Articles on Science

Major discoveries – Timeline

Laws of Physics

Articles on Science

Ethane in the Sky

On March nights, we were treated to the glorious spectacle of Hyakutake spreading across the sky. Astronomers from all over the Earth turned to look at the comet. They were well rewarded. The Hubble Space Telescope got the first-ever look from the earth at the icy nucleus that inside a comet. (It is just around 2 kilometers in size.) For the first time, a comet was seen giving off X-rays faint, but because Hyakutake passed only 15 lakh kilometers from the earth. They were caught by an X-ray telescope. Other astronomers have been busy looking the spectrum of the comet. This usually gives a lot of information about the compounds that are in the comet (Jantar Mantar, Sep-Oct ’95). Michael Mumma and Michael Di Santi found methane in the comet, forming almost 1% of the comet’s ice. Until now, methane has only been found in the planets (such as Saturn) and their moons (like Titan). Happy at their success, Mumma and DiSanti then looked even more carefully at the spectral then even more carefully at the spectral lines of the comet. They found the lines of ethane, a compound which has not been found in space before. Ethane is another 1% of the comet’s ice. They are looking at the lines again. Who knows, they might even find something like naphthalene!

Rotation of the Earth

How fast does the Earth rotate? In 24 hours, obviously, for that is how we define our day. That is how fast the surface of the Earth, where we live, rotates. What about other parts of the earth. That seems silly question. Because if, say, from 100 kilometers below the surface, the inside of the earth rotated at a different speed, the friction between that and the outside would split the crust apart. So the whole Earth has to rotate at the same speed. But that question is not silly after all. Have you experimented with a rotating egg? Below the crust of the Earth lies the mantle, and below that lies core. The core of the earth is believed to be iron. It is liquid up to some depth, but the innermost core is solid iron. It must be something worth seeing, huge iron crystal 2400 kilometers in diameter, as massive as the Moon. That gives the Earth the freedom it needs.

The solid core, separated core, separated from the outer hulk of the Earth by the liquid in between, can rotate at a different speed. This was pointed out by two American scientists, Gary Glatzmeier and Paul Roberts. They did some calculation and suggested that the inner core should be rotating slightly faster than 24 hours. Geologist Xiaodong Song and seismologist Paul Richards have found that Glatzmeier and Roberts were absolutely right. The inner core goes around in 1 second less than a day. To put it more dramatically, the inner core that was below the western tip of Africa in the year 1900 is under our feet today. How did they find this out? The seismic waves of earthquake (Jantar Mantar, Nov- Dec ’93) travel right through the Earth. When a tremor takes place in Antarctica and is recorded in Alaska, the seismic wave has traveled through the Earth’s core. Because this wave is going eastwards, it should be carried slightly faster through the core. Another wave, going westwards, for instance when an earthquake in New Zealand is recorded in Norway, will be slower. Checking such earthquake data, Song and Richards found that the westward waves were indeed a fraction of a second slower than the eastward ones, and from that they calculated the speed of the core’s rotation. Several questions arise. As is known to physicists, such a difference in speed gives rise to a current (in this case, of billions of amperes) flowing between the solid inner core and the liquid outer core. Such a current must be causing a magnetic field as well. In this linked to the magnetic field of the Earth?

Life on Mars

Four and a half billion years ago, a rock was formed on Mars by some volcanic process. Half a billion years later, this rock was broken into smaller pieces by a meteorite impact nearby. Some ground water also entered the rock. 16 million years ago, an asteroid hit Mars somewhere near where this rock was. The impact threw pieces of the rock into space. One 2 kilogram piece of rock orbited the Sun until 13,000 years ago, when it came close to the Earth. This piece crashed onto an Antarctic glacier. Over 13,000 years, it reached the Allan Hills region of Antarctica, buried inside the ice. In 1984, this meteorite was discovered and named ALH84001. A large number of people worked out this history of the meteorite that we just narrated.

This year, a team led by David McKay of the American space organization NASA, suggested that there seemed that there seemed to be signs that life may have existed on this rock in some bygone era: The meteorite has some organic molecules, of the same family as naphthalene (which is used in mothballs). When bacteria decay, such compounds are produced. Many meteorites do have such compounds. The meteorite has iron oxide (magnetite) of the sort which some bacteria on Earth secrete. It has iron sulphide, which is produced by some anaerobic bacteria (those that don’t use oxygen). The meteorite has some balls of carbonate material, which may be formed by some material, which may be formed by some living thing. On the other hand, almost all earth bacteria are 100 times larger than this material. The meteorite may contain very small fossils (less than hundred millionth of a millimeter). Nanobacteria are this size.

In 1961, another meteorite was found to have signs of life. But soon these were discovered to be grains of pollen and particles of furnace ash. The signs of life turned out to be from Earth itself. This could be the case for the Antarctic meteorite too. What makes scientist more hopeful is that some of these items mentioned are within cracks, and the cracks could only have been formed before the meteorite came to rest in Antarctica. So maybe, just maybe, the signs of bacterial life that we see are from when the rock was on Mars. In 1976, the Viking spacecraft failed to find any such bacteria on Mars. But maybe they landed in the lifeless part of Mars. Or maybe bacteria were present on Mars millions of years ago, but aren’t there now. Scientists are looking at ALH84001 very, very carefully. And even US President Bill Clinton has promised support for a new NASA spacecraft to Mars.

