Chapter 15 : Some Natural Phenomena

MULTIPLE CHOICE QUESTIONS

1. An electroscope is a device which is used to find if an object is
(a) charged          (b) magnetic     (c) free of cracks     (d) hot
Answer: (a) charged

[ An electroscope detects the presence of electric charge on an object by showing movement (divergence) of its metal leaves.]

2. Electric current is to be passed from one body to another. For this purpose the two bodies must be joined by
(a) cotton thread   (b) plastic string   (c) copper wire  (d) rubber band
Answer: (c) copper wire

[ Electric current flows through conductors. Copper wire is a good conductor, so it allows current to pass between two bodies.]

3. The movement of the earth’s plates causes
(a) cyclones   (b) lightning  (c) earthquakes (d) thunderstorms
Answer: (c) earthquakes

[ The movement of Earth’s tectonic plates releases energy, causing vibrations in the ground called earthquakes.]

4. Two charged objects are brought close to each other. Choose the most appropriate statement from the following options:
(a) they may attract
(b) they may repel
(c) they may attract or repel depending on the type of charges they carry
(d) there will be no effect

Answer: (c) they may attract or repel depending on the type of charges they carry

[ Like charges repel, unlike charges attract. So, the effect depends on whether the charges are same or opposite.]

5.Which of the following is not likely to cause Tsunami?
(a) A major nuclear explosion under sea   (b) Earthquake
(c) Volcanic eruption                               (d) Lightning
Answer: (d) Lightning

[ Tsunamis are caused by sudden displacement of large water volumes due to earthquakes, volcanic eruptions, or underwater explosions. Lightning occurs in the air and does not displace ocean water.]

6.The earth’s plate responsible for causing earthquakes is
(a)the crust of the earth
(b)the mantle of the earth
(c)the inner core of the earth
(d)the outer core of the earth

Answer: (a) the crust of the earth

[ Earthquakes occur due to sudden movements of tectonic plates in the Earth’s crust.]
7.Consider the list of terms given below:
(i) Seismic Zone   (ii)Fault Zone   (iii) Mantle     (iv) Inner Core

The boundaries of the earth’s plate are known as
(a)(i) & (ii)      (b)(i) & (iii)        (c)(iii) & (iv)     (d)(ii), (iii) & (iv)

Answer: (a) (i) & (ii)

[ Boundaries of Earth’s plates are called Seismic Zones (earthquake-prone areas) and Fault Zones (where rocks fracture and move). Mantle and inner core are not plate boundaries.]

8.The outermost layer of earth is called
(a) mantle      (b) outer core      (c) crust      (d) inner core
Answer: (c) crust

[ The crust is the thin, solid outermost layer of the Earth. The mantle, outer core, and inner core lie beneath it.]

9.Major earthquakes are less likely to occur in
(a) North East India     (b) Rajasthan     (c) Rann of Kutch     (d) Orissa
Answer: (d) Orissa

[ Major earthquakes are less likely in Orissa (now Odisha) because it lies in a lower seismic zone (Zone II/III), unlike Northeast India, Rajasthan (Kutch), and Rann of Kutch which are in high-risk zones.]

10.Consider the list of terms given below
(i) Tsunami    (ii) Landslide     (iii)Floods      (iv) Lightning
Earthquakes can cause
(a) (i), (ii) & (iii)       (b) (ii) & (iv)      (c) (ii), (iii) & (iv)        (d) (iii) & (iv)

Answer: (a) (i), (ii) & (iii)

[ Earthquakes under the sea can cause tsunamis; on land, they can trigger landslides and floods (due to dam breaks or ground shaking). Lightning is not caused by earthquakes.]

VERY SHORT ANSWER QUESTIONS

11. State whether the following are True or False.
(a) Earthquakes occur all the time all over the world.
(b) The plates of the outermost layer of the earth are always in continuous motion.
(c) Tremors on the earth can also be caused by the eruption of a  volcano.
(d) The process of electric discharge cannot occur between clouds and the earth.
(e) Bathing outdoors should be avoided during thunderstorm.
Answer: (a) True – Earthquakes occur all the time all over the world, though most are too weak to feel.

(b) True – The plates of Earth’s crust (outermost layer) are always in slow, continuous motion.

(c) True – Volcanic eruptions can cause tremors on the Earth.

(d) False – Electric discharge (lightning) commonly occurs between clouds and the Earth.

(e) True – Bathing outdoors during a thunderstorm should be avoided as water and pipes can conduct electricity.

12. Is it possible to predict the occurrence of an earthquake?
Answer: No, earthquakes cannot be predicted. Scientists can only find likely zones, but not the exact time or place of occurrence.

13. If a charged plastic straw is brought near another uncharged plastic straw, what will happen?
Answer: The uncharged plastic straw will be attracted towards the charged straw due to induced charges (electrostatic induction).

14. The aluminium strips in an electroscope as shown in fig. 15.1 are replaced by plastic strips and a charged body is brought in contact with the metal clip. What will happen? 

