Some Natural Phenomena Class 8 Notes NCERT and MCQs

23-08-2023 17:14 PM IST Priyanka Chaudhary

The chapter discusses natural phenomena such as lightning and earthquakes along with steps that we can take to minimize the destruction caused by these phenomena.

Lightning

Lightning is also an electric spark but on a huge scale. It is caused by the accumulation of charges in the clouds. Lightening can be explained in terms of charges produced by rubbing.

Charging objects by rubbing

Some objects can be charged by rubbing with other objects. For example, rubbing plastic refill with polythene charges the refill and polythene. Similarly, rubbing of plastic comb with dry hair charges both of them. The electrical charges produced by rubbing are called static charges. The ancient Greeks knew as early as 600 B.C. that when amber (amber is a kind of resin) was rubbed with fur, it attracted light objects such as hair.

Types, interaction, and transfer of charges

• There are two kinds of charges — positive charge and negative charge.
• Like charges repel and unlike charges attract each other.
• When charges move, they constitute an electric current.
• Electrical charge can be transferred from a charged object to another through a metal conductor.
• The process of transfer of charge from a charged object to the earth is called earthing. In buildings, earthing protect us from electrical shocks due to any leakage of electrical current. 

The process of lightning

The process of an electric discharge between clouds and the earth or between different clouds causes lightning. The process of lightning is explained below.

• Upward movement of air currents and downward movement of water droplets during the development of a thunderstorm causes the separation of charges.
• Positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. Positive charges accumulate near the ground also.
• When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity is no longer able to resist their flow.
• Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning. The process is called an electric discharge.

Lightning Safety

During lightning and thunderstorms, an open place is not safe. If you are outside and travelling in a car or bus you are safe, but if you are travelling in an open vehicle or sitting in an open field, try to find out a space nearby free from buildings, trees, poles, etc. Do not lie on the ground, squat low.

Carrying Umbrella during thunderstorms is not advisable. In forests, take shelter below short trees.

If you are inside the house, use mobile phones, avoid being in contact with metal pipes, telephone cords, or electrical wires. Avoid bathing during thunderstorms. Unplug all the electrical appliances. Lights can remain on.

Lightning conductors

A lightning conductor is a device that can protect buildings from the effects of lightning. During the construction of buildings, a metallic rod bigger than the building is installed in the walls. The rod has one end out in the air and the other end deep in the ground. It allows the transfer of charge to the ground.

Earthquake

It is a sudden shaking or trembling of the earth which lasts for a very short time. We are not yet able to predict earthquakes accurately. Earthquakes occur all the time, all over the earth. Earthquakes are caused by the disturbance deep down inside the uppermost and fragmented layer of the earth called the crust. Each fragment of the crust is called a plate.

Seismic or fault zones

The boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones. Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan, the Indo–Gangetic Plain and some areas of South India are areas most threatened by earthquakes in India.

Richter scale

Richter scale measures the power of the earthquake in terms of its magnitude. An earthquake measuring 7 or more on the Richter scale can cause severe damage to life and property. Richter scale is not linear. An earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4.

Protection against earthquake

• The buildings in the seismic zones should be Quake safe.
• If you are at home during an earthquake, take shelter under a table and stay there till the shaking stops. If you are in bed, protect your head with the pillow.
• If you are outdoors during an earthquake, find a clear spot away from buildings, trees and power lines. If you are in a car or any other closed vehicle, do not come out and move to a clear spot.

MCQs based on NCERT Class 8 Science 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

Ans. b

Explanation:

Non-conducting materials can be charged easily by friction like a plastic scale, inflated balloon, woolen cloth, etc. Copper rod is a conducting material.

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.

Ans. b

Explanation:

When the glass rod is rubbed with the silk cloth, the glass rod becomes positively charged as it losses electrons and the silk cloth becomes negatively charged as it acquires the electrons.

3. Name the scale on which the destructive energy of an earthquake is measured.

a. Richter scale

b. Mercalli scale

c. Kelvin scale

d. None of the above

Ans a

Explanation:

Richter scale is the scale on which the destructive energy of an earthquake is measured. Mercalli scale signifies the intensity of an earthquake based on its effects.