Some Natural Phenomena

'This flash of lightning is an electric spark that releases an enormous amount of electrical energy in an instant. However, in homes and other buildings, electrical energy is released in a controlled way by the flow of electric currents.'

Lightning is an electric discharge during a thunderstorm.

1.0Lightning - An Introduction

A bolt of lightning flash across dark cloudy skies, followed a few moments later by the deafening sound of thunder, is still one of the most awesome physical events unleashed by nature. What is the cause of lightning? Why is it so dangerous? So powerful it is that many early civilizations reasoned these events must be the actions of gods. To the Romans, lightning was the sign that Jove, the king of the gods, was angry at his enemies. In the legends of native North Americans, lightning is flashed from the eyes of an enormous thunderbird, while thunder is boomed from the flapping of its huge wings (see figure).

• Thunderbird is a legendary creature in certain North American indigenous people's history and culture. It is considered a "supernatural" bird of power and strength. It is described as a large bird, capable of creating storms and thunder while it flies. Clouds are pulled together by its wing beats, the sound of thunder made by its wings clapping, the light flashing from its eyes when it blinks. Now, of course, we understand that lightning is caused by the accumulation of charges in the clouds. We need not be afraid of lightning, but we have to take precautions to protect ourselves from the deadly sparks.
• Our human body contains various ions like sodium ion, potassium ion, chloride ions etc. due to these ions human body is a good conductor of electricity.

The sparks that the Greeks knew about:

The first electrical phenomena was discovered in about 600 B.C. by 'Thales of Miletus', one of the founders of Greek science. He showed that when a piece of amber is rubbed with a woollen cloth (or fur), it acquires the power of attracting light objects like feather, dust, pieces of leaves, bits of straw, etc.

Amber is a hard yellowish-brown translucent fossil resin (gum like) found on the shores of the Baltic sea. The Greek name for amber was 'electrum' and this is the origin of all familiar words like electricity, electric charge, electron, etc. You might have seen that when you take off woollen or polyester clothes, your hair stand at the ends. If you rub your hands against a pure woollen blanket in dark, you can see a and hear a crackling sound. In 1752, the American scientist and inventor Benjamin Franklin (1706-1790) attempted to prove that lightning in the sky was the same electricity as the spark observed when you reach for a metal door handle after shuffling across a carpet. He performed his famous kite experiment to explore whether lightning was a form of electricity (see figure). Luckily, he did not get killed, and he succeeded in drawing electricity from the clouds. He observed that lightning behaves the same way that electricity produced in the laboratory does. It soon became apparent that electricity is present in all substances.

The existence of two kinds of electricity was first suggested by C.F. Du Fay (1698-1739 A.D.). He gave two kinds of electricity the name 'vitreous' and 'resinous'.

Benjamin Franklin (1706-1790) named the two different kinds of charge positive and negative. He introduced the words 'positive' in place of 'vitreous' and 'negative' in place of 'resinous', although the choice of these names was arbitrary.

We shall now study some properties of electric charges. We shall also see how they are related to the lightning in the sky.

• In the $18^{\text{th }}$ century, it was Franklin who named the two kinds of electricity ; positive and negative. He collected electric charge from thunder storm clouds through wet string from a kite.

2.0Frictional Electricity(Charging by Rubbing)

Most substances release electricity when rubbed against one another. This is most noticeable when the rubbed substance is a very good insulating material. You have probably noticed that after rubbing a plastic comb through your hair on a dry day, the comb attracts strands of your hair or small pieces of paper. The electricity developed on the surface of insulating bodies when rubbed against each other is called frictional electricity or static electricity. The term static electricity is used because here the charges that accumulate on the surface of the materials being rubbed, are at rest.

• When two bodies are rubbed against each other, they get charged. The charges on them are opposite in nature i.e., one becomes negative while other becomes positive. Oppositely charged particles or unlike charges attract each other.
• When two bodies are rubbed against each other, they get charged. Here, the balloon gets charged as it is rubbed against the dry hair. Note that the wall is not charged as we have not rubbed the balloon or any other thing against the wall. Thus, we can say a charged object can get attracted towards an uncharged object. The above activity works best on a dry day because excessive moisture can provide a pathway for charge to leak off a charged object.
  
  (a) Balloon rubbed against dry hair
  
  (b) Charged balloon and uncharged wall attract each other

Q. When a plastic comb is rubbed against dry hair, the plastic comb becomes negatively charged and dry hair becomes positively charged. Why are positive and negative charges called opposite in nature?

