NCERT Solutions for Science class 8 Chapter 15 – Some Natural Phenomena

(b) A copper rod

When insulating materials such as plastic scales, balloons, and woollen clothes rub against each other, they may become electrically charged. Charging by friction is used to charge only non-conducting materials (poor conductors of electricity). As copper is a highly conducting material, it cannot be charged easily by friction.

(b) becomes positively charged while the cloth has a negative charge.

When an object is charged by rubbing it against another object, the two objects acquire opposite charges. By convention, it is considered that the charge acquired by the silk cloth is negative and the charge acquired by the glass rod is positive. Therefore, the glass rod becomes positively charged, whereas the silk cloth becomes negatively charged.

(a) F

Like charges (positive-positive or negative-negative) repel each other, whereas unlike charges (positive-negative) attract each other.

(b) T

Both objects have opposite charges on them. A charged glass rod (positively charged) attracts a charged plastic straw (negatively charged) because opposite charges attract each other.

(c) F

When lightning strikes, the lightning conductor conducts all of the atmospheric charges directly to the Earth, keeping the building safe. As a result, a lightning conductor protects a building against lightning strikes.

(d) F

Despite the fact that the causes of earthquakes are known, no instruments have been developed to detect them in advance. As a result, earthquakes cannot be predicted in advance.

When we take off a sweater during winter, the woollen sweater gets charged due to friction between the body and the sweater. The discharge of these electric charges produces tiny sparks of light and a crackling sound.

 Electricity can flow through the human body because the human body is a conductor of electricity. When we come into contact with a charged object, our body conducts its charges to the Earth. That is why if we touch a charged body with our hand, it loses its charge. This is referred to as an electric discharge.

The Richter scale is used to determine how destructive an earthquake is. The scale ranges from one to ten. A seismograph would record a reading of magnitude 3 on the Richter scale.

If the earthquake has a magnitude of 3 on the Richter scale, it is unlikely to cause significant damage. Earthquakes of a magnitude greater than 5 are generally considered destructive in nature.

Three protective measures against lightning are:

(i) Always remain in a covered area or stay indoors. If you are moving in a car, stay there until the lightning is over and keep the windows shut.

(ii) Do not bathe in running water during lightning.

(iii) Do not touch any electrical appliances, wires, telephone cables, metal pipes etc. during lightning.

A charged balloon repels another charged balloon because both carry the same type of charges and the same charges repel each other. 

When an uncharged balloon is brought near to a charged balloon, the uncharged balloon acquires the opposite charge on its surface by the process of induction. Thus, an uncharged balloon is attracted by another charged balloon because opposite charges attract each other.

 An electroscope is a device which is used to detect the charge on a body. It works on the principle that charges (positive-positive or negative-negative) repel each other while unlike charges (positive-negative) attract each other. 

This device has a metal rod. At one end of the metal rod, two strips of aluminium foil are fixed and at the other end, there is a metal disc. The bottom of the rod and strips of aluminium foil are placed inside a conical flask for protection. When the metal disc of the electroscope is touched with a charged body (ebonite or glass rod), the aluminium strips diverge or move away from each other.  This happens because some of the charges of the charged body are transferred to the aluminium strips through the metal rod.  The nature of charges on both the aluminium strips is similar. Hence, both the strips of the aluminium foil move away from each other. If the body is not charged, then the two strips of aluminium remain as they are. They do not repel each other. 

The extent of divergence depends upon the amount of charge on the electroscope. When the aluminium strips repel each other proves that the body is charged because repulsion is the sure test to detect if the body is charged or not through an electroscope.

 The following are the three Indian states where earthquakes are more likely to occur:

(i) Jammu and Kashmir

(ii) Assam

(iii) Gujarat

 If we are outside our home and an earthquake strikes, then the following precautions must be followed: 

(i) We should move to an open space away from tall buildings, tall trees, electric wires and poles.

(ii) If we are driving, we should slow down the vehicle and move it slowly to a clear spot away from buildings, electric poles, trees, etc. and stay there till the earthquake stops.

In a thunderstorm, no one should carry an umbrella. Electric charges can travel from the cloud through the metallic rod of an umbrella during a thunderstorm accompanied by lightning. This could cause an electric shock to the person carrying it. As a result, carrying an umbrella during a lightning storm is dangerous.