Quick Revision Notes

Class 8 Science - Some Natural Phenomena

MP Board Education

### Introduction

Natural phenomena are events caused by nature that often have a massive impact on life and property on Earth. Two of the most destructive natural phenomena studied at this level are Lightning and Earthquakes.

### Concept 1: Charging by Friction (घर्षण द्वारा आवेश)

• Electric Charge: It is an intrinsic property of matter. When objects are rubbed together, they acquire electric charge.
• Charging by Friction: The process of rubbing two objects to transfer electrons and produce static electricity.
• Types of Charges:
  1. Positive Charge (+): Charge acquired by a glass rod when rubbed with silk.
  2. Negative Charge (-): Charge acquired by a plastic straw or ebonite rod when rubbed with fur or polythene.
• Basic Law of Electrostatics:
  • Like charges repel each other (Positive-Positive or Negative-Negative).
  • Unlike charges attract each other (Positive-Negative).

### Concept 2: Transfer of Charge (आवेश का स्थानांतरण)

• Electroscope (विद्युतदर्शी): A simple device used to detect whether an object is charged or not.
  • Working Principle: Charges are transferred from the charged object to the metal foils through a metal rod. Since the foils get the same type of charge, they repel each other and spread apart.
• Earthing (भू-संपर्कन): The process of transferring the charge from a charged object to the earth.
  • Importance: It protects us from electrical shocks due to leakage of electrical current.

### Concept 3: Lightning (तितली / तड़ित)

• Definition: An electric spark on a massive scale caused by the accumulation of electric charges in the clouds.
• How Lightning Occurs:
  1. During a thunderstorm, air currents move upward while water droplets move downward.
  2. These vigorous movements cause separation of charges.
  3. The positive charges collect near the upper edges of the clouds, and negative charges accumulate near the lower edges.
  4. Near the ground, positive charges accumulate too.
  5. When the magnitude of accumulated charges becomes very large, the air (normally a poor conductor) can no longer resist it.
  6. Negative and positive charges meet, producing streaks of bright light and sound. This is called lightning.
• Lightning Conductor (तड़ित चालक): A device used to protect buildings from the effects of lightning. It consists of a metallic rod taller than the building, installed on the walls, with the lower end deeply buried in the ground to safely direct the electric current to the Earth.

### Concept 4: Earthquakes (भूकंप)

• Definition: A sudden shaking or trembling of the earth that lasts for a very short time. It is caused by a disturbance deep inside the earth's crust.
• Structure of the Earth:
  • Crust: The outermost layer of the earth. It is not in one piece; it is fragmented into several plates called tectonic plates.
  • These plates are in continuous slow motion. When they bump into one another, or a plate goes under another due to collision, they cause a disturbance in the earth's crust. This disturbance appears as an earthquake on the surface.
• Seismic Zones (भूकंपीय क्षेत्र): Boundaries of the tectonic plates where earthquakes are more likely to occur. In India, areas like Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic plane are most threatened.
• Focus and Epicentre:
  • Focus: The point inside the earth's crust where the earthquake originates.
  • Epicentre: The point on the earth's surface directly above the focus. Maximum damage usually occurs here.

### Key Measurement & Safety Rules

1. Measuring Earthquakes

• Seismograph (सिस्मोग्राफ): An instrument used to record the seismic waves generated by an earthquake. It is essentially a vibrating rod or a pendulum that starts recording when tremors occur.
• Richter Scale (रिफ्टर पैमाना): The magnitude of an earthquake is expressed on a scale known as the Richter scale.
  • It is not linear. An earthquake measuring 6 on the Richter scale has 100 times more destructive energy than one measuring 4.

2. Safety Measures During an Earthquake

• If Indoors:
  • Take shelter under a table and stay there till the shaking stops.
  • Stay away from tall and heavy objects that may fall.
  • If you are in bed, stay there and protect your head with a pillow.
• If Outdoors:
  • Find a clear spot, away from buildings, trees, and overhead power lines.
  • If you are in a car or a bus, do not come out. Drive slowly to a clear spot.

3. Safety Measures During Lightning and Thunderstorm

• Open vehicles, like motorbikes, tractors, construction machinery, and open cars are not safe.
• Staying in open fields, tall trees, parks, or elevated places is dangerous.
• Safest Place: Inside a house or a building.
• If no shelter is available and you are in an open field, squat low on the ground with your hands on your knees and head between the hands (minimize contact with the ground to avoid a direct lightning path).