Category Modern Science

Scientists have developed techniques through which gases like nitrogen, oxygen, hydrogen, and helium can be converted into liquids. These techniques involve cooling of the gas to a certain temperature called the critical temperature and then it is compressed to a very high pressure. Due to this cooling and compression the molecules of the gas come closer and the gas gets converted into a liquid. Air is a mixture of nitrogen, oxygen and other gases and can be liquefied by cooling it to about -200°C at normal atmospheric pressure. Under high pressure it can be liquefied at about -141°C.

The technique used for the liquefaction of air is shown above. Through this technique the air from the atmosphere is compressed to a high pressure. This air is then allowed to expand rapidly. As a result, the air gets cooled to a very low temperature. Its heat is lost due to the sudden expansion. This cool air is compressed further by which it gets converted into liquid.

Liquid air is very cool. It is a mixture of liquid oxygen which boils at -183 °C, liquid nitrogen which boils at -196°C and liquid argon which boils at -186 °C. It is bluish in colour and is kept in special vacuum flasks. It is mainly used in research laboratories to produce low temperatures.

Liquid hydrogen boils at -253°C. It is cooler than liquid air. Liquid helium is still cooler. It boils at -269°C. All the liquid gases should be handled with care. If they fall on your skin they may damage the body cells. If a rubber tube is inserted in liquid air it becomes as hard as a wooden stick. 

               Optical microscopes cannot magnify more than about 2500 times because the light rays can not produce a sharp image. The electron microscope is such a powerful instrument which can magnify minute objects by as much as a million times. It is used to study micro-organisms such as viruses, tissues and bacteria. We know that light travels in the form of waves. Similarly, waves are also associated with the moving electrons. These are known as matter waves. Electron microscope was constructed by making use of matter waves associated with electrons. Wavelengths of light waves are longer than that of the waves associated with electrons. Due to this reason an electron microscope has a higher resolving power and greater magnifications as compared to an optical microscope.

               The electron microscope works like an optical microscope with a condenser and objective and eyepiece (projector) lenses. The lenses are powerful magnets or electrodes.

               In an electron microscope a beam of electrons is focussed onto the object. With the help of electromagnetic lenses an enlarged image of the object is produced on a fluorescent screen. This image is photographed on a photographic film or plate. With the help of this photograph the object structure is studied in detail. Most of the big research laboratories make use of electron microscopes.

            Photostat or xerography is a means of copying documents, letters or pages of books without using liquid inks. This could be called ‘dry writing’.

            A photostat machine makes use of static electricity. It relies upon the special properties of an element called selenium. When the light falls on selenium’s surface its electrical resistance drops sharply.

            To copy a page, the operator places it face down, upon a horizontal glass window. The button is pressed on the photocopier and a bright light comes on to light up the page. Its image is projected onto a highly polished selenium-coated cylindrical drum through a lens. The drum is charged with static electricity.

            The place where the light reflected from the white parts of the page falls on the selenium drum, the electrical resistance of the drum drops. Selenium’s charge leaks away to the ground. The light does not reach the drum from the black areas of the page. The drum on these areas retains the electrostatic charge. Now the drum is covered with a special powdered black ink. The ink adheres to the drum where there is still an electrostatic charge and so the image of the document gets formed in powdered ink on the selenium drum.

             Now a sheet of plain white paper is pressed close to the drum. The paper develops a charge opposite to that of the drum by induction. This charge attracts the ink powder from the drum. The ink jumps from the selenium drum to the paper, thus transferring the image to the paper. The paper is heated before it leaves the machine. This melts the ink which sticks permanently to the paper, giving a reproduction of the original document.

            In another type of electrostatic copiers, the image of the page is projected directly on to the paper, which charges its surface. The paper then passes through a bath of toner and the particles cling to the charged parts of the paper to produce the copy.

           Solar energy comes from the Sun to the Earth in the form of light and heat. It can be converted into electricity by solar cells. Today, scientists are engaged in developing new techniques for the benefit of mankind.

           Experiments have shown that when sunlight falls on certain metals like potassium and silicon, electrons are emitted from their surfaces. These electrons are known as photoelectrons and this phenomenon is called photoelectric effect. Photoelectrons so emitted can be used to produce electric current.

