How do trawlers fish?

               There is perhaps no human activity older, more varied or stranger, than fishing. He tricks and catches fish in different ways, such as using his bare hand, or fishing even with harpoon guns in whaling! But the method most used today is the one by which it produces the biggest share of commercial fishing known as trawling. Do you know how do trawlers fish?



               Trawlers fish with a bag-size net. It is let out on long warps or ropes. The fish are swept in at the wide, open end and then get trapped at the narrower, closed end. The trawler may be between 100 to 1500 metres long or more. In this system, the motorized fishing boats trawl by towing a large net in three different ways to keep the mouth of the net open. Firstly, a beam can be placed across the head of the net; secondly a pair of boats can be used - one at each side of the net to tow it and thirdly, some floating weights, called otter boards can be attached to the sides of the mouth of the net.



               However, the beam trawl is only used on a few small fishing crafts, and on the other hand, pair trawling is used to catch fishes from the bottom of the sea to enormous depths, sometimes at the range of 1500 metres or more. When the net is full, powered winches haul it on the board through a ramp. The otter trawl is widely used and is employed on almost every fishing technique except the smaller trawlers.



               The net gathers in everything including eggs, newly hatched fishes and algae. But this system is considered to be very destructive and alarming in the context of overfishing along the seas. Sometimes an entire fleet of fishing vessels is headed by a large factory ship fitted out just for processing of the catch. A single “sweep” of the net often taken in terms of tonnes of fish provides an idea of the quantity of fish caught in rich seas. Deep sea fishes like sardines and herrings together account for eighteen percent of the world’s catch.



               Today, the large motor fishing vessels are fitted with sonar or echo-sound equipments to locate a shoal of fish.





 


Where would a ball fall when thrown inside a running train?

               You might say that the ball would fall behind the person who throws it because he would have moved forward with the moving train. But in fact this is not correct.



               You can perform a simple experiment to answer this question. You would be surprised to find that the ball lands right in your hand when thrown upward inside the moving train. Do you know why it happens so?



               In a moving train everything inside the train also moves with the speed of the train, for example, the fans, passengers, you and the ball in your hand. When you throw up the ball, a part of the speed of the train is imparted to it. It acquires a vertical motion in addition to its horizontal motion. The passengers in the train cannot see its horizontal motion but only its upward and downward movements.



               Imagine a man outside the train, who is watching your experiment. As we have said the ball possesses both vertical and horizontal motions, both these motions combined together make the ball travel along a parabolic path. The observer outside the train will see the ball moving in a parabolic path but a passenger in the train will see only the up and down motions of the ball.



               Now the question arises whether the ball follows the parabolic path or just moves up and down? Out of these two which one is right? In fact, all motion is relative to the observer. There is nothing like absolute motion and hence the motion of the ball is different for the two observers. 


How does an Electric Bell function?


               When you push the button of an electric door bell or calling bell it keeps on ringing as long as the button remains pressed.



               Do you know how does it function? An electric bell is a simple device based on the magnetic effects of electric current. It is used in offices, houses, industries and for fire alarms.



               It consists of a U-shaped electromagnet and a soft-iron armature. The armature has a small hammer for striking the gong. This hammer hits the gong repeatedly and produces sound. The gong is made of a metal. For operating the bell, a push button is pressed. In an electric bell, the button is a switch that connects the supply of electricity to the bell.



               When the button of the bell is pressed, the current flows through electromagnet winding, armature, contact spring and the contact screw. The flow of the current magnetizes the soft-iron core of the electromagnet. This attracts the armature, causing the attached hammer to strike the metal gong and thereby produce sound.



               As the armature moves forward due to magnetic attraction the contact spring moves away from the contact screw. This breaks the circuit and the current stops flowing. As a result, the soft-iron core loses its magnetism. It, therefore, no longer attracts the armature which, then, is pulled back by the contact spring to its original position. As soon as the armature comes to its original position the electric circuit is again completed and the soft iron becomes magnetized. It again attracts the armature and thereby the hammer strikes against the gong and produces sound. As long as the push button remains pressed, the circuit is alternately broken and completed causing the hammer to strike the gong. Thus an electric bell keeps ringing.



