NCERT Science Notes - Class 8
Chapter 12 - Some Natural Phenomena

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 12 - Some Natural Phenomena. The chapter explores the intriguing natural events of lightning and earthquakes. This chapter explains the causes and effects of these phenomena, detailing how lightning is formed through the accumulation of charges in clouds and how earthquakes result from the sudden release of energy in the Earth’s crust. Students will learn about safety measures to protect themselves during these events. Engaging activities and experiments, such as creating a simple electroscope and understanding seismic waves, make this chapter both educational and interactive, enhancing students’ comprehension of these powerful natural occurrences.

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NCERT Science Notes - Class 8
Chapter 12 - Some Natural Phenomena

Topics Covered:

• Two destructive natural phenomena:
  1. Lightning
  2. Earthquakes
• Discussion will focus on:
  • Understanding these phenomena.
  • Steps to minimize destruction caused by them.

12.1 - Lightning

The Sparks that the Greeks Knew About : Discovery of Electric Charges

• Ancient Greek Observation (600 B.C.):
  • Greeks discovered that rubbing amber (a type of resin) with fur caused it to attract light objects like hair.
• Everyday Example:
  • When removing woollen or polyester clothes, hair stands on end.
  • In the dark, sparks and crackling sounds may be observed.
• Benjamin Franklin's Contribution (1752):
  • Demonstrated that lightning and the sparks from clothes are the same phenomena.
  • This realisation took 2000 years to occur.
• Understanding Scientific Progress:
  • Boojho wonders why the discovery took so long.
  • Paheli explains that scientific discoveries result from the cumulative effort of many people, sometimes taking a long time.
• Electric Charges and Lightning:
  • Next, the properties of electric charges will be explored, showing their connection to lightning.
  • Example activity: Rubbing a plastic scale on dry hair attracts small pieces of paper, demonstrating static electricity.

12.2 Charging by Rubbing

Activity 12.1 : Charging by rubbing

• Materials:
  • A used ballpen refill
  • A piece of polythene
  • Small pieces of paper, dry leaf, husk, mustard seeds
• Procedure:
  1. Rub the ballpen refill vigorously with polythene.
  2. Bring the rubbed end of the refill close to small pieces of paper, dry leaf, husk, and mustard seeds.
  3. Avoid touching the rubbed end with your hand or any metal object.
  4. Observe and record the effect on the items.
• Observations:
  • The rubbed plastic refill acquires an electric charge.
  • The charged refill attracts small pieces of paper, dry leaf, husk, and mustard seeds.
• Key Concept:
  • Rubbing certain materials together can transfer electric charge, making them charged objects.
  • Polythene and hair also become charged during this process.
• Other Examples:
  • A plastic comb rubbed with dry hair also becomes charged.

Activity 12.2 - Charging Objects by Rubbing

1. Rub each object listed in Table 12.1 with the materials mentioned.
2. Bring the object close to small pieces of paper.
3. Observe whether the object attracts the pieces of paper, indicating it has been charged.
4. Record the observations for each object in the table.

Activity 12.3: Types of Charges and Their Interaction

(a) Balloons Experiment

1. Setup:
   • Inflate two balloons and hang them so they do not touch each other.
   • Rub both balloons with a woollen cloth.
2. Observation:
   • Upon releasing the balloons, they will repel each other. This demonstrates that like charges repel each other.

1. Setup:
   • Rub one pen refill with polythene and place it in a glass tumbler.
   • Rub the second refill with polythene.
2. Observation:
   • Bring the second charged refill close to the first one in the tumbler.
   • Result: The two refills will repel each other. This shows that charges produced by rubbing the same materials (polythene) are of the same type (like charges).

1. Setup:
   • Rub a refill and place it gently in a glass tumbler.
   • Bring an inflated charged balloon near the refill.
2. Observation:
   • The charged balloon and the refill will attract each other. This indicates that they carry different types of charges.

• Like Charges: When two objects made from the same material are rubbed and charged, they repel each other.
• Unlike Charges: When charged objects made from different materials (like a balloon and a refill) are brought close, they attract each other.

Summary of Observations on Electric Charges

1. Repulsion:
   • A charged balloon repelled another charged balloon.
   • A charged refill repelled another charged refill.
2. Attraction:
   • A charged balloon attracted a charged refill.

