LECTURE - 1 : CLASS IX : SCIENCE : CHAPTER 4 : WORK AND ENERGY

WORK:

What is done when a force produces motion. 

The work done by a force on a body depends on two factors. 

(i) magnitude of the force, and

(ii) distance/displacement through which the body moves in the direction of force. 

Work done in moving a body is equal to the product of force exerted on the body and the distance/displacement moved by the body in the direction of the force.

Work = Force x distance/displacement moved the direction of force.

W = F x S

Unit of work

The SI unit of work is Joule.

1 joule of work= When a force of 1 Newton moves a body through a distance of 1 m in its own direction then the work done is known as 1 Joule. 

Work is a scalar quantity.

Work done against gravity

Whenever work is done against gravity, the amount of work done is equal to the product of weight of the body and the vertical distance through which the body is lifted. 

Work done in lifting a body = weight of body x vertical distance

W = m x g x h = mgh

W= work done, m= mass of the body, g = acceleration due to gravity, h = height through which the body is lifted. 

1. How much work is done by a force of 10 N in moving and objects through a distance of 1 m in the direction of force?

Soln. We know work done W = F x s

Here, F = 10 N , s = 1 m

So, Work done = 10 x 1 J = 10 J

2. Calculate the work done in lifting 200kg of water through a vertical height of 6 m (g = 10 m/s²).

Soln. We know work done against gravity, 

W = mgh

Here, mass of water, m = 200 kg

Acceleration due to gravity, g = 10 m/s²

And height, h = 6 m

W = 200 x 10 x 6 = 12000 J = 12 kJ

3. A car weighing 1000 kg and travelling at 30 m/s stops at a distance of 50 m decelerating uniformly. What is the force exerted on it by the brakes? What is the work done by the brakes?

WORK DONE BY A FORCE ACTING OBLIQUELY

When the movement of the body is at an angle to the direction of the applied force, then the work done in pulling the body will be equal to the horizontal component of the force (F cosθ) and the displacement of the body. 

W = F cosθ x s

F = applied force, 

θ= angle between the direction of force and the direction of motion, 

s = displacement. 

When the force acts at right angles to the direction of motion (zero work)

When the displacement of the body is perpendicular (at 90°) to the direction of force no work is done.

W = F cosθ x s  

θ = 90°  but cos 90° = 0

W = 0

To keep a body moving in a circle there must be a force acting on it is directed towards the centre this force is called centripetal force. The work done on a body moving in a circular path by the centripetal force is zero.

The work done in the case of earth moving around the sun is zero as well as the work done in the case of a satellite moving around the earth is also zero. 

Work done when the force acts opposite to the direction of motion (negative work)

If the angle between the direction of force and the direction of motion is 180° , then the work done is negative. 

As cos θ = cos 180° = - 1

W = F cos 180° x s = - F x s

Positive, Negative and Zero Work

The work done by a force can be positive, negative or zero. 

1. Work done is positive when a force acts in the direction of motion of the body.

2. Work done is negative and a force acts opposite to the direction of motion of the body.

3. Work done is zero when a force acts at right angles to the direction of motion of the body. 

Examples of positive, negative and zero work

1. We kick a football lying on the ground, then the football stars moving. Here,  we have the applied force in the direction of the motion of football. So the work done on the football in this case is positive.

2. A football moves on the ground slows down gradually and ultimately stops. This is because a force due to friction of the ground acts on the football. The force of friction acts in a direction opposite to the direction of motion of football. So in this case the work done by the force of friction on the football is negative.

3. The satellite move around the earth in a circular path. In this case, the gravitational force of earth acts on the satellite at right angles to the direction of motion of satellite. So the work done by the Earth on the satellite moving around it in circular path is zero.

ENERGY: The ability to do work is called energy. The amount of energy possessed by a body e is equal to the amount of work it can do to where its energy is released. Energy is a scalar quantity. 

UNIT OF ENERGY: The SI unit of energy is Joule (J). The energy required to do one joule of work is called 1 Joule of energy. 

1 kilo Joule (1 kJ) = 1000 J

DIFFERENT FORMS OF ENERGY:

The main forms of energy are

1. Kinetic energy, 2. Potential energy, 3. Chemical energy, 4. Heat energy, 5. Light energy, 6. Sound energy, 7. Electrical energy, 8. Solar energy, 9. Nuclear energy.

KINETIC ENERGY: The energy of a body due to its motion is called Kinetic energy. 

Formula for kinetic energy:

If a body of mass m starts to move from rest to a velocity v, then its kinetic energy is equals to 

K.E. = ½mv²

Proof of the kinetic energy:

If a body of mass m starts to move from rest to a velocity v, 

Work = Force x Displacement

W = F x s

But we know v² = u² + 2as

u = 0, v² = 2as => s = v²/2a

Again , F = ma

W = F x s = ma x v²/2a = ½mv²

*If the mass of a body is doubled, its kinetic energy also gets doubled. 

If the mass of a body is halved, is kinetic energy also gets halved. 

The velocity of a body is doubled, its kinetic energy becomes four times. If the velocity of a body is halved, then its kinetic energy becomes one fourth. 

As the kinetic energy of a body depends on its mass and velocity, therefore heavy bodies moving with high velocities have more kinetic energy. 