     

Answer: If the aluminum strips are replaced by plastic strips, they will not diverge . This is because plastic is an insulator, so it cannot conduct charge from the metal clip .   

15. Plastic straws A and B are rubbed with dry cotton cloth. What will happen if they are brought near each other?

Answer: When plastic straws A and B are rubbed with a dry cotton cloth, both get the same type of electric charge (negative charge). Since like charges repel each other, the two straws will push away from each other when brought close.

SHORT ANSWER QUESTIONS

16. During the construction of a building the lightning conductor was left hanging in the air by mistake. Would the lightning conductor be still effective? Explain.
Answer: No, it won’t be effective. A lightning conductor must be connected to the ground. Hanging in air, it cannot safely carry the electric charge to Earth.

17. If air and cloud were good conductors of electricity, do you think lightning could occur? Explain.

Answer: No, lightning could not occur.

For lightning, air must act as an insulator. If air and clouds were good conductors, charges would flow continuously without building up, so no sudden electric discharge (lightning) would happen.

18. Identify the lightning conductor and the copper plate in Fig. 15.2.

     

Answer: The lightning conductor (A) is the pointed rod at the top of the building . The copper plate (B) is the metal plate buried deep in the earth for earthing .   

19. If the materials used for constructing a building were good conductors, do you think lightning will strike the building. Will the lightning conductor be still required to be installed in the building?
Answer: Yes, lightning would still strike the building because it is tall. A lightning conductor would still be needed to safely pass the current to the ground. Without it, the current could flow through the building and cause damage or fire.

20. You might have observed on a dry day that when you touch the screen of a television or computer monitor (with picture tube), you get a slight shock. Why does it happen?
Answer: On a dry day, static electricity builds up on the TV or computer screen due to friction from the picture tube. When you touch it, the static charge discharges through your body, giving you a slight shock.

21. Explain how does lightning conductor protects a building from getting struck by lightning.
Answer: A lightning conductor gives an easy path for electric charge to go to the ground. So, if lightning strikes, the charge safely flows to Earth instead of damaging the building.

22. In an electroscope if a negatively charged body is brought in contact with the metal clip, the strips of the electroscope diverge. If now another charged object carrying equal amount of positive charge is brought in contact with the clip, what will happen?
Answer: The diverged strips will collapse or come closer together. This happens because the positive charge neutralises the existing negative charge on the electroscope. When the net charge becomes zero, the strips no longer repel each other and fall back to their normal position.

23. The strips of an electroscope diverge when a charged body is brought in contact with the metal clip. Now the clip is touched gently by our hand. What will happen to the strips? Explain.

Answer: The strips will collapse or come closer together. When we touch the clip with our hand, the charge from the electroscope passes through our body to the earth (earthing). The electroscope becomes uncharged, so the strips stop repelling and fall back.                                                                      

LONG ANSWER QUESTIONS

24. Explain how lightning takes place?
Answer: Lightning takes place due to the accumulation of electric charges in clouds. As water droplets and air move inside a cloud, positive and negative charges separate. Positive charges gather at the top and negative charges at the bottom. When the charge difference becomes very large, it breaks down the insulating air, causing a sudden electric discharge in the form of lightning between clouds or between a cloud and the earth.

25. Mention three precautions that you will take to protect yourself if earthquake strikes when you are inside the house.
Answer: If an earthquake strikes while you are inside the house, then the three precautions you should take are:

(i) Take cover under a strong table or desk : Stay there until the shaking stops to protect yourself from falling objects.

(ii) Stay away from windows, glass doors, and heavy furniture : These can break or fall and cause serious injuries.

(iii) Do not run downstairs or use the lift : Stairs and lifts can collapse or get damaged. Instead, stay inside and protect your head with your arms or a pillow.

26. Explain why it is safer to use a wireless telephone instead of a landline telephone during lightning.
Answer: During lightning, it is safer to use a wireless (cordless or mobile) phone instead of a landline phone because a landline phone is connected to wires that run outside the house. These wires can carry the electric current from a lightning strike directly into the phone and harm the person using it. A wireless phone is not physically connected to such outside wires, so the risk of electric shock is much lower.

27. What precautions would you take if lightning occurs while you are outside the house?
Answer: If lightning occurs while you are outside, follow these precautions:

(i) Stay away from open fields, tall trees, and poles: Lightning often strikes the tallest object.

(ii) Avoid water bodies like ponds, rivers, or swimming pools: Water conducts electricity and can be dangerous.

(iii) Do not lie flat on the ground : Instead, crouch down with your head low and hands on your knees, keeping as little contact with the ground as possible.

(iv) Stay away from metal objects : Such as fences, umbrellas, or bicycles, as metal attracts lightning.

28. If the metal clip used in the electroscope is replaced by an ebonite rod and a charged body is brought in contact with it, will there be any effect on the aluminium strips? Explain.

Answer: No, there will be no effect on the aluminium strips. This is because ebonite is an insulator, not a conductor. It does not allow electric charge to pass from the charged body to the metal strips. Without reaching the strips, the charge cannot make them diverge.