CBSE Class 8 Science Some Natural Phenomena Notes

CBSE Class 8 Science Rain Thunder and Lightning Chapter Notes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations. 

RAIN, THUNDER AND LIGHTNING
A Game of Light and Sound in The Sky, Lightning and Thunder The game of light and sound is going on in the sky since eternity. Initially man took it as a natural phenomenon which brought rain later. Some people took this as a wrath of God but wiser people took this phenomenon as a prelude to bleassings from the God. Man was afraid of lightning but happy for the rain. It is true that thunderbolts were feared since they brought devastation to life on earth and caused forest fires, but the rain brought prosperity. All this was happening in nature but the exact nature of the happenings in the sky were not known to man. 

LEARNING ABOUT SPARKS
It was sometime in 600 BC, when a Greek Philosopher, by the name of Thales while rubbing amber with a piece of fur found the amber attracting light objects such as small bits of straw. He also found amber generating a spark. Amber in Greek is called elektron. After a gap of about two thousand years Dr. William Gilbert, physician to the Queen of England confirmed the experiment made by Thales and in addition showed that similar property developed by rubbing glass with silk, ebonite with cat's skin cat's fur with dry hands and sealing wax (lac) with flannel. It was he who coined the word electric, from the Greek word elektron, the name for amber. 

LEARN THAT
1. Amber is a resin which dripped from Pines millions of years ago and got burried under the soil. Over a long period of time this resin became hard like stone and looked like brown coloured glass. Amber was used for making cigar holders and combs for the rich.

2.Ebonite is the hard wood from Ebony trees. It is dark brown or black due to deposition of gum-like substance in its cells. At one time this wood was being used for making piano keys and handles for brush and walking sticks. 

STATIC ELECTRICITY
electric charge at rest Rub a plastic scale or a comb made from plastic through your dry hair. Bring them over bits of paper. Bits of paper get attracted to the plastic. Substances like amber and plastic, develop a "CHARGE" on them on rubbing. This charge attracts the bits of paper. The charge was given the name 'electric charge' and the bodies on which electric charge developed were called electrified bodies. The electric charge developed on these materials ramain upon the body of the material and it does not move as a current through metallic wires as we find it in our homes in the case of current electricity. This kind of electric charge is therefore called "Static Electric Charge". Static means which is at rest and is not moving. The static charge is also called 'static electricity' and its study is known as electrostatics. 

UNCHARGED AND CHARGED BODIES

Objects around us do not show the presence of any charge upon them. If you move your plastic pen over bits of paper, they remain undisturbed. This shows that there is no charge either on the plastic or on the bits of paper. These are uncharged bodies. 

ACTIVITY

Rub a plastic pen with a piece of paper or a polythene bag or your handkerchief and bring it near the bits of paper. The bits are attracted towards the pen. Pen is now a charged body. The bits of paper remain uncharged.

Take an inflated rubber balloon. Rub it against your woollen pullover and put it against the wall. The balloon gets stuck to the wall. The balloon gets electrically charged on rubbing and gets stuck to the uncharged wall. Repeat the above activities with materials like plastics (a comb, a ruler, a pen, a rod, a sheet, a polythene), glass, lac, rubber, cellophane paper, and a knife blade, aluminium foil and a copper plate. You may observe that objects made of metals like iron, aluminium and copper do not get charged whereas other materials develop a charge on them. We will learn later about the reason for metals not
developing a charge on them. For present you may learn that metals do not hold charge on them since they are the good conductors of electricity.

LEARN THAT
1. Some materials develop charged on them on being rubbed.
2. Some materials like metals do not hold charge when rubbed.
3. It is important to take care that for performing experiments or activities concerning electricity the materials being used are dry. Presence of moisture will not give satisfactory results.

KINDS OF CHARGES, two kinds of electric charges We have observed that a charged object attracts another uncharged body. A rubbed plastic pen is charged and it attracts the uncharged bits of paper. Charged balloons stick to the uncharged wall. Let us find out the way, the two charged bodies behave when they are brought near each other. For performing the experiments we make use of:
(i) a glass rod and a piece of silk, (ii) an ebonite rod and a piece of cat's skin,
(iii) a piece of silk thread, (iv) a heavy based wooden stand with a hook.

