NCERT Solutions for Class 8 Science Chapter 15 | Some Natural Phenomena

Class 8 Science

Chapter 15 | Some Natural Phenomena

Select the correct option in questions 1 and 2.

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 woolen cloth

Answer: A copper rod

Reason

Copper is a highly conductive material. Generally speaking, objects that get charged by frictions are bad conductors of electricity.

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

Answer:

becomes positively charged while the cloth has a negative charge

Reason

When we rub one object against another object both of them acquire opposite charges. It is a convention to call the charge acquired by the glass rod positive and the charge acquired by the silk cloth negative.

3. Write T against true and F against false in the following statements.

a) Like charges attract each other – false
b) A charged glass rod attracts a charged plastic straw – true
c) Lightning conductor cannot protect a building from lightning – false
d) Earthquakes can be predicted in advance – false

4. Sometimes a crackling sound is heard while taking off a sweater during winters. Explain.

Answer:

When the sweater rubs against the body, they both acquire a small charge. It is this charge that causes the crackling sound heard while taking off the sweater.

5. Explain why a charged body loses its charge if we touch it with our hand.

Answer:

When we touch a charged object with our hand while standing on earth, the object loses its charge to the earth through our body. This process in which charge is transferred from a charged object to earth is called earthing.

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?

Answer:

The destructive energy of an earthquake is measured on the Richter Scale. The Richter scale measures the severity of an earthquake on a scale of 1 to 10. An earthquake that measures 3 on Richter scale is likely to be recorded by a seismograph; however, it will cause hardly any damage to life or property. Generally speaking, earthquakes that measure 5 or less on the Richter scale are harmless.

7. Suggest three measures to protect ourselves from lightning.

Answer:

• Stay inside a house or building. If you are traveling do not get out of the vehicle. Make sure that all doors and windows are shut.
• Do not carry an umbrella during a thunderstorm.
• If you are in an open space, stay away from all tall objects like trees, poles etc. Squat low on the field. This position will make you the smallest target to strike.
• If you are in a forest, take shelter under short trees.

8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon.

Answer:

A charged balloon is repelled by another charged balloon because the charge on both balloons is of the same kind. Like charges repel each other. When a charged body is brought near an uncharged body, the uncharged body will acquire charges due to the induction of charges. Since this charge is of a different kind than the charge on the charged body, they attract.

10. List three states in India where earthquakes are likely to strike.

Answer:

Kashmir, Rajasthan, parts of Gujarat and the whole of North-East India lie in seismic zones. Hence earthquakes are more likely to occur in these states.

11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Answers:

I will move to an open space away from trees, buildings or overhead power lines. In order to protect myself from falls, I will drop to the ground.

If I am in a car or a bus, I will ask the driver to drive the car slowly to an open space. I will not come out until the tremors stop.

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?

Answers

No. Carrying an umbrella during a thunderstorm is dangerous because electric discharge from the clouds can travel to the earth through the metallic rod of the umbrella. This will give us an electric shock.