Explanation: Positive and negative charges are said to be opposite because an object with an equal amount of positive and negative charge has no net charge i.e., they cancel out each other. In the above example also, if we touch the negatively charged plastic comb again with the positively charged hair, they both become neutral.

3.0Modern Theory Of Electric Charges

Modern theory of nature of electric charges is based on the models of the atom that Ernest Rutherford (1871-1937) and Niels Bohr (1885-1962) proposed in the early 1900s. In their theories, an atom is composed of two types of charges: positively charged protons in a nucleus surrounded by negatively charged electrons. The amount of negative charge on an electron is exactly equal to the amount of positive charge on a proton. Because atoms have equal numbers of protons and electrons, the amount of positive charge on all the protons in the nucleus of an atom is balanced (or cancelled) by the negative charge on all the electrons moving around the nucleus. Therefore, atoms are electrically neutral, which means they have no overall electric charge.

• According the modern theory every matter is composed of atoms which contains electrons, protons and neutrons. Protons are bound together strongly only electrons are available for inducing charge.
• An atom becomes negatively charged when it gains extra electrons. If an atom loses electrons it becomes positively charged. An electrically charged atom is called ion.

Source of frictional electricity:

When two substances are rubbed together, some electrons are removed from the atoms on the surface of one and transferred to the other. The substance which gains electrons becomes negatively charged and the one which losses electrons becomes positively charged. Thus, the excess or deficiency of electrons make a substance negative or positive.

Q. Have you ever taken clinging clothes from a clothes dryer? Why this happens? How can this cling can be reduced or avoided?

Explanation: In a cloth dryer, when clothes are rubbed against each other, some clothes gain electrons while some clothes loose electrons. Thus, they get oppositely charged. Since opposite (or unlike) charges attract each other, the clothes cling to one another. This tendency of clothes to stick (cling) to other clothes because of development of opposite charges on them is called static cling.

Electrons move between clothes in a dryer. A positively charged sock will stick to a negatively charged T-shirt. To reduce or avoid the static cling of clothes, put the synthetic materials and the natural materials for separate drying. Combining synthetic materials in the dryer, such as nylon, often leads to static cling with natural materials, such as cotton. Also, do not dry clothes for too long as overdrying also leads to static cling.

4.0Conductors And Insulators

Electrical conductivity depends on how tightly the electrons are bound to the nucleus of an atom. Some materials have electrons that are tightly bound to the nucleus and are not free to travel within the substance. These materials are called insulators. Materials that have electrons in the outermost regions of the atom that are free to travel are called conductors.

In solids, electric charge is due to the transfer of electrons between objects. Protons cannot be removed easily from the nucleus of an atom and thus, they do not move from one object to another.

T- Metals are good conductors as they have large number of free electrons. These electrons act as if they no longer belong to any one atom, but to the metal as a whole; consequently, they move freely throughout the piece of metal.

Figure shows how charges behave when they are placed on a conductor with how they behave on an insulator.

Distribution of charge on the surfaces of metals and insulators

Q. If a neutral body is made positively charged, is there any change in its mass?

Explanation: In charging any neutral body, the mass of a body changes, though the change is extremely small or negligible. If a neutral body is made positive, it means electrons are removed from it. Thus, the mass of body decreases. List of objects acquiring two kinds of charges on rubbing

5.0Interaction Between The Charges

Electric charge is a scalar quantity. It can be of two types : positive and negative. Like charges repel each other while unlike charges attract each other. Two charged objects near one another may experience attraction or repulsion from each other because each object exerts a force on the other object. This force is called the electric force.

The electric force between two charged objects depends on the distance between them and the amount of charge on each object.

The electric force between two electric charges gets stronger as the charges get closer together and vice-versa. The electric force between two objects that are charged, increases if the amount of charge on at least one of the objects increases and vice-versa.

• In the 1780s, Charles Coulomb found that the electric force between two charges is proportional to the product of the two charges. If one charge is doubled, the electric force doubles. If both charges are doubled, the electric force becomes four times.
• Coulomb also found that the electric force is inversely proportional to the square of the distance between the charges. Thus, when the distance between two charges is doubled, the force between them decreases by a factor of four i.e., the force becomes one-fourth of the initial value.

