Class 8 Science Chapter 15 Notes- Some Natural Phenomena Notes

In this post, we will discuss Class 8 Science Chapter 15 notes pdf Some Natural Phenomena Notes. Class 8th science ch 15 notes help students in revision. In Some Natural Phenomena notes class 8, we will learn about Some Natural Phenomena like lightning and earthquakes. You can also download Some Natural Phenomena notes class 8 pdf for further study. NCERT notes for the Class 8 Science Chapter 15 are designed by our subject expert team.

In this Class 8th Science Ch 15 Notes pdf, we will learn about Some Natural Phenomena Notes.

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Class 8 Science Chapter 15 notes Overview

In Class VII, you learn about winds, storms and cyclones. You learned that cyclones can cause a lot of damage to human life and property. You also learnt that we can protect ourselves from these destructive phenomena to some extent. In Class 8 Science Chapter 15 notes, we shall discuss two other destructive natural phenomena. These are lightning and earthquakes.

Natural Phenomenon:- A Natural Phenomenon is anything that occurs on its own in nature without any kind of human intervention. For example, storms, winds, tides, volcanic eruptions and cyclones all can be categorized as natural phenomena.

Lightning

Lightning is an electric spark that occurs in nature on a major scale. It is caused by the accumulation of charges in the clouds.
It can be deadly and cause the destruction of life and property.
In ancient times people were unaware of the cause of lightning and hence they were scared of it. They thought that the wrath of the gods was visiting them. We need not be afraid of lightning, but we have to take precautions to protect ourselves from the deadly sparks.
Nowadays, scientists have evolved some precautions that can help us prepare and protect ourselves from this natural phenomenon.

Benjamin Franklin was an American scientist who in 1752 discovered and proved that lightning and spark produced from these clothes are all same things.

What are electric charges?

We know that every atom is made up of particles such as electrons, protons and neutrons.
All these particles share a common property in that they carry electric charges.
Electrons have a negative charge on them while protons have a positive charge and neutrons have no charge.
An object will be called electrically neutral if it is carrying a balanced proportion of positive and negative charges.
An object is called a charged object if there is an imbalance of electrons and protons in it.

Charging by Rubbing

When we rub two objects with each other they get charged due to a transfer of electrons between them. In this process, one object has an excess electron while fur has a shortage of electrons.

For example, When a plastic refill is rubbed with polythene, it acquires a small electric charge. Similarly, when a plastic comb is rubbed with dry hair, it acquires a small charge. These objects are called charged objects.

Types of Charges and Their Interaction

We know that charged objects may have a shortage or excess of electrons.
Objects having an excess of electrons are called negatively charged objects while an object having a shortage of electrons is called a positively charged object.

So we can say that we have two types of charge

positive charge and
Negative charge

Objects having the same kind of charges repel each other while objects with different kinds of charges attract each other.
For instance, when a glass rod is rubbed with silk cloth it becomes positively charged while the silk cloth becomes negatively charged.
These charged objects are now capable of attracting other charged and uncharged objects.

The force of attraction or repulsion experienced by charged objects is called electrostatic force.

Transfer of Charge

Electrical charge can be transferred from a charged object to another through a metal conductor.

Charges can transfer from one object to another with the help of conduction and induction:

Conduction: when a charged object comes in contact with a conductor it results in the transfer of charges through the conductor.
Induction: When a charged object is brought near a neutral object, it results in a shifting in the position of the electrons in the other object.

Electroscope:- A device that can be used to test whether an object is carrying a charge or not. This device is known as an electroscope.

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

Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current.

The Story of Lightning

During the development of a thunderstorm, the air currents move upward while the water droplets move downward.
These vigorous movements cause the separation of charges.
As a result, positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges.
There is an accumulation of positive charges 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 (Fig. 15.5). The process is called an electric discharge.

Class 8 Science Chapter 15 Notes Accumulation of charges leading to lightning — Accumulation of charges leading to lightning

Lightning Safety

During lightning and thunderstorm, no open place is safe.

Hearing thunder is an alert to rush to a safer place.
After hearing the last thunder, wait for some time before coming out of the safe place.
If somebody is there in a car or bus, they should stay inside and keep all the doors and windows closed.

Do’s and Don’ts during a Thunderstorm

Outside

Open vehicles, like motorbikes, tractors, construction machinery, and open cars are not safe.
Open fields, tall trees, shelters in parks, and elevated places do not protect us from lightning strikes.
Carrying an umbrella is not at all a good idea during thunderstorms.
If in a forest, take shelter under shorter trees.
If no shelter is available and you are in an open field, stay far away from all trees. Stay away from poles or other metal objects.
Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between the hands (Fig). This position will make you the smallest target to be struck.

Inside the house

During a thunderstorm contact with telephone cords, electrical wires and metal pipes should be avoided. It is safer to use mobile phones and cordless phones. However, it is not wise to call up a person who is receiving your phone through a wired phone.
Bathing should be avoided during thunderstorms to avoid contact with running water.
Electrical appliances like computers, TVs, etc., should be unplugged. Electrical lights can remain on. They do not cause any harm.

