Materials that allow electric current to pass through them are good conductor of electricity. Ex: Copper and Aluminium.
Materials that do not allow electric current to pass through them are poor conductor of electricity. Ex: Rubber, plastic and wood.
Do liquids conduct electricity?
The circuit shown tests whether liquids conduct electricity. When the liquid between the two ends of the tester allows the electric current to pass, the circuit of the tester becomes complete. The current flows in the circuit and the bulb glows. When the liquid does not allow the electric current to pass, the circuit of the tester is not complete and the bulb does not glow.
The bulb glows when the electric current is passed through it. It is due to heating effect of current. The filament of the bulb gets heated to a high temperature and it starts glowing. If the current through a circuit is too weak, the filament does not get heated sufficiently and it does not glow.
To detect a weak current, LED is used instead of a bulb. Or the weak current can be detected by its magnetic effect. If there is a weak current in the circuit, there will be a deflection in the compass needle.
It is found that lemon juice conducts electricity. If we replace lemon juice with distilled water, there is no current flowing through the circuit. Therefore, distilled water is a poor conductor of electricity. When salt is added, the solution conducts electricity. The water that we get from sources such as taps, hand pumps, wells and ponds is not pure. It may contain several salts dissolved in it. Small amounts of mineral salts are naturally present in it. This water is thus a good conductor of electricity. Most liquids that conduct electricity are solutions of acids, bases and salts.
Chemical effects of electric current:
Two carbon rods also known as electrodes are dipped in a salt solution. Salt solution is a conducting solution. The passage of electric current through conducting solution causes chemical reactions. Few chemical reactions that can be seen in this experiment are formation of bubbles on the electrodes, deposits of metal on electrodes and color change of the solution.
Electroplating:
Copper sulphate solution is placed in the beaker and few drops of sulphuric acid is added to make it more conducting. Two copper plates are dipped in the solution and electricity is passed through it. When electric current is passed through the copper sulphate solution, copper sulphate dissociates into copper and sulphate. The free copper gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it. From the other electrode, a copper plate, an equal amount of copper gets dissolved in the solution. Thus, the loss of copper from the solution is restored and the process keeps going. This means that copper gets transferred from one electrode to the other.
The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating. It is a very useful process. Some of the uses are:
- It is widely used in industry for coating metal objects with a thin layer of a different metal. The layer of metal deposited has some desired property, which the metal of the object lacks. For example, chromium plating is done on many objects such as car parts, bath taps, kitchen gas burners, bicycle handlebars, wheel rims and many others.
- Chromium has a shiny appearance. It does not corrode. It resists scratches. However, chromium is expensive and it may not be economical to make the whole object out of chromium. So the object is made from a cheaper metal and only a coating of chromium over it is deposited.
- Jewellery makers electroplate silver and gold on less expensive metals. These ornaments have the appearance of silver or gold but are much less expensive.
- Tin cans, used for storing food, are made by electroplating tin onto iron. Tin is less reactive than iron. Thus, food does not come into contact with iron and is protected from getting spoilt.
- Iron is used in bridges and automobiles to provide strength. However, iron tends to corrode and rust. So, a coating of zinc is deposited on iron to protect it from corrosion and formation of rust.
Force – a push or a pull:
Force: A push or a pull on an object is called force. Actions like kicking, picking, opening, shutting, hitting, lifting, flicking, pushing, pulling involve either pull or push or both. These actions cause movement in an object which is due to the force.
Forces are due to an interaction:
Interaction of one object with another object results in a force between two objects. For example, if you want to push a table, you have to apply some force on that table with your hands which will make the table move. The pushing of table with hands is interaction.
Exploring forces:
When two forces are applied in the same direction, the forces add to one another. For example, when two people push a table in the same direction, the table moves easily because the force applied by both the people adds up.
When two forces are applied in opposite direction on the same object, the total force is the difference between the individual force. For example, if two people are pushing a table from opposite directions, the table doesn’t move if the force applied by each person is same. If one person exerts more force than the other one, then the table moves in their direction.