• The differences in behavior indicate that:
  • The charge on the balloon is of a different kind from the charge on the refill.
  • This leads to the conclusion that there are two kinds of electric charges:
    • Positive Charge: Acquired by a glass rod when rubbed with silk.
    • Negative Charge: Associated with materials like plastic when rubbed with polythene.

• Charges of the same kind repel each other.
• Charges of different kinds attract each other.

• When a charged glass rod is brought near a charged plastic straw (rubbed with polythene), there is attraction. This implies that:
  • The plastic straw carries a negative charge.

• The electric charges generated by rubbing are static and do not move on their own.
• When charges move, they create an electric current.
  • This current is responsible for making bulbs glow or wires heat up, as discussed in earlier classes.

12.4 - Transfer of Charge

Activity 12.4: Building a Simple Electroscope

Materials Needed:

• An empty jam bottle
• A piece of cardboard slightly larger than the bottle’s mouth
• A metal paper clip
• Two strips of aluminum foil (4 cm × 1 cm each)
• A charged refill (or other charged objects)

1. Prepare the Lid: Pierce a hole in the center of the cardboard large enough to insert a metal paper clip.
2. Shape the Paper Clip: Open out the paper clip as shown in the figure (Fig. 12.4). This will act as a conductor for transferring the charge.
3. Attach Aluminum Foil Strips: Cut two strips of aluminum foil and hang them from the lower end of the paper clip, making sure they hang freely and don't touch each other.
4. Insert the Paper Clip: Insert the paper clip through the hole in the cardboard so that it stands perpendicular to the cardboard, with the foil strips hanging inside the bottle.
5. Charge a Refill: Rub the refill with polythene to charge it, then touch the charged refill to the metal part of the paper clip.
6. Observe the Foil Strips: Watch for the reaction of the aluminum foil strips once the charge is transferred.

• After charging the refill and touching it to the paper clip, the aluminum foil strips will repel each other.
• If you touch other charged bodies (like a charged balloon or another refill) to the paper clip, the foil strips will behave similarly and continue to repel each other.

• When you transfer charge from the refill to the paper clip, the charge is distributed through the paper clip and onto the aluminum foil strips. Since both strips acquire the same type of charge (either positive or negative), they repel each other due to the principle that like charges repel.

• This simple device, known as an electroscope, can detect whether an object is charged. If the foil strips repel each other after a charged object is brought near the paper clip, it indicates that the object is charged.
• The reason the foil strips repel each other is because they have acquired the same type of charge from the charged object, causing repulsion between them.

Electroscopes and Earthing

1. Electroscope Functionality:
   • The aluminum foil strips receive the same charge from the charged refill via the paper clip, which acts as a conductor.
   • Strips with similar charges repel each other, causing them to spread apart widely.
   • This device, known as an electroscope, can be used to test if an object is charged.
2. Charging and Discharging:
   • Touching the end of the paper clip with your hand results in a change in the foil strips, causing them to return to their original position.
   • When the paper clip is touched, the foil strips collapse because they lose charge to the earth through your body.
   • This phenomenon is known as discharging.
3. Earthing:
   • The process of transferring charge from a charged object to the earth is called earthing.
   • Earthing is an important safety measure in buildings, protecting against electrical shocks caused by current leakage.

12.5 - The Story of Lightning

12.6 - Lightning Safety

General Precautions

• Safe Places:
  • Stay indoors during thunderstorms; houses and buildings provide safety.
  • If traveling in a vehicle (car or bus), ensure windows and doors are shut for protection.
• Alert Signals:
  • Hearing thunder serves as a warning to find shelter.
  • Wait some time after hearing the last thunder before exiting your safe place.

Outside

• Unsafe Situations:
  • Avoid open vehicles (motorbikes, tractors, open cars) and open fields.
  • Do not shelter under tall trees, park shelters, or elevated places.
  • Carrying an umbrella is not advisable.
• In the Forest:
  • Take shelter under shorter trees if available.
• If in an Open Field:
  • Stay away from all trees and metal objects.
  • Do not lie flat on the ground; instead, squat low with your hands on your knees and head between your hands to minimize your profile as a target.