               If a steel core is used instead of a soft-iron, then the steel core will become a permanent magnet due to passage of electric current through the winding. Consequently, the armature will stay attracted even when the contact spring moves away from the contact screw, so the hammer will strike the gong only once.



 


What is Osmosis?


               It is a well known fact that when resins are put in water they get swollen. This swelling takes place due to the entry of water through the membrane of the resins. Similarly, if grapes are put in sugar solution they shrink. Swelling of resins and shrinking of grapes take place due to a process known as osmosis. Do you know what this osmosis is?



               Osmosis is a process in which a solution of lower concentration passes into a solution of higher concentration through a semipermeable membrane. A semipermeable membrane is one that allows some, but not all, substances to pass through it. This contains very small pores. When resins are put into water, the covering acts as a semipermeable membrane. Water is less concentrated than the substance present inside the resins and so the water moves into the resins through its semipermeable membrane. Similarly, fluid from grapes moves out through the semipermeable membrane, as the concentration of sugar solution is more than that of the grapes. There is a tendency for solutions separated by a membrane to become equal in molecular concentration.



               In osmosis, the movement is always from a dilute solution into a solution of higher concentration. This reduces the concentration of the stronger solution. The rate of osmosis depends upon the comparative strengths of the two solutions. The greater the difference, the faster the rate of osmosis. This process continues until both solutions are of equal strength. When this equilibrium is reached, osmosis stops.



               Osmosis is an ongoing process among the living beings. The membranes of cells are semipermeable. Plants absorb water and dissolved minerals from the soil by osmosis; they use osmosis to move the water and dissolved minerals through the plant, cell by cell. Osmosis also maintains turgor pressure. Turgor pressure is the pressure of water on the cell. It gives the cell form and strength. When there is a decrease in turgor pressure, the plant will soon wilt and lose its regular stiffness.



               Osmosis allows the transfer of water and dissolved nutrients in the human body from the blood into the cells.



 


How do electrically heated appliances work?


Electric heaters, immersion heaters, electric irons, electric kettles, etc. are appliances which produce heat through electricity. All these appliances are based on the heating effects of electric current. When electric current is passed through a wire, it gets heated up. Heating of a wire depends upon two facts: first, on the resistance of the wire and then on the amount of electric current passed. The heat produced in the wire is directly proportional to the resistance of the wire and that of the square of the current. The amount of heat produced also depends upon the time for which the current passes through the wire.



Based upon this property of current, many domestic electric appliances have been developed. The working principle of all these appliances is almost the same, the difference lies only in their construction. An electric heater consists of a coil of nichrome wire which is in the form of a spring. This coil is mounted on an insulating base plate made of clay. When electric current is passed through the coil, it gets heated up. Room heaters are also made in a similar way, the only difference being that nichrome wire is wound around an insulating rod and a reflector is mounted at the back of the coil which reflects the heat radiation.



Immersion heaters also consist of a nichrome wire which is enclosed in a metal tube. To isolate the wire from the metal tube, an insulating powder is filled in the tube. This powder acts as an insulator for electricity but conducts heat. When the two terminals of the wire are connected to an electric source, the current starts flowing through the wire and it gets heated up. The immersion heater is put inside a bucket full of water to heat the water.



An electric iron is used to remove the wrinkles from washed clothes. This appliance also consists of a ribbon of nichrome wire which is enclosed between two sheets of mica. This spreads the heat uniformally along the base plate of an electric iron. Mica sheets are mounted on a heavy metal plate. This metal plate, when pressed against the surface of the cloth, removes the wrinkles from the cloth.



Electric irons are of two types: automatic and manual. Automatic one is fitted with a thermostat control which regulates the temperature. Manual irons do not have such a device. When the iron is cold, thermostat provides and maintains a constant temperature by the use of a device that cuts off the supply of heat when the required temperature is exceeded.



An electric kettle is used to prepare tea or coffee. It also consists of a heating element fitted at the bottom of the vessel and is isolated from it. Water is put into the vessel which gets heated when current is passed through the heating element.



For all electrically heated appliances, it is very essential to have an earth connection. Immersion heaters should not be switched on, until there is water in the bucket. The electric bulb is also a similar device whose filament gets heated up when the electric current is passed through it and it produces light.