ACTIVITY

1. Rub a glass rod with silk and suspend it freely through the hook with a silk thread on the wooden stand. Rub another glass rod with silk and bring its one end near the suspended charged glass rod. The suspended glass rod gets repelled.

2.Rub an ebonite rod with cat's skin and suspend it freely through the hook with a silk thread on the wooden stand. Rub another ebonite rod with cat's skin and bring its one end near the suspended charged ebonite rod. The suspended ebonite rod gets repelled.
From the above activities we learn that electrical charge developed on similar materials on rubbing repel each other

ACTIVITY

Rub a glass rod with silk and suspend it freely through the hook with a silk thread on the wooden stand. Rub another ebonite rod with cat's skin and bring it near the suspended charged glass rod. The suspended glass rod is attracted towards the charged ebonite rod.

From the above activity we learn that the electrical charges developed on dissimilar materials on rubbing may attract one another.
Thus, there must be some difference in the charge being developed on glass rod on rubbing it with silk and the charge being developed on ebonite on rubbing it with cat's skin.

Naming a Charge
Since we do not see the electric charge and distinguish one kind of charge from the other by observation, the charges have been named as positive (+ ive) charge and the negative (– ive) charge. It was also decided that:
• the charge on the glass rod be described as positive (+ ive) charge.
• the charge on the ebonite rod be described as negative (– ive) charge.
While naming a charge as positive (+ ive) or negative (– ive), the terms being used for naming them are used only as symbols. Positive does not denote addition and negative does not denote substraction. They do not have any mathematical value or numerical significance.

LEARN THAT
1. An object being rubbed with another object may develop a charge on it. This is called charging by friction.
2. A charged object attracts an uncharged object.
3. Two objects bearing similar charges repel one another. (Like charges repel each other).
4. Two objects bearing dissimilar charges attract one another. (Unlike charges attract each other).
5. Signs of (+) and (–) are symbols to denote two different charges.

ATTRACTION AND REPULSION

1. When two bodies are brought face to face and they repel each other, we are sure that :
• both the bodies are bearing a charge upon them, and
• the charge on both is similar.

2. When two bodies are brought face to face and they attract each other, then
• one of the two bodies may be bearing a charge and the other may not be bearing any charge on it OR
• both the bodies may be bearing dissimilar charges upon them.

Thus, repulsion between two bodies is the sure test of the presence of electrical charge on both of them.

ACTIVITY

Testing charge on a thermocol ball: Take a small ball from thermocol and suspend it with the help of a piece of thread and a thumbtack from the side of a table. Put the fan 'off' so that the ball remains undisturbed.

Charge a glass rod by rubbing it with silk. Bring the rod near the ball. The ball is attracted to the rod. The ball is not bearing any electrical charge on it, hence it is attracted to the charged rod.
Now, touch the ball with the charged glass rod. Charge from the rod gets transferred to the ball by touch. It gets positively charged.
Bring a freshly charged glass rod to the charged ball. The ball gets pushed and it drifts away from the rod. This shows that similarly charged bodies repel each other. Thus, attraction between the two bodies is NOT the surest test of the presence of electrical charge on them while facing each other.

CHARGE BY INDUCTION.

ACTIVITY

A glass rod is charged positively by rubbing it with silk. This charged rod is placed facing a thermocol ball which is uncharged (neutral). The positive charge in the glass rod induces the negative charge particles in the thermocol ball to move towards the rod, and the positive charge will move to the side away from the side facing the glass rod. On touching the positively charged side of the thermocol ball with a finger, the positive charge is earthed, that is it moves through our body to the earth. Only the negative charge is held in the pith ball, and so it gets negatively charged. This phenomenon is called charge by induction. Similarly, a thermocol ball can also be charged with positive charge by induction, if we make use of a charged ebonite rod instead of a glass rod.

Equal and opposite charges are produced at the same time.