POTENTIAL ENERGY: 

The energy of a body due to its position or change in shape is known as potential energy. 

A body may possess energy even when it is not in motion due to its position or shape. 

The sum of the potential and kinetic energies of a body is called its mechanical energy. 

Formula for potential energy

Work done on a body against a force, is stored in the body as potential energy. 

Therefore, workdone on a body against gravitational force will be stored as the gravitational potential energy (U). 

Suppose a body of mass m has been raised to a height h from the ground against gravitational force. The workdone occurs against gravitational force equal to the weight of the body mg. 

Workdone, W = force x displacement

W = mg x h = mgh

POWER:

Power is defined as the rate of doing work. Therefore, power is equals to work done/time taken. 

If W work is done in t time, then power P = W/t

Hence, we can say power is equals to work done per unit time. 

When work is done, an equal amount of energy is consumed. Therefore, power can also be defined as the rate at which energy is consumed. Power is a scalar quantity. 

Q. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.

A. Suma is swimming in a pond.

B. A donkey is carrying a load on its back.

C. A wind-mill is lifting water from a well.

D. A green plant is carrying out photosynthesis.

E. An engine is pulling a train.

F. Food grains are getting dried in the sun.

G. A sailboat is moving due to wind energy.

Answer:

A. Suma is swimming in a pond: - She is pushing the water in the backward direction, which is an action performed by her.

However, due to reaction, the water pushes her in the forward direction. Work is done by Suma.

B. A donkey is carrying a load on its back: - In this case, force of gravity on the load is acting in the downward direction, whereas the displacement will be in the horizontal direction i.e., the force and displacement are perpendicular to each other.

There is no displacement in the direction of the force of gravity, and therefore no is work done as there is no displacement.

C. A wind-mill is lifting water from a well: - The work is done by the wind mill in lifting the bucket of water from the well. The work is done against the force of gravity.

D. A green plant is carrying out photosynthesis: - No work done is done in this case. As both force and displacement are 0.

E. An engine is pulling a train: - In this case, an engine is pulling a train parallel to the ground.

The force exerted by the engine is in the direction of displacement of the train.

Thus, the force and displacement are in the same direction. Therefore, work is done.

F. Food grains are getting dried in the sun: - No work is done in this case as food grains remain at rest.

G. A sailboat is moving due to wind energy: - The force exerted by the wind on the sail move the boat in the direction of force, hence, positive work is done by wind energy

LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 18 : POLLUTION OF AIR AND WATER

WATER POLLUTION: 

What is water pollution?

The contamination of water of rivers, lakes and ponds etc with unwanted and harmful substances is called water pollution. 

What are water pollutants?

The unwanted and harmful substances which make the water contaminated are called water pollutants.

Major water pollutants:

The major pollutants which cause water pollution are sewage, fertilizers, pesticides and industrial wastes. Hot water from power plants and industries is also considered a pollutant.

Sources of water pollution:

There are several sources of water pollution. They are as follows.

1. Dumping of untreated sewage into rivers causes water pollution.

2. Use of fertilizers in agriculture causes water pollution.

3. Use of pesticides in agriculture causes water pollution.

4. Discharging of untreated industrial wastes into rivers and lakes causes water pollution.

Harmful effects of common pollutants

1. Untreated sewage from homes

The waste water containing human excreta and carried in sewers or underground pipes is called sewage. Sewage contains several harmful microorganisms such as bacteria, protozoa, fungi, viruses and parasites which cause serious diseases in humans. The drinking of river water contaminated with can cause water borne diseases such as typhoid diarrhoea dysentery and jaundice.

2. Fertilisers used in agriculture

The water pollution caused by fertilizers used in agriculture decreases the amount of dissolved oxygen in water bodies and leads to the death of fish and other aquatic animals. 

3. Pesticides used in agriculture

The farmers use toxic chemicals called pesticides on standing crops in the fields to protect them from pests and diseases. Some of these pesticides dissolve in rain water and run into rivers, lakes and ponds and pollute their water. This poisonous pesticides kill the aquatic animals like fish. The non biodegradable pesticides in the polluted water are absorbed by aquatic plants. This aquatic plants are then eaten by aquatic animals like fish. During this pesticides are transferred from aquatic plants to fish and when a person eats the fish grown in this polluted water the pesticides present in the fish enter his body and damage his health.

4. Untreated industrial wastes

The toxic chemicals present in industrial wastes are the compounds of harmful metals such as Mercury, lead, arsenic, nickle and cadmium. The water of rivers and lakes polluted with toxic industrial waste can kill fish and other aquatic animals which live in this river or lake. The toxic metal compounds present in this polluted water can enter the human body through the food chain like eating fish grown in this polluted water. They can damage our nervous system and cause diseases such as blood poisoning and cancer.

Harmful effects of water pollution

The various harmful effects of water pollution are as follows.

1. Drinking of water polluted with untreated sewage can cause diseases such as Cholera, Typhoid, Diarrhoea, Dysentery and Jaundice.

2. Water of rivers and lakes polluted with fertilizers can cause the death of aquatic animals like fish.

3. Water of rivers and lakes polluted with pesticides can kill aquatic animals like fish and also damage our health through food chain.