 


How do a mixer and grinder work?


               Mixer and grinder are very useful domestic appliances. With the help of these appliances we can grate, grind and prepare mango shake, milk shake, cold coffee etc. in a short period of time. Butter can be extracted from cream by using this apparatus. Pulses and spices can also be ground easily with its help.



               This apparatus consists mainly of two parts. One is the base of the apparatus which is fitted with a high speed motor. This motor makes 15-20 thousand revolutions per minute. It also consists of a variable switch by which the speed of the motor can be adjusted with the other part of the apparatus known as a mixer and grinder. This is usually made of stainless steel or plastic in the shape of a jar. It is fitted with blades which revolve with the speed of the motor. This rotating blade minces the food material into small pieces.



               Modern mixer and grinders also consist of other attachments such as a juicer with the help of which we can extract the juices of apples, oranges, tomatoes and other fruits and vegetables. In this attachment juice pours out on one side and pulp from the other side. Most modern grinders and mixers can be fitted with various other attachments such as a slice grater, meat mincer, dough maker etc. Nowadays we have grinders by which even wheat or maize can be ground.



               These electrically operated machines have minimized the tedious work in a kitchen. Not only do these machines save time but also provide neat, clean and tasty food for us. Moreover, these machines do not consume much electricity.


What is ammonia gas?

                Ammonia gas is a colourless non-poisonous gas that has pungent smell and strong irritating effect on the eyes, nose, throat and lungs. It is highly soluble in water and a compound of nitrogen and hydrogen. One atom of nitrogen on combining with three atoms of hydrogen forms one molecule of ammonia. This gas can be liquefied by compressing or by cooling at -33°C. At normal temperature and pressure, 700 volumes of ammonia can dissolve in one volume of water. Its solution in water is basic in nature and is known as Ammonium Hydroxide.



               There are several methods of making ammonia gas. In the laboratory, this gas is prepared by heating a mixture of ammonium chloride and lime. On a large scale, this gas is manufactured by causing a chemical reaction between hydrogen and nitrogen. This method is called Haber’s Process. In this process one part of nitrogen and three parts of hydrogen are mixed and compressed to 500°C in the presence of iron which acts as a catalyst. Ammonia can also be obtained by distilling coal into coke and coal gas.



                Ammonia is very useful to us. It is used in the manufacture of many of its compounds and also for making nitric acid which is used to dissolve many dry cells. This is also used in the dyeing and printing industry.



                Ammonium sulphate is an important compound of ammonia. It is made from ammonia and sulphuric acid. It is used as a fertilizer because it provides nitrogen for the soil. Ammonium nitrate is used in fertilizers, explosives and for making nitrous oxide, also called, ‘Laughing Gas’.



               The ammonia used as a household cleaner is a strong solution of gas in water. It is also used as cooling agents in refrigerators.





 


How do you distinguish between force, work, energy and power?


               In a football match it is our common experience to see that when a player kicks a resting ball it moves in a certain direction. Similarly when the goal keeper grabs the ball firmly with his hands it stops moving and remains at rest till it is released again. Do you know why it happens so? In both these actions force is applied on the ball.



               Force is a physical quantity which, when applied to a body tries to displace or displaces it. This quantity is equal to the product of the mass of the body and its acceleration. The unit for measuring force is Newton or Dyne. Force is required to set any body in motion and this force is applied in a particular direction. The force is an external agency capable of changing the rest or motion in an object or a body. When force is applied on a body and it gets displaced, we say that work has been done on the body by force. The amount of work done by the force is equal to the product of the force and the distance covered by it. A work done by force is measured in Joules. In short, force is a vector quantity possessing both magnitude and direction. 



 



 





 



           



   The capacity of doing work is called energy. Everything in the universe has some energy by which it can do some work. We experience energy in many forms such as mechanical energy, heat energy, light energy, electrical energy, magnetic energy, chemical energy, nuclear energy etc. 



 



 



 



 



 



 



             Mechanical energy is of two types: potential and kinetic. Potential energy is due to the position of the body while kinetic energy is due to the motion. One form of energy can be converted into the other form of energy. Winding a watch spring stores potential energy. This stored energy gets converted into kinetic energy when the watch starts running. Although energy can be converted from one form to the other, yet the total quantity remains the same. 



