Lightning

We are familiar with natural phenomena like winds, storms, and cyclones. In this chapter, we will explore two other powerful and potentially destructive natural phenomena: lightning and earthquakes. Unlike some weather events, earthquakes are difficult to predict accurately. Understanding these phenomena and taking appropriate safety measures can help minimise damage and protect lives.

Lightning is a massive electric spark that occurs in the sky during thunderstorms. Similar, smaller sparks can be observed when electrical wires become loose or when a plug is loose in its socket. Ancient civilisations were often afraid of lightning, interpreting it as divine anger due to a lack of scientific understanding.

However, scientists now understand that lightning is caused by the accumulation and sudden discharge of electric charges within clouds, or between clouds and the ground. While we no longer need to fear lightning superstitiously, its immense electrical energy makes it dangerous, and safety precautions are essential.

Historically, the connection between small static sparks and lightning was not recognised for a long time. Ancient Greeks knew that rubbing amber with fur attracted light objects. Similarly, rubbing materials like wool or polyester on hair can cause hair to stand on end and sometimes produce sparks and crackling sounds in the dark. In 1752, Benjamin Franklin scientifically demonstrated that these small sparks and lightning are fundamentally the same electrical phenomenon.

To understand lightning, we first need to learn about electric charges and their properties.

Charging By Rubbing

Objects can acquire an electric charge when they are rubbed against certain other materials. For instance, rubbing a plastic scale on dry hair allows the scale to attract small pieces of paper. Similarly, rubbing a plastic ballpen refill with polythene or a plastic comb with dry hair causes them to acquire a small electric charge. These objects are then referred to as charged objects.

In the process of rubbing, both objects involved become charged. For example, when a refill is rubbed with polythene, the refill acquires a charge, and the polythene also acquires a charge.

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	Does not attract	Not Charged
Steel spoon	Polythene, woollen cloth	Does not attract	Not Charged (Metal conducts charge away)

This simple process of rubbing transfers electric charge between objects, making them electrically charged.

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:

• Repulsion between Like Charges: When two objects carrying the same type of charge are brought near each other, they push each other away, i.e., they repel each other. For example, two balloons that have both been rubbed with a woollen cloth (acquiring the same type of charge) will repel each other when brought close. Similarly, two plastic refills rubbed with polythene will repel each other.
• Attraction between Unlike Charges: When two objects carrying different types of charge are brought near each other, they pull each other towards one another, i.e., they attract each other. For example, a charged balloon and a charged refill (rubbed with different materials) will attract each other.

By convention, the charge acquired by a glass rod when rubbed with silk is defined as positive charge. The other type of charge is then called negative charge.

Based on the interaction rule (like charges repel, unlike charges attract), if a positively charged glass rod attracts a plastic straw rubbed with polythene, it means the plastic straw must carry a negative charge.

The electric charges generated by rubbing, like on a rubbed scale or balloon, are typically static charges; they stay in one place. However, when electric charges move through a conductor (like a wire), they constitute an electric current. The current that powers lights or heats wires is the flow of these charges.

Transfer Of Charge

Electric charge can be transferred from a charged object to an uncharged object, especially through materials that are good conductors of electricity, such as metals.

A simple device called an electroscope can be made to detect the presence of charge on an object. A simple electroscope can consist of a metal paper clip inserted through a cardboard lid into a bottle, with two small aluminium foil strips hanging from the paper clip inside the bottle. When a charged object touches the metal paper clip, the charge is transferred through the clip to the aluminium foil strips. Since both strips receive the same charge, they repel each other and spread apart. If the foil strips repel, it indicates that the object that touched the clip was charged. This device can be used to test if a body is carrying an electric charge.

If you touch the metal part of the electroscope (the paper clip) with your hand after the foil strips are charged and repelling, the strips will collapse and return to their original position. This happens because the electric charge from the strips is transferred through your body (which is a conductor) to the Earth. This process of transferring electric charge from a charged object to the Earth is called earthing.

Earthing connections are installed in buildings to provide a safe path for electrical current to flow into the ground in case of leakage, protecting people from electric shocks.

The Story Of Lightning

Building upon the understanding of charge accumulation and transfer, we can now explain the phenomenon of lightning.

During the formation and development of thunderstorms, strong air currents move upwards, while water droplets and ice crystals move downwards within the clouds. These turbulent movements cause friction, leading to the separation and accumulation of electric charges. Positive charges tend to gather near the top of the clouds, while negative charges accumulate near the lower edges of the clouds and also near the ground.

As the amount of accumulated positive and negative charges grows very large, the air between the charges, which is normally a poor conductor, can no longer prevent the flow of electricity. The negative and positive charges rapidly move towards each other, either within the same cloud, between different clouds, or between a cloud and the ground. This rapid movement of charge is called an electric discharge.

The sudden and massive electric discharge heats the air along its path to extremely high temperatures, causing it to glow brightly and expand rapidly. We see the bright streaks of light as lightning, and the rapid expansion and contraction of air produce the loud sound known as thunder.

Lightning strikes are powerful and can cause severe damage to life and property. While the fundamental phenomenon is understood, scientists continue to study lightning to improve prediction and protection methods.

Lightning Safety

During thunderstorms with lightning, it is crucial to take precautions to stay safe, as open areas are particularly dangerous. Hearing thunder indicates that lightning is within striking distance.