4. Water of rivers and lakes polluted with toxic industrial waste kills the aquatic animals like fish damages our nervous system and causes diseases such as blood poisoning and cancer.

Controlling water pollution

The various ways of controlling our minimising water pollution are as follows.

1. Sewage should be treated properly at sewage treatment plants to make it harmless before discharging it into nearby rivers.

2. Farmers should use correct amounts of fertilizers and pesticides in the fields so that excessive amounts of these harmful agricultural chemicals are not washed into the rivers, lakes and ponds.

3. All the industries should treat the toxic wastes produced by them suitably to make them harmless before discharging them into rivers.

4. Water pollution prohibition laws which exist for the industries producing toxic chemical waste should be implemented strictly by the concerned authorities.

5. Garbage or household wastes should not be thrown into open drains, rivers, lakes or ponds. Dead bodies should not be thrown into rivers. Dead bodies should be cremated or buried properly to prevent pollution of water.

6. People should be made aware of the harmful effects of water pollution so that they may co-operate and help in preventing it. 

Conservation of water

As there is scarcity of fresh water in our country, we should conserve water by using it carefully, economically and avoiding is wastage. The various ways to conserve water or save water at home are as follows.

1. Turn off the tap immediately after use. Get the leaking taps repaired immediately.

2. Brush your teeth by filling water in a mug. Leaving the tap running while brushing the teeth may waste several litres of water.

3. Wash the utensils by filling water in a basin and not under the running tap. 

4. Use water from washing rice, pulses, vegetables and fruits after wearing the plants at home. 

5. Do not use the full flash from the cistern in the toilet when a half flush is sufficient.

Potable Water

The water which is safe for drinking is called potable water. 

Ways to make water potable

The various methods which can be used to make water potable are as follows. 

1. Water can be filtered to make it potable by using porcelain candle filter or electric water filter. 

2. Water can be made absolutely safe for drinking by boiling for sufficient time.

3. Chlorination is a commonly used chemical method for purifying water.

Pollution of river Ganga

The water of river Ganga has been highly polluted because the villagers, town, and cities which are situated on the banks of this very long river through large quantities of garbage, untreated sewage, toxic industrial vestige, dead bodies of infants and many other harmful things directly into the river. 

GANGA ACTION PLAN

WWF has found that Ganga is one of the most polluted rivers in the world. At present, the pollution level in many stretches of river Ganga is so high that no aquatic animals like fish etc can survive in it. The river is said to be dead at those places. An ambitious plan to cleanse the highly polluted river Ganga and save it, was launched in 1985. It is called Ganga action plan. The aim of Ganga action plan is to reduce the pollution level in the water of river Ganga. However, the increasing population and industrialization has already damaged this mighty and sacred river beyond repair.

LECTURE - 3 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

SEISMIC ZONES OR FAULT ZONES:

The weak zones of earth's crust which is more prone to earthquakes are called seismic zones. It is also known as fault zones. 

In India, the areas most affected or threatened by earthquakes are Kashmir, Western and central Himalayas, the whole of North-east, Rann of Kutch, Rajasthan and Indoh gangetic plane. Some areas of South India is also prone to earth-quakes. 

SEISMOGRAPH: THE EARTHQUAKE MEASURING INSTRUMENT.

Seismograph is an instrument which measures and records the magnitude of an earthquake in terms of the shock waves it produces. 

Construction of Seismograph or Seismometer:

A seismograph consists of a heavyweight like a heavy metal ball suspended from a support with the help of a strong wire. The support is attached to the base of the seismograph and the base is rigidly fixed to a solid rock on the surface of the earth. A pen is attached to the the lower end of the hanging heavyweight.  It is used to draw graphs on the graph paper which is wrapped around a rotating drum. When earthquake occurs the pen draws graphs on the graphsheets due to the oscillation of the heavy object. From this graph we can measure earthquakes.

FOCUS AND EPICENTER OF EARTHQUAKE:

The place inside the earth's crust where the earthquake is generated, is called 'focus' of the earthquake. 

The point on earth's surface directly above the focus is called epicenter. 

MAGNITUDE OF EARTHQUAKE AND RICHTER SCALE:

The magnitude or intensity of an earthquake is expressed on the Richter Scale.

In a Richter scale 1 to 12 numbers are assigned to express the magnitude of an earthquake. 

Suppose the magnitude of an earthquake is 3 on the Richter scale, and magnitude of another Earthquake is 4 on the Richter scale, then the second earthquake is 10 times more intense than first earthquake. 

The magnitude of earthquake having 4 in Richter Scale is ten times more than the magnitude of the Earthquake having 3 on the Richter scale.

The Earthquakes having magnitudes higher than 7 on the Richter Scale are called destructive Earthquake. 

PROTECTION AGAINST EARTHQUAKE:

Earthquakes are highly destructive and it cannot be predicted. Therefore, we should always take some precaution against earthquake.

1. All the houses and other buildings in seismic zones should be designed and constructed in such a way that they can with stand measure earthquake tremors.

2. In highly systemic areas the use of mud and timber for building houses is better than using heavy construction materials. 

3. The cupboards and shelves should be fixed to the world's so that they do not fall easily when shaking occurs during an earthquake. Glass bottles should not be placed on high shelves and heavy objects should be placed low to the ground.