 



 



 



 





 



               Some people confuse between power and energy and think of both as the same. But it is not so. Total energy of a body is equal to the capacity of the work done by the body while power is the rate of doing work by the body. It is equal to the amount of work done in unit time. The system to measure unit of power is called horse power (hp) or watt. Horse power is the British unit of power. 



 



 





 



 



 



 



               One horse power is equal to 735.7 watts. The word ‘watt’ is derived from the International Systems of Unit and named after the British engineer James Watt. 


What is an embossing machine?


               Embossing is the process of producing raised patterns on a surface. This is one of the oldest methods to decorate metals. A technique widely used for making ornaments is in which a thin metal sheet is decorated by beating it on the underside. This type of embossing is usually done either by hand or with a die and a counter die. It is usually called repouses. The materials suitable for embossing are plastics, thin metals, papers and leathers etc.



               Crests, monograms, and addresses may be embossed on paper envelopes from dies set either in a small hand-screw press or in an ordinary letter press.



               For impressing embossed pattern on wallpapers, textiles, copper cylinders are engraved with the desired patterns to be raised.



               In this process the pattern is drawn or inscribed on the face of the die called male die. The surface is then machined away around the pattern so as to leave it raised. The counter-die termed as female die is engraved to match this die, so that when a thin strip of metal is placed between them and the die is forced into the counter-die, the pattern is left impressed or embossed upon this thin strip of metal. Die stamping has been used for many years for manufacturing metal parts. This method is also used in stationaries and letterheads. In this method paper is pressed between the dies and ink is applied to the top surface at the same time. Printers nowadays are using embossing machines for this purpose which produce raised patterns in a very short period of time. Blocked ornamental design on book covers or imitation tooling on letter work for instance, can be beautifully affected by means of powerful embossing presses.



               Small hand-operated embossing machines have become very popular. The letters and numbers are embossed on a strip of soft metal or more commonly used vinyl tape. These are then formed by the hand-operated embossing machine. A wheel is used for pressing which transfers the pattern onto the other strip.



               Modern embossing machines are equipped with latest electronic devices. They are replacing the hand-driven machines gradually. But still, a few traditional users of embossed material, such as ornaments prefer the old technique in making their ornamental designs.



 


What is a Hydrofoil?

               A hydrofoil is an underwater fin which consists of a flat or curved plane surface and is designed to lift a moving water vehicle by the reaction on its surface from the water through which it moves. Hydrofoils are used with ships and motor boats.



               The first hydrofoil was invented by an Italian called Forlanini in 1898. In 1918 a hydrofoil powered by an aircraft engine, gained the world’s water speed record. Hydrofoils were not widely used until the 1950s. After 1950 their use became common in military and commercial ships. By the 1970s hydrofoil craft were in operation in many places and speeds of upto 80 knots were achieved. During 1950s hydrofoils were developed in the United States, Canada and Russia.



               Now the question arises, how does a hydrofoil work?



               We know that water is 775 times heavier than air. And so very small hydrofoil wings can support relatively heavy boats. But since water puts great loads on boats, the hulls are usually built of high strength steel.



               The function of the hydrofoil is to raise the hull from the water so that the resistance caused by friction is reduced. This means the power needed to drive the boat at high speeds is reduced considerably. Another advantage of hydrofoil is that it can travel smoothly even in rough water.



               Hydrofoils are of such a shape that the flow of water over them causes a lift. As the boat’s speed increases it raises out of the water, supported on wing-like struts or foils. The hull lifts farther and farther out of the water until it is clear. Under this condition the only parts then in contact with the water are the hydrofoils and supporting struts and the propeller shaft.



               Hydrofoils are of various designs. While some boats have V-shaped or surface piercing hydrofoils, others have variable angle foils that can be adjusted. The purpose of all these is to lift the boat above water surface so that water friction does not produce any resistance. Hydrofoil boats can travel at a high speed. Nowadays hydrofoils are being used on a large scale in naval ships and commercial boats.



               The largest hydrofoil was launched by the Lockheed ship-building and Constructions Company, Washington, on 28 June, 1965. The 64.6 metre long hydrofoil has a service speed of 92 km per hour.






