NCERT Class 8 Science Chapter 15: Some Natural Phenomena

Brain Teasers

Thunderstorm and Lightning

Electric charge and Electric Current

How Lightning Conductor protect a building?

If the metal clip used in the electroscope is replaced by an ebonite rod and a charged body is brought in contact with it, will there be any effect on the aluminium strips? The aluminium strips will not show any repulsion. The charged body will not transfer any charge to the ebonite rod as ebonite rod is an insulator. As a result there will be no charge on the aluminium strips and no repulsion will occur.

Lightning

• Accumulation of charges in clouds

• Greek Concept

• Spark when plug is loose in the socket

• Greeks knew as early as 600 B.C. that when amber (amber is a kind of resin) was rubbed with fur, it attracted light objects such as hair.

• In 1752 Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomena

Charged Object

When a plastic refill is rubbed with polythene, it acquires a small electric charge. Similarly, when a plastic comb is rubbed with dry hair, it acquires a small charge. These objects are called charged objects

A charged balloon repelled a charged balloon.

A charged refill repelled a charged refill.

But a charged balloon attracted a charged refill.

It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive. It is observed that when a charged glass rod is brought near a charged plastic straw rubbed with polythene there is attraction between the two. Plastic straw would carry a negative charge is correct.

Plastic straws A and B are rubbed with dry cotton cloth – when they are brought near each other they will repel

If a charged plastic straw is brought near another uncharged plastic straw – they will attract each other

Static Charge vs. Electric Current

The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current

The current in a circuit which makes a bulb glow, or the current that makes a wire hot, is nothing but a motion of charges

The aluminium foil strips receive the same charge from the charged refill through the paper clip (remember that metals are good conductors of electricity). The strips carrying similar charges repel each other and they become wide open. Such a device can be used to test whether an object is carrying charge or not. This device is known as electroscope.

Electrical charge can be transferred from a charged object to another through a metal conductor

Foil strips lose charge to the earth through your body. We say that the foil strips are discharged. The process of transferring of charge from a charged object to the earth is called earthing

Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current.

Thunderstorm

The development of a thunderstorm, the air currents move upward while the water droplets move downward

Vigorous movements cause separation of charges

Positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. There is accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound- lightning - electric discharge

After lightning, thunder is seen – light travels faster than sound

Thunderstorm - Precautions

Open vehicles, like motorbikes, tractors, construction machinery, open cars are not safe. Open fields, tall trees, shelters in parks, elevated places do not protect us from lightning strokes. Carrying an umbrella is not at all a good idea during thunderstorms. If in a forest, take shelter under shorter trees. If no shelter is available and you are in an open field, stay far away from all trees.

Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between the hands. This position will make you the smallest target to be struck.

Lightning can strike telephone cords, electrical wires and metal pipes

It is safer to use mobile phones and cordless phones – wired phone is not advised

Bathing should be avoided during thunderstorms to avoid contact with running water. Electrical appliances like computers, TVs, etc., should be unplugged

Lightning Conductor

Lightning Conductor is a device used to protect buildings from the effect of lightning. A metallic rod, taller than the building, is installed in the walls of the building during its construction. One

end of the rod is kept out in the air and the other is buried deep in the ground

The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.

Lightning conductor does not allow the charge to accumulate on a building as it conducts the charge to the earth, protecting building from being struck by lightning.

Earthquakes

If a thunderstorm occurs there is always a possibility of lightning and cyclones accompanying it.

A major earthquake occurred in India on 8 October 2005 in Uri and Tangdhar towns of North Kashmir. Before that a major earthquake occurred on 26 January 2001 in Bhuj district of Gujarat.

An earthquake is a sudden shaking or trembling of the earth which lasts for a very short time

Earthquakes can cause floods, landslides and tsunamis. A major tsunami occurred in the Indian Ocean on 26 December 2004.

Pacific Ring of Fire

The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion – convergent, divergent and transform fault

Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion

The weak zones are also known as seismic or fault zones. In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo–Gangetic Plains. Some areas of South India also fall in the danger zone.

The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale. Earthquake intensity measured by Mercalli scale

Seismograph - tremors produce waves on the surface of the earth

Protection Against Earthquakes

Safe structure

Consult qualified architects and structural engineers.

In highly seismic areas, the use of mud or timber is better than using heavy construction material. Keep roofs as light as possible. In case the structure falls, the damage will not be heavy.

It is better if the cupboards and shelves are fixed to the walls, so that they do not fall easily.

Be careful where you hang wall clocks, photo-frames, water heaters etc., so that in the event of an earthquake, they do not fall on people.

Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have fire fighting equipment in working order.

The Central Building Research Institute, Roorkee, has developed knowhow to make quake-resistant houses

1. If you are at home

Take shelter under a table and stay there till the shaking stops.

Stay away from tall and heavy objects that may fall on you.

If you are in bed, do not get up.