Lightning Conductors

A lightning Conductor is a device used to protect buildings from the effect of lightning.

Working of a lightning conductor

A metallic rod, taller than the building, is installed in the walls of the building during its construction.
One end of the rod is kept out in the air and the other is buried deep in the ground (Fig).
The rod provides an easy route for the transfer of electric charge to the ground.
The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.

Earthquakes

Some natural phenomena such as thunderstorms and cyclones can be predicted by meteorologists. However, there are certain natural phenomena that are uncertain and cannot be predicted accurately. One of them is an earthquake.
An earthquake is a sudden shaking or trembling of the earth which lasts for a very short time.
The main cause of earthquakes is disturbances inside the crust of the earth.
Earthquakes occur all the time, all over the earth. They are not even noticed.

Earthquakes can lead:

Loss of life
Loss of property such as buildings, dams and bridges
Floods
Landslides
tsunamis

A major earthquake occurred in India on 8 October 2005 in Uri and Tangdhar towns of North Kashmir
A major earthquake occurred on 26 January 2001 in the Bhuj district of Gujarat.

What Causes an Earthquake?

Earth’s surface is divided into several layers the crust, mantle, inner core and outer core.

The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion.
When they brush past one another, or a plate goes under another due to collision (Fig. 15.11), they cause disturbance in the earth’s crust.
This disturbance shows up as an earthquake or tremors on the surface of the earth.

we are sure what causes an earthquake, but it is not yet possible to predict when and where the next earthquake might occur.

Other causes of an earthquake can be:

Volcanic eruptions
When a meteor hits the Earth’s surface
The nuclear explosion under the Earth’s surface
Most earthquakes are caused by the movement of the earth’s plates.

What are seismic zones or fault zones?

Most earthquakes are caused by the movement of plates. Hence earthquakes are most likely to occur within their boundaries. The areas that lie on the boundaries of these plates are called weak zones, seismic zones or fault zones.

What is the Richter scale?

A scale which is used to determine the magnitude or strength of an earthquake is called the Richter scale.
Destructive earthquakes have a Richter scale magnitude of more than 7.
Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5.

Richter scale is not linear. This means that an earthquake of magnitude 6 does not have one and half times the destructive energy of an earthquake of magnitude 4. In fact, an increase of 2 in magnitude means 1000 times more destructive energy. Therefore, an earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4.

What are seismic waves?

The earthquake or tremors produce waves on the surface of the earth. These are called seismic waves.

What is a seismograph?

The seismic waves are recorded by an instrument called the seismograph.

It contains a metal rod or a pendulum which can vibrate as the earthquake occurs.
The metal rod is attached to a pen which records the waves on the paper.
Scientists study these waves and then construct a map of the earthquake.
This also helps them in determining the power of the earthquake.

Protection against Earthquakes

We know that earthquakes cannot be predicted. We know that they can be highly destructive. so, it is important that we take necessary precautions to protect ourselves. People living in seismic zones, where earthquakes are more likely to occur, have to be specially prepared.

The buildings in these zones should be so designed that they can withstand major tremors. Modern building technology can make it possible.
It is advisable to make the structure simple so that it is ‘Quake Safe’.
Consult qualified architects and structural engineers.
In highly seismic areas, the use of mud or timber is better than using heavy construction material.
Keep roofs as light as possible.
It is better if the cupboards and shelves are fixed to the walls so that they do not fall easily.
Be careful where you hang wall clocks, photo-frames, water heaters etc. so that in the event of an earthquake, they do not fall on people.
Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have fire fighting equipment in working order.

How can people protect themselves from an earthquake?

If you are at home

Take shelter under a table and stay there till the shaking stops.
Stay away from tall and heavy objects that may fall on you.
If you are in bed, do not get up. Protect your head with a pillow.

If you are outdoors

Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.

What We Learn Class 8 Science Chapter 15 Notes

In Class 8th Science Ch 15 Notes pdf / Some Natural Phenomena Notes

Some objects can be charged by rubbing with other objects.
There are two kinds of charges — positive charge and negative charge
Like charges repel and unlike charges attract each other.
The electrical charges produced by rubbing are called static charges.
When charges move, they constitute an electric current.
An electroscope may be used to detect whether a body is charged or not.
The process of transfer of charge from a charged object to the earth is called earthing.
The process of an electric discharge between clouds and the earth or between different clouds causes lightning.
A lightning strike could destroy life and property.
Lightning conductors can protect buildings from the effects of lightning.
An earthquake is a sudden shaking or trembling of the earth.
An earthquake is caused by a disturbance deep inside the earth’s crust.
It is not possible to predict the occurrence of an earthquake.
Earthquakes tend to occur at the boundaries of the earth’s plates. These boundaries are known as fault zones.
The destructive energy of an earthquake is measured on the Richter scale. An earthquake measuring 7 or more on the Richter scale can cause severe damage to life and property.
We should take necessary precautions to protect ourselves from earthquakes.

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