Class 8 Science Notes Chapter 15 (Chapter 15) – Examplar Problems Book

Alright class, let's focus on Chapter 15, "Some Natural Phenomena," from your Science Exemplar book. This chapter is crucial not just for your school exams but also forms a basis for questions in various government competitive exams. Pay close attention as we break down the key concepts.

Chapter 15: Some Natural Phenomena - Detailed Notes

This chapter primarily deals with two major natural phenomena: Lightning and Earthquakes. Both are powerful and potentially destructive, so understanding the science behind them and the necessary safety precautions is vital.

Part 1: Lightning

1. Static Electricity and Charges:

   • Historical Context: The ancient Greeks knew that rubbing amber attracted light objects like feathers. This is due to electric charge.
   • Charging by Rubbing: When certain objects are rubbed together, they acquire an electric charge. One object loses electrons (becomes positively charged), and the other gains electrons (becomes negatively charged).
     • Example: Rubbing a plastic refill with polythene charges the refill negatively. Rubbing a glass rod with silk charges the glass rod positively.
   • Types of Charges: There are two types of electric charges:
     • Positive Charge (+): Acquired by objects that lose electrons.
     • Negative Charge (-): Acquired by objects that gain electrons.
   • Interaction of Charges:
     • Like charges repel each other (Positive repels Positive; Negative repels Negative).
     • Unlike charges attract each other (Positive attracts Negative).
   • Charged vs. Uncharged Objects: A charged object can attract an uncharged object.

2. Transfer of Charge:

   • Electric charge can be transferred from a charged object to another through a conductor (usually a metal).
   • Our body is a conductor of electricity. Touching a charged object can cause the charge to flow through our body to the Earth (if we are grounded), a process called earthing or grounding.

3. Detecting Charge - The Electroscope:

   • Definition: An electroscope is a device used to detect whether an object is charged or not and the nature of the charge (though simple ones primarily detect presence).
   • Simple Electroscope: Can be made using an empty jar, a metal rod (or paper clip), and two strips of aluminium foil.
   • Working:
     • When a charged object touches the metal end of the rod, the charge transfers through the rod to the 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.
     • If we touch the metal end with our hand while the strips are diverged, the charge flows to the Earth through our body, and the strips collapse (discharge).

4. The Story of Lightning:

   • Cause: Lightning is a massive electric discharge (spark) occurring between clouds, or between clouds and the Earth. It's caused by the accumulation of static charges in clouds due to friction between water droplets, ice crystals, and air currents.
   • Charge Separation: Generally, the upper parts of clouds become positively charged, and the lower/middle parts become negatively charged. The ground below a negatively charged cloud base becomes positively charged by induction.
   • Discharge: When the accumulated charge becomes very large, the air (normally an insulator) breaks down and allows the charge to flow rapidly, creating a bright flash (lightning) and intense heat.
   • Thunder: The rapid heating of air by the lightning discharge causes it to expand explosively, creating sound waves that we hear as thunder.
   • Lightning Strike: The discharge between the cloud and the Earth.

5. Lightning Safety:

   • Indoors:
     • Avoid contact with running water (bathing, washing dishes) during a thunderstorm, as lightning can travel through plumbing.
     • Avoid using corded phones and electrical appliances. Mobile phones and cordless phones are generally safer.
     • Stay away from windows and doors.
   • Outdoors:
     • Open fields, tall trees, elevated ground, and metal structures (like sheds or poles) are dangerous. Avoid them.
     • If in a forest, take shelter under shorter trees.
     • If no shelter is available, squat low on the ground with hands on knees and head between hands. Make yourself the smallest possible target. Do NOT lie flat on the ground.
     • Car or bus with windows closed is a safe place (metal body acts like a Faraday cage).
   • Lightning Conductor:
     • A device installed on tall buildings to protect them from lightning strikes.
     • Consists of a metal rod (taller than the building) connected via a thick copper strip to a metal plate buried deep in the Earth.
     • It provides an easy path for the lightning discharge to flow directly to the ground, bypassing the building structure and preventing damage.

Part 2: Earthquakes

1. What is an Earthquake?

   • A sudden shaking or trembling of the Earth's surface.
   • Caused by disturbances deep inside the Earth's outermost layer, the crust.

2. Causes of Earthquakes:

   • Tectonic Plates: The Earth's crust is not one single piece but is fragmented into several large and small rigid plates called tectonic plates.
   • Plate Movement: These plates are constantly moving very slowly (a few millimeters per year).
   • Stress Build-up: The movement causes stress to build up at the boundaries where plates interact (collide, slide past, or move apart).
   • Sudden Release: When the stress exceeds the strength of the rocks, there is a sudden slip or rupture along a fault line, releasing the stored energy as seismic waves, causing the ground to shake.
   • Other Causes: Volcanic eruptions, underground nuclear explosions, and sometimes the collapse of mine roofs can also cause tremors.

