physicscatalyst.com logo





Some Natural Phenomena Class 8 Notes - Chapter 15



Introduction

The natural phenomenon can be defined as the naturally occurring calamity or physical process.

Types of the natural phenomenon include

  • Thunder
  • Weather
  • Germination
  • Erosion

Some natural phenomena can be destructive such as

  • Earthquakes
  • Lightning
  • Volcanic eruption
  • Cyclones

Charged Bodies

What is Charge?

Charge is most commonly used to refer to electric charge. It is a fundamental property of matter like mass. It is a physical property because of which matter experience a force in an electromagnetic field.

Electric charges may be positive or negative in nature. If there is no net electric charge, the matter is considered neutral or uncharged.

Types of charges and their interactions

Charges are of two types

  1. Positive charge – When the matter has more protons than the number of electrons.
  2. Negative charge – When the matter has more electrons than protons. A negative charged body has excess of electrons.

This concept arose from the fact that when we rubbed a glass rod with silk, the glass rod gained a positive charge while the silk fabric gained a negative charge.

What are electrons, protons, and neutrons, and where it is present?

Electrons, protons and neutrons are fundamental particles. Electrons are negatively charged particles that move around the nucleus of an atom.

Protons are positively charged particles. Neutrons are electrically neutral which means that they carry no charge.

Both protons and neutrons are concentrated at the centre of an atom known as the nucleus of the atom. Electrons surround the nucleus.

How the object becomes positively charged and negatively charged?

When an object loses electrons, it becomes positively charged because it has more protons than electrons. After gaining electrons, the objects become negatively charged.
When a glass rod is rubbed with silk cloth, for example, it becomes positively charged, whereas the silk cloth becomes negatively charged.

charging by rubbing
Figure (a) shows a glass rod being rubbed with a silk cloth. Figure (b) shows that on roubbing with silk cloth glass rod becomes positive in charge because electrons are rubbed off glass rod. Due to this silk cloth becomes negative.

Charging by rubbing or friction

It means that when two objects rub against each other, electrons are transferred between them and they become charged.

For example, rubbing a plastic comb through dry hair charges it. This charged object (plastic comb) can now attract other charged and uncharged objects. The charged plastic comb is capable of attracting scraps of paper.


Properties of charge

Like charges repel each other. This means that two positive charges repel each other. Similarly, two negative charges would also repel each other.

Unlike charges, they attract each other. This means that a Positive-Negative charge would attract each other.

For example, a charged rubber balloon is repelled by another charged balloon, whereas an uncharged balloon is attracted by another charged balloon.

like charges repel

charge properties

unlike charges attract

charge properties

The reason for repulsion is that both the balloons contain the same type of charge, whereas attraction happens because both the balloons contain different types of charge.

Static charge vs Current Electricity

The electric charge produced by rubbing is known as a static charge, whereas charges that move together form an electric current. A static charge is one that does not move.

Current electricity on the other hand is study of moving charges.

Transfer of Charge

There are two ways to transfer charge from one object to another.

1. Conduction – When a charged object makes contact with a conductor, charges are transferred through the conductor. The object gains the same charge as the charged body. This method requires physical contact between the objects.

2. Induction – When a charged object is brought near a neutral object, the object gets induced and becomes charged. The object acquires the opposite charge to that of the charged body. This process doesn’t require physical contact.

The transfer of electric charges does not create or destroy charges and in this process charge remains conserved.

Electroscope

It is a device or an instrument that can be used to test whether an object is carrying a charge or not. An electrical charge can be transferred from one charged object to another through a metal conductor. It consists of a metal rod with a thin metal strip or leaf attached to it at the bottom.

Because gold and silver are good conductors of electricity, they are commonly used to make electroscopes.

How does the electroscope detect the charge?

  • A charged object is brought in contact with the open end of the wire.
  • The charges are transferred through the wire, which is a good conductor of electricity.
  • The gold plates also get charged and repel each other as they are similarly charged.

Discharged bodies

The body is said to be discharged if it loses its charge to the earth or any other body.

Explain why a charged body loses its charge if we touch it with our hand?

When a charged body is touched by our hand, our body conducts its charge to the earth as the human body is a good conductor of electricity.

The process of transfer of charges from a charged object to the earth takes place is called earthing.In general, every tall building has earthing to protect it from electrical shocks caused by electric current leakage.

Lightning – A natural destructive phenomenon

Lightning is an electric discharge seen in the sky between oppositely charged clouds or between charged clouds and the earth. Lightning is defined as the transfer of charge from cloud to another cloud or from one cloud to the earth.

Cause of lightning

Lightning is caused by the accumulation of charges. When negative and positive charges meet, they produce a streak of bright light accompanied by sound. It causes a lot of damage.

Mostly, lightning occurs within the clouds. Lightning is also caused due to static electricity.