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Finding A Safe Place

The safest places during a thunderstorm are enclosed structures:

• A house or a building provides good shelter.
• Inside a car or bus with the windows and doors shut is also a relatively safe place.

Unsafe locations include:

• Open vehicles like motorbikes, tractors, open cars, etc.
• Open fields, elevated areas, and isolated tall trees.
• Shelters in parks (unless they are fully enclosed buildings).
• Carrying an umbrella, especially one with a metal frame, is dangerous.

If you are outdoors and cannot find immediate shelter:

• In a forest, take cover under shorter trees, away from very tall ones.
• In an open field with no shelter, stay away from poles and metal objects. Do not lie flat on the ground. Instead, squat low with your hands on your knees and your head between your hands. This position minimizes your height and contact with the ground, reducing the risk.

Inside a building during a thunderstorm, avoid contact with telephone cords (wired phones), electrical wires, and metal pipes, as lightning can travel through these conductors. It is safer to use mobile phones or cordless phones, but avoid using wired phones to call someone who is also using a wired phone. Avoid bathing or using running water during a thunderstorm. Unplug electrical appliances like computers and televisions to protect them from power surges caused by lightning.

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Lightning Conductors

Tall buildings are particularly vulnerable to lightning strikes. A lightning conductor is a device installed to protect buildings from the destructive effects of lightning. It consists of a thick metallic rod, usually copper, installed during construction, running from above the highest point of the building down to the ground.

The pointed upper end of the rod extends into the air, and the lower end is buried deep in the ground, often connected to a metal plate. If lightning strikes the building, the lightning conductor provides an easy and safe path for the immense electrical charge to travel directly into the ground, thus protecting the building structure and its occupants. Metal columns, electrical wires, and water pipes within buildings can also offer some protection, but touching them during a thunderstorm should be avoided.

Earthquakes

Earthquakes are another powerful natural phenomenon that can cause widespread destruction. Unlike thunderstorms or cyclones, which can often be predicted with some warning, it is not yet possible to accurately predict when or where an earthquake will occur. This unpredictability makes them particularly dangerous.

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What Is An Earthquake?

An earthquake is the sudden shaking or trembling of the Earth's surface. These tremors are usually brief but can range from minor vibrations to violent shaking. Major earthquakes, though less frequent, can cause immense damage to buildings, bridges, dams, and infrastructure, leading to significant loss of life and property. Earthquakes can also trigger secondary disasters like floods, landslides, and tsunamis (large ocean waves caused by underwater earthquakes). A major tsunami in the Indian Ocean in 2004 caused widespread destruction in coastal regions.

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What Causes An Earthquake?

Earthquakes are caused by disturbances deep within the Earth's outermost layer, called the crust. In ancient times, various myths existed to explain earthquakes, lacking a scientific understanding of the Earth's structure and dynamics.

Scientific understanding reveals that the Earth's crust is not a single solid shell but is broken into large and small pieces called plates (also known as tectonic plates). These plates are in constant, slow motion. Earthquakes occur primarily at the boundaries where these plates interact.

When tectonic plates brush past each other, collide, or one plate slides under another, the movement is not always smooth. Stress builds up along the plate boundaries. When this stress is suddenly released, it causes the ground to shake, resulting in an earthquake. These boundaries, where earthquakes are more likely to occur, are called weak zones, seismic zones, or fault zones.

While most earthquakes are caused by the movement of tectonic plates, tremors can also be caused by other events like volcanic eruptions, meteor impacts, or underground nuclear explosions, although these are less frequent causes of major earthquakes.

The severity or destructive power of an earthquake is measured using the Richter scale. It is a logarithmic scale, meaning an increase of one unit on the scale represents a tenfold increase in the amplitude of seismic waves and about 32 times more energy released. A magnitude of 3 is minor, while earthquakes with magnitudes of 7 or higher are considered highly destructive. For example, the Bhuj (2001) and Kashmir (2005) earthquakes had magnitudes greater than 7.5, causing widespread damage.

The waves produced by an earthquake are called seismic waves. These waves are detected and recorded by an instrument called a seismograph. A seismograph records the intensity and pattern of shaking, allowing scientists to determine the earthquake's characteristics and pinpoint its origin (epicentre).

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Protection Against Earthquakes

Since earthquakes cannot be reliably predicted, preparedness is key, especially for people living in seismic zones. Building structures in these areas should be designed to be 'quake-safe' and withstand tremors. Consulting qualified architects and engineers experienced in seismic-resistant construction is advisable.

Building safety measures:

• Keep structures simple in design.
• In highly seismic areas, consider using lighter construction materials like mud or timber instead of heavy concrete.
• Keep roofs as light as possible.
• Fix cupboards and shelves securely to walls to prevent them from falling.
• Avoid hanging heavy objects like wall clocks or water heaters in places where they could fall on people during shaking.
• Ensure buildings have working fire fighting equipment, as earthquakes can cause fires.

During an earthquake, take immediate action to protect yourself:

• If indoors: Take shelter under a sturdy table and hold on until the shaking stops. Stay away from windows and heavy objects that might fall. If in bed, stay there and protect your head with a pillow.
• If outdoors: Move to a clear open area, away from buildings, trees, poles, and overhead power lines. Drop to the ground. If in a vehicle, stay inside, ask the driver to move slowly to an open spot, and wait inside until the tremors cease.

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