See me, touch me

There is a popular belief that loss of one sense strengthens others. Scientists have now found some evidence for this. Normally, sight and touch are handled by different areas of the brain- the first in the visual cortex and the second in the sensori motor cortex. A team of scientist led by Norihiro Sadato in Japan found that in blind people, the visual cortex may be taken over to perform other tasks. When blind people used their fingers to scan text written in Braille, scientists measured an increase in blood flow (and hence brain activity) in the visual cortex. When scanning meaningless dots on page, brain activity was restricted to the sensorimotor cortex. On the other hand, for sighted but blindfolded people (trained in Braille), weather the page contained Braille or meaningless bumps, the act of scanning the page with the finger only activated the sensorimotor cortex. This suggests that how pathways in the brain get “wired” during learning is much more complex than previously imagined.

Going Mobile

Telecommunication signals are sent, using different frequencies. Usually a “base” or carrier frequency is chosen, and the message itself “shifts” or modulates the carrier frequency. If many frequencies- a large bandwidth- is available for transmission, several messages can be multiplexed and sent in parallel. we took a look at the older telecom systems: the telegraph and telephones. In the May-Jun ‘96 issues, we examined the working of fancier things like fax and small PABXes. This issue looks recently introduced technology of mobile telecom equipment. Pagers and cellular phones are now available most major cities. These mobile systems attempt to solve one big disadvantage if the telephone system. Your telephone instrument is fixed to a particular location- you cannot move it around with you while you are traveling.

The instrument is connected to the exchange through wires. It is this connection that’ sets your telephone number. The number is used to switch incoming calls to you and to bill you for outgoing calls. What if you to call someone who is not near a telephone? In large hospital or a36.jpg factory, it is sufficient for person to carry a beeper. To contact him, the operator at the switchboard dials the number of the beeper. The beeper then makes a sound or vibrates to alert the person that he is needed. He goes to the nearest internal telephone and calls up the switch board to get the message. This is called paging. Such systems have been in use for long time. Paging systems use frequencies in the Very High Frequency (VHF) range- around 100 Megahertz (MHz or million cycles per second). Since they are very low power there is no interference with systems installed elsewhere and the same frequencies can be reused. Also, since only a few tones (beeps) are sent, the bandwidth needed by these systems is very small (only around 1 kilohertz). However these are local systems which cannot be accessed by people outside the particular building complex- a call can be made only from the switchboard.

Making paper

• Mechanical Pulper

When making paper from wood, the logs are trimmed and their bark removed. The wood is then ground by rotating grindstones in the presence of water to form wood pulp. Fiber quantity can be controlled by varying the water supply. For example, if the water supply is reduced so that more heat is generated, longer fibers are obtained. Mechanical pulping is typically used to produce pulp for newsprint.

• Chemical Pulper

In a chemical pulper logs of wood are sliced and chipped. Chips of the appropriate size are pressure cooked in the presence if either an acid or a base. The size of the chips, the strength of the acid or base and pressure applied, determines the fineness of the pulp.

• Pulp preparation

The pulp is washed to remove impurities, by rotating rapidly in cleaner so that rubbish collects at the bottom. Pure pulp comes out through an outlet at the top. Bleaching agents are added to make the paper extra white. Now the pulp can be pored out into flat sieves and dried. This crude paper can be shipped to different mills.

• At the paper mill

Pulp arrives to a paper mill in the form of sheets. These are disintegrated in water. Loading materials, usually china clay or chalk, are added to the paper to make it less transparent. Sizing agents, like resin, make the paper resident to soaking, so that one can write on it with water-based ink. Dyes and pigments are added to lend colors. The mixer thoroughly mixes everything. A beating machine now takes over. The beating affects the fibers of the pulp. This stage determines the sort of paper that will be produced. The end produce is called stock. The stock comes onto a continuous belt made of wire. Sideways shaking of the belt improves the way the fibers bond. Water is drained out from below the wire mesh. A huge roll rides on the upper surface of this web. By improving s design on this roll, a watermark can be impressed on the web. The web is further squeezed of its water as it goes through a pair of rollers. It then passes over steam-heated iron cylinders where most of the water is removed. If the cylinders are polished, the end product gets a glazed finish. Finally the paper is wound onto reels.

From the time we get up in the morning and reach out for the newspaper at our doorstep to the time in the night when we put aside a book or a magazine and go to sleep, we encounter paper in various forms-school books, bus tickets, paper money, office –files, letters- everything crucially dependent upon the existence of paper. It is so much a part of our life that a world devoid paper unimaginable. The invention of paper is credited to Tsai Lun, a Chinese courtier, who made the first paper about 2000 years ago from old finishing nets and rags. The Chinese made paper by boiling rags and plants, constantly stirring it to make a thick pulp.