  • During rain, the air current moves upwards and water droplets move downwards. This movement leads to the separation of charge in a cloud.
  • The positive charges accumulate near the upper side and negative charges collect near the bottom of the cloud, which causes a rearrangement of charges on the ground surface.
  • Thus, positive charges accumulate near the ground surface. When the accumulated charge becomes high, the flow of charges takes place.
  • The flow of charges takes place through the air. However, the air is a poor conductor of electricity, but the magnitude of accumulated charges becomes high, causing the air to get ionized.
  • When the negative charge meets the ground's positive charge, it results in streaks of bright light.
  • Lightning strikes the highest objects, like tall buildings, bridges, and monuments, and causes damage. To prevent this damage, lightning protection systems are installed.
accumulation of charge leading to lightening

Lightning Conductor

A metal rod (generally made of copper) placed on top of tall buildings with its lower end connected to the ground. It is used to protect buildings from the effects of lightning. When lightning strikes, the metal rod, being a good conductor, provides an easy passage for the transfer of charge to the ground. This way, the electric discharge flows from the clouds into the ground without damaging the clouds.

Things to do during lightning

  • Switch off the electrical appliances like computer, TV, refrigerator etc
  • If traveling in a car or bus, remains inside the vehicle and shut all its doors and windows.
  • Get inside as quickly as possible.
  • Check the forecast before going outside in the monsoon.

Things to avoid during lightning.

  • Do not roam here when there is lightning.
  • Avoid contact with running water.
  • Do not lie on the ground.
  • Do not sit in open vehicles.
  • Do not carry an umbrella.

Earthquake – Another natural destructive phenomenon

A sudden trembling or shaking of the earth for a short interval of time is caused by disturbances deep inside the earth's crust. It can cause large-scale destruction. It is not possible to predict the occurrence of an earthquake. An earthquake produces waves on the surface of the earth. Earthquakes may be caused by the sliding of tectonic plates. Sometimes, an earthquake may be followed by aftershocks that occur as the rocks settle down in their new position.

Why do earthquakes occur?

  • The earth’s crust is made up of fragments called plates, also called tectonic plates.
  • These plates are continuously moving.
  • Due to continuous motion, these plates slide past or collide with each other.
  • The rocks at the boundaries of these plates get interlocked and prevent the plates from moving, which results in pressure being formed on these rocks.
  • The increase in pressure leads to the slipping of rocks and causes the rocks to vibrate.
  • These vibrations travel up to the surface and cause earthquakes.

Focus, Epicentre and Fault zones

  1. Focus:-The point where the earthquake originates or starts is called the focus.
  2. Epicentre:-The point on the surface of the Earth immediately above the focus is known as the epicentre.
  3. Fault zones or seismic zones:-Weak zones (The boundaries of the tectonic plates) where earthquakes are most likely to occur are called fault zones or seismic zones.

Seismology, Seismic waves, and Seismograph

  1. Seismology:- The study of earthquakes is called seismology.
  2. Seismic waves:- The waves produced on the surface of the earth in an earthquake are called seismic waves.
  3. Seismograph:- The instrument is used to measure seismic waves.

How can the intensity of an earthquake be measured?

  • The destructive energy of an earthquake is measured on the Richter scale designed by an American Seismologist, Charles F. Richter, using a seismograph.
  • On the Richter scale, an earthquake measuring
    • 2 to 4 – It is a mild earthquake and does not cause any damage.
    • 4 to 8 – It is moderate to severe.
    • 8 to 9 – It is very severe and destructive earthquakes. It causes a lot of damage to life and property.
  • A major earthquake occurred in India on 8 October 2005 in Uri and Tangdhar towns in North Kashmir.
  • On 26 January 2001, in the Bhuj district of Gujarat, a major earthquake occurred which caused a lot of damage to life and property.
  • Both the Bhuj and Kashmir had a earthquake of magnitude greater than 7.5.

Protection against Earthquakes

People living in seismic zones have to be specially prepared. Firstly, the buildings in these zones should be so designed that they can withstand major tremors.

  • Steps to protect ourselves in an earthquake
  • Stay away from tall and heavy objects.
  • Take shelter under a table.
  • If you are outdoors, stay away from buildings, trees, and overhead power lines. Try to move to the open ground.

Frequently Asked Questions

What is a Lightning Conductor?

It's a device that protects buildings from the effect of lightning.

What is the effect of lightning conductor?

The lightning can be absorbed by the conductor and sent to the surface of the land when it falls on a building.

How do you determine the intensity of earthquake?

By using Richter scale we can determine the intensity of earthquake.

What is static electricity?

Static electricity is an electric phenomenon that involves the transfer of charged particles from one body to another.

link to this page by copying the following text
Also Read



Search Our Website





Class 8 Maths Class 8 Science