6.0Charging By Induction

We know that, solid conductors or metals contain large number of free electrons that can move about from atom to atom. When a negatively charged ebonite rod is brought near a neutral metallic ball, many free electrons are repelled by the ebonite rod and move to the far side of the ball. The separation of charges on the metallic ball is caused by the presence of the negative charge on the ebonite rod [see figure (a)]. A charge separation will also result from the presence of a positively charged glass rod [see figure (b)]. In both examples using the neutral metallic ball, the charge induced on the near side of the ball is the opposite of the charge on the approaching rod.

• The charges developed on an object without any contact with another charge object are called induced charges. The process of charging a conductor from a charged body without touching them together is called induction.
  
  Development of induced charges on metallic bodies

Charging a single metal sphere by induction

To charge a single metal sphere by induction, (suppose positive), a negatively charged rod is brought near the sphere. Due to this, electrons are repelled and thus, drifted towards the right end (far end) of the sphere (see figure). Now, the far end is earthed and the electrons are moved to the earth. The earth connection is then removed, leaving the sphere positively charged. Now, remove the negatively charged rod. The positive charge will spread uniformly over the sphere.

Q. What happens when a positively charged sphere is Earthed (grounded)? What happens if the sphere is negatively charged?

Explanation: When a positively charged sphere is Earthed or grounded using a conducting wire, the electrons from the Earth reach the surface of sphere [see figure (a)]. This neutralises some positive charge of the sphere. This process continues till the whole positive charge gets neutralised by the incoming electrons from the ground i.e., net charge on the sphere becomes zero. If the sphere is negatively charged, the electrons will move from the sphere to the Earth [see figure (b)]. Again, the sphere becomes neutral i.e., net charge on the sphere becomes zero.

Positively charged

Negatively charged

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.

Attraction of charged body for uncharged conductor

Let us consider an uncharged body which is a conductor (say metal). If it is brought near to a negatively charged rubber rod, then the negative charge on the rod repels the negative free electrons in the metal to its far end (see figure). A positive charge is developed on the near end of the metal. Since positive charge is nearer to the negatively charged rod, it is attracted more strongly than the repulsion of negative charge. Thus, the metal gets attracted towards the negatively charged rubber rod.

Q. Why does walking across a carpeted floor and then touching something sometimes result in a shock?

Explanation: As you walk on the carpeted floor, electrons are transferred to your shoes from the carpeted floor [see figure (a)]. These electrons then spread over the surface of your skin. As you bring your hand close to something like a doorknob, electrons on your hand repel the electrons in the doorknob. Because the doorknob is a good conductor, its electrons easily move away from your hand. The part of the doorknob closest to your hand then becomes positively charged [see figure (b)]. The process of separation of positive and negative charges on the doorknob is called induction. If the amount of charge on your hand and that on the knob is large, the attractive electric force between the electrons on your hand and the induced positive charge on the doorknob pulls electrons from your hand to the doorknob. Negative and positive charges meet, producing a streak of light called spark and you feel a mild electric shock [see figure (c)].

7.0Transfer Of Charge

If a charged body is put in contact with an uncharged conductor, the uncharged conductor becomes charged due to transfer of charged particles (electrons) between the charged body and the uncharged conductor. This is called 'conduction'. (a) If charged body is positive, it will withdraw some electrons from uncharged body and the uncharged body becomes positively charged [see figure (a)]. (b) If charged body is negative, it will transfer some electrons to uncharged body and the uncharged body becomes negatively charged [see figure (b)].

8.0Leaf Electroscope

The leaf electroscope is a useful instrument in the study of electrostatics. It is an instrument used in electrostatics to detect the presence of electric charge on a body and it gives a rough idea of the amount of charge present on the body.

Construction : It contains a vertical metal rod, with a round metal ball or knob on top, housed in a box. The metal rod and the box are insulated to each other by hard rubber or amber. Two very thin leaves of gold (or aluminium) are attached to bottom end of the rod.

Detection of presence of charge on a body using electroscope

When the electroscope is uncharged, the two leaves hang parallel and vertically downwards due to their own weight [see figure(a)]. Suppose a negatively charged rod touches the knob. Because the metal is a good conductor, electrons travel down the rod into the leaves. Both leaves become negatively charged as they gain electrons [see figure (b)].

Because the leaves have similar charges, they repel each other. When the positively charged glass rod is brought into contact with the metal knob of an uncharged electroscope, electrons flow out of the metal leaves and onto the rod. The leaves repel each other because each leaf becomes positively charged as it loses electrons [see figure (c)].