A sieve was then dipped into the pulp and removed horizontally with a layer of pulp on it. Excess water drained away through the mesh of the sieve. The layer of pulp was then pressed and dried to obtain paper. In principle, the same process is used today . The word paper comes from the Egyptian papyrus a water reed. It was used as writing material more than 5000 years ago by the Egyptians. Papyrus is different from paper: it is made of players of reed placed across each other (like a mat), pressed and then rags or plants are completely realigned (like mattress). The bonds that form between the cellulose are chemical- hydrogen bonds. For its weight, paper is stronger than steel. The Sumerian civilization (in present-day Iraq) before the Egyptians used clay tablets. In Tamil Nadu, palm leaf manuscripts were in use. Parchment (made of material like tree bark) was used in Europe until the 14th century. Of the copies if the Gutenberg Bible; the first printed book, 180 were printed on paper and 30 on parchment. None of these are boded chemically like paper is. Writing was quite difficult on all the other materials. A hard style had to be used to carve out the letters. The pulpiness of paper means that something quite soft, like a pencil, can be used to write on it. Paper-making spread to the west when Chinese paper makers were captured by the Arabs in the 18th century. It reached Europe in the twelfth century when Jean Montgolfier escaped from the Saracens (in Syria) and returned to France to set up paper mill.

He learned this art while working as a slave in a paper mill. While the basic process, making pulp and drying of that pulp to make paper remains the same, paper-making had seen technological advances. So much so that these new advances were exported back to china from the west- the technology has truly made a complete circuit of the globe! Paper- making, which started as a small scale industry confined to homes, is now almost completely mechanized. The most common type of paper-making machine is the Fourdrinier, named after two brothers who built the machine in 1803. Paper became so popular for writing that there was soon a shortage of rags! Newspapers had already started way back around the year 1600, but now they became widespread.

A more copious source for pulp was looked for. The French naturalist, de Maurer, observed that wasps chew wood and make it into a pulp with their saliva. The pulp is spread out in the shape of a comb and when it dries, a reasonably tough, paper- like nest is formed.

Nowadays most paper is made from wood pulp. Your school notebook, newspapers, magazines, posters, envelops- all are made from wood pulp. Jantar Mantar is printed on glazed newsprint, which is also made from wood pulp. The glazing comes from polishing the paper when it is made into rolls. Wood pulp fibre is longer than that of rags. Rags and plant material continue to be used for making bond paper, currency notes, drawing paper and blotting paper. Before the 19th century, paper was used only for writing.

Do you know when envelops were first made? In 1841. Paper bags came in 1850. Large, machine-made paper boxes appeared in 1894. (Cartons are 20th century invention) Have you wondered whether bread has always been wrapped in paper? The practice dates to 1910. What has made paper-making a huge industry is the realization that by treating the paper differently during the process, very different qualities of paper can be produced. To wrap, say, Amul butter, you require paper which is dense, smooth and greaseproof. This is done by adding wax while making paper. If paraffin is added, the paper becomes a bit stiffer, which is useful for papers cups.

By using a resin instead, paper suitable for wrapping bread is produced. A different resin produced paper for lining bottle caps! During paper-making, a coating can also be introduced on to the paper. By coating one side with abrasive, you get sandpaper. Coating one side with gum is useful for postage stamps, stickers, and so on. By appropriate treatment, the paper can be made resident to tearing, water vapour, gases, oil, grease, insects, rats….It is any wonder that paper is the man- made product which is made in the largest quantities?

K. V. Subrahamanyam Max Planck Institute, Saarbrucken

Two Hundred Years Ago

The Europeans built forts, how their armies were used by local kings to fight their neighbours. The foreigners- especially the English- became more and more powerful. By two hundred years ago, the British began to feel that their trade was paramount, and to protect their trade it would be necessary for them to rule India. What was the Indian trade so important to the English that they decided to rule over a country thousands of kilometers away?

• The industrial revolution
  First the British got rich by selling Indian spices and Indian- made clothes in England. This was the time Industrial Revolution began in Britain. Textile factories were set up in England. For these textile factories, cotton was bought from Indian or American farmers and sold in England. Clothes made in Manchester were sold in Calcutta, Bombay and Madras. Other raw material such as indigo, opium, sugarcane, tea and coffee, were grown in India and taken away to Britain. 
  To take away these raw materials, the British began to build railway lines and roads across India. For these, they needed iron, coal, wood and so on. To mine these minerals and take away wood from the forests, they had to keep officers in different parts of the country. To have control over what was grown here, it was not very convenient to have to go through an Indian prince, however accommodating he might be to the British. The East India Company began to take an interest in ruling India.
  
  
  
• Dissatisfaction
  

  Many Indian rulers realized what was happening and fought the British. Hyder Ali and Tipu Sultan of Mysore, Mahadji Scindia of Gwalior and Nana Phandnavis of Pune were among them. But they were all states and finally lost to the Englishmen.
  
  So the English occupied much of India. Where other kings ruled, an English Resident was placed to watch over their activity. The Britishers had to fight a lot retain their rule. But still things were not peaceful for them. The Indian kings and princes revolted because the British would decide whom to make a king and when to remove him. But the farmers and landowners also revolted because the tax (in the form of crops) taken from them was heavy and very strictly enforced. If the tax were not paid, the land would be auctioned and sold.
  
  The tribals living in the forests of India revolted because the British entered what had until then been their territory. Their forests were being taken away from them for laying railway sleepers. Both Hindus and Muslims were worried that the English would destroy their religion and make everybody into Christians.