ACTIVITY

Make a cap of fur fitting an ebonite rod. Tie a piece of silk thread to the fur cap. Rub the cap over the rod. Remove the cap from the rod with the help of the silk thread, without touching the cap with the hand. Test the presence of electric charge on the rod with the help of a thermocol ball. Then, touch the ball with the rod to charge it with the charge on the rod. Bring the fur cap near the charged ball. The ball is attracted towards the cap showing that the charge on the cap is dissimilar to the charge on the ball, or say, the ebonite rod. Thus, when two bodies are rubbed together equal and opposite charges are produced in them at the same time.

ADDITIONAL LEARNING
Atoms and the Electrical Charge Scientists did not know what electricity was. They thought electricity to be like a fluid which flows along the metal wires, and that is why they called it electric current. Recently, it has been learnt that the electric charge on a body is due to the electric effect of tiny particles in an atom called 'electrons.'Each atom of a matter has a nucleus with a positive charge (protons), around which one or more tiny particles, called electrons, orbit. These electrons have a negative electric charge on them. Since every atom has an equal positive and negative charges on it, it is normally neutral. When two materials are rubbed together the electrons are, sometimes, pulled out of their orbits. The electrons get transferred from one material to the other and an electric charge develops on the rubbed material. Transfer of electrons takes place in the following manner :

When we rub ebonite (a dark coloured hard wood) with an animal fur, the ebonite becomes negatively charged. This is because during rubbing the electrons from fur get transferred to the ebonite. The ebonite gains electrons which far outnumber the protons in it, and, thus they develop a negative charge on its body. The fur, on the other hand, loses electrons with the rusult that the protons in it outnumber the electrons, and it becomes positively charged. Similarly, a glass rod when rubbed with a piece of silk cloth becomes positively charged and the silk becomes negatively charged.

Today, with the advancement of scientific knowhow, we know that the positive charge is obtained when the electrons are lost and negative charge is obtained on gaining of electrons.

TRANSFER OF CHARGE
On touching an uncharged body with a charged body, charge is transferred from the charged body to the uncharged body. Again, on touching a charged body with our bare hands, the charge from the charged body is transferred to the earth through our body. This transfer of charge from a charged body to the earth is called "earthing." The body from which the charge has been earthed loses the charge on it. Such a body on losing the charge on it called "discharged".

ACTIVITY

Structure and Working of An Electroscope
An electroscope is an apparatus to test an electric charge. It is more sensitive than the thermocol ball or the tinfoil. An electroscope has brass rod passing through a tight fitting cork in a glass bottle. A pair of thin gold-leaves are attached to the lower end of the brass rod. To protect the gold-leaves from external electric charge the lower half of the bottle is lined with tinfoil which is 'earthed' by connecting it to the table. When a body is earthed, it means that it is made netural by transferring the charge on it to the earth. This kind of electroscope is called a 'gold-leaf' electroscope since thin gold foils are used in it.

 In place of an electroscope an improvised apparatus may be used. Place a metallic bar on a stand. Take a strip of ordinary or cellophane paper of the size 4 cm × 0.5 cm, fold it half and hang it from the metallic bar. This may work like a gold leaf electroscope.

WORKING OF THE GOLD-LEAF ELECTROSCOPE

ACTIVITY

1. Bring a rubbed ebonite rod with fur and touch the metal disc of the electroscope with it. The negative charge from the ebonite rod gets transferred to the gold leaves. The gold leaves diverge (open up) showing that both are bearing the same charge. They repel each other.
2. Touch the disc of the electroscope with bare hands. The gold leaves fold and come to lie parallel to each other.The charge from the electroscope and its gold leaves is 'earthed' on touching them with hands.
3. Again, charge the electroscope with negative charge from a rubbed ebonite rod. The gold leaves diverge.
Now, bring a positively charged glass rod rubbed with silk and touch the disc with it. The gold leaves fold back,showing that there is no charge on them. This time, the negative from ebonite rod has been neutralised with positive charge from the glas rod.