• More the amount of charge present on a body, more will be the deflection of the electroscope leaves and vice-versa. This gives a rough idea of amount of charge present on a body.

9.0The Story of Lightning

It is now possible to explain lightning in terms of the charges produced by rubbing.

• Lightning is an electric discharge between clouds and Earth's surface that takes place due to the development of charges on the clouds.

How clouds get charged?

Many theories attempt to explain the development of charges on clouds. During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges. In such a condition, a churning cloud formation causes water vapour molecules to collide, resulting in a transfer of electrons between these molecules. The transfer of an electron from one water molecule to another leaves the molecules oppositely charged. Cooling causes water vapour molecules to condense into water droplets. The atoms in these droplets hold onto electrons more readily than atoms in water vapour, and thus the droplets become negatively charged. Being heavier, these negatively charged water droplets accumulate at the bottom of the cloud, causing the bottom of the cloud to become negatively charged. The top of the cloud, containing the rising water vapour, becomes positively charged.

Why air becomes a conductor during lightning?

As explained above, the positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. Also, there is accumulation of positive charges near the ground due to phenomenon of induction. When the magnitude of the accumulated charges becomes very large, the attractive electric force between the negative electrons on the clouds and the induced positive charge on the ground pulls electrons from the cloud to the ground. Negative and positive charges meet, producing a streak of light and sound. We call these streaks of light as lightning. The process is called an electric discharge because the charge from the cloud is discharged to the ground.

• The heavier droplets hold onto electrons falls towards the lower part of the cloud and lighter water vapors collect at upper part of the cloud.

Q. Does carrying an open umbrella during a thunderstorm increase your odds of being hit by lightning?

Explanation: As a lightning flash travels toward the ground from a nearby cloud, it looks for the tallest object. This is because all the objects attached to the ground acts as ground or earthed objects. So, lightning takes the shortest path to travel and thus, falls on the tallest object. If you're holding an umbrella in an area surrounded by taller buildings, it's not so risky. But if you are the only tall object in the place surrounding you, the lightning flash will fall on you. Thus, carrying umbrella is not a good idea at all during thunderstorms.

10.0Lightning Safety

Remember, no place on the earth is safe against thunder storms and lightning, especially when you are outdoors. All thunder storms are dangerous despite their usually small area of coverage (typically 24 km in diameter). If you hear a thunder, it is an alert to rush to a safer place. Wait for some time (at least 30 minutes) before coming out of the safe place even after hearing the last thunder.

Lightning can affect a building in three ways (a) direct hit (b) through external wires and pipes, and (c) through the ground. Lightning can also travel through reinforcement in concrete walls, flooring, etc.

What are the safe places during lightning? (1) A house or a building is a safe place. (2) Find shelter in a fully enclosed metal vehicle such as a car, truck or a van with the windows completely shut.

• 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.

11.0Do's and Dont's During A Thunderstorm

Outside the house

(1) When you see lightning and hear thunder of a storm even before you could count 30, it is a sure warning signal that you should rush indoors to protect yourselves. (2) If no shelter is available in the neighbourhood of where you are, then seek protection by getting into a hard topped vehicle like car, van, bus, etc. with windows closed. You are much safer inside a car than outside because metal frame of a vehicle usually provides protection, but be careful not to touch metal. (3) Keep distance from vulnerable locations like isolated trees or isolated tall structures. Lightning can strike the same place twice and can spread out nearly 20 m after striking the ground. (4) If in a forest, take shelter under shorter trees. (5) If no shelter is available and you are in an open field, stay far away from all trees. Stay away from poles or other metal objects. Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between the hands (see figure).

Inside the house

(1) Do not use wired phones as lightning can strike telephone wires. Mobile phones are the safest to use. (2) Unplug all the electrical and electronic equipment's as soon as the thunder and lightning seem likely. Do not forget to disconnect radios and TVs from external antennas. Electrical lights can remain on. They do not cause any harm. (3) Avoid contact with all sorts of pipes and avoid taking a shower during thundering and lightning. (4) If any part of a building is hit by lightning, call fire department immediately. Check whether anybody is hurt. If someone is struck by lightning, at once come to his or her rescue. It is a very wrong notion that those struck by lightning carry an electrical charge.

