15 seconds between a lightning flash and the thunder. Find the distance of the lightning. A few minutes later, the gap becomes 3 seconds. Find the new distance, and state whether the storm is moving closer or away and what she should do. (3 marks) 10. Arrange the following events in chronological order and name the state in which each occurred: the Bhuj earthquake (26 January 2001), the Kashmir earthquake (8 October 2005), the Sikkim earthquake (18 September 2011). Name three regions of India that lie in the most seismic zones. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a simple electroscope made from a glass jar, a cardboard lid, a paper clip and two strips of aluminium foil. Show what happens to the strips when a charged refill touches the clip. (4 marks) 12. Draw a labelled diagram showing the accumulation of charges in a thundercloud and the discharge between the cloud and the Earth. (3 marks) 13. Draw a labelled diagram of a lightning conductor on a tall building, showing the metal rod, the wire running down the wall and the metal plate buried in the earth. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. List five safety measures to be followed during lightning, both outdoors and indoors. Explain why it is unsafe to stand under a tall tree or carry an umbrella in an open field, and why a car or bus with its windows closed is relatively safe. (5 marks) 15. Explain what causes earthquakes, using the idea of plates. Why are some regions of India, such as the Himalayas, the North-East and the Rann of Kutch, more prone to earthquakes? Why can earthquakes not be predicted accurately like weather? (5 marks) 16. Suggest five steps to make homes safer in earthquake-prone areas, including the design of buildings and the arrangement of heavy objects. Describe what you should do if an earthquake strikes while you are (i) at home (ii) outdoors. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part; never experiment with electricity from mains or during a storm.