LEARN THAT

The gold leaves in an electroscope :
1. Diverge when both the leaves bear similar charge on them.
2. Fold back when 'earthed' on touching with hands showing that there is no charge on them.
3. fold back when 'neutralised' showing that the charge on the ebonite is different from the charge on the glass rod.

ALSO LEARN THAT
1. More the charge, more is the divergence of the gold leaves.
2. All animal bodies, including our body, are a good conductor of electricity.
3. Any charge (may be negative or positive) flowing from a charged body is neutralised by equal and opposite charge from the earth.
4. Earth bears both the charges upon it.
5. Earthing is provided in all electrical wirings in the buildings. All electrical gadgets and plugs (sockets) are connected to the earthing to enable the leaked electric current, if any, to pass on to the earth instead of harming the operator of the gadgets.

LIGHTINING, Conduction of Electricity Through Air and Water

Clouds are very small droplets of water in the form of vapour. Clouds roam about in the sky with the wind. Generally, a patch of cloud develops an electric charge on it by friction. The clouds develop positive as well the negative charge on them. The reason for this has not been understood so far. As a result of friction he upper layers of cloud (which are away from earth) get positively charged and the lower layers of cloud (which are facing earth) get negatively charged.

Dry air and pure water are bad conductors of electricity, hence clouds continue to carry the charge on them till the intensity between the two charges is too high. 

When two patches of cloud bearing different charges come face to face they get attracted to one another and the electrons from negatively charged cloud jumps to the positively charged cloud. The jumping of electrons between the clouds result in a big spark between the two clouds. The heat from the spark results in sudden expansion of air setting the air iin violent waves which are heard by us as thunder. The spark is seen as a flash of lightning, first and then followed by a thunder, a little later.

When the weather is dry and it is not raining, the sparking remains limited to the clouds. While raining the electric charge from the clouds may come down to the earth. The bright and long spark is seen travelling along a zig-zag path from the clouds to the earth.
American statesman and a scientist Benjamin Franklin was the first to explain the relation between the presence of electric charge in the clouds and the occurrence of lightning in the year 1752. He and his son flew a home made kite during a thunderstorm. A bolt of lightning struck the kite and the thread laced with fine metallic wire. The charge from the kite travelled down to a key tied to the thread. Franklin was saved from the shock. He showed that lightning comes froma charge on the clouds and it is the same as the electrical charge and the spark as we find on objects, after rubbing them.

Lightning striking the earth causes great damage to the buildings and the living beings on the earth. It may also cause forest fires. At times the charged clouds dip so low so as to transfer the charge on them to the trees and the buildings which get damaged or burnt, killing life around. Lightning also affects radio and T.V. signals resulting in the disturbance in sound and distortions in picture. Electric charge from clouds may also be transferred to the radio, T.V. sets and telephones through antenna, cable or telephone pole and may damage the clostly instruments.

Lightning and Lightning Conductors : Lightning conductors are used to protect buildings from lightning. Lightning conductor is a metallic (preferably copper) strip running from under the earth to the top of the building, which is to be protected. One end of the strip is buried deep in moist earth, since earth has the capacity to neutralise both the electric charges. The other end of the strip goes upto the top of the building and has a sharp pointed end pointing towards the sky. Whenever a cloud with an electric charge in it passes by the building, an equal and opposite charge is induced at the pointed end of the lightning conductor. Since the pointed end cannot hold a big charge, the charge begins to leak into to cloud thus neutralising the charge in the cloud. This reduces the danger of lightning striking the building. If, however, the lightning strikes a building, the discharge finds an easy path through the conductor to the earth, without causing any damage to the building.

Other Safety Measures : During thunderstorm
• Disconnect all your heavy electrical apliances, including telephone. The electrical discharge may damage them.
• Do not take bath under running tap water.
• Do not stand under trees or touch them or any other metallic poles.
• Walking under an umbrella is not safe.
• Best is, not to move out.