12.0Lightning Conductor

It is a device which protects buildings from the lightning either by neutralising or by conducting. Construction: It consists of number of pointed conductors fixed to a high point on the building and connected to a thick copper wire which runs down the side of building and ends on a metal plate buried in the ground. Working: When a negatively charge cloud passes over the building it induces a positive charge on the point ends of lightning conductor and equal negative charge at the metal plate. Charges are always concentrated at sharp points, they strongly repel the positive ions of the air particles. These positive ions move towards the negatively charged cloud. On reaching the cloud, they neutralise some negative charges on the cloud thereby reducing the amount of charges on the cloud. Thus, possibility of electric discharge through air from cloud to earth is reduced. Even if the electric discharge from cloud takes place, the negative charge of the cloud passes through copper wire to metal plate in the ground. And, thus, the building is saved from the damage due to electric discharge.

• The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent from lightning. But do not touch them during a thunderstorm. If you have a taller building near your house, then your house is quite safe against lightning stroke. This is because lightning falls on the tallest building near the thundercloud.

13.0Earthquakes

Thunderstorm, lightning and cyclones are natural phenomena that can cause large scale destruction of human life and property. Fortunately, we can predict them to some extent. If weather department warn about a thunderstorm developing in some area, there is always a possibility of lightning and cyclones accompanying it. So, we get time to take measures to protect ourselves from the damage caused by these phenomena. But, there is one natural phenomenon which we are not yet able to predict which is an earthquake. It can cause damage to human life and property on a huge scale. What is an earthquake? An earthquake occurs when a sudden release of energy causes the ground to shake and vibrate, associated with the passage of waves of energy released at the source. Earthquakes can be extremely devastating and costly events, sometimes killing hundreds or thousands and destroying entire cities in a few seconds. Earthquakes are also associated with secondary hazards, such as tsunami, landslides, fire, famine, and disease that also exert their toll on humans and other animals. A major tsunami occurred in the Indian Ocean on $26^{\text{th }}$ December 2004. All the coastal areas around the ocean suffered huge losses. Earthquakes are occurring all the time, on the entire earth but they are not even noticed as their intensities are quite low. 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.

14.0Structure Of The Earth

The earth is organised into three major layers which are as follows : (1) Crust (2) Mantle (3) Core

The crust : The outermost layer of earth which is composed of crystalline rocks is called crust. The crust is like the skin around an apple or an orange and its thickness varies from place to place. The thickness is maximum under the continents and minimum under the oceans. The crust is about three quarters (3/4) covered with water and enveloped by a layer of atmosphere. Since the formation of earth, it's crust has been constantly fractured and reorganised forming new continents and oceans in the process.

The mantle: The interior region of earth between the crust and the central core is called mantle. The clear picture of mantle is still not known, but it is believed to be made up largely of iron and magnesium silicates. The pressure inside the mantle increases with the depth. Under the high pressures in the mantle, the rocks are normally solid. Some of the rocks near the bottom of mantle tends to flow like coal tar due to the extremely high temperatures.

The core: The central part of the earth having a radius of about 3400 km and physical properties different from those of the surrounding parts is called the core. At the end of mantle, a sudden increase in density occurs, and here the core of the earth begins. The core consists of two parts : (1) The inner core, which is a solid sphere made up of iron. (2) The outer core, which is a shell of molten iron surrounding the inner core.

The temperature at the centre of the earth is about $4000^{\circ }\mathrm{C}$ and pressure is about 3.7 million atmosphere. Due to the extremely high pressures in the inner core, the iron can remain in the solid form in it despite the high temperature. The pressure in the outer core is lower than the inner core and hence, the iron in it becomes molten. About 1600 kilometers of the outer core behaves like liquid.

The crystalline rocks in the Earth's crust are light having density of about $3\mathrm{g}/{\mathrm{cm}}^3$. Under continents, the thickness extends from 35 km to 60 km . Under oceans, the thickness of crust is only about 10 km. The density of mantle increases slowly with the depth. It starts from about $4\mathrm{g}/{\mathrm{cm}}^3$ and reaches a value of $6\mathrm{g}/{\mathrm{cm}}^3$ at the bottom of mantle. The mantle extends from the base of the crust towards the centre to a depth of 2900 Km . The density of inner core is about $18\mathrm{g}/{\mathrm{cm}}^3$. The density of outer core varies from about $10\mathrm{g}/{\mathrm{cm}}^3$ just below the mantle to about $16\mathrm{g}/{\mathrm{cm}}^3$ just before the inner core starts.

