NCERT Solutions Class 8 Science Chapter 15 Some Natural Phenomena

NCERT Solutions for Class 8 Science for chapter 15 Some Natural Phenomena

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

Answer: The correct option is (b).

Explanation: Charging by friction is possible only  in non-conducting materials. As the copper rod is a conducting material therefore, it cannot be charged by friction.

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: The correct option is (b).

Explanation: When  an object is rubbed with another object, the two objects acquire charges. By convention, it is considered that the charge acquired by the glass rod is positive and charge acquired by the cloth is negative.

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 attracts 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)

Answer:

(a)    False

Explanation: Like charges always  repel each other while  unlike charges attract each other.

(b)    True

Explanation: A charged glass rod has positive charges on its surface  while  a charged plastic straw has negative charges on its surface.

(c) False

Explanation: The lightning conductor transfers all the atmospheric charge to the Earth. Hence, lightning does not affect the building.

(d) False

Explanation: There is no instrument in the world which  can predict the earthquake in advance.

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

Answer: The sweater gets charged when  it is taken off. It is because  of the friction between the sweater and the body. Thus, crackling sound is produced during this process.

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

Answer: When  a person touches a charged object, its charge is conducted by his body  to the earth. Hence, the charged body  loses its charge.

6. Name the scale on which the destructive energy of an earthquake is measure An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Answer: Richter scale is used to measure the destructive energy of an earthquake. This scale has the readings from 1 to 10. Seismograph is used to measure the reading of magnitude 3 on the Richter scale. When  Richter scale shows  a reading of magnitude 3, the earthquake will  not cause much  damage. An earthquake of magnitudes higher than 5 is considered destructive in nature.

7. Suggest three measures to protect ourselves from lightning.

Answer:

(i) Keep away  from electrical wires, telephone cables, metal pipes, etc.

(ii) If you are driving car, then remain there until the lightning is over. Close the windows of the car immediately.

(iii) During lighting, do not take bath in running water.

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

Answer: Two charged balloons have  similar nature of charges. As like  charges always  repel each other hence, two charged balloons will repel each other. When  a charged balloon is brought near an uncharged balloon, the uncharged balloon acquires charges on its surface  due to induction. Since unlike charges attract each other, a charged body  always attracts an uncharged body. Therefore, an uncharged balloon is attracted by another charged balloon.

9. Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer:

An electroscope is used to detect a charged body. A simple electroscope is shown  in the following figure.

It consists of a metal rod. At one end of the rod, two leaves  of aluminium foil  are fixed  and at the other end, there is a metal disc. The leaves  of aluminium foil  are placed  inside  a conical  flask and the flask  is corked to isolate the leaves  from air. When  the metal disc is touched with a charged body, the aluminium strips move  away from each other. This happens because  some  of the charges of the body  are transferred to the strips through the metal rod. This method of charging a body  is called  charging by conduction. The nature of charges on both the leaves  and the charged body  are the similar. Hence, both the leaves  of the aluminium foil  will move  away  from each other. If the body  was not charged, then the leaves  of the foil  would remain as they were before. They would  not repel each other.

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

Answer: Jammu  and Kashmir, Gujarat and Assam  are three major states of India where earthquakes are more likely to strike.

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

Answer:

(i) If you are travelling in a bus or a car, then do not come  out when  an earthquake strikes.

(ii) Rush to find  an open  field  away  from tall buildings, installations, tall trees, and electric wires and poles  during an earthquake.

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.

Answer: No. One should  not carry an umbrella during thunderstorm. The charge flows  from the clouds  during thunderstorm and it can travel through the metallic rod of the umbrella. This may give  an electric shock  to the person who is carrying it. Thus, it is not safe to carry an umbrella during lightning.