Class -8, Science, Chapter - 15 Some Natural Phenomena Notes, Ncert Solutions & Frequently Asked Questions

                     Science

Class - 8, Chapter - 15

Some Natural Phenomena 

Notes, Ncert Solutions & Frequently Asked Questions 

-- Notes --

⭐ Natural Phenomena:-

• Any naturally occurring calamity or physical process is called natural phenomenon.

• Two destructive natural phenomena: lightning and earthquakes.

⭐ Methods of Charging:-

• A body can be charged by rubbing due to friction or by induction.

• The electrical charges produced by rubbing are called static charges.

⭐ Charging by rubbing:- 

• When objects like plastic get rubbed with hair, it acquires a charge. Similarly, a glass rod gets charged when rubbed with a silk cloth.

• They get charged as they gain or lose electrons and that is why they can attract or repel the small pieces of paper depending on the type of charge they carry.

⭐ Charged objects:-

• Objects that carry a charge by means of rubbing or other processes are called as charged objects.

⭐ Interaction between like and unlike charges:- 

• Like charges repel, while unlike charges attract.

⭐ Convention of Positive and Negative charges:- 

• It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive.

⭐ Transfer of charges:- 

• Charges can be transferred through good conductors (metals) to another conducting material.

• The charge transfer is due to the movement of the electron from one atom to another.

⭐ Electroscope:- A device that can detect the presence of charge is called as an electroscope.

⭐ Discharged Objects:- When objects lose their charge by transfer, they are known as discharged objects.

⭐ Earthing:- The process of transferring charges from a charged object to the earth is called earthing.

⭐ Lightning:- 

• During thunderstorms, air currents move upwards and water droplets move downwards. This causes separation of charges between clouds and between clouds and earth.

• When the magnitude of charges increases, air (normally a bad conductor) starts conducting and allows the flow of electricity. This is called as lightning, as this flow of charge is accompanied by bright streaks of light and sound.

⭐ Electric Discharge:- The process of flow of charge from cloud to cloud or from cloud to earth due to the separation of positive and negative charges is called an electric discharge.

⭐ Lightning safety: Steps to follow:- 

• Outside the house:-

a) Find a safe place or shelter under small trees. 

b) If inside a car or vehicle, stay inside with doors and windows shut. 

c) Stay away from metal poles. 

d) Do not lie on the ground. Instead, crouch with your head in between your hands.

• Inside the house:- 

a) Avoid contact with telephone and electrical wires. 

b) avoid bathing. 

c) Unplug electrical appliances.

⭐ Lightning Conductors:- 

• Lightning conductors help to protect buildings during lightning.

• They consist of a metal rod that is taller than the building which is installed within the walls during construction. They run all the way to the earth and act as a direct passage for electric discharge during lightning.

⭐ Earthquakes:- 

• Earthquake is the sudden trembling of the surface of the earth due to disturbance deep inside the earth’s crust.

• Causes large-scale damage to life and property.

• Cannot be predicted.

• Causes of earthquake

• Earthquakes are caused due to movement or collision of tectonic plates in the uppermost layer of the earth’s crust.

• Movement of plates:- Earth’s crust is fragmented and each such fragment is known as a plate. These plates are constantly in motion and sometimes can collapse under another, causing an earthquake on the surface.

⭐ Structure of the earth:- 

⭐ Seismic/fault Zones:- Boundaries of the plates on the earth’s crust are weak zones where earthquakes are most likely to occur. These are known as seismic or fault zones.

⭐ Power of earthquake:- 

• Power of an earthquake is expressed in terms of magnitude on a scale. This scale is called as a Richter Scale.

• An earthquake with magnitude > 7 on the Richter scale is considered destructive.

• This scale is not linear. i.e. an increase of 2 in magnitude implies 1000 times more destructive energy.

⭐ Seismic Waves:- Tremors deep inside the earth produce waves, which are called seismic waves.