Plate tectonics

The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. The continental and oceanic crust consists of enormous slabs, which geologists describe as tectonic plates. Tectonic plates are huge pieces of crust and rigid upper mantle that fit together at their edges to cover earth's surface.

As illustrated in figure (a), there are about 12 major plates and several smaller ones. These plates move very slowly, only a few centimeters each year which is similar to the rate at which fingernails grow.

• The movement of plates causes earthquakes, the plate are weak in their boundaries, so they are referred to as weak zone where earthquakes are more likely to occur. Plate tectonics is the theory that describes how tectonic plates move and shape Earth's surface. They move in different directions and at different rates relative to one another and they interact with one another at their boundaries. Each type of boundary has certain geologic characteristics and processes associated with it. A divergent boundary occurs where tectonic plates move away from each other. A convergent boundary occurs where tectonic plates move toward each other. A transform boundary occurs where tectonic plates move horizontally past each other.
  
  Plate tectonic map of the Earth

15.0How Earthquake Occurs

1. Stress and Strain: Most earthquakes are the result of the movement of Earth's crust produced by plate tectonics. As a whole, tectonic plates tend to move gradually. Along the boundaries between two plates, rocks in the crust often resist movement. Over time, stress builds up. Stress is the total force acting on crustal rocks per unit area. When stress overcomes the strength of the rocks involved, movement occurs along fractures in the rocks. The vibrations caused by this sudden movement are felt as an earthquake. The characteristics of earthquakes are determined by the orientation and magnitude of stress applied to rocks, and by the strength of the rocks involved. There are three kinds of stress that act on Earth's rocks: compression, tension, and shear. Compression is stress that decreases the volume of a material, tension is stress that pulls a material apart, and shear is stress that causes a material to twist. The deformation of materials in response to stress is called strain (see figure).
   
   (a) No strain
   
   (b) Compression
   
   (c) Tension
   
   (d) Shear

Compression causes a material to shorten. Tension causes a material to lengthen. Shear causes distortion of a material. Even though rocks can be twisted, squeezed, and stretched, they fracture when stress and strain reach a critical point. At these breaks, rock can move, releasing the energy built up as a result of stress. Earthquakes are the result of this movement and release of energy.

2. Faults: Crustal rocks fail when stresses exceed the strength of the rocks. The resulting movement occurs along a weak region in the crustal rock called a fault. A fault is any fracture or system of fractures along which earth moves. Figure shows a fault. The surface along which the movement takes places is called the fault plane. The orientation of the fault plane can vary from nearly horizontal to almost vertical. The movement along a fault results in earthquakes.
   
   A major fault passes through these rice fields on an island in Japan

Reverse and normal faults : Reverse faults form as a result of horizontal and vertical compression that squeezes rock and creates a shortening of the crust. This causes rock on one side of a reverse fault to be pushed up relative to the other side. Reverse faulting can be seen near convergent plate boundaries.

Movement along a normal fault is partly horizontal and partly vertical. The horizontal movement pulls rock apart and stretches the crust. Vertical movement occurs as the stretching causes rock on one side of the fault to move down relative to the other side. The crust is being stretched apart in that area. The two areas separated by the reverse fault would be closer after the faulting than before, and that two areas at a normal fault would be farther apart after the faulting than before the faulting. Normal faulting can be seen near divergent plate boundaries.

Strike-slip faults: Strike-slip faults are caused by horizontal shear. The movement at a strike-slip fault is mainly horizontal and in opposite directions, similar to the way cars move in opposite directions on either side of a freeway. A strike-slip fault can be seen near a transform plate boundary.

Different faults that causes movements of earth 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 (see figure). Some areas of South India also fall in the danger zone.

16.0Earthquake Waves

Most earthquakes are caused by movements along faults. As stress continues to build in the rocks, they reach a limit after that they break, and the vibrations from the energy that is released to produce an earthquake.

The vibrations of the ground during an earthquake are called seismic waves.

Types of seismic waves

Every earthquake generates three types of seismic waves: (i) Primary waves (ii) Secondary waves (iii) Surface waves.

Primary waves: Also referred to as P-waves, primary waves squeeze and push rocks in the direction along which the waves are travelling [see figure (a)]. Note how a volume of rock, which is represented by small dark grey squares, changes its length as a P-wave passes through it.

Secondary waves: Secondary waves, called S-waves, are named with respect to their arrival times. They are slower than P-waves, so they are the second set of waves to be felt. S-waves have a motion that causes rocks to move at right angles in relation to the direction of the waves, [see figure (b)]

Both P-waves and S-waves pass through Earth's interior. For this reason, they are also called body waves.