• 1857
  

  The biggest opposition to the English was in 1857, when a large portion of north India rose against the foreigners. This revolt was started by Indian troops in the English army, but soon kings, landowners, farmers, tribals, workers all took part in this rebellion.
  
  On the evening of 10 May 1857, in the cantonment of Meerut, sepoys who had fought for the British and defeated the Indian kings were now annoyed with the Englishmen. They were not treated well in the British army. Even their wages often delayed. On top of that, there were rumours that the new bullets issued to them used cow and pig fat.
  
  That very night, the rebel troops marched for Delhi. By morning, they had crossed the Yamuna and reached Delhi. The Mughal emperor Bahadurshah Zafar was imprisoned in Red Fort by British. The sepoys declared him their emperor and convinced him to revoke British rule. “We will again establish the Mughal empire”. That was their cry.
  
  As the news spread, there were riot in Meerut. Huge crowd attacked Englishmen’s house and offices. Many Britishers were killed. In other places too- in the cantonments of Aligarh, Mainpuri, Bulandshahr, Attock, Mathura, the sepoys rebelled.
  
  In spite of this huge success of the revolters, the English slowly brought the situation back under their control. The main reason why this uprising lost to the British was that each region fought the English separately. There was no combined strategy. So the English could deal with the revolters of each region separately. Another reason was the lack of modern weapons with the Indian ruler’s armies. Modern guns and cannon, and ammunition for them, came from abroad. How long could the rebels, with their old guns, their bows and arrows, spears and sword, stand against the British? It took more than a year, but finally the British crushed the revolt.

• The Indian empire
  

  The 1857 rebellion was biggest threat to their strength that the British faced. After defeating it, they changed their policies. In 1858, many Indian zamindars were given their own jagir .They were promised that their property would be protected. Pandits and maulvis were assured that British government would not interface with India’s would be taken into the administration of the country.
  In 1857 the British had seen their empire slipping out of their hands. Now they tried to make sure that by pleasing the important people in India, by coming to an understanding with them, they could continue to rule. Their grip on India was tightened so that they could stay on for next 90 years.

Long distance travel

Imagine that you live on a planet with out an atmosphere. (Perhaps we have colonized the Moon.) You will be using spacesuits when you step out of your airtight buildings. In such a world, there would be no air friction. How could you use this?

Let us say you want to send a package to a friend on the other side of the planet. You would hold it at a certain height and if you gave it is the right speed horizontally you would put it into a circular orbit around the planet at that height. Your friend would then catch it as it came around to where she was living. (You would need to tell her it was coming, for she wouldn’t always be waiting for packages from you.)

The only energy you would spend would be what it cost you to accelerate the package to the right speed. Even this energy could be regained at your friend’s end with an appropriate stopping mechanism. Along the orbit no fuel is needed: the package cruises along like any satellite that orbits the Earth. It is in free fall.

Of course, the package could be your vehicle. This suggests a system of space ways instead of roads. Buildings on the planet would be kept off the space ways along which vehicles in planetary orbits would be whizzing along. Transport would cost next to nothing. The orbits could be as low as you want. In principle, they could be so low that they are almost touching the ground. But of course, to prevent encountering hillocks on the way, they could be a bit higher.

This fizzix fact (fantasy?) tells you something. On Earth, most of the fuel we spend in operating our vehicles goes into combating air friction (assuming the roads are good!) Whenever we decelerate or stop, the kinetic energy of the vehicle is simply dissipated into friction.

Let’s look at the physics in some detail. An object moves in a straight line with a constant velocity if no force acts on it. A force will change its velocity. Even to just change the direction in which an object is moving, without changing its speed, a force must be applied. The gravitational pull of a planet on a satellite is the force which continuously keeps changing its direction of motion, bending its orbit into a circle.

• On a small planet

  An object of mass m moving with a speed v in a circular orbit of radius r is under going an acceleration. The gravitational pull of the planet is GMm/r2, where M is the mass of the planet and G is a constant.
  For a low, hugging –the –ground orbit on an Earth-like planet, this speed is roughly 7.9 km/second, a prohibitively large speed. It is so large that an accelerating ramp for vehicles that are accelerated at one g would be 3000 km long! When an object is dropped on the surface of the earth, it falls with an acceleration of 9.8 m/sec2. This acceleration is referred to as one g. Twice this acceleration would be two g’s.
  
  Inside a vehicle moving at two g’s a person would be pressed against her seat with a force that would be twice her weight. In a vehicle moving at 5 g’s, a Person would be squashed against her seat by a force that is 5 times her weight. A human being accelerated by a few g’s will find it very uncomfortable and will not be able to stand it for long. Very high g’s will be lethal. So this method of transport may be impractical.
  
  On a Moon-like planet though, v is smaller, about 1.7 km/sec. Even this will be too large for human passengers, but might be okay for packages which can withstand large g’s. On smaller bodies like asteroids, we would certainly use this mode as the required velocity will be small enough to reach.
  - K.S Balaji
  The other school

Sounds good to me!