• Avoid Contact:
  • Stay away from telephone cords, electrical wires, and metal pipes during thunderstorms.
  • Use mobile and cordless phones instead of wired phones.
  • Avoid bathing to prevent contact with running water.
  • Unplug electrical appliances (computers, TVs, etc.) but keep electrical lights on, as they do not pose a danger.

• Protection Device:
  • A lightning conductor is installed on buildings to safeguard against lightning.
  • It consists of a metallic rod taller than the building, with one end exposed and the other buried in the ground.
  • The rod provides a direct path for electric charge to reach the ground safely.
• Building Materials:
  • Metal columns, electrical wires, and water pipes in buildings offer additional protection, but do not touch these during a thunderstorm.

12.7 - Earthquakes

• Comparison with Thunderstorms:
  • Thunderstorms and lightning can be predicted to some extent, allowing for warnings and protective measures.
  • Weather departments can forecast the development of thunderstorms, which often accompany lightning and cyclones.
• Unpredictability of Earthquakes:
  • Earthquakes are difficult to predict accurately.
  • They can cause extensive damage to human life and property.
• Historical Examples:
  • A significant earthquake occurred on 8 October 2005 in Uri and Tangdhar towns of North Kashmir.
  • Another major earthquake took place on 26 January 2001 in Bhuj district of Gujarat.

Earthquake

Definition:
An earthquake is a sudden shaking or trembling of the earth that lasts for a very short duration.

Causes:

• Earthquakes are caused by disturbances deep inside the earth’s crust. These disturbances may occur due to:
  • Movement of tectonic plates.
  • Volcanic activity.
  • Accumulation of stress along fault lines.

• Minor Earthquakes: Earthquakes occur all the time around the world but are often too small to be noticed.
• Major Earthquakes: These are less frequent but can cause significant destruction.

• Damage: Major earthquakes can lead to:
  • Destruction of buildings, bridges, and dams.
  • Loss of life and injuries to people.
• Secondary Effects: They can also trigger:
  • Floods.
  • Landslides.
  • Tsunamis.

• A significant event was the tsunami that occurred in the Indian Ocean on 26 December 2004, which caused massive devastation in coastal areas around the ocean, resulting in huge losses in life and property.

Activity 12.6: Understanding the Impact of the 2004 Tsunami

Objective:
To locate affected regions and gather personal accounts of the tsunami's devastation.

Materials Needed:

• An outline map of the world
• Pencil or marker for marking locations

1. Map Locating:
   • Take the outline map of the world.
   • Locate the Eastern Coast of India: Identify and mark the eastern coast of India.
   • Andaman and Nicobar Islands: Locate and mark the Andaman and Nicobar Islands on the map.
2. Mark Affected Countries:
   • Identify and mark other countries around the Indian Ocean that could have suffered damage from the 2004 tsunami. These may include:
     • Indonesia
     • Thailand
     • Sri Lanka
     • Maldives
     • Myanmar
     • Australia (specifically northern regions)
   • You may also want to include the eastern coast of Africa (e.g., Somalia) and the Arabian Peninsula.
3. Collect Personal Accounts:
   • Talk to your parents, grandparents, or other elders in your family or neighborhood to gather personal accounts of the devastation caused by the tsunami in India.
   • Questions to Ask:
     • What do you remember about the tsunami?
     • How did it affect your community?
     • Were there any rescue or relief efforts?
     • What changes did the tsunami bring to your area in the aftermath?
4. Document Findings:
   • Write down the stories and accounts you collect.
   • Highlight key facts and emotions shared by the individuals to understand the impact of the tsunami.

What Causes an Earthquake?

Myths About Earthquakes

• Boojho asks about a myth: His grandmother told him that the earth is balanced on the horn of a bull, and when the bull shifts it to the other horn, an earthquake occurs.
• In ancient times, people did not understand the true causes of earthquakes, leading to various mythical explanations, including the one shared by Boojho’s grandmother. Such myths were common worldwide.