Lightning is Useful : Lightning which was feared at one time came to be known as useful in many ways. The use of lightning is :

1. High discharge of electric charge through air helps in fixing nitrogen. Nitrogen from the air combines with the oxygen forming nitric oxide and nitrogen dioxide gases. Nitrogen dioxide gas gets dissolved in rainwater forming dilute nitric acid. Nitric acid comes to the soil with the rain where it reacts with calcium and other elements forming their nitrates. The nitrates are excellent fertilizers which help plant growth.

2. Ozone is formed when electric charge pases through the oxygen in the air. Lightning is most effective in the formation of ozone in nature. Ozone, as we know, forms a thin layer in the upper atmosphere. This layer is protective for the life on earth. Ozone screens out harmful ultraviolet rays coming from the sun.

3. It is believed that lightning played an important role in the origin and evolution of life on earth.

EARTHQUAKE, a natural phenomenon
Beside thunderstorm, we have learnt about the cyclones. To some extent our weatherman predicts the coming of rain and a cyclone. But, earthquake is one natural phenomenon which cannot be predicted.
An earthquake is the sudden shaking of the earth caused by deep layers of the rocks inside. The rocks under the earth slide over one another from time to time, bringing about disturbance of the surface of the earth. This disturbance is the earthquake. It is measured on a scale called 'Richter Scale'. The Richter Scale measures the magnitude of disturbance on a scale from 1 to 10. The damage begins when the earthquake is 5 or above and disastrous beyond 6.5 or Richter Scale.

The earthquake may cause damage to the buildings, animal and plant population on the earth. Earthquake at a place surrounded with sea may cause TIDAL WAVES in the sea around. The waves move as a high wall of water to distant islands and may brings disaster for the people living along the coastline. Japanese name for tidal waves (or seismic waves) is Tsunami. Last Tsunami occured on 26th Dec, 2004 after an earthquake in Indonesia. Causing more than 10,000 deaths in India along the East coast. Tsunami in India was the result of 20 m high, towering all of seawave, striking the shores, drowning people and the dwellings and taking away all on the way alongwith them while receding.
Today, there is a fashion to construct malls and mutistorey residential and commercial complexes. These are prone to big damage from earthquakes. For the construction of any building precautions be taken to make it "quake safe", and for this a qualified architect must be consulted. The foundation of the building should be deep, broad and rolling. Roofs should be of light material and the fixtures should be light.

SUMMARY

1. Electric charge bears energy.
2. Charged bodies bear an electric charge on them.
3. Uncharged bodies are neutral. They do not bear any charge.
4. There are two kinds of charges.
5. The charges have been named as positive and negative charge since they are not visibly differentiated.
6. An ebonite rod when rubbed with cat's skin develops negative charge on it.
7. A glass rod when rubbed with a silk develops a positive charge on it.
8. Metals do not develop a charge on them since they are the good conductors of electricity.
9. A charged body attracts an uncharged body.
10. Bodies bearing dissimilar charges on them attract each other.
11. Bodies bearing similar charge on them repel each other.
12. Electric charge on a body is due to the loss or gain of electrons from the atoms of the matter.
13. A body may be charged by (a) friction, (b) conduction, (c) induction.
14. Equal and opposite charges are produced at the same time.
15. Benjamin Franklin was the first to study electric charge in clouds. He gave the idea of 'lightning conductor' for the safety of the highrise buildings.

TICK THE CORRECT OPTION

Question. Charges are of :
(A) Two types
(B) One types
(C) Many types
(D) *
Answer.A

Question. When a glass rod is rubbed with silk, silk attains a :
(A) Positive charge
(B) Negative charge
(C) Remains neutral
(D) Either positive or negative
Answer.B

Question. The safest place during a thunderstorm is :
(A) Under a tree
(B) Near electricity poles
(C) Inside a car
(D) Lying on the ground
Answer.C

Question. An earthquake measuring 9.0 on the Richter scale causes :
(A) No damage
(B) Little damage
(C) Tremendous damage
(D) Devastation
Answer.D

Question. Lightning ocurs because of :
(A) Rain
(B) Electric discharge
(C) Wind
(D) Angry Gods
Answer.B

Question. Electric charges are :
(A) Only positive
(B) Only negative
(C) Either positive or negative
(D) Insulators
Answer.C

