Nature presents us with various phenomena, some of which can be incredibly powerful and destructive. This chapter focuses on two such natural disasters: lightning and earthquakes. We will explore their causes, effects, and importantly, the measures we can take to minimise the harm they inflict.

Lightning

We sometimes see sparks from electrical wires, especially if they are loose and moved by wind, or from loose plugs in sockets. These are small examples of electrical discharge.

Lightning is essentially an electrical spark on a massive scale, occurring in the atmosphere.

The Sparks That The Greeks Knew About

The ancient Greeks, around 600 B.C., observed that when amber (fossilized resin) was rubbed with fur, it could attract light objects like hair. This was an early encounter with static electricity.

A similar effect can be observed today when you take off woollen or polyester clothes in the dark; your hair might stand on end, and you might see sparks and hear crackling sounds. This phenomenon, known as triboelectric effect, results in the accumulation of static electric charges.

It wasn't until 1752 that the American scientist Benjamin Franklin demonstrated that lightning and the sparks from clothing are the same fundamental phenomenon – the discharge of accumulated electrical charges. The long gap between the initial observation by the Greeks and Franklin's realisation highlights that scientific discoveries can take considerable time and effort.

To understand lightning better, we need to learn about electric charges and their behaviour.

A simple example of charging by rubbing is seen when a plastic scale rubbed on dry hair attracts small pieces of paper.

Charging By Rubbing

Objects can acquire electric charge by being rubbed against certain other materials. This process is called charging by rubbing or triboelectric charging.

For instance, rubbing a plastic ballpoint pen refill vigorously with a piece of polythene causes the refill to acquire a small electric charge. This charged refill can then attract light objects like small pieces of paper, dry leaves, husk, or mustard seeds.

Similarly, rubbing a plastic comb through dry hair charges the comb. In this process, both the object being rubbed (refill, comb) and the material used for rubbing (polythene, hair) become electrically charged.

Experimenting with various objects and rubbing materials allows us to observe which combinations result in charging:

Objects Rubbed	Materials Used for Rubbing	Attracts/does not Attract Pieces of Paper	Charged/Not Charged
Refill	Polythene, woollen cloth	Attracts	Charged
Balloon	Polythene, woollen cloth, dry hair	Attracts	Charged
Eraser	Wool	May attract slightly/not attract	Might be slightly charged/Not charged easily
Steel spoon	Polythene, woollen cloth	Does not attract	Not Charged (metals are conductors, charge doesn't stay)

Metals are good conductors of electricity, meaning charges can move easily through them. Hence, charging metal objects by simply rubbing them while holding them is difficult, as the charge is quickly transferred away through your body to the earth (earthing).

Types Of Charges And Their Interaction

Experiments with charged objects reveal that there are two distinct types of electric charges, and they interact with each other in specific ways:

• Inflating and charging two balloons by rubbing them with a woollen cloth, and then hanging them near each other, shows that they repel each other.
• Charging two pen refills by rubbing them with polythene, and then bringing one charged refill near another charged refill suspended in a tumbler, also shows that they repel each other.
• However, if a charged balloon is brought near a charged refill, they attract each other.

These observations lead to the conclusion that there are two kinds of electric charges. Objects with the same type of charge (like charges) repel each other, while objects with different types of charge (unlike charges) attract each other.

By convention, the charge acquired by a glass rod when rubbed with silk is called positive charge. The other type of charge is called negative charge. Since a charged glass rod attracts a charged plastic straw rubbed with polythene, the plastic straw must carry the opposite type of charge, which is negative.

The charges generated by rubbing, like those on a charged refill or balloon, are usually static charges because they remain on the surface of the object and do not move readily. When electric charges move from one place to another, they constitute an electric current. The current in electrical circuits, which powers appliances, is the motion of charges.

Transfer Of Charge

Electric charge can be transferred from a charged object to another object, particularly through materials that are good conductors of electricity, like metals.

A device called an electroscope can be used to detect the presence of charge on a body. A simple electroscope can be made using a paper clip and two strips of aluminium foil suspended inside a jar.

When a charged object (like a rubbed refill) is brought into contact with the metal paper clip, charge is transferred from the charged object, through the paper clip (a conductor), to the aluminium foil strips. Since both foil strips receive the same type of charge, they repel each other and move apart. The extent to which they repel indicates the amount of charge. If an object touches the paper clip and the foil strips move apart, it means the object is charged.

