Class 8 Science Chapter 15 Some Natural Phenomena

Lightning                                                                                                      Earthquake

 

 

Lightning is a natural phenomenon that has fascinated people for ages. Several people thought and researched about the cause of lightning and its process. Benjamin Franklin discovered that there is an electric discharge between clouds that produces a spark, and it is the electric spark between the clouds and the earth that appears as lightning. His famous kite experiment proved this fact.
The occurrence of lightning is as follows. The formation of clouds involves friction between water particles in the atmosphere. The friction charges the particles. Among the positive and negative charges, the negative charge accumulates at the bottom of the cloud and the positive charges in its top. As the accumulation of the charge increases, the cloud will create a positive charge on the ground nearby. As the amount of charge increases, the negative charge on the cloud tends to make a path towards the ground, and it results in a narrow streak of electrical discharge, which we call lightning.
An electroscope is a device that detects the type of charge on a body. Like charges repel and unlike charges attract each other.  This is used in an electroscope. An electroscope consists of a glass jar fitted with a cork lid and a metallic wire passing through it. There are two metallic strips at the bottom of the wire. The upper end of the wire is connected to a metal disc. A body that is positively charged is touched to the metal disc, so that the charge is transferred to the metal strips through the wire, and they diverge from each other on gaining a like charge. Now, if a negatively charged object is brought into contact with the disc, the strips converge towards each other, indicating the unlike charge on the body. Similarly, if a positively charged body is brought in contact with the metal disc, the divergence of the metal strips increases, indicating the like charge on the body. If the metal disc of the electroscope is touched with the hand, it loses its charge to the ground by transfer of charge through the human body. This is called earthing.

Protection from Lightning

Lightning and Safety

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Lightning is caused due to the heavy electric discharge from the clouds to the earth. If lightning is not controlled, it can cause heavy damage to life and property. Hence, protective measures are of utmost importance during lightning strikes. Interiors of buildings are safe places to protect ourselves from lightning strikes. Taking shelter in interiors and not staying in open areas is one of the best measures you can take. You should not take shelter under trees when in the open, or go to an open area if outside. If you are in a vehicle, you should not come out of it; you should also close the doors and windows of the vehicle.
Tall buildings are provided with lightning rods. A lightning rod a metal rod that is erected at the top of a building. It is connected to a thick metallic plate, made of a good conducting material like copper and buried under the earth, through a thick copper wire. If lightning is likely to strike the building, it is attracted by the lightning rod, and the electrical discharge is transferred through the wire to the ground. Thus, it keeps the building safe.

Earthquakes

Earthquakes

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A natural phenomenon that cannot be predicted is an earthquake. The earth consists of three major layers, called the crust, the mantle and the core. The core is further divided into the inner core and the outer core. The mantle consists of semi-solid material above which the crust floats. The crust consists of oceans and continents. The crust is divided into several parts, called tectonic plates. The regions where one tectonic plate slides against another are referred to as fault zones, and these are the regions where an earthquake is likely to occur. Hence, these zones are referred to as seismic zones.
The place in the interior of the earth where an earthquake occurs is the focus, and the region on the surface of the earth that is the closest to focus is likely to experience the largest damage. This region is called the epicentre of the earthquake.
The instrument that measures the severity of an earthquake is a seismograph. It basically consists of a drum that rolls and a pendulum with a stylus that traces the waves of an earthquake on a sheet like a graph paper. The severity of an earthquake is measured on the Richter scale. A major earthquake measures 7 or more on the Richter scale.

Some More Notes

Some Natural Phenomena

Lightning

We always see sparks on a electric pole when wires become loose. This phenomenon is quite common when a wind is blowing and shaking the wires. We also often see sparks when a plug is loose in its socket. Lightning is also an electric spark, but on a huge scale.

Lightning is an atmospheric discharge of electricity usually accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. Lightning is caused when two clouds collide or slide against one another. This involves same process which happens when you rub a comb on your dry hair and the hair gets an electric charge.