The strength of force is determined by its magnitude. Force has both magnitude and direction.
Force can change the state of motion:
When a force is applied on an object it may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases. If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object. For example, when we push a moving ball in the same direction it is moving, the ball moves with greater speed and we put an obstacle in the opposite direction, the ball either stops or decreases its speed.
Force can also change the direction of the motion of an object. While playing cricket when the batsman hits the ball, it moves in another direction.
A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Thus, a force may bring a change in the state of motion of an object.
Sometimes exerting force doesn’t change the state of the motion. It happens when the force exerted is low.
Force can change the shape of an object:
Applying force can change the shape of an object. For example, playing with a clay, or pressing an inflated balloon.
Forces have many effects. They may make an object move from rest, change the speed, direction, shape of an object. An object cannot move by itself; it cannot change speed by itself, it cannot change direction by itself and its shape cannot change by itself. It requires force.
Contact Forces:
Muscular force: The force resulting due to the action of muscles is known as muscular force. It is caused due to the action of muscles in our body. While lifting a bucket, or pulling a rope, muscle energy is required. Since muscular force can be applied only when it is in contact with an object, it is also called a contact force. Animals like donkeys, bullocks, camels and horses use muscular force to perform various tasks.
Friction: There are few scenarios in our daily lives where there is no force acting on an object yet their speed gradually decreases and they come to rest after sometime. For example, boat comes to rest if we stop rowing it, car or scooter come to rest once the engine is switched off, the moving ball stops rolling after some time. In all these scenarios, the force responsible for changing the state of motion of objects is the force of friction. The force of friction acts in the opposite direction of the motion. Since the force arises due to the contact between two surfaces, it is also a contact force.
Non-contact forces:
Magnetic force: When a magnet is placed near an iron nail, the iron nail is pulled towards the magnet. Two magnets of like poles repel each other and magnets of opposite poles attract each other. This force is called magnetic force. A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force.
Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. Example, when a plastic scale is rubbed on the hair and brought near thermocole balls, they get attracted towards the scale and stick. The scale is said to have acquired charge. The scale exerts force even though they are not in contact. Thus, electrostatic force is non-contact force.
Gravitational force: The force exerted by earth on the object is known as gravitational force. It is an attractive force which pulls the objects towards it. When we throw an object upward, it comes back down. When we open a top, the water flows downward.
Pressure:
Pressure is the force acting on a unit area of a surface.
Pressure=force/area
The smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank.
Pressure exerted by liquids and gases:
Liquids and gases both exert pressure on the walls of the container they are contained in. They exert pressure in all the directions.
Atmospheric Pressure:
The air around us is atmosphere. The atmospheric air extends up to many kilometers above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure.
Friction:
Friction always works in the opposite direction of applied force. If force is applied along the left, then friction acts along the right. If force is applied along the right, then friction acts along the left. Vehicles slow down due to friction, moving ball slows down due to friction.
Factors affecting friction:
Friction is caused by the irregularities on the two surfaces in contact. All surfaces have irregularities on them. Even smooth surfaces have minute irregularities. These irregularities on the two surfaces lock into one another. To move any surface, we have to apply a force to overcome interlocking. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved. For example, when we try to push a heavy box, it is difficult but when it is placed on a plastic, it becomes easier to move that box.
Static friction is the force required to overcome friction to move an object from rest. The force required to keep the object moving with the same speed is a measure of sliding friction. When the box starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. Therefore, sliding friction is lesser than static friction and it is easier to move an object which is already in motion.
Friction: A Necessary Evil:
Friction is very important in our daily lives. If an object started moving, it would never stop if there were no friction. We wouldn’t be able to hold anything, the vehicles wouldn’t be able to start or stop, no building could be constructed. Even though friction is important, it has its drawbacks too. It wears out materials like screws, ball bearings, sole of the shoes. Friction also produces heat. When we rub our palms vigorously, it produces heat. When you strike a matchstick against the rough surface, it catches fire. When a machine is operated, heat generated causes much wastage of energy.