If you touch the metal paper clip of a charged electroscope with your hand, the foil strips collapse back together. This happens because the charge on the foil strips is transferred through your body to the Earth. This process of transferring electric charge from a charged object to the Earth is called earthing.

Earthing is a safety measure implemented in buildings to prevent electrical shocks. Metal wires connected to the Earth provide a path for any accidental leakage of electrical current to flow safely into the ground.

The Story Of Lightning

Armed with the knowledge of electric charges and their behaviour, we can now understand the phenomenon of lightning.

During the development of thunderstorms, complex processes involving the movement of air currents and water droplets within clouds lead to the separation and accumulation of electric charges. Although the exact mechanism is not fully understood, it is observed that positive charges tend to collect near the top of the clouds, while negative charges accumulate near the bottom. Additionally, positive charges also build up on the ground surface below the clouds.

As the amount of accumulated charge increases, the insulating capacity of the air between the clouds, or between the clouds and the ground, is overcome. When the difference in charge becomes very large, a sudden, rapid flow of charge occurs, known as an electric discharge.

This electric discharge produces a brilliant streak of light and a loud sound (thunder). The visible streak of light is what we call lightning. Electric discharge can happen between different clouds or between a cloud and the Earth.

While we now understand the science behind lightning, it remains a powerful and potentially deadly natural phenomenon that can cause significant damage to life and property. Therefore, taking appropriate safety precautions during thunderstorms is essential.

Lightning Safety

Safety during thunderstorms and lightning is crucial. The primary rule is that no open place is truly safe during lightning activity. Hearing thunder indicates that lightning is occurring or imminent in your area, serving as an immediate alert to seek safe shelter. It is advisable to wait for some time after the last thunder is heard before leaving the safe place, as lightning can still occur.

Finding A Safe Place

The safest places during a thunderstorm are inside a house or a substantial building. If you are travelling, being inside a car or a bus with windows and doors closed also provides a safe enclosure (the metal body of the vehicle acts as a Faraday cage, conducting current around the occupants).

Do’s And Don’ts During A Thunderstorm

Outside Safety:

• Avoid open vehicles like motorbikes, tractors, construction machinery, or open cars.
• Stay away from open fields, tall isolated trees, shelters in parks, and elevated places.
• Do NOT carry an umbrella, as its metallic rod can attract lightning.
• If in a forest, seek shelter under shorter trees, away from tall ones.
• If caught in an open field with no shelter, the safest position is to squat low on the ground, putting your hands on your knees and your head between your hands (reducing your height and contact with the ground). Do NOT lie flat on the ground.

Inside The House

Even indoors, certain precautions are necessary during a thunderstorm:

• Avoid contact with telephone cords, electrical wires, and metal pipes (plumbing), as these can conduct lightning current.
• Using mobile phones or cordless phones is generally safe. Avoid using wired telephones.
• Avoid bathing or showering to prevent contact with running water, which can conduct electricity.
• Unplug electrical appliances like computers, TVs, and other electronics. Keep electrical lights switched on; they are generally safe.

Lightning Conductors

A lightning conductor is a protective device installed on buildings to safeguard them from lightning strikes. It consists of a metal rod, typically copper or aluminium, installed vertically on the highest point of the building, taller than the structure itself. One end of the rod is pointed upwards towards the sky, and the other end is connected via a thick metal strip to a metal plate or rod buried deep within the earth.

If lightning strikes the building, it is likely to strike the lightning conductor (the tallest, pointed metallic object). The conductor provides a low-resistance path for the enormous electrical charge from the lightning strike to safely flow into the ground, preventing severe damage to the building and protecting its occupants. Metal components within the building structure, like supporting columns, electrical wires, and water pipes, also provide some protection, but direct contact should still be avoided during a storm.

Earthquakes

While thunderstorms and lightning can be predicted to some extent (weather forecasts can warn about developing storms), earthquakes are natural phenomena that are currently very difficult, if not impossible, to predict accurately in terms of exact time and location.

Despite their unpredictability, earthquakes can cause immense destruction. Examples include the major earthquakes in Uri and Tangdhar in Kashmir in 2005, and in Bhuj, Gujarat, in 2001, which resulted in widespread loss of life and property. Earthquakes can also trigger other devastating events like floods, landslides, and tsunamis (large ocean waves, like the one in the Indian Ocean in 2004).