4. The objects such as heavy mirrors, photo frames, wall clocks and water heaters should be mounted securely on the world at such places in the house that they do not fall on the people in the house in the event of an earthquake.

5. All buildings should have fire fighting equipment in working order because some buildings may catch fire during an earthquake due to electric short circuits. 

PRECAUTIONS TAKEN WHEN EARTHQUAKE OCCURS:

A. If we are at home when an earthquake occurs.

We should take the following precautions.

1. We should take shelter under a study table for a kitchen counter and stay there till the shaking due to earthquake stops and we should cover our head with hands.

2. We should stay away from tall and heavy objects so that if they topple they may not for on us. We should also stay away from glass windows that may shatter due to vibrations.

3. If we are in bed, we should just move on the bed to be close to a wall but not get up from the bed. We should protect our head with a pillow.

B. If we are outdoors when an earthquake occurs.

We should take the following precautions.

1. We should stay at a clear spot away from buildings, trees and overhead power lines etc. We should also seat on the ground.

2. If we are in a car or bus, we should not come out of it. The car or bus should be driven slowly to a clear spot away from buildings, trees and overhead electric wires etc. We should not come out of the vehicle till the tremors stop.

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 18 : POLLUTION OF AIR AND WATER

POLLUTION AND POLLUTANTS:

Pollution: The presence of harmful or toxic substances in our surroundings is called polition. Our surroundings includes air, water, soil, sound etc. 

Pollutant: An unwanted and harmful and poisonous substances that contaminates the environment is called pollutant. Pollutant are also called contaminant. 

AIR POLLUTION:

 Air:  Air contains Nitrogen, Oxygen, carbon di oxide and other gases like argon and water vapour. 

Carbon monoxide, sulphur di oxide, nitrogen di oxide and other oxides of nitrogen, smoke, dust they are the most common pollutants of air. Fly ashes are also an air pollutant. 

Air pollution: 

The contamination of air with harmful gases like Sulphur dioxide, nitrogen oxides, carbon monoxides, smoke, dust etc is called air pollution. 

Air pollutant: Carbon monoxide, sulphur di oxide, nitrogen di oxide and other oxides of nitrogen, smoke, dust they are the most common pollutants of air. Fly ashes are also an air pollutant. 

SOURCES OF AIR POLLUTION:

1. Burning of fossil fuels (kerosene, petrol, diesel) , wood, cow dung cakes in home produces smoke, which is an air pollutant.

2. Exhaust gas from motor vehicles are also polluting the air.

3. Industrial factories has chimney which produces thick smoke also pollutes air.

4. Oil refineries which produces petrol, diesel and other petroleum products also pollutes air. Chemical factories also pollutes air.

5. Cement, asbestos factories and stone crushers also pollutes air.

6. CFC (chloro-fluoro carbons) used in refrigerator and air conditioner harm the ozone layer of atmosphere, which protects us from ultra violet rays. 

7. Smoking also causes air pollution. 

COMMON AIR POLLUTANTS:

1. Sulphur di oxide: SO₂ 

When petrol and diesel burns it produces sulphur di oxides. It creates respiratory problems in human beings. It can damage the lungs permanently. Due to the presence of Sulphur di oxide, acid rain occurs. 

Effect: Sulphur dioxide in air causes respiratory problems. It may even cause permanent lung damage. It causes acid rain and acid rain damages trees, plants, soil, aquatic animals, statues, buildings and historical monuments.

SMOG: 

Smog is a deadly air pollutant which is formed by the combination of smoke and fog. When smoke, and dust mixed with water droplets and water vapour present in atmosphere (Fog), it produces smog. Sulphur di oxide also contributes in smog. 

Effect: Smog causes cough and and other lung diseases especially in children.

Nitrogen oxides: 

Burning of fuels like petrol and diesel produces nitrogen oxides. Nitrogen oxide attack breathing system and lead to lung congestion. They also attack skin. Nitrogen oxides present in polluted air produces a acid rain. It also contributes to the formation of smog. 

Effect: Nitrogen oxides attack breathing system and lead to lung congestion. They also attack skin.

Carbon monoxide: CO

Incomplete combustion of fuels produces carbon monoxide. The exhaust gases of motor vehicles also produces carbon monoxide.

Effect: Carbon monoxide combines with the hemoglobin of our blood and reduces the oxygen carrying capacity of blood this lack of oxygen causes respiratory problems or breathing problems. it causes suffocation. 

Chlorofluorocarbons: 

Chlorofluorocarbons are used in refrigeration air conditioning and aerosol sprays. Chlorofluorocarbons are depleting the useful ozone layer of the upper atmosphere. Ozone layer prevents the harmful ultraviolet radiations of the sun from reaching to the earth. Effect: Destruction of ozone layer by CFC allow extremely harmful ultraviolet radiations of the sun to reach the earth. This ultraviolet radiations can cause skin cancer cataract and destruction of plants including crops. 

Suspended particulate matter or SPM: 

The finely divided solid or liquid particles suspended in air are called suspended particulate matter. Some of the examples of suspended particulate matter are dust, smoke and fly ash. 

1. Dust: Dust consists tiny particles of Earth.

Effect: Dust produces allergic reaction in human body and aggravates diseases like bronchitis. Dust covers the leaves of plants and trees and prevents photosynthesis.