As amount of charge generated during collision of clouds is huge so we witness it as flashing lights in the sky. This flashing light is called lightning. Once a cloud gets electrically charged it releases the charge to earth. In other words the earth receives the charge and neutralizing the cloud in the process.

Lightning Safety:

During lightning and thunderstorm no open place is safe.

• Hearing thunder is an alert to rush to a safer place.

• After hearing the last thunder, wait for some time before coming out of the safe place.

Do’s and Don’ts during a Thunderstorm:

Outside: 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 umbrella is not a good idea at all during thunderstorms. If in a forest, we should take shelter under shorter trees. If no shelter is available and we are in an open field, stay far away from all trees. We should stay away from poles or other metal objects. We should not lie on the ground. Instead, we should squat low on the ground.

Inside the House: Lightning can strike telephone cords, electrical wires and metal pipes. During a thunderstorm contact with these should be avoided. It is safer to use mobile phones and cordless phones. Bathing should be avoided during thunderstorms to avoid contact with running water. Electrical appliances like computers, TVs, etc., should be unplugged. Electrical lights can remain on. They do not cause any harm.

Lightning Conductors: 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 rod provides easy route for the transfer of electric charge to 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.

You can notice trident shaped iron rod on top of temples, TV towers and cell phone towers. These are lightning conductors.

Earthquake

An earthquake is a sudden shaking or trembling of the earth lasting for a very short time. It is caused by a disturbance deep inside the earth’s crust. Earthquakes occur all the time, all over the earth. They are not even noticed. Major earthquakes are much less frequent. They can cause immense damage to buildings, bridges, dams and people. There can be a great loss to life and property. The earthquakes can cause floods, landslides and tsunamis.

Causes of Earthquake

The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a tectonic plate. These plates are in continual motion. When they brush past one another, or a plate goes under another due to collision, they cause disturbance in the earth’s crust. It is this disturbance that shows up as an earthquake on the surface of the earth.

Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion is carried out. However, most earthquakes are caused by the movement of earth’s plates.

Since earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. 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 Plane. Some areas of South India also fall in the danger zone

Power of Earthquakes: The power of an earthquake is expressed in terms of a magnitude on a scale called Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale. Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5.

Protection against Earthquakes

It is, important that we take necessary precautions to protect ourselves all the time. People living in seismic zones, where the earthquakes are more likely to occur, have to be specially prepared. First of all, the buildings in these zones should be designed so that they can withstand major tremors. Modern building technology can make it possible. It is advisable to make the structure simple so that it is ‘Quake Safe’.

• Consult qualified architects and structural engineers. In highly seismic areas, the use of mud or timber is better than the 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.

Summary

• Some objects can be charged by rubbing with other objects.

• There are two kinds of charges — positive charge and negative charge

• Like charges repel and unlike charges attract each other.

• The electrical charges produced by rubbing are called static charges.

• When charges move, they constitute an electric current.

• An electroscope may be used to detect whether a body is charged or not.

• The process of transfer of charge from a charged object to the earth is called earthing.

• The process of electric discharge between clouds and the earth or between different clouds causes lightning.

• Lightning strike can destroy life and property.

• Lightning conductors can protect buildings from the effects of lightning.

• An earthquake is a sudden shaking or trembling of the earth.

• Earthquake is caused by a disturbance deep inside the earth’s crust.

• It is not possible to predict the occurrence of an earthquake.

• Earthquakes tend to occur at the boundaries of earth’s plates. These boundaries are known as fault zones.

• Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property.

• We should take necessary precautions to protect ourselves from earthquakes.

Question 1:

Which of the following cannot be charged easily by friction?
(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth

Answer :

(b) A copper rod
Only non-conducting materials can be easily charged by friction. Copper is a highly conducting materials. Therefore, a copper rod cannot be charged easily by friction.

Question 2:

When a glass rod is rubbed with a piece of silk cloth the rod
(a) and the cloth both acquire positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.
(d) becomes negatively charged while the cloth has a positive charge.

Answer :

(b) The rod becomes positively charged, while the cloth has a negative charge.
When an object is charged by rubbing it against another object, the two objects get oppositely charged. By convention, it is considered that the charged acquired by the glass rod is positive and charged acquired by the cloth is negative. Therefore, the rod becomes positively charged and the cloth becomes negatively charged.

