Class 8 Science Notes Chapter 15 (Some Natural Phenomena) – Science Book

Here are the notes of Chapter 15, 'Some Natural Phenomena' for class 8 students. This chapter deals with two major destructive natural events: Lightning and Earthquakes. Understanding the science behind them and the necessary precautions is vital, not just for your exams but for safety as well.

Chapter 15: Some Natural Phenomena - Detailed Notes

1. Lightning

• Static Electricity & Electric Charge:

  • Objects can be charged by rubbing against each other. This is due to the transfer of tiny particles called electrons.
  • Example: Rubbing a plastic refill with polythene charges the refill; rubbing a glass rod with silk charges the rod.
  • Types of Charges: There are two types of electric charges: Positive (+) and Negative (-).
  • Interaction of Charges:
    • Like charges repel each other (e.g., positive repels positive, negative repels negative).
    • Unlike charges attract each other (e.g., positive attracts negative).
  • Conventionally: The charge acquired by a glass rod rubbed with silk is called positive. The charge acquired by an ebonite rod rubbed with fur (or a plastic refill rubbed with polythene) is called negative.
  • When charges move, they constitute an electric current.

• Transfer of Charge:

  • Electric charge can be transferred from a charged object to another through a conductor (usually a metal).
  • Earthing: The process of transferring charge from a charged object to the Earth is called earthing. This is used as a safety measure in electrical circuits and appliances.

• Detecting Charge - The Electroscope:

  • An electroscope is a device used to detect whether an object is charged or not, and sometimes the nature of the charge.
  • Simple Electroscope: Can be made with an empty jar, a cardboard lid, a metal wire (like a paper clip), and two small strips of aluminium foil.
  • Working: When a charged object touches the metal end of the wire, the charge travels down to the aluminium foil strips. Since both strips receive the same type of charge, they repel each other and move apart (diverge). The extent of divergence indicates the amount of charge.

• The Story of Lightning:

  • Lightning is a massive electric discharge (spark) occurring in the atmosphere.
  • Cause: During a thunderstorm, air currents move upward while water droplets move downward. Vigorous movements cause separation of charges within the clouds, and between clouds and the Earth.
  • Typically, the upper part of the clouds accumulates positive charge, and the lower part accumulates negative charge. There is also an accumulation of positive charge on the ground beneath the cloud.
  • When the magnitude of the accumulated charges becomes very large, the air (normally a poor conductor) cannot resist their flow. Negative charges from the cloud rush to meet positive charges on the ground (or in another cloud), producing streaks of bright light and sound – this is lightning.
  • The process is called an electric discharge.

• Lightning Safety:

  • Lightning strikes can cause severe damage to life and property.
  • Safe Places: Inside a house or a building is safest. If travelling by car or bus, stay inside with windows and doors shut.
  • Unsafe Places: Open areas, tall trees, metal sheds, construction sites, parks, elevated grounds. Do NOT take shelter under isolated trees.
  • If in an open area with no shelter: Stay far from trees, poles, or metal objects. Do not lie flat on the ground. Instead, squat low, placing hands on knees with your head between the hands. This makes you the smallest possible target.
  • Indoors: Avoid contact with telephone cords, electrical wires, and metal pipes during a thunderstorm. Avoid bathing. Unplug electrical appliances like TVs and computers.

• Lightning Conductors:

  • A device used to protect buildings from the damaging effects of lightning.
  • Structure: A metal rod (usually copper), taller than the building, installed on the walls during construction. One end is kept out in the air, and the other is buried deep in the ground.
  • Working: The rod provides an easy path for the electric discharge (lightning) to pass to the ground without damaging the building. The charge flows directly from the cloud to the rod and then safely into the Earth.

2. Earthquakes

• What is an Earthquake?

  • An earthquake is a sudden shaking or trembling of the Earth lasting for a very short time.
  • It is caused by a disturbance deep inside the Earth's crust.

• What Causes an Earthquake?