3. Key Terms:

   • Fault Zones / Seismic Zones: Boundaries of the tectonic plates where earthquakes are more likely to occur. Major fault zones in India include the Himalayas, Kashmir, North-East, Rann of Kutch, and parts of the Indo-Gangetic plain.
   • Focus (Hypocenter): The point inside the Earth's crust where the earthquake rupture originates.
   • Epicenter: The point on the Earth's surface directly above the focus. Damage is usually greatest near the epicenter.
   • Seismic Waves: Waves of energy that travel through the Earth's layers, resulting from an earthquake.

4. Measuring Earthquakes:

   • Seismograph (or Seismometer): An instrument that detects and records seismic waves. It essentially consists of a vibrating rod or pendulum that begins to vibrate when tremors occur. A pen attached to it records the vibrations on a moving paper chart (seismogram).
   • Richter Scale: A logarithmic scale used to measure the magnitude (energy released) of an earthquake.
     • It is not a linear scale. An increase of 1 unit on the scale represents a tenfold increase in the amplitude of the seismic waves and roughly 32 times more energy released.
     • Earthquakes of magnitude > 7 are considered major and can cause severe damage.
   • Modified Mercalli Scale: Measures the intensity of an earthquake based on the observed effects on people, buildings, and the environment.

5. Effects of Earthquakes:

   • Damage to buildings, bridges, dams, infrastructure.
   • Landslides and avalanches.
   • Floods (if dams break).
   • Tsunamis (if the earthquake occurs under the ocean floor).
   • Loss of life and property.

6. Protection from Earthquakes:

   • Building Design:
     • In highly seismic zones, buildings should be designed to be earthquake-resistant.
     • Use light construction materials (timber, mud) rather than heavy ones. Cupboards and shelves should be fixed to walls.
     • Consult qualified architects and structural engineers.
     • Ensure fire-fighting equipment is functional.
   • During an Earthquake:
     • Indoors:
       • Drop, Cover, and Hold On: Drop to the ground, take cover under a sturdy table or desk, and hold on until the shaking stops.
       • Stay away from windows, mirrors, heavy objects, and fireplaces.
       • Do NOT use elevators.
     • Outdoors:
       • Move to an open area away from buildings, trees, overhead power lines, and poles.
       • If in a vehicle, stop safely in an open area and stay inside.

Multiple Choice Questions (MCQs)

1. When a glass rod is rubbed with a piece of silk cloth:
   a) The glass rod gets negatively charged, and the silk gets positively charged.
   b) Both the glass rod and the silk cloth get positively charged.
   c) The glass rod gets positively charged, and the silk gets negatively charged.
   d) Both the glass rod and the silk cloth get negatively charged.

2. Which of the following statements about electric charges is correct?
   a) Like charges attract each other.
   b) Unlike charges repel each other.
   c) A charged body can attract an uncharged body.
   d) An uncharged body always repels a charged body.

3. An electroscope is used to:
   a) Measure electric current.
   b) Measure the magnitude of an earthquake.
   c) Detect the presence of electric charge on a body.
   d) Protect buildings from lightning.

4. Lightning occurs due to:
   a) Magnetic discharge between clouds.
   b) Wind currents colliding violently.
   c) Electric discharge between charged clouds or between clouds and the Earth.
   d) Nuclear reactions within clouds.

5. Which of the following is the safest place during a thunderstorm?
   a) Under a tall tree.
   b) Inside a car with windows closed.
   c) Near a metal pole.
   d) On elevated ground.

6. The primary cause of most earthquakes is:
   a) Volcanic eruptions.
   b) Movement of Earth's tectonic plates.
   c) Underground nuclear explosions.
   d) Collapse of large buildings.

7. The point on the Earth's surface directly above the origin of an earthquake is called the:
   a) Focus
   b) Seismic Zone
   c) Epicenter
   d) Fault Line

8. Which instrument is used to record the waves generated by an earthquake?
   a) Barometer
   b) Electroscope
   c) Richter Scale
   d) Seismograph

9. The magnitude of an earthquake is commonly expressed on the:
   a) Mercalli Scale
   b) Celsius Scale
   c) Richter Scale
   d) Kelvin Scale

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

Answer Key for MCQs:

1. c
2. c
3. c
4. c
5. b
6. b
7. c
8. d
9. c
10. c

Make sure you revise these notes thoroughly. Understand the 'why' behind each phenomenon and safety measure. This will help you tackle application-based questions effectively in your exams. Let me know if any part needs further clarification.