Question. In a neutral object, there are :
(A) Equal number of atoms
(B) More positive charges than negative charge
(C) Both (A) and (B)
(D) Wind
Answer.A

Question. The air in the path of lightning goes up to a temperature of about :
(A) 300°C
(B) 3,000°C
(C) 300,000°C
(D) 30,000°C
Answer.B

Question. A lightning conductor :
(A) Conducts light
(B) Stops lightning
(C) Protects buildings from the damagking effects of lightning
(D) Prevents clouds from coming near a building and thus protects it
Answer.C

Question. If you are caught in a thunderstorm you should :
(A) Go and stand on a high ground
(B) Stand under a tree
(C) Take shelter indoors
(D) All of the above
Answer.C

Question. Charged objects exert a ________ on each other :
(A) Cloud
(B) Lightning
(C) Force
(D) Power
Answer.B

Question. We hear a thunder because :
(A) A lot of charge goes in lightning
(B) Because lightning is very bright
(C) Because the air heats up and expands all of a sudden
(D) Clouds bang against each other
Answer.C

Question. Lightning always follows :
(A) A straight path
(B) The easiest path
(C) A thunder
(D) Rain
Answer.C

Question. Two objects rubbed against each other :
(A) Will lose electrons
(B) Will repal each other
(C) Will attract each other
(D) A thunder
Answer.A

Question. The gold leaf electroscope can be used to :
(A) Detect charge only
(B) Detect or measure charge only
(C) Detect, measure and find the nature of charge
(D) Stand under a tree
Answer.C

Question. Which of the following can be used to find out whether a body is charged or uncharged?
(A) Lightning conductor
(B) Gold leaf electroscope
(C) Turning fork
(D) The easiest path
Answer.B

Question. Atmospheric electricity was discovered by :
(A) Hertz
(B) Benjamin Franklin
(C) William Gilbert
(D) All of these
Answer.B

Question. When a glass rod is rubbed with a silk piece, then the glass rod :
(A) Loses electrons
(B) Gains electrons
(C) Neither loses nor gains electrons
(D) None of these
Answer.A

Question. When an ebonite rod is rubbed with flannel, then the ebonite rod :
(A) Loses electrons
(B) Gains electrons
(C) Neither loses nor gains electrons
(D) All of these
Answer.B

Question. The gold leaf electroscope has two strips of foils made of _______ which are fixed to the inside of the glass jar.
(A) Gold
(B) Silver
(C) Tin
(D) Steel
Answer.A

Question. The lightning conductor is :
(A) Erected on the building
(B) Buried inside the earth
(C) Installed near the window
(D) All of these
Answer.A

Question. The study of earthquakes is called :
(A) Seimology
(B) Phonology
(C) Richter scale
(D) Both (A) & (B)
Answer.C

Question. Magnitude of destructive earthquake generally measures ________on Richter scale :
(A) > 7
(B) < 7
(C) = 7
(D) ± 7
Answer.A

Question. When two bodies P and Q are rubbed against each other and P acquires a charge +q, then the charge on Q is :
(A) Greater than + q
(B) – q
(C) Less than – q
(D) More than +q
Answer.B

Question. Bodies can be charged by :
(A) Conduction only
(B) Induction only
(C) Both conduction and induction
(D) None of these
Answer.C

Question. When a body is charged by induction, the end farther away from the charged body acquires the :
(A) Positive charge
(B) Negative charge
(C) Same charge
(D) All of these
Answer.C

Question. Which of the following is a very good conductor of electricity?
(A) Wood
(B) Rubber
(C) Graphite
(D) None of these
Answer.D

Question. Lightning is a heavy flow of charges between two oppositively charged :
(A) Clouds
(B) Particles
(C) Atoms
(D) Graphite
Answer.A

Question. Which of the following cannot be charged easily by friction?
(A) Plastic scale
(B) Inflated balloon
(C) Woollen cloth
(D) All of these
Answer.B

Question. Earthquake is major :
(A) Richter scale
(B) Seismology
(C) Phonology
(D) None of these
Answer.A

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