2. Smoke: Smoke is mainly tiny particles of carbon in air.

Effect: Smoke attacks our lungs and causes respiratory diseases.

3. Fly ash: The minute particles formed by the burning of coal and carried into Air by the gases produced during burning is called fly ash.

Effect: Fly ash particles present in air cause irritation to the eyes skin nose throat and respiratory tract. Continued breathing in air containing fly ash causes diseases like bronchitis and lung cancer. 

EFFECTS OF AIR POLLUTION:

The two extremely harmful effects of air pollution on the environment are acid rain and greenhouse effect. 

ACID RAIN: Acid rain is that rain which contain small amounts of acids formed from acidic gases like sulphur dioxide and nitrogen oxides present in polluted air. Acid rain contains very dilute solutions of sulphuric acid and nitric acid. Acid rain causes great damage to living and non-living things.

Effects: There are several harmful effects of acid rain. They are as followings.

1. Acid rain destroys forests.

2. Acid rain kills aquatic animals such as fish.

3. Acid rain corrodes the statues buildings and historical monuments and damages them slowly. 

TAJ MAHAL AND ACID RAIN:

The Taj Mahal at Agra is a beautiful historical monument made of pure, white marble. Extreme air pollution around Taj Mahal area is this colouring its white marble and also corroding it slowly. 

Marble cancer:

The acids present in acid rain react with the marble (calcium carbonate) of Taj Mahal monument and corrode it slowly. The slow corrosion of marble of a monument by acid rain is also known as marble cancer.

Discolouring of Taj Mahal:

The suspended particulate matter such as soot particles emitted in the smoke from Mathura oil refinery is discoloring the pure white marble of Taj Mahal by turning it yellowish.

Preventive measure taken to save Taj Mahal:

The supreme court of India has taken several steps to save Taj Mahal from the damage being caused by air pollution and acid rain. It has ordered all the industries in Agra area to switch over to cleaner fuels like CNG and LPG to reduce air pollution. It has also asked vehicles to be run on CNG or unleaded petrol in the Taj Mahal area. 

GREENHOUSE EFFECT:

1. Greenhouse: 

The greenhouse is a structure or building made of glass walls and glass roof in which the plants that need protection from cold weather are grown. The glass walls and glass roof of a greenhouse allow the sun's rays to go in freely but do not allow the inside heat reflected by soil, plants and other things in the greenhouse to go out.

2. Greenhouse effect:

Due to the presence of carbon dioxide layer around the earth, our atmosphere acts like the glass roof of an ordinary greenhouse and allows sun's heat rays to be trapped within the earth's atmosphere. The warming up or heating up of the earth's atmosphere due to the trapping of sun's heat rays by carbon dioxide gas in the atmosphere is called greenhouse effect.

3. Global warming:

The rise in the temperature of Earth's atmosphere due to excessive greenhouse effect produced by increasing amounts of carbon dioxide gas in the atmosphere is called global warming.

4. Effects of global warming:

1. Global warming can cause sea levels to rise dramatically.

2. Global warming can reduce rainfall in some areas of the earth leading to droughts. 

3. Global warming is a serious threat to the existence of life on earth.

5. Greenhouse gases:

Those gases which can trap sun's heat rays in the earth's atmosphere by producing greenhouse effect leading up to the heating of Earth's atmosphere are called greenhouse gases. Carbon dioxide, Methane, Nitrous oxide, Chlorofluorocarbons and Water vapour are all greenhouse gases.

HOW TO CONTROL AIR POLLUTION:

There are several ways to control air pollution. 

1. The air pollution can be controlled by using smokeless waves like LPG, PNG, solar cookers and biogas for cooking food. 

2. The air pollution from motor vehicles can be reduced by using CNG or compressed natural gas is filled in place of petrol and diesel.

3. The air pollution from factories, thermal power plants, oil refineries and chemical industries can be controlled by washing down their smoke and waste gases with jets of water.

4. The air pollution can be controlled by using non polluting sources of energy like solar energy wind energy hydro energy for generating electricity.

OUR ROLE IN CONTROLLING AIR POLLUTION:

1. We should not burn dry leaves, papers and garbage in the open.

2. We should save electricity.

3. We should grow more trees and also take care of the existing trees in our neighborhood.

4. We should join "say no to crackers" campaign.

5. We should use bicycle for covering short distances.

6. We should use the school bus or public transport bus for going to school. 

LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

EARTHQUAKES:

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

"Earthquakes can cause immense damage to houses, other buildings, bridges, dams and people etc. A lot of people get killed when they get buried under the debris of collapsed houses and other buildings during an earthquake. Earthquakes can also cause floods, landslides and tsunamis."

Earthquake is a destructive natural phenomenon which can not be predicted in advance. 

STRUCTURE OF EARTH:

The inside of earth is made up of three main layers:  Core, Mantle, Crust. 

Core: The innermost part of the earth is called its core. The core of earth is made up mostly of iron. The core of earth is extremely hot. Most of the earth's core called outer core is liquid (molten iron) whereas the inner part of the core called inner core is under such high pressure that it is solid (solid iron).  