How does a colour television function?

               A television is an electronic device which produces audio and visual effect simultaneously. It is not only a means of entertainment but also a great source of education. The basic theory of television was developed by the English scientists, Ayrton and Perry in 1806. The idea developed by them was called Electric vision.



               Televisions are of two types: black and white, and colour. Colour television functions quite like a black and white television set but its working is much more complex.



               The colour television has mirrors inside the camera which divide the light into three parts. There are three filters inside the camera, one for each part of the light. One filter allows only red light to pass, another allows only green and the third only blue. Each colour goes to a different camera tube and each tube has a separate glass plate and electron beam. From the three tubes three signals go to the transmitter.



               The colour television transmitter multiplexes three signals into one. To this resultant signal a black and white signal is added. This combined signal is sent to the broadcasting antenna. From here this signal reaches our television.



               In colour sets three electron beams - red, green and blue, scan the screen which when mixed together give full colour picture. The screen of the picture tube is coated with  million tiny dots of phospher, each arranged into a group of three. A phospher is a substance that emits light when an electron beam falls on it. Each of the three phosphers emits three colours - red, blue and green. So the blue phospher emits blue light when the electron beam carrying the blue light signal falls on it, and so on. The colour produced at each group of dots depends on the intensity of the electron beams. To make sure that each beam produces the right colour, the beams pass through holes in a shadow mask behind the screen. These three colours can be produced in different proportions to give all the other colours of the original.



               In a modern television set, all its functions can be regulated by remote control system which includes sound adjustments, colour perfection, channel changing and so on. 




How does a sodium lamp work?


               Sodium lamp is used for street lighting. It is also used in research laboratories as a monochromatic source of light, as it produces bright yellow light which is quite pleasant to the eyes. Do you know how does this lamp work?



               A sodium lamp is operated on alternating current. It consists of a U-shaped glass tube with two electrodes of tungsten spiral coated with barium oxide. The tube is evacuated and neon gas at low pressure of about 10 mm of mercury is filled along with a small quantity of metallic sodium or sodium vapour. This discharge tube is enclosed in an unsilvered vacuum jacket to avoid heat loss. For electric discharge a voltage of 400 volts is applied to the electrodes with the help of a transformer. Initially neon gas gets discharged and red light is produced, due to this sodium atoms get excited and produce yellow light. Because the ionization potential of sodium is higher than neon gas, the lamp produces more of sodium light.



               The working temperature of the lamp is about 250 degree centigrade. If this temperature is not maintained constantly the intensity of emitted light would be considerably varied. The sodium light contains only two wavelengths, viz 5890°A and 5896°A. Sodium lamp is also used for outdoor illumination as the characteristic yellow light is less absorbed by fog and mist than white lamp.



 


How are acoustically sound buildings designed?


               Acoustics is the science of the production, transmission and effect of sound. Cinema halls, lecture halls and auditoriums are designed in such a way that speeches or music programmes can be heard clearly by the audience. While designing such buildings it is always taken into account that no echo is produced. Do you know how the buildings with good acoustical quality are designed? Architectural acoustics is now an integral part of modern architecture.



               While designing such buildings it is kept in mind that the sound of the speaker is neither too loud nor too low so that it is clearly audible to everyone in the hall. Normally some materials such as plaster reflect the sound. Other materials, such as carpets, clothing, draperies and human bodies absorb sound. Thus in an auditorium a perfect balance has to be maintained by placing these things in such a way that the reflection and absorption of the sound is evenly spread.



               Two properties of sound help the builders a great deal in designing the buildings of good acoustical qualities. These properties are echo and reverberation. An echo is a sound that has been reflected from a surface. Substances which reflect sound produce strong echoes. In an auditorium, we hear the sound from two sources: directly from the speaker, and from a surface. It has travelled farther than the direct sound. This means that it reaches our ears after the direct sound. In a properly designed room, the echo and direct sound are heard almost at the same time, thus ensuring that there’s no disturbance and the sound heard is clear and distinct. But in a poorly designed room, the time gap between the two is quite long and as a result sound is not heard clearly.