What Is An Earthquake?

An earthquake is characterised by a sudden shaking or trembling of the Earth's surface. These tremors can last for a very short duration. Earthquakes are caused by disturbances originating deep within the Earth's crust, the outermost solid layer.

Earthquakes happen frequently around the world, but most are minor and not even felt. Major earthquakes are much rarer but can cause catastrophic damage to buildings, bridges, dams, and infrastructure, leading to huge human and economic losses.

What Causes An Earthquake?

Contrary to old myths, earthquakes are caused by scientific processes within the Earth. The Earth's outermost layer, the crust, is not a single continuous shell. Instead, it is broken into large, irregular pieces called tectonic plates or simply earth's plates.

These plates are not stationary; they are in constant, slow motion. Earthquakes are primarily caused by the movement and interaction of these tectonic plates. When plates:

• Brush past each other (transform boundary)
• Collide and one goes under the other (convergent boundary/subduction)
• Move apart (divergent boundary)

The stress built up by these interactions is released as seismic energy, causing tremors that travel through the Earth's crust and manifest as shaking on the surface.

While most earthquakes are caused by plate movements, less frequent causes can include volcanic eruptions, meteor impacts, or underground nuclear explosions.

Since earthquakes are caused by the interactions at the boundaries of these plates, the regions along plate boundaries are more prone to seismic activity. These areas are referred to as weak zones, seismic zones, or fault zones.

In India, major seismic zones include Kashmir, the Western and Central Himalayas, the entire North-East region, parts of Gujarat (Rann of Kutch), Rajasthan, and the Indo-Gangetic Plain. Some areas in South India also face earthquake risk.

The energy released by an earthquake is measured using a scale called the Richter scale. The magnitude on the Richter scale indicates the earthquake's power. Earthquakes with a magnitude of 7 or higher on the Richter scale are considered highly destructive and can cause extensive damage. For example, the Bhuj and Kashmir earthquakes both had magnitudes greater than 7.5.

The Richter scale is logarithmic, meaning that an increase of one unit in magnitude corresponds to a tenfold increase in the amplitude of seismic waves and roughly a 32-fold increase in the energy released. Thus, an earthquake of magnitude 6 is significantly more destructive than one of magnitude 4 (releasing about $32 \times 32 \approx 1000$ times more energy).

The tremors generated by an earthquake produce seismic waves that travel through the Earth. These waves are recorded by an instrument called a seismograph. A seismograph detects ground motion and records the pattern of seismic waves on a chart, allowing scientists to study the earthquake's characteristics and estimate its intensity and potential for destruction.

Despite advancements in science, accurately predicting when and where the next earthquake will strike remains a major challenge.

Protection Against Earthquakes

Since earthquakes are unpredictable and potentially devastating, taking necessary precautions beforehand is vital, especially for people living in seismic zones.

Before an Earthquake (Preparedness):

• Buildings in seismic zones should be designed and constructed to be "quake-safe" or earthquake-resistant. Modern building technology allows for such constructions.
• Consult qualified architects and structural engineers for building designs in earthquake-prone areas.
• In highly seismic regions, building materials like mud and timber may be preferred over heavy construction materials, as they cause less damage if the structure collapses.
• Roofs should be kept as light as possible.
• Fix cupboards and shelves securely to walls to prevent them from falling.
• Avoid hanging heavy objects like wall clocks, photo frames, or water heaters over beds or places where people sit frequently.
• Ensure that all buildings have working fire fighting equipment, as earthquakes can cause fires.

During an Earthquake (Immediate Safety Measures):

• If indoors:
  • Seek shelter under a sturdy table or desk and hold on until the shaking stops.
  • Stay away from windows and objects that might fall (mirrors, heavy furniture, light fixtures).
  • If in bed, protect your head with a pillow and stay in bed.
• If outdoors:
  • Move to a clear open area away from buildings, trees, overhead power lines, and any structures that could collapse or fall.
  • Drop to the ground in the open spot.
  • If in a car or bus, stay inside the vehicle. Ask the driver to pull over to a clear open area away from hazards, stop, and stay there until the tremors cease. Do not get out of the vehicle during the shaking.

Being prepared and knowing what to do during an earthquake can significantly reduce the risk of injury.

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