Class 8: SOME NATURAL PHENOMENA

While winds, storms, and cyclones cause severe destruction, this chapter focuses on two other formidable natural phenomena: lightning and earthquakes. We explore their scientific causes and safety precautions.

Historical Context: Ancient Greeks (around 600 B.C.) noted amber rubbed with fur attracts hair. In 1752, Benjamin Franklin proved that clothing static sparks and lightning are the same phenomenon at different scales.

Activity-1: Rubbing ball-pen refills, plastic combs, inflated balloons, or drinking straws against hair, polythene sheets, or woollen cloth makes them attract bits of paper, dry leaves, and husk.

Objects rubbed acquire static electric charges and are called charged objects.

Lab Activity: Testing various rubbed objects (refill, balloon, comb, eraser, steel spoon) against materials like paper, polythene, and wool. While plastic and rubber objects acquire charges easily, a steel spoon held in hand does not retain charge because electric charges easily conduct away through our body into the earth.

Activity-2: Two balloons rubbed with wool repel each other. Two refills rubbed with polythene repel each other. However, a rubbed balloon and a rubbed refill attract each other.

Conclusion: There are two kinds of charges. Like charges repel, while unlike charges attract each other. By convention, glass rubbed with silk is positive; plastic rubbed with polythene/wool is negative.

Charges generated by rubbing are static (they stay in place). When charges move, they constitute an **electric current**.

Activity-3: Suspending a silver foil-wrapped thermocol ball and bringing a charged glass rod near it. Initially, attraction occurs due to **electrostatic induction** (inducing opposite charges). Upon contact, charges transfer, and repulsion ensues (like charges repel).

Activity-4 (Electroscope): Making an electroscope using a jam bottle, cardboard lid, metal paper clip, and two aluminium foil strips. Touching a charged refill to the clip passes the charge down to the foil strips. Because both strips receive identical charges, they repel each other and diverge.

Earthing: The process of transferring electrical charge from a charged object to the earth. When you touch the paper clip, foil strips collapse because charge drains through your body into the ground.

Buildings are fitted with earthing systems to protect residents from electrical leakage shocks.

During thunderstorms, fast air currents move clouds, accumulating high static charges on cloud surfaces due to friction with air particles. When massive charges build up, air (normally an insulator) breaks down, causing massive electric discharge between clouds or between clouds and the earth, producing bright lightning streaks and thunder.

Safe Places: Low buildings, cars/buses with closed windows. Avoid open fields, tall isolated trees, landline phones, and electric poles.

Lightning Conductors: A tall metallic rod installed on buildings with one end in the air and the other buried deep in the ground, safely routing lightning strikes harmlessly into the earth.

Earthquakes: Sudden shaking or trembling of the earth caused by deep subterranean disturbances in the crust. Major historical disasters in India include Bhuj (2001) and Kashmir (2005).

Submarine earthquakes can trigger giant destructive ocean waves called tsunamis (e.g., 26 December 2004 Indian Ocean tsunami).

The earth's outermost layer (crust) is fragmented into moving pieces called tectonic plates. When plates collide or brush past one another, they create seismic disturbances along boundaries known as fault zones or seismic zones (e.g., Himalayas, North-East India, Rann of Kutch).

Seismograph: An instrument featuring a vibrating pendulum and stylus that records seismic waves on moving graph paper.

Richter Scale: Measures earthquake magnitude logarithmically. An increase of 2 units on the Richter scale represents a 1000-fold increase in destructive energy!

Earthquake Safety: Construct simple quake-safe structures using mud and timber in seismic zones. Keep heavy furniture anchored.

During Tremors: Take shelter under a sturdy table indoors. Move to clear open spaces outdoors away from tall buildings, trees, and power lines.