• Modern Understanding: Tremors are caused by disturbances deep inside the earth’s uppermost layer, known as the crust (Fig. 12.9).
• Plate Tectonics:
  • The outermost layer of the earth is fragmented into pieces called plates (Fig. 12.10).
  • These plates are constantly in motion.
  • Disturbances occur when:
    • Plates brush past one another.
    • One plate goes under another due to collision (Fig. 12.11).
  • It is these disturbances that manifest as earthquakes on the earth's surface.

Prediction of Earthquakes

• Boojho's Inquiry: He questions whether scientists can predict the timing and location of earthquakes, given their knowledge of causes.
• Current Limitations: While scientists understand what causes earthquakes, they cannot predict when or where the next one will occur.

• Paheli's Observation: She notes that underground explosions can also cause tremors.
• Additional Causes:
  • Volcanic eruptions.
  • Meteor impacts.
  • Underground nuclear explosions.
• Primary Cause: However, the majority of earthquakes are caused by the movement of tectonic plates.

• Weak Zones: The boundaries of tectonic plates are considered weak zones where earthquakes are more likely to happen. These areas are known as seismic or fault zones.
• Regions in India at Risk:
  • Kashmir.
  • Western and Central Himalayas.
  • Northeast India.
  • Rann of Kutch.
  • Rajasthan.
  • Indo-Gangetic Plain.
  • Some areas of South India also fall into danger zones (Fig. 12.12).

Measuring Earthquake Power

• The power of an earthquake is measured using the Richter scale.
• Magnitude Levels:
  • Destructive earthquakes typically have magnitudes higher than 7 on the Richter scale.
  • Notable examples include the Bhuj and Kashmir earthquakes, both of which had magnitudes greater than 7.5.

Seismic Waves and Measurement

Seismic Waves

• When tremors occur, they produce waves on the earth's surface known as seismic waves.

• Seismograph: Seismic waves are recorded by an instrument called a seismograph (Fig. 12.13).
  • The seismograph consists of a vibrating rod or a pendulum that vibrates during tremors.
  • A pen is attached to the vibrating system, which records the seismic waves on moving paper.

• By analyzing the recorded seismic waves, scientists can:
  • Construct a complete map of the earthquake (as shown in Fig. 12.14).
  • Estimate the earthquake's potential for destruction.

• The Richter scale is a logarithmic scale, which means it is not linear.
  • An increase of 2 in magnitude does not equate to one and a half times more destructive energy.
  • Specifically, each whole number increase on the Richter scale represents a 1000-fold increase in destructive energy.
  • For example:
    • An earthquake with a magnitude of 6 has 1000 times more destructive energy than one with a magnitude of 4.

Protection Against Earthquakes

Importance of Precautions

• Earthquakes cannot be predicted and can be highly destructive, making it crucial to take necessary precautions for safety, especially for those living in seismic zones.

• Quake-Safe Structures:
  • Buildings in seismic zones should be designed to withstand major tremors.
  • Use modern building technology to enhance structural integrity.
  • Simple Structures: Keep building designs simple to reduce risk during earthquakes.
  • Consult Experts: Work with qualified architects and structural engineers for safe designs.
• Material Considerations:
  • In highly seismic areas, prefer mud or timber over heavy construction materials.
  • Use light roofing materials to minimize damage if structures collapse.
• Securing Items:
  • Fix cupboards and shelves to walls to prevent them from falling during tremors.
  • Be cautious about placing wall clocks, photo frames, and water heaters in locations where they could fall on people during an earthquake.
• Fire Safety:
  • Ensure all buildings, particularly tall ones, are equipped with functioning fire-fighting equipment, as earthquakes may cause fires.

• The Central Building Research Institute, Roorkee, has developed techniques for constructing quake-resistant houses.

1. If You Are at Home:
   • Take shelter under a sturdy table and remain there until shaking stops.
   • Stay away from tall and heavy objects that could fall.
   • If you are in bed, remain there and protect your head with a pillow.
2. If You Are Outdoors:
   • Move to a clear area away from buildings, trees, and overhead power lines. Drop to the ground.
   • If you are in a car or bus, remain inside. Ask the driver to drive slowly to a clear area, and stay inside until the shaking stops.

NCERT Science Notes - Class 8 | Science | Chapter 11 - Some Natural Phenomena

NCERT Science Notes - Class 8 | Science | Chapter 8 - Force and Pressure