Question 3:

Write T against true and F against false in the following statements.
(a) Like charges attract each other. (T / F)
(b) A charged glass rod attracts a charged plastic straw. (T / F)
(c) Lightning conductor cannot protect a building from lightning. (T/F)
(d) Earthquakes can be predicted in advance. (T / F)

Answer :

(a) False
Like charges repel each other. It is the unlike charges that attract each other.
(b) True
A charged glass rod has positive charges on its surface while a charged plastic straw has negative charges on its surface. Since unlike charges attract each other, a charged glass rod attracts a charged plastic straw.
(c) False
During a lightning, the lightning conductor conducts all the atmospheric charges to the Earth directly, leaving the building safe. Hence, lightning conductors protect a building from lightning.
(d) False
Although the causes of earthquakes is known, but no instrument could be invented to detect it till now. Hence, earthquakes cannot be predicted in advance.

Question 4:

Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.

Answer :

When a sweater is taken off, the woollen sweater gets charged because of the friction between the sweater and the body. Hence, one can hear a crackling sound during the given process.

Question 5:

Explain why a charged body loses its charge if we touch it with our hand.

Answer :

When we touch a charged object, our body conducts its charges to the earth. That is why a charged body loses its charge, if we touch it with our hand. This phenomenon is known as electric discharge.

Question 6:

Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Answer :

The destructive energy of an earthquake is measured by the Richter scale. This scale has the readings from 1 to 10.
The reading of magnitude 3 on the Richter scale would be recorded by a seismograph.
If the Richter scale gives a reading of magnitude 3, then the earthquake is not likely to cause much damage. Generally, earthquake of magnitudes higher than 5 is considered destructive in nature.

Question 7:

Suggest three measures to protect ourselves from lightning.

Answer :

Protective measures against lightning are as follows:
(i) Stay in a completely closed place. If you are moving in a car, then remain there until the lightning is over. Close the windows of the car immediately.
(ii) Do not touch any electrical wires, telephone cables, metal pipes, etc.
(iii) Do not bath in running water. This may cause an electric shock.

Question 8:

Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

Answer :

The nature of charges present on the surface of charged balloons are similar. Since like charges repel each other, two charged balloons repel each other. When a charged body is brought near an uncharged body, the uncharged body acquires charges on its surface caused by the induction of charges. The charges are of opposite nature in relation to the charged body. Since unlike charges attract each other, a charged body always attracts an uncharged body. Hence, an uncharged balloon is attracted by another charged balloon.

Question 9:

Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer :

An electroscope can be used to detect whether a body is charged or not. The following figure shows a simple electroscope.

It consists of a metal rod. At one end of the rod, two leaves of aluminium foil are fixed and at the other end, there is a metal disc. The leaves of aluminium foil are placed inside a conical flask and the flask is corked to isolate the leaves from air.
When the metal disc is touched with a charged body, the aluminium strips move away from each other. This happens because some of the charges of the body are transferred to the strips through the metal rod. This method of charging a body is called charging by conduction. The nature of charges on both the leaves and the charged body are the similar. Hence, both the leaves of the aluminium foil will move away from each other. If the body was not charged, then the leaves of the foil would remain as they were before. They would not repel each other.

Question 10:

List three states in India where earthquakes are more likely to strike.

Answer :

The three states in India where earthquakes are more likely to strike are Jammu and Kashmir, Gujrat, and Assam.

Question 11:

Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Answer :

Some of the precautions are as follows:
(i) Try to find an open field away from tall buildings, installations, tall trees, and electric wires and poles.
(ii) If travelling in a bus or a car, then do not come out when an earthquake strikes. Ask the driver to drive in an open field.

uestion 12:
The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

Answer :

No. We should not carry an umbrella in a thunderstorm. During thunderstorms, which are accompanied with lightning, electric discharge from the clouds can travel through the metallic rod of the umbrella. This may give an electric shock to the person who is carrying it. Hence, it is not safe to carry an umbrella during lightning.