In a bus, in theatre, during your school assembly, and almost everywhere else, people can be seen singing songs or listening to songs. Indeed, singing and listening to songs is a favorite pastime for many of us. So how do we hear these songs? How do the songs come out of the loudspeakers (or speakers, as we sometimes call them)? How do the speakers work? What is a “good quality” speaker? In each of our ears, we have an ear drum. When the ear drum vibrates, our brain interprets it as sound. Why does the ear drum vibrate? Well, when the air pressure close to the ear drum increases, it pushes the ear drum inwards and when the air pressure close to the ear drum decreases; it pulls the ear drum outward. Sound traveling through the air creates these waves of compressed air (higher pressure) and rarefied air (lower pressure). So when these waves reach our ear, our ear drum also moves in and out at the same rate of frequency as these waves, and our brain recognizes the sound.

Speakers use the same principle to generate sound, except that the process is in reverse. A membrane or diaphragm in the speaker vibrates in and out and pushes and pulls the air near it. The pulsating movement of the air then travels outward as sound. indeed it is the same principle that we use when we talk. Our vocal chords vibrate and generate sound. The term “speaker” refers to something a bit more that what we usually associate it with. The typical cone that we see inside a radio or type recorder is actually termed a “driver”. One or more such driver and the enclosure/box they are mounted in together constitute a “speaker”.

Why do we need more than one driver? The answer is simple, sound has a range of frequencies and a single driver may not do good job of reproducing all of the frequencies. So while a single driver can be used, usually two or three drivers are present in each speaker, each of which is optimized to a smaller range of frequencies. For example, high frequency noises, usually from stringed instruments, are better reproduced in small sized drivers. These small sized drivers are called “tweeters”. Their small size allows them to more easily sustain high frequency vibrations. Larger sized drivers may be more sluggish and less effective in sustaining high frequency vibrations. Mid-range frequencies, such as human voices are better reproduced in medium sized drivers which are simply called “mid range” drivers.

Finally, low frequency noises, which also correspond to large wavelengths, such as those from deep sounding drums, are best reproduced using large drivers, called “woofers”.

The box or enclosure the drivers are placed in also plays an important role in making sure that the drivers are held securely in place. Well designed enclosures prevent unnecessary vibrations interfering with the sound from the speakers.

What makes the drivers vibrate? Electricity that is encoded to carry the sound signal arrives at the drivers through wires. At the drivers, the electricity encoded with sound keeps changing the polarity of an electromagnet present at the center of the driver. These changes in polarity occur in a manner consistent with the sound encoded in the electricity. The electromagnet at the center of the driver is surrounded by a permanent magnet. Due to the interaction between the changing polarity of the electromagnet and the permanent magnet, the electromagnet moves back and forth. Attached to the electromagnet is the membrane that makes up the cone of the speaker and hence the cone vibrates as will and sound is produced.

So what is a good quality speaker? One important aspect of the quality of speaker is its ability to accurately reproduce sound. In order to find out if a speaker is of good quality, it is important to have a good source of sound signals. So let us assume that we have a well recorded CD, in other words we are assuming that the CD has an accurate copy of the original music.

So if this CD is played on a CD player, the speaker that makes the music sound as lose to the original recording as possible is the best speaker. In other words clarity of sound reproduction over the range of audible frequencies is the primary measure of the quality of the speaker. You can find this out easily. If you have a CD that has a song which has several instruments (stringed instruments, drums, etc.) and some human voices as well, in better quality speakers you will be able to hear each of the instruments clearly and also hear the voices clearly. In poorer quality speakers, some of the instrument will get hidden in the sounds of the other instruments, the sounds of the drums will tend to reverberate and over all the various sounds will feel mixed up and unclear.

You will still hear the song but it will be a poor quality sound reproduction. Generally speaking, most speakers do a better job o f reproducing mid range and higher frequencies. Even “cheap” single driver speakers will usually do a reasonable job of reproducing human voices and stringed instruments. However, very few speakers do a good job of producing deep sounding drums typical cheap speakers will reverberate while trying to reproduce drums. Sometimes you can hear this clearly as cars drive by with music being played at a high volume – you might notice the speakers jarring when the drums occur. Good speakers will produce a good, clean, single beat each time the drum is hit. Poorer speakers will continue to vibrate and produce additional sounds which are not there in the recording even after the drum beat. So far we have looked at the clarity of sound reproduction.

Over and above the clarity of sound reproduction, different speakers sound different based on the material used for construction of the etc. for example, some speakers may have a tendency to sound slightly shrill, while others may have a more polished sound. In some ways judging speakers can be quite subjective. What seems pleasing to my ears may not seem as pleasant to yours. While it is easy to agree on the clarity of different speakers, variation in tone (shrillness etc.) can be difficult to grade. The tone of some speakers may be better suited for certain types of music as well. So you taste in music and you listening preferences make you the ultimate judge of the speaker that sounds best to you! How speakers work is one aspect of the audio world. But how about “THX”, “surround sound”, “Dolby pro-logic” etc. What are all those? Some other time!

