My Science School

Lakes in cold region freeze in winter. What happens to the fish and other animals living in the lake?

A lake may freeze when it is very cold but fortunately for the fish and other animals living in it, it freezes only at the surface. The water below the surface remains unaffected. This happens because of the odd behaviour of water. Like any substance water also contracts and becomes denser as it cools; but once it cools to 4  its behaviour changes. Below 4  water actually starts becoming lighter. At 0 , water freezes to ice. Ice is lighter than water and it floats on water. The ice layer insulates the water below from the cool air above. This keeps the water below the surface, warm and in liquid state.

 

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My friend had a small magnet and he warned me not to use it to pull things towards it too often as that would exhaust its magnetic powers.

Can a magnet become demagnetized by overuse?

In materials like iron, the atoms themselves are little magnets. In the natural or neutral state, these atoms all point in different directions. In a magnet, the atoms all point in the same direction.

To de-magnetise a magnet, the alignment of the atoms needs to be disturbed in such a way that they are pointing randomly in different direction, as in the neutral metal. This can be done in either of three ways:

(a) by heating the magnet over a gas flame.

(b) by hammering it for a few minutes, or

(c) by dropping the magnet from a height of over 100 ft.

The magnet cannot lose its magnetic properties due to over use.

 

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The electric train gets electricity from an overhead wire. But how does a train running underground obtain electricity? The electric train has a device called a pantograph through which it draws electricity from a live wire above the tracks. In a suburban train 3 or 4 pantographs are used to run electric motors located in various compartments.

But in many underground train systems, the current is not obtained from an overhead wire. Instead, a third rail that runs parallel to the existing two, is laid and charged with electricity. The train picks up electricity from this track. This is done to reduce the height of the tunnel. In India, the Calcutta metro railway is an example of such an arrangement. Abroad, it is used in the London Underground.

 

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My friend says his uncle has invented a machine that once started can run forever without any assistance or additional energy. Are there such machines?

Many scientists have tried to build perpetual motion machines but without success. The force of friction ultimately brings every moving thing to a stop. So far the only machine that comes close to being a perpetual motion machine, is an artificial satellite circling the earth. It circles the planet for years but ultimately friction with dust particles slows it down and it falls into the upper atmosphere where t burns up.

 

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Glass is a bad conductor of heat. If a glass vessel is heated directly over a flame, the heat is not readily conducted to other parts of the vessel and even at the point of contact with the flame, the inside surface does not get as hot as the outside surface.

Consequently the outside surface expands more than the inside surface and the unequal expansions cause the glass to crack.

To avoid this the glass vessel is usually heated by keeping it on a wire gauze made of a good conductor of heat such as iron or copper. This ensures that heat is transferred indirectly and slowly to the glass vessel and the vessel does not crack.

 

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The ink is stored in the barrel of the pen. When we start writing, the ink flows down and collects at the hole near the middle of the nib. When we press the nib against the paper while writing, the slit in the nib opens slightly and behaves as a very narrow tube or capillary. The ink is pulled through the tube by capillary force and flows onto the paper through the tip of the nib.

Why isn’t it possible to write on an extremely smooth surface with a ballpoint pen?

The tip of the ballpoint pen consists of a small ball which projects from a metallic socket. Fast-drying, oil-based ink is supplied to it through a narrow tube. When we write, the ball rotates and transfers the ink onto the paper. If the surface on which we want to write is very smooth, there will not roll efficiently. Consequently the ink will not get transferred to the paper. Sometimes, the ball may be rotating but the ink may not be able to adhere to the surface.

 

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The speed of sound in air varies. At sea level it is about 1220 kmph but at higher altitudes, where air is cooler, it slows down becoming about 1,070 kmph at a height of 11,000 metres. Because the speed of sound changes with height, planes that fly faster than the speed of sound use a Mach number to measure their speed. The Mach number is the aeroplane’s speed divided by the speed of sound at the height the aeroplane is flying.

Mach 2 indicates a speed of twice the speed of sound. At Mach 1 the plane is flying at the speed of sound. Mach 0.5 is half the speed of sound, or subsonic.

Many fighter aircraft are capable of flying at high Mach numbers.

The unit is named after Ernst Mach, an Austrian physicist.

 

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The freezing of water starts at the walls of the tray and at the open surface as these are the areas in contact with a cooler material. As water is a bad conductor, the cooling process of freezing moves from outside to the inside. Water has some air dissolved in it. These air molecules occupy the spaces between the water molecules. When the water starts freezing, i.e. when liquid water changes to solid state, the air molecules are forced out from the inter-molecular spaces. They can now move only to the interior where some liquid still exists. As finally, the whole volume of water freezes, the air molecules get trapped in the middle as tiny bubbles. It is these air bubbles that make the centre of the ice cube opaque. The rest of the cube, being pure ice, is transparent.

 

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Coal is a porous, black carbon-rich, naturally-occurring material which is mined from the earth. It is formed by the decomposition of remains of plants which died millions of years ago. Though rich in carbon, coal contains a lot of impurities including sulpher. When used as a fuel it poisons the air with sulphur dioxide.

Charcoal is a black, porous material obtained by heating wood in a closed space, with no air. This type of charcoal contains mostly carbon. Another type is made by heating bones.

Charcoal is used as a filter to clean gases and liquids and as a fuel. It is also as a drawing instrument.

 

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The blood takes oxygen to each and every part of the body but the oxygen is not in gaseous form. It is dissolved in the blood. If air is injected directly into a vein or artery, say through an injection, the air will not easily get dissolved and will travel in the bloodstream in the form of a bubble. This bubble may finally reach the brain and get lodged in a blood vessel disrupting the flow of blood in the brain. This can lead to serious complications, even death.

It is to avoid the possibility of introducing an air bubble in the bloodstream that the doctor expels a little liquid from syringe. If air is trapped in the needle it will be driven out by the liquid.

 

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After a nuclear explosion has occurred, harmful radioactive material is released into the atmosphere. The debris in due course descends to the surface of the earth and pollutes whatever material it comes in contact with.

This debris, called ‘fallout’ is produced by all nuclear bombs – atom bomb or hydrogen bomb – even if they have been exploded for peaceful purposes. The debris once released into the atmosphere is carried by winds around the world and slowly brought down to the earth either by gravity or by rainfall. The fallout that occurs immediately after an explosion is highly radioactive and will kill living things located near the blast site. The delayed fallout may occur after a few weeks or a number of years. This causes long-term radiation damage to all living things: plants, animals and humans over a vast area. Some of the fallout elements may get into the bodies of living organisms.

The Pokhran explosions were done underground by Indian scientists to keep the fallout to a minimum.

 

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