Increasing and reducing friction:
It is important to increase friction sometimes. The sole of our shoes is grooved which provides a better grip for the shoes on the floor and we can move safely. The treaded tyres of cars, trucks and bulldozers provide better grip with the ground. Friction is increased for better grip. In bicycles and automobiles, the brake system uses brake pads to increase friction and stop the wheels.
Sometimes we may also want to minimize friction. Powder is sprinkled on a carrom board, oil is poured on the hinges of door, for smooth movement. Friction is reduced to increase efficiency. When oil, grease or graphite is applied between the moving part of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other. Interlocking of irregularities is avoided to a great extent. Movement becomes smooth. The substance which reduces friction is called lubricants. In some machines, air cushions are used to reduce friction.
Wheels reduce friction:
When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction. Rolling reduces friction and it is smaller than sliding friction. That is why it is easier to roll a body than sliding it. Therefore, luggage is fitted with rollers, ball bearers are used between hubs and axles are used in ceiling fans and bicycles.
Fluid friction:
Gases and liquids are called fluids. Fluids exert force of friction on objects in motion through them. The frictional force exerted by fluids is also called drag. The frictional force on an object in a fluid depends on the speed with respect to the fluid, shape of the object and nature of the fluid. When objects move through the fluid, they have to overcome friction acting on them, during which they lose energy. To minimize these efforts, they are given different shapes. Birds and fishes have to move about in fluids all the time. Their bodies must evolve to shapes which would make them not to lose much energy in overcoming friction. All vehicles are designed to have shapes which reduce fluid friction.
What makes things visible?
We can see objects only when light from an object enters our eyes. The light may have been emitted from the object or reflected by it. Thus, we can’t see objects in the dark.
Laws of Reflection:
Incident Ray: The light ray which strikes any surface is known as incident ray.
Reflected Ray: The light ray that comes back from the surface after reflection is known as reflected ray.
Normal: The ray that is incident at 90 degrees to the surface.
Angle of incidence (∠i): The angle between the normal and incident ray is called the angle of incidence.
Angle of reflection (∠r): The angle between the normal and reflected ray is called the angle of reflection.
Laws of reflection:
- the angle of incidence is always equal to the angle of reflection.
- the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
Lateral Inversion: in an image formed by a mirror. The left side of the object appears on the right side and the right side appears on the left side. This is known as lateral inversion.
Regular and diffused reflection:
Regular reflection: Reflection from a smooth surface like that of a mirror is called regular reflection. Images are formed by regular reflection.
Irregular or diffused reflection: When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection. It is caused by the irregularities in the reflecting surface.
Reflected light can be reflected again:
When we go to a trial room, we see mirrors on three sides of the room. We can see multiple reflections of ourselves. Periscope is a device that makes use of two plane mirrors to see objects which are not visible directly. Periscopes are used in submarines, tanks and also by soldiers in bunkers to see things outside.
Multiple Images:
Multiple Images can be formed when you place mirrors at different angles. The idea of number of images formed by mirrors placed at an angle to one another is used in a kaleidoscope to make numerous beautiful patterns. Kaleidoscope can be made at home. Three rectangular mirrors of same size are joined together in a form of prism. It is fixed in a circular cardboard tube. One end of the tube is closed with a cardboard disc which has a hole in the centre through which we can see. A circular plane glass plate is fixed at the other end and several small pieces of colored glass are placed. It is then closed by a round glass plate. When we peep through the hole, we will be able to see a variety of patterns in the tube. Interesting feature of a kaleidoscope is that we will never see the same pattern again. Designers of wallpapers and fabrics and artists use kaleidoscopes to get ideas for new patterns.
Sunlight:
Sunlight consists of several colors. They show dispersion. Splitting of light into its colors is known as dispersion of light. Rainbow is a natural phenomenon showing dispersion.
Eyes:
We see things only when light coming from them enters our eyes. The eye has a roughly spherical shape. Outer coat of the eye is white. It is tough so that it can protect the interior of the eye from accidents. Its transparent front part is called cornea. Behind the cornea, we find a dark muscular structure called iris. In the iris, there is a small opening called the pupil. The size of the pupil is controlled by the iris.