Surface waves: The third and slowest type of waves are surface waves, which travel only along earth's surface. Surface waves can cause the ground to move sideways and up and down like ocean waves, [see figure (c)]. These waves usually cause the most destruction because they cause the most movement of the ground and take the longest time to pass.

Generation of Seismic Waves

The first body waves generated by an earthquake spread out from the point of failure of crustal rocks. The point where the waves originate is the focus of the earthquake. The focus is usually several kilometers below the Earth's surface. The point on Earth's surface directly above the focus is the epicenter (see figure). Surface waves originate from the epicenter and spread out.

Seismograph (Or Seismometer)

The seismic waves are recorded by an instrument called the seismograph [see figure]. The instrument is simply a vibrating rod, or a pendulum, which starts vibrating when tremors occur. A pen is attached to the vibrating system. The pen records the seismic waves on a paper which moves under it. All seismograph include a frame

By studying seismic waves, scientists can construct a complete map of the earthquake. They can also estimate its power to cause destruction.

When an earthquake occurs, the frame vibrates but the hanging pendulum and attached pen do not. The pendulum and pen record the relative movement, as the recording device moves under them.

Richter scale

The Richter scale, devised by a geologist named Charles Richter, is a numerical rating system that measures the energy of the largest seismic waves, that are produced during an earthquake. It is called the magnitude of the earthquake. The numbers in the Richter scale are determined by the height, called the amplitude, of the largest seismic wave. Each successive number represents an increase in amplitude by a factor of 10. For example, the seismic waves of a magnitude-8 earthquake on the Richter scale are ten times larger than those of a magnitude-7 earthquake. The difference in the amount of energy released by earthquakes is even greater than the difference between the amplitudes of their waves. Each increase in magnitude corresponds to about a 32 -fold increase in seismic energy. Thus, an earthquake of magnitude-8 releases about 32 times the energy of a magnitude-7 earthquake.

17.0Protection Against Earthquakes

We must take necessary precautions to protect ourselves from earthquakes all the time. People living in seismic zones, where the earthquakes are more likely to occur, have to be specially prepared. (1) Designing of buildings : The buildings in the seismic zones should be designed so that they can withstand major tremors. Modern building technology can make it possible. (2) In highly seismic areas, the use of mud or timber is better than the heavy construction material. The roofs should be made as light as possible. In case the structure falls, the damage will not be so large. (3) It is better if the cupboards and shelves are fixed to the walls, so that they do not fall easily. (4) The positions of the wall clocks, photo-frames, water heaters, etc., should be safe so that in the event of an earthquake, they do not fall on people. (5) It is necessary that all buildings, especially tall buildings, have fire fighting equipment in working order as they may catch fire due to an earthquake. If an earthquake strikes, take the following steps to protect yourself :

At home: (1) Take shelter under a table and stay there till shaking stops. (2) Stay away from tall and heavy objects that may fall on you. (3) If you are in bed, do not get up, just protect your head with a pillow.

At outdoors: (1) Try to find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground. (2) If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.

Magnitude is the size of earthquake. The amplitude is the height of the wave relative the base live.

18.0Mind Map

19.0Some Basic Terms

• Electric spark :- The negative charges and positive charges meet which results in production of streak of light accompanied by sound.
• Accumulation :- Together or pile up especially little by little.
• Arbitrary :- Not seeming to be based on any reason or plan and sometimes seeming unfair.
• Composed :- Made or formed from several different parts.
• Deficiency :- The quality or state of being defective or of lacking some necessary quality or element.
• Static cling : The tendency of a material (such as fabric) to adhere to another material or surface because of a buildup of static electricity.
• Insulating material :- A material that reduces or prevents the transmission of electricity.
• Neutralised :- To make chemically neutral.
• Vigorous :- Carried out forcefully and energetically.
• Shock :- A serious medical condition of extreme weakness caused by damage to the body.
• Thunderstorm :- A storm of heavy rain accompanied by lightning, thunder, wind, and sometimes hail.
• Lightning :- A flash of bright light in the sky produced by electricity moving within or between clouds, or between clouds and the ground.
• Cyclones :- A large air mass that rotates around a strong center of low atmospheric pressure.
• Density :- The mass of a substance per unit volume.
• Squeezed :- To force something into or through a small space.