⭐ Seismograph:- 

• An instrument that records seismic waves is called a seismograph.

• Consists of a vibrating rod or pendulum that starts vibrating when tremors occur.

⭐ Protection from an earthquake: Steps to follow:- 

• Outdoors:-

a) Find a clear spot, away from overhead lines and buildings 

b) if in a car then go to a clear spot and stay inside the car till the tremors stop.

• Indoors:- 

a) stay under a table till tremors stop 

b) Avoid staying near tall heavy objects.

• Structural measures:- 

a) in highly seismic areas build mud houses with light roofs in order to minimize damage 

b) Fix cupboards and shelves to the walls 

c) During an earthquake, some buildings catch fire and therefore must have proper working firefighting equipment.

-- NCERT Solutions --

Select the correct option in Questions 1 and 2.

Question 1.- Which of the following cannot be changed easily by friction?

a) A plastic scale

b) A copper rod

c) An inflated balloon

d) A woollen cloth

Answer:- b) A copper rod

Question 2.- When a glass rod is rubbed with a piece of silk cloth the rod

a) and the cloth both acquire a positive charge.

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

c) and the cloth both acquire a negative charge.

d) becomes negatively charged while the cloth has a positive charge

Answer:- b) becomes positively charged while the cloth has a negative charge.

Question 3.- Write 'T' against true and 'F' against false in the following statements.

a) Like charges attract each other.

b) A charged glass rod attracts a charged plastic straw.

c) Lightning conductor cannot protect a building from lightning.

d) Earthquakes can be predicted in advance.

Answer:- 

a) False

b) True

c) False

d) False

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

Answer:- The electric discharge takes place between the body and the sweater due to friction. At the time of electric discharge some energy is released. In this case energy is released in the form of crackling sound.

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

Answer:- Human body is a conductor of electricity. When a charged body is touched with hand, our body conducts its charges to the earth. Hence, by this way charged body loses its charge.

Question 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?

Solution:- The destructive energy of an earthquake is measured on a scale called the Richter Scale.

Yes, it would be recorded by seismograph because this scale has the readings from 1 to 10. No it is not likely to cause much damage as earthquakes of magnitude higher than 5 is considered destructive in nature.

Question 7.- Suggest three measures to protect ourselves from lightning.

Answer:- 

i) Do not use TV or cable phone during lightning.

ii) Stay indoor or under covered area.

iii) Don’t take bath during lightning.

Question 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 both carry same type of charges. On the other hand, an uncharged balloon is attracted by another charged balloon as they have opposite charges. We know that same charges repel and opposite charges attract each other.

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

Answer:- Electroscope is a device which is used to detect the charge on a body. It works on the principle that like charges repel each other while unlike charges attract each other. It consists of a metal rod with thin metal strip or leaf attached to it at the bottom.

At the top, the metal rod enters in a metallic cup or disc. The bottom of the rod and the metal leaf are enclosed in a glass box for protection. When the disc of the electroscope is touched with a charged ebonite or glass rod, the metal leaves open out or diverge.

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

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

Answer:- Kashmir, Rajasthan and Gujarat.

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

Answer:- We will take the following precautions to protect ourselves:

i) Move to open space. Find a clean spot away from buildings, trees and overhead power lines. Drop to the ground.

ii) If we are in a car or a bus, will not come out. Drive slowly to a clear spot and remain in it till the earthquake stops.

Question 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, it is not advisable to carry an umbrella at all. Its metallic objects are more prone to lightning stroke. So an umbrella increases the risk.

-- Frequently Asked Questions --

Question 1.- What is an ‘Electroscope’?

Answer:- A device that can detect the presence of charge is called as an electroscope.

Question 2.- What are the different layers of Earth?

Answer:- The earth is made up of three different layers: the crust, the mantle and the core.

Question 3.- What is a ‘Seismograph’?

Answer:- An instrument that records seismic waves is called a seismograph.

-- By Durgesh Pandey

(Eklavya Coaching Institute)