Category Physics

Why do watches have jewels?

Whenever somebody purchases a wrist watch, he often enquires about the number of jewels in it. The number of jewels is inscribed or printed on the outer cases or the dial of the watch. It is commonly believed and rightly too that the larger the number of jewels, the better and more durable the watch is. Would you like to know what are jewels and why are they fitted into watches?

              A good watch is one which gives correct time and does not easily go out of order. If you open a watch, you will notice that the internal mechanism of a watch is very complicated. It has many big and small parts of different kinds. A watch contains roughly 211 parts. Amongst these parts is a small wheel which is always in motion.

             Along with it, there is a hair-like wire which is called a spring. When we wind the spring, the watch starts ticking. The energy stored in the spring by winding is used for running the watch. In addition to this wheel, there are many other wheels, which are constantly rotating. These wheels move the hour, minute and second hands. The axles of these wheels rest on pivots. When the wheels rotate, friction is generated between the axles and the pivots. Due to this friction, pivots and axles can wear out soon and then the watch will show incorrect time, and finally stop working. To reduce this friction, small pieces of some very hard but smooth materials are used as pivots. These small pieces are called jewels. In general, the materials to be used as jewels are ruby and sapphire. They are precious stones next to diamond in hardness. The axles of the wheels of the watches pivoted on these jewels go on rotating without experiencing much friction. Because of hardness, these jewels do not wear out soon and the watch does not go out of order easily. Hence jewels are used in watches to prolong their life. 

Are the molecules of all substances in motion?

              Every substance is made up of very tiny particles called molecules. A molecule is the smallest part of matter that can exist independently and exhibits all the properties of that substance. For example, if we take a molecule of sugar, it will exhibit all the properties of sugar such as taste, shape, colour etc. Molecules of different substances differ in size. Some are only few billionths of a centimetre in size while some other molecules are thousand times larger. Molecules of gases are small in size. One cubic centimetre of air contains about (25,000, 000, 000, 000, 000, 000) molecules.

             Even though a tiny piece of matter contains a very large number of molecules, there are empty spaces in between them. Molecules of matter are constantly in random motion. As the temperature increases, the motion of the molecules also increases. The hotter the substance, the faster is the motion of the molecules. Molecules of a gas move very fast in comparison to the molecules of liquids and solids. Even the molecules of ice are in motion. Isn’t it surprising then that though molecules of all substances are in motion, substances don’t appear shaky? Why? The reason is that there exists between the molecules a force of attraction which keeps them together. If this force is not present, the molecules of matter will scatter away in all directions.

             When the molecules of a solid are heated, their speed increases and the force of attraction between them decreases. If we continue adding heat, the speed of the molecules would be still faster and the solid will change into the liquid state and, finally, to the gaseous state. 

 

 

Why is gold so costly?

From the very ancient times man has a deep fascination for this shiny yellow coloured metal which symbolizes affluence. The statues and temples of our gods and goddesses are decorated with gold ornaments. Kings and Emperors always gave great importance to gold. Man has always shown a great affinity for it. What is so special about gold that it has always been costly and tends to become costlier?

             The increasing preciousness of a substance depends on many factors. First is its rare availability. Second factor is its utility, third is its beauty and fourth is the least reactivity with air and water. Gold is an element that possesses all the four qualities. It is a rare metal found in very small quantities. Its colour and shine are very attractive. It is not affected by heat, cold, wind and humidity. No wonder, it is a costly metal.

            Gold is a shining metal of yellow colour, found in nature in very small quantity. It is found in both free and combined states. To extract gold from its ores is a costly process. An exception was a small rock of seventy and a half kilogram found in Victoria, Australia which yielded seventy kilograms of pure gold. But such rocks are rare. Extremely small quantity of gold is obtained from gold mines. It is traceable in sea water also, but its extraction from sea is uneconomical. It also occurs as a fine grain is sand and gravel.

 

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How is the rainbow formed?

            A rainbow is an arc of multi-coloured light that appears when the sun shines through the rain.

            How is the rainbow formed? Sunlight which appears to be white is actually composed of seven colours. These colours are – Violet, Indigo, Blue, Green, Yellow, Orange and Red, abbreviated as VIBGYOR, each letter of this word represents one colour. Splitting of sunlight into its constituent colours is known as dispersion and the strip of colours is known as spectrum.