The iris is the part of that eye which gives it its distinctive colour. The iris controls the amount of light entering into the eye. Behind the pupil of the eye is a lens which is thicker in the centre. The lens focuses light on the back of the eye, on a layer called retina. Retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve. There are two kinds of cells (i) cones, which are sensitive to bright light and (ii) rods, which are sensitive to dim light. Besides, cones sense colour. At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot. The impression of an image does not vanish immediately from the retina. It persists there for about 1/16th of a second. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving. The movies that we see are actually a number of separate pictures in proper sequence. They are made to move across the eye usually at the rate of 24 pictures per second (faster than 16 per second). So, we see a moving picture. Eyelids protect our eyes from object entering the eye. The minimum distance at which the eye can see objects distinctly varies with age. The most comfortable distance at which one can read with a normal eye is about 25 cm. Some persons can see near objects clearly but cannot see distant objects so clearly. On the other hand, some persons cannot see near objects clearly but they can see distant objects quite well. With suitable corrective lenses, these defects of the eye can be corrected.
Care of Eyes:
To take proper care of our eyes, we should take some measures:
- Have a regular checkup of eyes. If advised, use suitable spectacles.
- Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
- Do not look at the sun or a powerful light directly.
- Never rub your eyes. If particles of dust go into your eyes, wash your eyes with clean water. If there is no improvement go to a doctor.
- Wash your eyes frequently with clean water.
- Always read at the normal distance for vision. Do not read by bringing your book too close to your eyes or keeping it too far.
Visually challenged persons can read and write:
Visually challenged people can try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply.
What is a Braille System?
The most popular resource for visually challenged persons is known as Braille. The present system was adopted in 1932. There is Braille code for common languages, mathematics and scientific notation. Many Indian languages can be read using the Braille system. Visually challenged people learn the Braille system by beginning with letters, then special characters and letter combinations. Methods depend upon recognition by touching. Each character has to be memorized. Braille texts can be produced by hand or by machine. Type writer – like devices and printing machines have now been developed.
Lightning:
We see little sparks of electricity when a plug is loose in its socket. Lightning is an electric spark at a huge scale. Lightning is caused by the accumulation of charges in the clouds.
When we take off woolen or polyester clothes in the dark, we see a spark and hear crackling sound. In 1752 Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomena.
Charging by rubbing:
When a plastic comb is rubbed with dry hair, it acquires a small charge. The plastic comb becomes a charged object and hair also gets charged. Therefore, objects can be charged by rubbing
Types of charges and their interaction:
Suppose two objects are charged by rubbing, they acquire two kinds of charge. One object acquires positive charge and the other object acquires negative charge. Like charges repel each other and unlike charges attract each other. For example, when a glass rod is rubbed with a silk cloth, it acquires a positive charge. A plastic straw is also charged by rubbing it with a polythene. When the glass rod is brought near the plastic straw, they are attracted to each other. As they are attracted, the charge acquired by plastic straw is negative. The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current.
Transfer of charge:
An electroscope is a device used to test whether an object is carrying a charge or not. A simple electroscope can be made with an empty jar bottle. Place a cardboard on top of the jar and pierce it and insert a paper clip as shown in the figure. Pieces of aluminium foil are hung on the paper clip. A charged refill is touched with paper clip.
The aluminium foil strips receive the same charge from the charged refill through the paper clip (metals are good conductors). The strips carrying similar charges repel each other and they become wide open. Thus, we find that electrical charge can be transferred from a charged object to another through a metal conductor. When we touch the paper clip with our hand the foil strip collapse. The foil strips lose charge to the earth through our body. We say that the foil strips are discharged. The process of transferring of charge from a charged object to the earth is called earthing.
The story of lightning:
During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges. The positive charges accumulate on the upper edges of cloud and the negative charges accumulate near the lower edges. There is accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning. The process is called an electric discharge. The process of electric discharge can occur between two or more clouds, or between clouds and the earth.