Mantle: The central region of earth (between the core and crust) is called mantle. Mantle is mostly made of dense, solid rocks. Some of the mantle is, however, a mixture of solid rocks and hot molten rocks (liquid rocks) like the lava from a volcano. Heat coming from the core of earth warms the mantle. This heating sets up huge convection currents in the mantle. The giant convection currents occuring in the mantle can make the mantle move very slowly.

Crust:

The outermost layer of earth is called crust. The crust of earth is made of comparatively lighter rocks than that of mantle. The crust of earth is thicker where there is land. The crust of Earth is not in one piece. It is divided into many plates and those are called continental plates and oceanic plates. The plates of earth on which continents exist are called continental plates, whereas those plates of earth on which oceans exist are called oceanic plates. The solid plates which make up the earth's crust are floating on the partially molten rocks of mantle beneath. Due to convection currents taking place in the mantle, the plates of earth's crust are moving around very, very slowly. The reason the earthquakes occur is that the earth's crust is made of a number of plates which are able to move. 

Why do Earthquakes occur:

The Earthquakes occur when the moving plates of the earth's crust:

(i) slide past one another, and

(ii) collide with one another. 

When the entangled rocks of the crust plates break open suddenly with a big jolt, the Earthquake occurs releasing a tremendous amount of energy. 

A tremendous amount of energy is released when two plates collide with eache other. This large amount of energy produces a wave throughout the earth, which is known as shock wave. It is also known as Seismic Waves. 

(iii) Tremors on the earth can also be caused due to the strike of large mateorites to the earth and it can be due to the  explosion of nuclear devices. 

SEISMIC ZONES ( FAULT ZONES )

The weak zones of earth 's crust which are prone to earthquake are called seismic zone.

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

Lightning:

Lightning is an electric Spark which is caused by the accumulation of electric charges in the clouds. 

Electric charges:

Electric charges are the property of matter. When Amber is rubbed with silk cloth, both the Amber and the silk cloth become charged. Similarly when a rubber balloon is rubbed with a woolen cloth The rubber balloon gets electric charges due to friction. 

Uncharged and charged objects:

An object having no electric charge on it is called an uncharged object. An object having electric charge on it is called a charged object. 

If we take a glass rod and bring it near some tiny pieces of paper it will not have any effect on them, but if the glass rod is first rubbed with a piece of silk cloth and then brought near the tiny pieces of paper then the glass rod attracts the tiny pieces of paper towards itself. It means that when the glass rod is rubbed with silk cloth then it gets electric charge. 

The process of giving electric charge to an object is called charging the object. 

Charging an object by rubbing:

The charging of an object by rubbing it with another object is called charging by friction. The electric charges generated by rubbing are static electric charges. These electric charges remain bound on the surface of the charged object. 

Friction charges both the objects which are rubbed together. 

When two objects are rubbed together then both the objects get charged by friction. 

When a glass rod is rubbed with silk cloth, glass rod becomes positively charged and the silk cloth becomes negatively charged. 

Two types of electric charges and their interaction:

There are two types of electric charges positive charges and negative charges. 

A positive charge repels another positive charge but attracts a negative charge. Similarly a negative charge repels another negative charge, but attracts a positive charge. 

Q. When a charged glass rod rubbed with silk cloth is brought near a charged plastic straw rubbed with polythene there is attraction between the two. What is the nature of charge on the plastic straw?

Ans: when a glass rod is rubbed with silk cloth, glass rod becomes positively charged, if it attracts a plastic straw, then plastic straw must be negatively charged. 

Electroscope:

The electric charge on an object can be detected by using an instrument called electroscope. 

The electroscope is a device for detecting electric charge on an object. 

Working of an electroscope:

Charge a glass rod by rubbing its own and with a piece of silk cloth. Touch the charged glass rod with the top and of metal clip. We will see that the two aluminium leaves move away from each other. We say that the aluminium leaves diverge or open up. 

When we touch the top end of metal clip with the positively charged glass rod, then some of its positive charge is transferred to the top and of metal clip. Since the metal clip is a good conductor of electricity, it conducts the positive electric charge to the two aluminium leaves held on its other end. In this way the two aluminium leaves get charged with the same kind of electric charge. And therefore they repel each other. 

Electric discharge: production of sparks

The passage of electric current in air due to movement of electric charges is called electric discharge. During electric discharge the positive and negative electric charges cancel out each other and and electric Spark and the crackling sound are produced. In nature, electric discharge within a cloud during thunderstorm produces use electric sparks known as lightning along with a loud sound called thunder. And electric discharge between a thunder cloud and the earth also produces lightning followed by thunder. 

LIGHTNING:

The bright flash of light which we see in the clouds is called lightning. 

A cloud is a visible mass of condensed water vapour floating in the atmosphere high above the ground. A heavy dark rain cloud is also called storm cloud. When a storm cloud develops in the sky strong winds move upwards through the cloud and make the water drops present in the cloud to rub against one another. This rubbing together of water drop produces extremely large electric charges in the cloud due to friction. The small water drops acquire a positive charge and being lighter move to the upper part of the cloud with rising wind. On the other hand the larger water drops acquire a negative charge and being here come down in the lower part of the cloud. In this way the top of the cloud becomes positively charged where is the bottom of the cloud becomes negatively charged. 

