Category Science

               Some stars that have been shining steadily for millions of years suddenly undergo a fantastic change. This change comes in a very unpredictable and violent way. Within a few hours or a couple of days their brightness increases by 10,000 times or more. An ordinary observer might think that a new bright star has appeared in the sky. The increase in brightness occurs when an explosion throws up a small amount of the star’s matter—probably less than a hundred thousandth of its matter. This matter is actually a shell of gas that expands brilliantly in the outer space as soon as the ignition and explosion take place. A nova reaches its maximum brilliance in a few hours or a few days, and then after a few weeks or months it returns to its normal brightness. The decline of the brightness begins at various rates which often fluctuate. After a few years the brightness of the novae remnant becomes steady and a gas cloud may be observed around it which expands at a rate of hundred kilometres per second. Some novae have been known to erupt more than once and are termed as recurrent. All recurrent novae flare up at long intervals. About 20 or 30 novae are believed to occur in our galaxy every year. 

               It has been found that the novae reach absolute visual luminosities to the extent of about 10,000 to 1,000,000 times than that of the sun. The total energy emitted during a large novae outburst is of the order of  ergs, equal to the radiation from the sun in 10,000 years. Should the sun ever become novae, the earth would be destroyed in a few hours or days. However, Sun is unlikely to become so.

               A supernova is a much more spectacular event than a nova. In a supernova explosion, there is a complete self-destruction of the star or at least one-tenth of its matter is thrown off. This may result in an increase in brightness which reveals an entire galaxy as the increase is a billion times more.

               The remains of a supernova that occurred in 1054 A.D is still seen today as the crab nebula which has become one of the most fascinating objects in the sky. Some supernovae including the above were bright enough to be seen in the broad daylight. It seems that a supernova occurs once in about every 300 years. All supernovae are shattered to pieces in their explosions, collapsing into neutron stars.

          ‘Every action has an equal and opposite reaction’ – Newton enunciated this principle long ago which is commonly known as Newton’s third law of motion. And a jet engine works on this principle. Its working can be compared to the action of a swimmer who swims forward by pushing water backwards. To put it in the Newtonian law, here the action is pushing of the water backwards and the opposite reaction is the forward movement of the swimmer. In a similar fashion a jet engine ejects (pushes backward) gases at the rear with a great speed and the resulting opposite reaction to this action is the moving of the aircraft in the forward direction in an equal speed. But where does this gas come from and how is it released with such a great force?

          All jet engines have fuel inside them which when burnt in the engine produces a great amount of hot gases almost instantly. It is like an explosion. These hot gases blast out of the back with a great force and the engine reacts by being pushed forward with an equal force. This forward force is called thrust. To get an idea of this movement, we can observe the motion of an air-filled balloon when the air is released suddenly. The balloon zips away rushing out the air in one direction. The rushing out of air is responsible for pushing the balloon in the opposite direction with a thrust.

          The rockets also work on the same principle. The main difference between jets and rockets is the source of oxygen to burn the fuel. A jet engine takes in oxygen from the air around it through an intake nozzle. But a rocket carries its own oxygen which may be in the form of Liquid oxygen in a tank or may be part of a solid fuel the rocket burns. The jet engines have compressors to compress or squeeze the sucked air together before it is mixed up with the fuel and burned in the combustion chamber thereafter. The compression is done to increase the force of explosion within the engine.

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                  It is a well-known fact that a solid block of iron which may be as small as a needle readily sinks in water but something as big as a ship floats on water. It is interesting to know why it so happen?

                   According to Archimedes principle, the apparent loss in weight of a body totally or partially immersed in a liquid is equal to the weight of the liquid displaced. The centre of gravity of the body and that of the displaced liquid (centre of buoyancy) must lie in the same vertical line which is called the centre line of the body. In the case of a solid iron block, the weight of the liquid displaced is less than the weight of the block — so it sinks. 

                   The case is different in case of an iron ship due to its special construction. Its body is shaped in such a manner that it displaces a large quantity of water, and therefore, experiences an upward thrust greater than its weight. Therefore, when it floats on water, its weight is equal to the weight of water displaced by its immersed portion. In other words, when the ship enters the water the volume of the water displaced is much greater than the volume of actual iron immersed, and as a solid it cannot displace more than its own weight of a liquid. The ship sinks only to the extent the weight of the displaced water is equal to the weight of the ship. Thus the remaining portion of the ship stays out of water.

               When we hear the sound of a police car speeding past us with its siren blaring or a train roaring past another train, we experience something strange happening to the pitch of the sound. The sound seems to get higher as the car approaches and lower as it goes past. This despite the fact that the actual pitch of the sound remains the same; it just seems so because sound waves reach us faster as the car gets nearer. The effect is known as the Doppler Effect after the Austrian physicist Christian Doppler who first studied it in 1842.

               The Doppler Effect is thus described as the apparent change in frequency of sound, light or radio waves caused by the motion of the source, observer or medium. The basis of the ‘Doppler Effect’ is the fact that sound travels in the form of waves. The pitch of a sound depends on its frequency. The frequency is the number of sound waves striking the ear every second. When the source of the sound is approaching, each wave sent out by the source has a shorter distance to travel than the wave that was sent out earlier from a longer distance. Each wave reaches the listener a little sooner than it would have if the source had not been moving. The waves seem to be more closely spaced. They have a higher frequency or higher pitch. As the train passes away the observer, each wave starts a little further away. Each wave seems to be longer than it would ordinarily be. Hence the pitch is lowered.

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          The ammeter is an instrument used for measuring electric current. The current is measured in amperes. There are three main kinds of ammeters: (i) moving coil ammeter (ii) moving iron ammeter and (iii) a hot-wire ammeter.

          The moving coil ammeter is like a galvanometer. It has a strip of soft iron which causes to move in the magnetic field created by the current flowing coil. 

          The moving-iron ammeter has two pieces of iron inside a coil. One of the iron pieces can move. The other piece cannot move. The current passing through the coil produces a magnetic field. The force of the field moves one piece of iron away from the other. A needle attached to the moving piece on a scale indicates the current. These ammeters can measure both direct and alternating current. 

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