Lightning safety:
During lightning and thunderstorm no open place is safe. We should rush to a safe place when we hear thunder. We should wait for some time before coming out to an open place when we hear the last thunder. A house or a building is a safe place. If you are travelling by car or by bus, you are safe inside with windows and doors of the vehicle shut.
If we’re outside, then open vehicles like bike, tractors are not safe. We should stay away from tall trees, poles and 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. This position will make you the smallest target to be struck.
Inside the house, telephone cords, electric wires and metal pipes should be avoided. Wired phones should also be avoided. Bathing should be avoided during thunderstorms to avoid contact with running water. Electrical appliances like computers, TVs, etc., should be unplugged. Electrical lights can remain on. They do not cause any harm.
Buildings are protected from lightning by using lightning conductor. It is a metallic rod which is taller than the building installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other is buried deep in the ground. The rod provides easy route for the transfer of electric charge to the ground. The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.
Earthquakes:
Natural phenomena like cyclone, thunderstorm, lightning can be predicted. The weather department can warn about a thunderstorm developing in some area. If a thunderstorm occurs, 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 an earthquake cannot be predicted, and it can cause damage to human life and property on a huge scale. A major earthquake occurred in India on 8th October 2005 in Uri and Tangdhar towns of North Kashmir. Before that, a major earthquake occurred on 26th January 2001 in Bhuj District of Gujarat.
An earthquake is a sudden shaking or trembling of the earth lasting for a very short time. It is caused by a disturbance deep inside the earth’s crust. Major earthquakes are less frequent but cause immense damage to buildings, bridges, dams and people. Earthquakes can cause floods, landslides, and tsunamis. A major tsunami occurred in the Indian Ocean on 26th December 2004. All the coastal areas around the ocean suffered huge losses.
Tremors are caused by the disturbances deep down inside the uppermost layer of the earth, called the crust. The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another, or a plate goes under another due to collision, they cause disturbance in the earth’s crust. It is this disturbance that shows up as an earthquake on the surface of the earth. Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion is carried out.
Since earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones. In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo – Gangetic Plane. Some areas of South India also fall in the danger zone.
The power of an earthquake is expressed in terms of magnitude on a scale called Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale. Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5.
Protection against earthquakes:
In highly seismic zones, the buildings should be quake safe. The use of mud or timber is better than the heavy construction material and roofs should be kept as light as possible. The cupboards and shelves should be fixed to the walls. Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have firefighting equipment in working order.
At home:
- Take shelter under a table and stay there till shaking stops.
- Stay away from tall and heavy objects that may fall on you.
- If you are in bed, do not get up. Protect your head with a pillow.
Outdoors:
- Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
- 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.
Sound
Sound plays a very important part in our life. It helps us to communicate with one another. We hear various kinds of sounds in our surroundings. There are musical instruments which produce sound, animals produce sound, tress produce sound etc.
Sound is produced by a vibrating body:
The to and fro or back and forth motion of an object is known as vibration. Vibrating objects produce sound. In some cases, we can see the vibration. In most of the cases, we can’t see the vibrations because the amplitude is very small. When we pluck a tightly stretched band, it vibrates and produces sound. When it stops vibrating, it does not produce any sound. The most common example of vibration which produces sound is musical instruments like Veena and Guitar.
Sound produced by humans:
In humans, sound is produced by the voice box or the larynx. Voice box is present at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.
The vocal cords in men are about 20mm long. In women these are about 5mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.
Sound needs a medium for propagation:
Sound needs a medium to travel. It can travel through solid, liquid and gases. Sound can travel through wood, iron, water, air etc. Sound can’t travel in vacuum.
We hear sound through our ears:
The shape of the outer part of the ear is like a funnel. When sound enters in it, it travels down a canal at the end of which a thin membrane is stretched tightly. It is called the eardrum. The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear.
Amplitude, Time period and Frequency of a vibration:
Oscillatory motion: The to and fro motion of an object is called oscillatory motion.