When the amount of opposite electric charges on the top and bottom of a storm cloud becomes extremely large then electric charges start flowing with high speed through the air between them. When the positive and negative charges of a cloud meet they produce and intense Spark of electricity between the cloud in the sky. This electric Spark as a flash of lightning in the sky. The electric sparks of lightning heat the nearby air in the sky to very high temperatures due to this heat the air in the sky expands repeatedly and produces a loud sound which we call thunder. 

Lightning is usually occurs within a cloud in the sky. It is called sheet lightning. Lightning also occurs between a cloud and the earth or tall all objects of the earth. It is then called fork lightning.

Storm clouds carry electric charges. Now if a storm cloud having negative charges at its bottom passes over a tall building it induces positive charges on the roof of the building. When the electric charges on the bottom of the cloud become extremely large, then this tremendous electric charges present on the bottom of the charged cloud suddenly starts to flow to the roof of the building and we see a flash of lightning coming towards the building.

Lightning strikes are more frequent in the hilly areas because in such areas clouds are comparatively closer to the ground than in the plains. 

When we take off woolen or synthetic clothes like polyester or nylon class sometimes we hear a crackling sound and if it is a dark or as during night we can even see tiny sparks. 

Dangers of lightning:

When lightning strikes the earth it can cause a lot of destruction by damaging property building, trees and killing people. The damage caused to buildings and other structures by lightning can be prevented by installing lightning conductors on them. 

Lightning conductor:

Lightning conductor is a device used to protect a building from the effects of lightning. 

The tall buildings and other tall structures are protected from lightning strikes by using a device calling lightning conductor. A lightning conductor is made of a thick strip of metal made of copper. The top end of lightning conductor is pointed like a sharp spike and it is fixed above the highest point of the building.

 From the top of a building the thick metal strip runs along the outer wall of the building to the ground. The lower end of metal strip is joined to a metal plate and buried deep in the ground near the base of the building. 

If lightning strikes it will hit the top of the lightning conductor rather than the building. The electric energy of lightning passes through the metal strip and gets discharged safely into the ground through the buried metal plate. Since no electric energy produced by lightning passes through the building no damage is caused to it.

Measures to protect ourselves from lightning:

We can take the following measures to protect ourselves from lightning strikes during a thunderstorm. 

1. No open space is safe during lightning and thunderstorms. A house is a safe place during a lightning. 

2. Open vehicles like motorbikes, scooters, tractors and construction machinery are not safe during lightning and thunderstorm. 

3. When in open space, a person should never stand under a tree to take shelter during a thunderstorm because there is danger of lightning striking the tree and burning it up. 

4. When in open space, a person should not lie on the ground during thunderstorm and lightning. A person should squat low on the ground during lightning. 

5. We should avoid raising an umbrella over our head during lightning. 

6. The TV antennas and dish antennas fixed on the tall buildings are especially prone to lightning strikes. We should, therefore, switch off our TV sets during frequent lightning otherwise TV sets may get burnt. 

7. Lightning can strike metal pipes fixed in buildings. So, during a thunderstorm when lightning is taking place, we should avoid touching the metal pipes fixed in a house or building.

LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 14 : CHEMICAL EFFECTS OF ELECTRIC CURRENT

The case of distilled water, tap water, sea water and rainwater

Distilled water, tap water, sea water and rain water

1. Distilled water is non conductors of electricity

2. When some salts are dissolved in distilled water, it becomes a good conductor of electricity

Q. How can we make distilled water a conductor of electricity?

Ans. We can make distilled water a conductor by adding

(i) salts (ii) acids (iii) bases

Tap water:

As tap water contains small amount of various salts dissolved in it, it is a conductor of electricity. 

We should never operate an electric appliances like heater, iron, mixer, grinder etc with wet hands nor we should touch electric switches. 

In case of a fire, before fireman use big water hoses to throw water on a burning houses, the electric supply to that home is cut off first. 

Drinking water:

Drinking water contains some amount of salts in it hence it is also a conductor of electricity.

Sea Water: Sea water is salty, it means a large amount of salts are dissolved in water, so they are very good conductor of electricity. 

Rain water:

Due to the presence of small amount of acids in rain water, it is also a conductor of electricity. 

Which one is better conductor drinking water or sea water and why?

Sea water contains more salts than that of drinking water, so sea water is a better conductor. 

Q. Is it safe to carry out electric repairs during a rainy day?

Ans: As rain water is a good conductors of electricity, it is not safe for an electrician to carry out electric repair during a rainy day 

Chemical effects of electric current

When electric current is passed through acidified water by using carbon electrodes, then there is a chemical reaction takes place and Hydrogen and Oxygen will be released as water will be decomposed into hydrogen and oxygen. 

ELECTROLYSIS

The chemical decomposition produced by passing electric current through a conducting liquid is called electrolysis. For example, When electric current is passed through acidified water by using carbon electrodes, then there is a chemical reaction takes place and Hydrogen and Oxygen will be released as water will be decomposed into hydrogen and oxygen. 

Describe an activity to demonstrate the chemical effect of current. 

Take two carbon rods and cleaning their metal caps they are fitted with wires and connected to a battery with a switch on the positive side. The carbon rods will act as electrodes, one is cathode (negative electrode) and anode (positive electrode). Take 250 ml of acidified water in the beaker and emerge the carbon rods into them. When we close the switch, electric current will pass through the acidified water and bubbles of gases will be produced at the two electrodes. The formation of gas bubbles at the two electrodes shows that a chemical change or chemical reaction takes place. This is a description of an activity which shows the chemical effects of electric current. 