  • The outermost layer of the Earth (the crust) is fragmented into pieces called plates.
  • These plates are in continual slow motion (drifting).
  • Earthquakes are primarily caused by the movement of these tectonic plates.
  • When plates brush past one another, collide, or one goes under another, it causes disturbances in the crust.
  • These disturbances manifest as tremors on the Earth's surface.
  • Other causes can include volcanic eruptions, meteor impacts, or underground nuclear explosions, but plate tectonics is the main cause.
  • The boundaries of the plates are weak zones where earthquakes are more likely to occur. These weak zones are also known as seismic zones or fault zones.

• Seismic Zones in India:

  • Major fault zones in India include Kashmir, Western and Central Himalayas, the whole of the North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic Plain. Some areas of South India also fall in danger zones.

• Measuring Earthquakes:

  • The power (magnitude or intensity) of an earthquake is expressed on a scale called the Richter scale.
  • It is a logarithmic scale. An increase of 1 on the scale means 10 times the amplitude of the seismic waves, and roughly 32 times more energy release.
  • Really destructive earthquakes usually have magnitudes higher than 7.0 on the Richter scale.
  • The tremors produce waves on the surface of the Earth called seismic waves.
  • These waves are recorded by an instrument called the seismograph.
  • The focus is the point inside the Earth's crust where the earthquake originates. The point on the surface directly above the focus is called the epicenter. Damage is usually greatest near the epicenter.

• Protection Against Earthquakes:

  • Since earthquakes cannot be predicted accurately, preparedness is key to minimizing damage.
  • Building Construction:
    • In highly seismic areas, building structures should be designed to withstand major tremors. Consult qualified architects and structural engineers.
    • Use light construction materials like timber or mud rather than heavy ones. Roofs should be as light as possible.
    • Cupboards and shelves should be fixed to the walls.
    • Wall hangings like clocks, photo frames, and water heaters should be hung carefully to prevent falling.
  • Personal 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.
      • Do NOT stand near windows, mirrors, or hanging objects.
      • If in bed, do not get up; protect your head with a pillow.
    • If outdoors:
      • Find a clear spot away from buildings, trees, and overhead power lines.
      • Drop to the ground.
      • If in a car or bus, do not come out. Drive slowly to a clear spot and remain inside until the shaking stops.

Multiple Choice Questions (MCQs)

1. When a glass rod is rubbed with a piece of silk cloth:
   a) The rod and the cloth both acquire positive charge.
   b) The rod becomes positively charged while the cloth has a negative charge.
   c) The rod and the cloth both acquire negative charge.
   d) The rod becomes negatively charged while the cloth has a positive charge.

2. Which of the following is the safest place during a thunderstorm?
   a) Under a tall tree
   b) In an open field
   c) Inside a car with windows and doors shut
   d) Near metal sheds

3. An electroscope is a device used for:
   a) Measuring electric current
   b) Measuring the Earth's tremors
   c) Detecting if an object is charged
   d) Protecting buildings from lightning

4. The process of transferring charge from a charged object to the Earth is known as:
   a) Charging
   b) Discharging
   c) Earthing
   d) Induction

5. Lightning occurs due to:
   a) Nuclear reactions in the clouds
   b) Movement of tectonic plates
   c) Wind blowing very fast
   d) Electric discharge between clouds or between clouds and the Earth

6. The primary cause of most earthquakes is:
   a) Volcanic eruptions
   b) Movement of Earth's tectonic plates
   c) Heavy rainfall and floods
   d) Underground nuclear tests

7. The instrument used to record seismic waves is called a:
   a) Richter scale
   b) Electroscope
   c) Seismograph
   d) Barometer

8. Which scale is used to measure the magnitude of an earthquake?
   a) Celsius scale
   b) Kelvin scale
   c) Richter scale
   d) Seismic scale

9. In India, which of the following regions is NOT considered a major seismic zone?
   a) Kashmir Valley
   b) Rann of Kutch
   c) North-Eastern states
   d) Most parts of Madhya Pradesh (relatively less prone compared to others listed)

10. During an earthquake, if you are indoors, what is the recommended immediate action?
    a) Run outside immediately.
    b) Stand near a window to see what's happening.
    c) Take shelter under a sturdy table or desk.
    d) Use the elevator to get to the ground floor quickly.

Make sure you understand the concepts behind these phenomena, especially the causes and safety precautions. Good luck with your preparation!