Frequency: The number of oscillations per second is called frequency of oscillation. It is expressed in hertz and is denoted by Hz.
The sounds are differentiated by their amplitude and frequency.
Loudness and Pitch:
The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.
The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is high, we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch. A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud. Usually the voice of a woman has a higher frequency and is shriller than that of a man.
Audible and Inaudible sounds:
A human ear can’t detect a sound below 20Hz and above 20kHz. The range of audible frequencies is from 20Hz to 20kHz.
Noise and Music:
The unpleasant sound is called noise. The sounds produced by horns of trucks or buses, sound from constructing sites etc. Musical sound is one which is pleasing to ear. The sound coming from musical instruments are melodious and pleasing to hear.
Noise Pollution:
Presence of excessive or unwanted sounds in the environment is called noise pollution. Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers etc. Lack of sleep, hypertension (high blood pressure), anxiety and many more health disorders may be caused by noise pollution. A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.
Measures to limit noise pollution:
To the sources of noise pollution, silencing devices must be installed in aircraft engines, transport vehicles, industrial machines, and home appliances. In residential area, the noisy operations must be conducted away from any residential area. Noise producing industries should be set up away from such areas. Use of automobile horns should be minimised. TV and music systems should be run at low volumes. Trees must be planted along the roads and around buildings to cut down on the sounds reaching the residents, thus reducing the harmful effects of noise pollution.
When we see the sky from a city and sky from the village, they appear to be different. In village, we see countless bright stars. It is due to clear atmosphere in village. We see few objects in the sky which twinkle, they are called stars. The stars that do not twinkle are called planets. The moon is the brightest object in the sky. The stars, the planets, the moon and many other objects in the sky are called celestial objects.
The Moon:
The moon appears to change its shape every day. The day on which the whole disc of moon is visible is called full moon day. Every night after that, the moon’s size becomes thinner and thinner. After fifteen days, the moon is not visible. That day is known as new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again, the moon grows larger every day. On the fifteenth day once again, we get a full view of the moon. The various shapes of the bright part of the moon as seen during a month are called phases of the moon.
The moon doesn’t produce its own light, whereas the Sun and other stars do. We see the moon because the sunlight falling on it gets reflected towards us. We, therefore, see only that part of the moon, from which the light of the Sun is reflected towards us. The moon revolves around the earth. The earth along with the moon revolve around the Sun. The size of the illuminated part of the moon visible from the Earth increases each day after the new moon day. After the full moon day, the sunlit part of the moon visible from the Earth decreases in size every day. The moon completes one rotation on its axis as it completes one revolution around the Earth.
The Moon’s Surface:
The moon’s surface is said to be dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. Some of these are as high as the highest mountains on the Earth. The moon has no atmosphere and no water. Neil Armstrong was the first person to land on moon on 21st July, 1969.
The Stars:
There is a large number of stars in the sky. Stars emit light of their own. The Sun is also a star. The stars are millions of times farther away than the Sun. Therefore, the stars appear to us like points. The distance of sun or stars from earth is calculated in light years. It is the distance travelled by light in one year. The distance of the Sun from the Earth may be said to be about 8 light minutes or 150 million km. The distance of Alpha Centauri is about 4.3 light years or 40,000,000,000,000 km. The stars are not visible during the day because of the bright sunlight. When we keep looking at the stars for a long period of time, we see that stars move from east to west. A star which rises in the east in the evening, sets in the west in the early morning. The Sun appears to rise in the east and set in the west as the Earth rotates from west to east on its axis. There is one star which does not move and is situated in the direction of the earth’s axis. It is known as pole star.
Constellations:
The stars forming a group that has a recognisable shape is called a constellation.
Constellations can be easily identified from the sky. One of the most famous constellations which we can see during summer is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi.
There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl. The pole star can be located using Ursa Major. Imagine the line connecting the two front stars of the Ursa Major, extend this line to the first bright star. That is the pole star. The constellation appears to revolve around the pole star. The Pole Star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere.
