Some Natural Phenomena - Class 8 : NCERT Exercise Questions



 

Q.1 Which of the following cannot be charged easily by friction?
(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth.
Sol. (b) A copper rod

Q.2 When a glass rod is rubbed with a piece of silk cloth the rod
(a) and the cloth both acquire positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.
(d) becomes negatively charged while the cloth has a positive charge.
Sol. (b) becomes positively charged while the cloth has a negative charge.

Q.3 Write T against true and F against false in the following statements:
(a) Like charges attract each other                                                                  (T/F)
(b) A charged glass rod attract a charged plastic straw                                  (T/F)
(c) Lightning conductor cannot protect a building from lightning                (T/F)
(d) Earthquakes can be predicted in advance                                                 (T/F)
Sol. (a) Like charges attract each other                                                          (False)
(b) A charged glass rod attract a charged plastic straw                                  (True)
(c) Lightning conductor cannot protect a building from lightning                 (False)
(d) Earthquakes can be predicted in advance                                                 (False)

Q.4 Sometime, a crackling sound is heard while taking off sweater during winters. Explain.
Sol. Sweaters are usually made of wool and the inner clothes like shirts are made up of cotton and synthetic fibres. Because of friction between these two, static charge gets accumulated on the sweater. Hence, when sweater is taken off, an electric discharge occurs between sweater and the body or clothes which are responsible for the crackling sound.

Q.5 Explain why a charged body loses its charge if we touch it with our hand.
Sol. When a charged body is touched with our hand, it loses its charge because it gets transferred to earth via our body. This happens because our body is a good conductor of electricity. This phenomenon is known as earthing.

Q.6 Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?
Sol. The scale which is used to measure the destructive energy of an earthquake is known as Ritcher Scale. This scale has readings from 1 to 10. Seismograph will be able to record an earthquake of magnitude 3, which is basically of low intensity and will not cause any damage.

Q.7 Suggest three measures to protect ourselves from lightning.
Sol. Following are three measures to protect ourselves from lightning:
(i) If travelling by car or by bus, shut the windows and doors.
(ii) Stay away from poles, metal objects, telephone cords, wires, etc.
(iii) Do not lie on the ground. Instead, squat low on the ground.

Q.8 Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?
Sol. When two charged balloon are brought near each other, they repel because like charges repel each other. On the other hand, when an uncharged balloon is brought near a charged balloon they attract each other because unlike charges attract each other.

Q.9 Describe with the help of a diagram an instrument which can be used to detect a charged body.
Sol. Take an empty jam bottle. Take a piece of cardboard slightly bigger in size than the mouth of the bottle. Pierce a hole in it so that a metal paper clip could be inserted. Open out paper clip as shown in figure. Cut two strips of aluminium foil about 4 cm × 1 cm each. Hang them on the paper clip as shown. Insert the paper clip in the cardboard lid so that it is perpendicular to it.
Now, touch a charged body to the end of the paper clip. The charge will transfer from paper clip to aluminium strips. Since, both the strips receive same charge they will repeal each other and become wide open. On the other side, touching uncharged body to paper clip will bring no movement in the aluminium strips. Such a device which can be used to detect whether an object is carrying charge or not is known as electroscope.

Q.10 List three states in India where earthquakes are more likely to strike.
Sol. The three states in India where earthquakes are more likely to strike are Gujarat, Jammu & Kashmir and Assam.

Q.11 Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
Sol. Some precautions which can be taken when someone is outside and an earthquake strikes are:
(i) Try to find clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
(ii) If travelling in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot and do not come out till the tremors stop.

Q.12 The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.
Sol. No, I would not carry an umbrella with me if there is a prediction about a thunderstorm. The reason is, an umbrella has a metallic rod, which serves as a good conductor for lightning to reach human body thereby causing him an electric shock. Hence, we should not carry an umbrella during lightning.



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