               A reverberation is defined as a close group of echoes i.e. echoes and re-echoes. Each successive echo is quieter than the previous one. One can minimize echoes and reverberations by building rooms with sound absorbent materials. But then the sound in such a room would have a dead quality. A certain amount of reverberation is also required for good quality of sound. In general, the reverberations should last for 1 to 2.5 seconds. This is called the reverberation time.



               Another difficulty encountered while designing an auditorium is the volume of sound. People sitting at the back of the auditorium should be able to hear as clearly as those in front. For this purpose sometimes sound has to be amplified by loudspeakers. Often this is not a very satisfactory arrangement as loudspeakers do not reproduce sound very accurately.



               In designing good sound quality rooms, we must consider pitch or frequency also. Sounds with different pitches can be reflected from surfaces in different degrees. Resonance also must be avoided. Due to resonance one particular frequency sounds much louder than the others. The frequency of sound waves makes sound high or low. If the high frequencies are loud, we hear shrill sounds, and if the low frequencies are too loud, we hear dull sounds.



               Ancient Greeks were the first people to build their theatres with good sound qualities. They placed their audience on steep hillsides where sound could travel to them directly. These theatres were called amphitheatres. The speaker’s stage was parallel to the first row of seats at the bottom. And thus every member of the audience could see and hear well. The Hollywood Bowl in California is a modern-day amphitheatre. Modern hi-fidelity equipments can reproduce sound with life-like clarity.



 


What are cathode rays?


Cathode rays are streams of electrons emitted from the negatively charged electrode or cathode when an electric discharge takes place in a vaccum tube. They are called cathode rays as they are emitted from the cathode.



To produce the cathode rays, a glass tube fitted with two electrodes at its open end, is used. Electrodes are connected to a D.C. source of high voltage. The electrode which is connected to the positive terminal of the electric source is called anode, and the one connected to the negative terminal is called cathode. The glass tube is connected to a vacuum pump. When the pressure inside the tube falls to about  mm of mercury and the high voltage supply to the electrodes is switched on, a particular type of rays emanate from the cathode. These are the cathode rays which produce fluorescent effect in the tube. These rays move towards anode. Experiments have proved that the properties of these rays do not depend upon the gas present in the tube. The charge on the electrons and their mass remain the same. These rays have some specific properties as follows:




  1. These rays travel in straight lines.

  2. Their direction is always perpendicular to the surface of the cathode.

  3. They possess mechanical energy so exert pressure.

  4. When these rays fall on certain substances, they produce a fluorescent effect.

  5. When these rays hit some substance, the temperature of the material rises.

  6. They can penetrate through thin metallic foils.

  7. They can ionize the gases on which they fall.

  8. The velocity of these rays lies between  to  of that of the velocity of light.

  9. These rays get deflected by the magnetic field.

  10. They are also affected by electric fields.



Cathode rays are very useful to us. When they fall on a metal like platinum or tungsten they produce X-rays. X-rays are very useful in science, industry and medical sciences. Cathode ray tube is also used as an indicator in radar systems in which electric signals can be seen on a fluorescent screen.



 


What is Bernoulli’s Effect?


               Bernoulli’s effect is an important derivation in mechanics and fluid dynamics. It was first described by the Swiss mathematician Daniel Bernoulli. This is also known as Bernoulli’s principle. He published this theory in 1738 applying mathematical calculus to that science.



               According to Bernoulli’s effect in any small volume of space through which a fluid is flowing steadily, the total energy comprising the pressure, gravitational potential and kinetic energy is always constant. In fact, this theory propounds the law of conservation of energy for flowing fluids. It also states, if the velocity of a horizontally flowing liquid or gas increases, its pressure decreases. This effect has many applications in mechanics.



               Bernoulli’s effect has helped a great deal in the development of aerodynamics and applied in the design of Airfoil. An aeroplane wing, seen from the tip, is flat at the bottom and curved at the top. As the wing travels through the air, the air must travel either over or under the wing. Air moving over the wing goes a longer distance so it must travel faster. Because the air moving over the wing is travelling faster, there is less air pressure on the top of the wing. This means that there is more pressure on the bottom of the wing, which pushes the wing upward, causing the aeroplane to stay up in the air. 


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