Dr. Prathap Haridoss,
Indian institute of Technology Madras,
Chennai

Sounding good

- Sufal Swaraj
Bundesanstalt fur material forcing and prufung, Berlin, Germany

All of us listen to the some or other type of music. We also hear the news and other programs on radio. Most of us take sound for granted not once we wonder how and where does sound come from. Why do your two hands make sound when you hit them together hard. Or why does the neighbor’s window –glass make the thrashing sound when it breaks when you hit it by your cricket ball.

Let us try to understand sound. Sound is any disturbance that can travel through medium like air or water and then be heard by human ear. But what are these disturbances and why do they travel through a medium? When a body vibrates, the vibration causes a periodic disturbance in the surrounding air or any other medium. These disturbances are pressure waves. It means the vibration leads to periodic pressure changes in the medium.

Suppose you have guitar with you and you pluck one of the strings of it. It does produce sound, how does it do so? The movement of the strings in one direction pushes the molecules of the air, which are just before it. This produces crowding of the molecules in that region .And when the string moves back from its regional position, it leaves behind a space with less number of molecules. Meantime the molecules which ware compressed or crowded together transmit some of there energy to the other molecules around them. The molecules return back to there original place, which had become less dense earlier. In other words, the vibratory motion set up by the guitar string causes alternately in space a crowding together of them molecules of air (a condensation) and an emptying out of the molecules (a rarefaction).

Taken together a condensation and a rarefaction make up a sound wave. This kind of a wave is called longitudinal, because the vibratory motion is forward and backward along the direction that the wave is following. Because such a wave travels by disturbing the molecules, a medium is absolutely essential for sound waves to travel. We know that sound waves cannot travel through vacuum.

How is it that we hear these sound waves? The vibrations or condensation and rarefaction cause sufficient vibrations on our eardrums for our brains to ‘hear it’. Our ears do not hear all the vibrations. Sound is generally audible to the ear if the frequency (number if vibrations per second) of sound waves lies between 20 and 20000 vibrations per second. The range varies from one individual to another. Sound waves with frequencies less than those of audible waves are called subsonic; those with frequencies above the audible range are called ultrasonic.
For us sound is what we can hear. But for scientists, sound is considered to be the waves of vibratory motion, weather or not they are heard by the human ear.

Now one can ask how fast these vibrations travel. Well, the velocity of sound is not a constant. It is different for different media. In the same medium, the velocity of sound is different at different temperature. Sound travels more slowly in liquid than is solids. Since the ability to conduct sound is dependent on the density of the medium, solids are better conductors than liquids; liquids are better conductors than gases. The velocity also varies with temperature. But normally the speed of sound is considered to be 344 meters per second.

Sound waves can be reflected, refracted and absorbed as light waves can be. The reflection of sound waves can result in an echo. Reflection of sound wave back to its source in sufficient strength and with a sufficient time lag to be separately distinguished is called an echo. If a sound wave returns within 1/10 sec, the human ear is incapable of distinguishing it from the original one.

Let us do quick calculation to find out how far the reflecting surface must be to take longer than 1/10 seconds. The velocity of sound is 344 m/s at normal room temperature. Can you confirm that reflecting wall must be more than 16.2 m from the person for an echo to be heard? In this case the sound requires 1/20 sec to reach the reflecting surface and the same time to return.

Echo is very useful, especially if you are Bat! Bats navigate by listening for the echo of their high frequency cry. Scientists have designed many instruments using principle of echo. For example, sonar and depth sounders work by analyzing the echo time lag of sounds waves. Radar sets broadcast certain waves of particular frequency, pick up the portion reflected back by objects, and electronically determine the distance and direction of the objects.

When a surface reflects sound it partially absorbs and partially reflects the energy. As the process is repeated the sound becomes weaker and weaker and eventually ceases. The absorption of sound by objects is also the reason why don’t listen an echo in hall full of people. But you listen to it when it is empty. What is that makes the sound of music pleasant when an expert is playing? If somebody who does not know the instrument plays, why is the sound thought of as ‘noise’? Musical sounds are distinguished from noise in that they are composed of regular, uniform vibrations as in case of the expert playing. Noise is nothing but collection of irregular and disordered vibrations. But nowadays, composers use musical sounds as well as noises!!

One musical tone distinguished from another on the basis of pitch, intensity or loudness, and quality. Pitch describes how high or low a tone is and depends upon the rapidity with which a sounding body vibrates, i.e., upon the frequency of vibrations, the higher the tone. The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration.

A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distances from source increases. Loudness is measured in units called decides. The sound waves given off different vibrating bodies differ in quality. A note from a saxophone, for example, differs from a note of the same pitch and intensity produced by a guitar.

Atoms and the bomb

The atomic tests that have been carried out to Pokharan in Rajasthan have initiated a lot of discussion among both scientists and the general public. What exactly is an atomic explosion and how does it work? Do you know that the very name itself is misleading? What occurs is actually a nuclear reaction. However, when the first atomic tests were conducted by American scientists more than 50 years ago, it was thought that most people would not understand the meaning of the word nuclear. So they called it an atomic bomb, or, more simply, just an atom bomb. Only at the end of the last century was it established that atoms do exist. Ernest Rutherford in England showed that atoms have a small central core (about 100,000 times smaller than the atom itself) called the nucleus. The nucleus is positively charged and surrounded by negatively charged particles called electrons.