"If electric current is passed through acidified water, then bubbles of oxygen gas and hydrogen gas are produced at the two electrodes."

1. Oxygen will be produced at positive electrode (anode)

2. Hydrogen will be produced at negative electrode (cathode)

The fresh fruits and vegetables conduct electricity to some extent due to the presence of various salt solution in them. 

Describe an activity to demonstrate the change in colour caused by the chemical effect of electric current.

We cut a potato into two halves. Taking one piece of cut potato and inserting to iron nails into it a little distance apart from one another. The iron nails are the two electrodes in this case. We connect the two terminals of a battery to the two iron nails by including a compass and a switch in the circuit as shown in the figure. When we pass the electric current through cut potato piece by closing the switch, we observe a deflection in the compass needle which indicates that potato is conducting electricity to some extent. Latest continue to pass electric current through water two piece for about half an hour, we shall notice a greenish blue spot on the cut surface of potato around the iron nail which is connected to the positive terminal of the battery. There is however no coloured spot around the other name which is connected to the negative terminal of the battery. The formation of a greenish blue spot around the positive electrode inserted in the surface of a cut potato shows that the chemical effect of current can bring about change in the colour of a conducting solution.

Applications of the chemical effect of electric current

1. Electroplating metals

2. Purification of Metals

3. Production of certain metals from their ores

4. Production of chemical compounds

5. Decomposing chemical compounds

Electroplating of metals

The process of depositing a thin a layer of a desired metal over a metal object with the help of electric current is called Electroplating.

The metal objects like taps, utencils, jewelry and other objects are electroplated with chromium, tin, nickel, silver, gold or copper. 

Activity for electroplating:

How does copper can be electroplated on the surface of an object made of iron?

We shall take a beaker and pour copper sulphate solution in it. Now we take a copper plate and connected to the positive terminal of the battery with a switch. Again we take iron object like an iron key and connect it to the negative terminal of the battery. Therefore, copper plate and iron key will act as electrodes. Now when we close the switch , electric current will flow through the copper solution and copper will be diposited on the surface of the iron key. Thus the iron key will be electroplated with copper. During the copper plating of an iron key, coppee metal transferred from copper plate to the iron key through the copper sulphate solution. This process is also known as copper plating.

The following points should be remembered while electroplating:

1) The metal object on which electroplating is to be done is made the negative electrode or cathode. Therefore it must be connected to the negative terminal of the battery.

2) The metal whose layer is to be deposited is made the positive electrode or anode. It is connected to the positive terminal of the battery. 

3) A water soluble salt of the metal to be deposited is taken as the electrolyte the electrolyte contains the metal to be deposited in the form of a soluble salt. For example, copper plating, we need copper sulphate solution. 

Uses of electroplating:

1. Electroplating is used to cover iron and steel objects with a thin layer of chromium metal. This chromium layer gives an attractive shiny surface and also protects iron and steel objects from rusting. For example, bicycle handlebars, bicycle bells wheel rims, bathroom fittings LPG stoves motorcycle parts are often electroplated with chromium. 

2. The less reactive and shiny metals like chromium, tin and nickel are electroplated on more reactive and dull looking metals like iron and Steel to protect them from corrosion and give them an attractive finish. 

3. Electroplating is used to give objects made of metal a coating of more expensive metal to make them look more attractive. For example, less expensive metals are electroplated with more expensive metals like silver and gold to make jewellery.

Purification of metals:

The chemical effect of electric current is also used in the purification of impure metals.

Impure metals are collected from their naturally occurring compounds called ores. In the purification of an impure metal by using the chemical effect of electric current or electrolysis, 

a. Thick rod of a metal is made positive electrode or anode and it is connected to the positive terminal of the battery.

b. A thin strip of pure metal is made negative electrode or cathode and it is connected to the negative terminal of the battery.

c. A water soluble salt of the metal to be purified is taken as electrolyte.

Take 250 ml of distilled water in a clean beaker. Dissolve two teaspoonfuls of copper sulphate in it. Add a few drops of dilute sulphuric acid to copper sulphate solution. A thick rod of a copper metal is made positive electrode by connecting it to the positive terminal of the battery. 18 plate of pure copper metal is made negative electrode or cathode by connecting it to the negative terminal of the battery. Switch on the electric current by closing the switch. Allow the current to pass for about half an hour. It will be observed that the A copper rod goes on becoming thinner and thinner where is the pure copper plate goes on becoming thicker and thicker. This is because the A copper metal of Android goes on dissolving in copper sulphate solution where is the pure metal from copper sulphate solution goes on depositing on copper plate cathode. Impurities present in a rod of copper fall to the bottom of the beaker. 

Production of metals:

The chemical effect of electric current is used in the production or extraction of certain metals from their naturally occurring compounds called ores. 

Production of compounds:

The chemical effect of electric current or electrolysis is used in the production of various chemical compounds like sodium hydroxide is produced by the electrolysis of an aqua solution of sodium chloride. 

Decomposition of compounds:

The chemical effect of electric current or electrolysis is used to decompose various chemical compounds into their elements.