            Rainbow is normally seen after the rains, when the atmosphere is full of small water droplets. These spherical raindrops act like tiny prisms. As the rays of the sun fall on these, they (the rays) get refracted as they enter the raindrops, and then get reflected from the far surface of the drop. They get refracted again as they come out of the raindrops. Each component of the rays of the sun gets refracted at a different angle, thus separating out from the others. That is how we get the separation of colours.

 

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How is dust useful to us?

          It is a general belief that dust is very harmful to us. But this is only one side of the picture. Dust also is very useful to us. Before we go through the uses of dust, it is essential to know what dust is and how it is formed.

          Every solid substance is composed of very small particles. When these small particles of matter are scattered, they become dust particles. For example, if we break a brick or a stone into small pieces, it will turn into small particles of dust. There are different ways by which dust is formed. When solids break, dust is formed. Smoke generated by the burning of coal, wood, petrol etc, also produces dust. Dust particles also come from dead plant and animal matter, sea salt, desert, volcanic ash etc. These particles get mixed with air and are called dust particles. Air carries dust particles from one place to another. The particles of the earth’s surface also fly in the air in the form of dust.

          The biggest utility of dust particles is that they help in the formation of rains. The water-vapour in the clouds condenses on dust particles in the form of water-drops. These drops fall on to the earth as rains. The absence of dust particles can delay the rain. Likewise, mist, fog etc. are also formed due to the presence of dust particles in the atmosphere.

          The dust particles present in the atmosphere scatter sun-rays in all directions. Due to this scattering, there is not complete darkness for one to two hours even after the sun set. The appearance of red colour at the sunrise and the sunset is due to dust particles and water vapour. The beautiful rays of the sun seen in the twilight are also due to these dust particles. Thus we see that dust particles which are regarded as absolutely harmful by people are in reality very useful.

Why does ice float on water?

          It is a matter of common experience that ice floats on water. Howsoever large the size of ice may be it will not sink. Even icebergs which are huge masses of ice keep on floating in the sea. Do you know why ice floats on water?

          The law of floatation of bodies was given by the Greek scientist Archimedes. According to this law, whenever a body is placed in water, it is acted upon by two forces – the weight of the body acting downwards and the buoyant force of water acting upwards. If the weight of the body is equal to or less than the up thrust of water, the body floats on water. In other words, if the weight of the body is equal to or less than the weight of the water displaced by it, it will float on water. On the other hand, if the weight of the body is more than the weight of the water displaced, the body will sink in water. Hence a body floats when its weight is equal to the weight of the water displaced. A piece of wood floats on water because its weight is less than the weight of the water displaced by it. Since the weight of wood is nearly half of the weight of an equal volume of water, half of wood is under water, while the other half is above it. Similarly, the weight of cork is nearly one-fifth of the weight of the water displaced by it, so approximately one-fifth of cork is under water while the rest remains above water. You can understand the floatation of ice also on the basis of this law.

          In general, when a liquid changes to solid, it contracts because its molecules come closer to each other. As a result of this, the volume of the substance decreases or its density increases. Hence a substance becomes heavier in solid state than in the liquid state.

          But water is a peculiar liquid. When it becomes ice, instead of contracting, it expands. The volume of ice becomes more than that of the water. The effect of the increase in the volume of ice is that its density becomes nine-tenth of water, that is, ice becomes lighter than water. So we get about 10 litres of ice made out of 9 litres of water. And as you know, ‘litre’ is a measure of volume. That is why nearly nine-tenth of ice is submerged in water, while only one-tenth is above water. The ice under water (9/10 of the whole) displaces water whose weight equals that of the whole ice. This law of floatation is derived from Archimedes principle. This also explains how the maximum portion of the giant icebergs remains sub-merged in water while only a tip (1/10 part) is above water.

          Bursting of water pipes in cold regions is due to the volume of ice being greater than that of water. As soon as water freezes, the volume is increased which produces strong force due to which pipes burst? In Finland, this property is utilized in breaking rocks. Water is filled in empty spaces available between rocks. When water freezes, it expands and generates enough pressure to cause cracks in the rocks.