Most of the atom is really empty. The electrons orbit around the nucleus, just as the planets orbit the sun. The nuclear is not a blob, but it is made up of protons and neutrons. While the proton is positively charged (equal and opposite to the electron), the neutron has no charge. However, a proton and a neutron are both around 2000 times heavier than an electron. All interactions which cause matter to be formed in large amounts (for example, liquids, solid pieces of metal), are governed by properties of the electrons. They energy due to these electric interactions is not very large: for example, you can melt ice or make steam just by cooling or applying heat in an ordinary kitchen at home. You do not need big laboratories for these processes.

The energies involved in nuclear reactions (involving protons and neutrons) turn out to be millions of times larger than these electronic energies. Almost within two decades of the discovery of the nucleus, it was clear to scientists that atoms had, trapped within their nuclei, enormous sources of energy. The clock was already ticking away towards the first atom bomb.

-Usha Didi

Nuclear binding

To understand this is a bit better, however we must first go into a little more detail about how exactly the protons and neutrons (together called nucleons) are bound together into the nucleus. Let us examine this concept of binding s little more closely.

• The nucleus
  

  Opposite charges do attract, and so it seems possible that the positively charged nucleus does hold together the negatively charged electrons in an atom. But what about the protons themselves? If they are all stuffed together in a tiny space and all carry positive charge, they should repel each other. Even before the atom is formed, then, the nucleus would break up since the electrical force between the like-charged protons would cause them to fly apart. What, then, prevents the nucleus from breaking up? This question was answered when a new force had been identified. Here we can not give you all the details of these truly exciting discoveries. We will merely rapidly sum them up. It was discovered that, apart from the well-known electrical force and gravitation (which allows you to stay on Earth), there are two more forces that have to do with the stability of nuclei in atoms. The first force is called the strong force. It is responsible for holding the nucleus together.
  
  The other force is called the weak force and is responsible for breaking up of nuclei in special kinds of atoms. The weak force is of interest when we talk about some forms of radiation and radioactivity, words which are now familiar through daily use in the field of medicine. It is the strong attractive force between neutrons and protons that overcomes the repulsion due to the proton’s electrical charge and causes the nucleus to remain together. Let us see what that means for a typical atom, say carbon. Carbon has 6 electrons, so it must have 6 protons to balance the electric charge. However if you find the mass of the carbon atom (how would you go about doing such a thing?!), it turns out that it is as heavy as 12 hydrogen atoms, that is, it is as though there are 12 protons in it. The explanation is that there are 6 neutrons in the carbon nucleus with mass very similar to that of protons. The 6 protons in the nucleus provide the positive charge to attract the 6 electrons outside, while the 6 neutrons provide the attractive strong force to prevent the (positively charged) protons inside the nucleus from flying apart due to their mutual repulsive electric force.
  
  Since electrons are very much lighter than protons or neutrons, the mass of atoms is more or less determined by the number of nucleons. If there are A number of nucleons in an atom, it is said to have a mass A in atomic units. As you get to larger (and heavier) nuclei, it is seen that more and more neutrons are required in order to overcome the enormous repulsion between the large numbers of protons in the nucleus. Here, there are more neutrons than protons in heavier nuclei. This simple fact will play major role in the understanding of the working of an atom bomb or a nuclear reactor. Consider a bowl of water standing on a table. The sides of the bowl act as a barrier, preventing the water from falling out. If the bowl is accidentally shaken, the water sometimes spills out. Some energy is given to that bowl in the process of shaking. Part of this energy is transferred to the water inside. The more energetic water can then overcome the barrier of the bowl walls and spill out.
  
  Once it crosses the barrier, it falls out due to gravity (its own weight). We can think of the protons and neutrons (forming nuclear matter) as a kind of liquid, like water, confined within a “bowl” which is the nucleus. Just as surface tension holds a drop of water together, the strong nuclear force holds the nucleons (both protons and neutrons) together inside a drop-shaded nucleus. This (very early) model of the nucleus is due to Niels Bohr. Of course, there is no hard edge to the nucleus (analogous to the walls of the bowl). What act as the wall is an energy barrier formed as a result of the forces between the nucleons? This is called the binding energy.
  
  Because of the energy barrier, the nucleons are confirmed within the nucleus. To break through this barrier (resulting in the breaking up of the nucleus itself) needs energy provided by shaking to spill the water in the bowl. The smaller the binding energy, the easier it is to break up the nucleus- the less stable the nucleus is. It turns out that nuclei of different atoms have different degrees of stability. A convenient way of looking at this is to consider the average binding energy B of each nucleon in the nucleus. We see from the figure that B is almost constant for most nuclei, being about 8 Me V. Typical energies involved in chemical reactions are of the order of a few electron-volts (ev); an Me V is a million times larger. Hence the energies involved in nuclear transmutations are millions of times larger. If we look at the binding energy curve more closely, we see that it peaks around A=60, which corresponds roughly to the Iron nucleus.
  
  The B value gradually decreases as A becomes larger than 60. This is precisely the characteristic that is exploited in nuclear fission which leads to both nuclear power generation as well as the atom bomb.

