1. Introduction – Some Natural Phenomena

In our daily life, we experience many natural phenomena like lightning, storms, thunder, and earthquakes. Some of these are beautiful to look at, but they can also be very dangerous. In this chapter, you will learn about two important natural phenomena – lightning and earthquakes. Understanding the science behind them helps us stay safe and also prepares you well for your CBSE Class 8 Science exams.

This Study Module is a part of the CBSE Class 8 Science – Chapter Wise Study Materials Based on NCERT. All the concepts are explained in simple language, as per the NCERT textbook, with exam-oriented points, so that you can revise quickly and write accurate answers in the examination.

2. Electric Charge and Charging by Rubbing

2.1 Electric Charge

Every object around us is made up of atoms. Atoms contain electrons (negatively charged) and protons (positively charged). Normally, an object has equal number of electrons and protons and is electrically neutral. When the number of electrons and protons becomes unequal, the object gets charged.

2.2 Types of Charges

• Positive charge: When an object loses electrons, it becomes positively charged.
• Negative charge: When an object gains extra electrons, it becomes negatively charged.
• Neutral: When the number of positive and negative charges is equal, the object is neutral.

2.3 Charging by Rubbing (Frictional Electricity)

One of the simplest ways to charge an object is by rubbing it with another object. For example, when you rub a plastic scale with dry hair or a balloon with wool, the objects attract small pieces of paper. This is due to production of static electricity.

During rubbing, electrons from one object get transferred to another. The object that loses electrons becomes positively charged, and the one that gains electrons becomes negatively charged. This process is called charging by friction.

2.4 Interaction of Charges

3. Electroscope – Detecting Electric Charge

An electroscope is a simple device used to detect the presence of electric charge on an object. Even though the NCERT textbook at this level introduces only a basic idea, you should remember its function clearly.

A simple electroscope can be made by:

• Taking a glass jar with a cardboard lid.
• Fixing a metal wire passing through the lid.
• Attaching two thin metal foils (like aluminium foil) to the lower end of the wire inside the jar.
• The upper end of the wire has a metal disc or knob.

When a charged object is touched to the metal disc, the electric charge spreads to the foils. Since both foils receive the same type of charge, they repel each other and move apart. This shows that the object is charged.

4. Lightning – A Powerful Natural Phenomenon

4.1 What is Lightning?

Lightning is caused by the build-up of electric charge in clouds. During a thunderstorm, strong air currents cause water droplets and ice particles in clouds to rub against each other and get charged. Some parts of the cloud become positively charged and some parts become negatively charged. The ground below may also become positively charged.

4.2 Formation of Lightning

1. Clouds get charged due to friction between water droplets and ice particles.
2. Heavy charges accumulate in different parts of the clouds.
3. When the difference of charges between the clouds, or between cloud and Earth, becomes very large, the air can no longer resist the flow of charge.
4. There is a sudden discharge of electricity, which we see as a bright flash called lightning.
5. This discharge heats the air rapidly and causes a loud sound called thunder.

4.3 Lightning Safety – Do’s and Don’ts

Lightning can cause fires, damage buildings, and even be fatal to living beings. That is why safety rules given in the NCERT textbook are very important for both exams and real life.

Safe Places During a Thunderstorm

• Stay indoors in a safe building.
• If inside a house, stay away from metal pipes, electrical appliances, and wires.
• Close doors and windows.
• If you are in a car, you may stay inside with windows closed. The metal body of the car can protect you.

Things to Avoid During a Thunderstorm

• Do not stand under a single tall tree, especially in open fields.
• Do not run in open areas, on rooftops, or in fields.
• Avoid using wired telephones, electrical appliances, or taking a shower.
• Do not carry metallic rods, umbrellas with metal tips, or other metal objects.

If You Are in an Open Field

If you are caught in an open area and you feel your hair stand on end, it may mean that lightning is about to strike near you.

4.4 Lightning Conductor (Basic Idea)

Tall buildings are often protected from lightning by installing a lightning conductor.

It consists of a long metal rod fixed on the top of the building and connected to a metal plate buried deep in the Earth. When lightning strikes, the charge is safely conducted to the ground through this metal rod instead of passing through the building.

5. Earthquakes – Another Natural Phenomenon

5.1 What is an Earthquake?

Earthquakes can cause great damage to buildings, roads, bridges, and can lead to loss of life and property. Just like lightning, earthquakes are also natural phenomena that we cannot prevent, but we can reduce damage by being prepared and following safety measures.

5.2 Causes of Earthquakes (Basic Idea)

The Earth’s crust is not a single solid piece. It is made up of large pieces called tectonic plates. These plates are constantly moving, though very slowly. When they rub against each other or collide, stress builds up in the rocks. When this stress is released suddenly, it causes an earthquake.

5.3 Important Terms Related to Earthquakes

• Focus (Hypocentre): The point inside the Earth where the earthquake originates.
• Epicentre: The point on the Earth’s surface directly above the focus.
• Seismic waves: The waves produced by an earthquake that travel through the Earth and cause shaking.

5.4 Measuring Earthquakes – Seismograph and Richter Scale (Basic Idea)

Earthquakes are measured with the help of an instrument called a seismograph. The strength or magnitude of an earthquake is expressed on the Richter scale. Higher the magnitude, more destructive is the earthquake. Earthquakes of magnitude 7 or above can cause serious damage in densely populated areas.

6. Earthquake Safety – Do’s and Don’ts

Since we cannot predict or stop earthquakes, it is very important to follow safety rules to reduce damage and save lives. These points are frequently asked in exams under “Do’s and Don’ts during an earthquake”.

6.1 If You Are Indoors

• Take shelter under a strong table or bed and hold on firmly.
• Stay away from windows, glass panes, mirrors, heavy objects, and tall furniture.
• Do not use lifts; use stairs if evacuation is necessary after the shaking stops.
• Turn off gas and electricity if possible after the tremors.

6.2 If You Are Outdoors

• Move to an open area away from buildings, trees, electric poles, and bridges.
• Stay away from the seashore if there is a possibility of tsunami after a strong earthquake.
• If you are in a moving vehicle, ask the driver to stop in an open area and stay inside the vehicle until the shaking stops.

7. Summary of Key Points – Exam-Oriented Revision