Orion is one of the most magnificent constellations in the sky. It can be seen during winter in the late evenings. It has seven or eight bright stars. Orion is also called the Hunter. The three middle stars represent the belt of the hunter.
The four bright stars appear to be arranged in the form of a quadrilateral. The brightest star in the sky, Sirius, can be located close to Orion. The line towards the east passing through three middle stars of Orion will lead to Sirius.
Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.
The Solar System:
The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it. The Earth revolves around the Sun. It is a planet and there are seven more planets that revolve around the Sun.
The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Till 2006 there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. It is no longer a planet of the solar system.
The Sun
The sun is the nearest star from us. It continuously emits huge amount of heat and light. The Sun is the source of almost all energy on the Earth. In fact, the Sun is the main source of heat and light for all the planets.
The planets
The planets look like stars, but they reflect the sunlight that falls on them, they do not emit their own light. The stars which do not twinkle are planets. A planet has a definite path in which it revolves around the Sun. This path is called an orbit. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun. Planet also rotates around their own axis and the time taken by a planet to complete one rotation is called its period of rotation. Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite. Earth can be said to be a satellite of the Sun. Usually, satellites are the body revolving around the planets. Moon is a satellite of the Earth. There are many man-made satellites revolving round the Earth. These are called artificial satellites.
Mercury (Budh):
It is the nearest planet to the Sun and the smallest planet of our solar system. Because Mercury is very close to the Sun, it is very difficult to observe it, as most of the time it is hidden in the glare of the Sun.
However, it can be observed just before sunrise or just after sunset, near the horizon. So, it is visible only at places where trees or buildings do not obstruct the view of the horizon. Mercury has no satellite of its own.
Venus (Shukra):
It is the brightest planet in the night sky. Sometimes Venus appears in the eastern sky before sunrise. Sometimes it appears in the western sky just after sunset. Therefore, it is often called a morning or an evening star, although it is not a star.
Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.
The Earth:
The Earth is the only planet in the solar system on which life is known to exist. There is existence and continuation of life on the Earth due to the right distance from the Sun, so that it has the right temperature range, the presence of water and suitable atmosphere and a blanket of ozone.
From space, the Earth appears blue green due to the reflection of light from water and landmass on its surface. The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. The Earth has only one moon.
Mars (Mangal):
The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.
Jupiter (Brihaspati):
Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed inside it. The mass of Jupiter is about 318 times that of our Earth. It rotates very rapidly on its axis. Jupiter has a large number of satellites.
It also has faint rings around it. You can easily recognise Jupiter as it appears quite bright in the sky. If you observe it with the help of a telescope, you can also see four of its large moons.
Saturn (Shani):
It’s yellowish in color and has beautiful rings around it. The rings can be observed by a telescope. Saturn also has a large number of satellites.
One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water.
Uranus and Neptune:
These are the outermost planets of the solar system. They can be seen only with the help of large telescopes. Like Venus, Uranus also rotates from east to west. The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side. The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets. The inner planets have very few moons. The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have large number of moons.
Other members of the solar system:
Asteroids:
Asteroids are small objects that revolve around the Sun. They are present in the large gap in between the orbits of Mars and Jupiter. Asteroids can be seen only through large telescopes.
Comets:
Comets revolve around the Sun in highly elliptical orbits. Their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun. Many comets appear periodically. Halley’s comet appears after every 76 years. It was last seen is 1986.
Meteors and meteorites:
The bright streaks of light in the sky known as shooting stars are not stars. They are meteors. A meteor is a small object that occasionally enters the Earth’s atmosphere. It enters with a very high speed . Due to atmospheric friction, it heats up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time. Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. Meteorites help scientists in investigating the nature of the material from which the solar system was formed.
Artificial Satellites:
There are number of artificial satellites orbiting the Earth. The artificial satellites are man-made. They are launched from the Earth and revolve around the Earth much closer than the Earth’s natural satellite, Moon. India has built and launched several artificial satellites. Aryabhatta was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc. Artificial satellites have many practical applications. They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing.
