NCERT Solutions For Class 8 Science Chapter 15 Some Natural Phenomena

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.

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.

1. If you are travelling in a bus or a car, then do not come out when an earthquake strikes.
2. Rush to find an open field away from tall buildings, installations, tall trees, and electric wires and poles during an earthquake.

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

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.

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.

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

1. Keep away from electrical wires, telephone cables, metal pipes, etc.
2. If you are driving car, then remain there until the lightning is over. Close the windows of the car immediately.
3. During lighting, do not take bath in running water.

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.

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.

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.

(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.