Nuclear fission and fusion

What exactly is meant by fission? The word is borrowed from biology, where fission means the breaking up of a living cell into two roughly equal parts. Imagine we have a nucleus with 200 nucleons and we can somehow break it into two equal parts. This can be thought of as happening in two steps.

1.The nucleus is broken up into 200 individual nucleons.

2.100 nucleons each are bound together into two nuclei.

The first step involves breaking up of a nucleus and needs energy to occur. How much energy does it need? We take a look at our binding energy figure. It tells us that for A=200, the binding energy per nucleon B is roughly 7 Me V. Hence the total binding energy = B×A = 7× 200 = 1400 Me V. This is the amount of energy we need to supply for this to happen.

The second step involves forming of two nuclei. Each releases energy. Observe that at A= 100, the B released is roughly 8 me V. Hence each nucleus releases 8A= 8×100 = 800 Me V so that two nuclei release 2×800 = 1600 Me V.

So the process, where a nucleus with 200 nucleons breaks up into two nuclei with 100 nucleons each, needs a net amount of energy needed equal to the difference in the above two steps. That is, the net amount of energy needed equals 1400-1600= -200 Me V. The minus sign means that energy is released in the process! This is the source of nuclear energy.

Let us summarize. We need (a lot of) energy to break up a heavy nucleus. A lot more energy is released when medium-heavy nuclei are formed. Hence, when a heavy nucleus breaks up into two medium-heavy nuclei, energy is released in the process. This is how energy is produced in nuclear fission. This energy powers nuclear reactors like those at Kalpakkam, as well as nuclear explosions, like those at Pokhran and Chagai last year. Looking at the binding energy of lighter nuclei, we can make an equally interesting statement. When two light nuclei (such as hydrogen) fuse together to form a slightly heavier nucleus, again, energy is released.

This is the basis of energy production n fusion reactions. This happens exactly as in the case of fission: at low A values, B increases with A and exactly the same argument as above can be used to show that more energy is released than absorbed. This is how energy is produced in the Sun.

The chain reaction

We have seen that energy is produced when fission of a heavy nucleus takes place. When this energy is produced in controlled amounts, the process results in peaceful energy production in a nuclear reactor. When it is an under controlled process, an atom bomb results.

We know that if there are Z electrons in an atom, then there must be Z protons as well. Then there are A-Z neutrons in the nucleus. There must be at least as many neutrons as protons in a nucleus for providing the strong attractive force. Typically there are more and more neutrons for heavier atoms to account for the increasing repulsion between protons.

Hence, a big nucleus never just breaks up into two equal fragments. Since the lighter nuclei have a smaller percentage of neutrons, a few neutrons are simply freely released as well. These extra neutrons are crucial for continuing the fission process.

The most common element used in nuclear fission is Uranium, which has 235 nucleons in it. We shall denote this by 235U. It is called radioactive, meaning that is decays easily (and is not very stable).

Left to itself, nothing much would happen, just like the bowl of water on the table.

The analogue of shaking the bowl is to hit the Uranium with a neutron. What happen is that Uranium absorbs the neutron. As the neutron becomes part of the nucleus, it loses its blinding energy (of about 7 Me V). The Uranium nucleus acquires this energy. It is said to be in an excited state, with more energy than usual. This energy is enough for it to overcome the fission energy barrier.

The Uranium breaks up into two roughly equal fragments and few neutrons. In doing so, it releases energy, as we have already discussed. More importantly, it also releases neutrons. These neutrons then hit another neighboring Uranium nucleus. This nucleus fragments, releasing more energy and more free neutrons. These neutrons get absorbed by more Uranium nuclei, leading to a cascading effect. Hence energy production occurs by what is called a chain reaction.

A simplified understanding is obtained by considering the nucleus as a liquid droplet. When hit by a neutron, the drop shape is disturbed. The energy released by the absorbed neutron causes the drop to vibrate and stretch. The deformed nucleus is unstable, and ultimately breaks and reforms into two spherical (stable) droplets. Some neutrons are lost and the two fragments are driven apart with considerable force due to the repulsion between their protons.

Producing power from fission

It is clear that, the faster the reactions take place, the faster will be the rate of energy production. If only a small number of interactions are allowed, what results is a controlled energy supply. This is what happens in a nuclear power reactor. Of course, the detailed engineering is extremely complex, but the principle is rather simple. Uranium is used as it occurs in its natural state. This consists of 99.3% 238U and 0.7% 235U. Only 235U undergoes thermal fission when it absorbs a slowly moving neutron. (238U can only fission when hit by fast neutrons). It is more efficient to have slow neutron fission; hence the neutrons that are produced are slowed down by moderators. Typical moderators are graphite rods, which alternate with Uranium rods in a reactor.

The process is started by one random accidental fission occurring. The neutrons coming out of one fission process pass through the graphite and get slowed down. These are absorbed by other 235U’s and the process continues. If too many neutrons are produced, the reaction occurs too quickly, and too much heat is produced; if too few neutrons are produced, the reaction will die out. The arrangement is made so that just about one neutron is produced per generation of interactions so that energy is slowly and steadily produced in the form of heat. This heat is taken away (usually by water in tubes surrounding the radioactive core region) through a heat exchanger and used to drive a turbine produces electricity from heat.

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Source: Portal Content team