           Photoelectric effect has been used for the making of solar cells. A solar cell, in fact, is a device that directly converts solar energy into electric energy. These wafers of silicon element are used for making solar cells. Generally, a four centimetre long and two centimetre broad and 0.14 millimetre thick silicon wafer is used for one solar cell. When sunlight falls on this wafer it gets converted into electric energy. Each solar cell is capable of producing about a half volt of electricity.

           To produce useful quantities of electric current, it is necessary to join together a great number of solar cells. The cells are joined together in big panels and are placed in the sunlight. A panel of about 20,000 solar cells can produce 500 watts of electric power. The electricity produced thus can be used either immediately or can be used to charge electric storage batteries for later use. Materials like cadmium sulphide and gallium arsenide are also being used for making solar cells.

           Solar cells have several different uses. In sunny places, solar panels are used to provide boost power to telephone signals. In space where the supply of solar energy is plenty, solar cells are particularly useful. Spacecrafts, space-laboratories and satellites have a number of solar panels to provide power for their equipments.

          The biggest solar energy furnace of the world is in California, USA. It uses almost 2000 mirrors, which focus the sun rays on to a boiler unit on top of a tower. This produces steam which is used to drive electricity generating turbines. In India, The Bharat Heavy Electricals Ltd. (BHEL) and Central Electronics Ltd. (CEL) are making cells that have found many applications.

A camera is the instrument that will give us picture of a person or a scene called a photograph that can be kept for sweet memory. It has become one of the most important means of communication and expression in modern times. Basically, a camera is a dark box fitted with a lens on one side. Just behind the lens there is an aperture which controls the amount of light admitted. A shutter exposes the film to the light for a short interval. A photosensitive film is mounted on the opposite side of the lens. This film has a layer of silver bromide coated on it.

The object, whose photograph is to be taken, is focused on the film. This is done with the help of another lens called a view–finder. On opening the shutter the light from the object enters the box through the lens. An inverted image of the object is recorded on the photo film.

The exposed film is taken out of the camera and developed by a chemical process. For developing the film a solution of quinol and metal is used. During this process the portion on which more light has fallen becomes darker and the image of the object appears in reverse tones. The developed film is put in the hypo solution for fixing the image. That is how the negative of the object is obtained. In the negative white portions appear dark and black portions appear white.

Now with the help of an enlarger, a print or positive is made from the negative. The light coming through the negative is focused on the photosensitive bromide paper. This paper, after developing and fixing, takes the shape of a positive. That is how we get the photograph of an object.

There are a lot of developments taking place in the field of photography. Nowadays, the techniques of making coloured photographs are being used on a large scale. Instant cameras take photographs in a few seconds. These instant cameras are called Polaroid cameras. Some special cameras produce motion pictures like TV and video cameras, the pictures are made electrically. Holography has given birth to three dimensional photography in which we can see the length, breadth and thickness of the object.

          We make use of dry cells in our torches, transistors and cameras. As soon as we press the switch of the torch the filament of the bulb gets heated up and it begins to glow. Similarly, when we switch on our transistors sound is produced. These devices get their energy through the electric current produced by the dry cells fitted in them. There are three main types of dry cells: carbon-zinc, alkaline and mercury.

          A carbon-zinc dry cell consists of a cylindrical vessel made of zinc. A carbon rod with a brass cap is placed at the centre of this vessel. Zinc acts as the negative electrode and carbon as the positive electrode. A paste of ammonium chloride, zinc chloride, manganese dioxide and carbon is filled in the vessel around the carbon rod. The paste is further surrounded by another paste of plaster of Paris, ammonium chloride and zinc chloride. Plaster of Paris makes the paste hard. The zinc vessel is closed with a layer of pitch. Finally, the vessel is wrapped with a thin cardboard and we have a carbon-zinc dry cell. This cell is a lechlanche type of primary cell in which the electrolyte ammonium chloride is dissociated into positive and negative ions. These are attracted towards the electrodes and produce voltage. When they are connected with a wire, current begins to flow. In this reaction hydrogen gas is also produced and it is converted into water by manganese dioxide. These cells produce electricity till the whole of the manganese dioxide is used up to convert hydrogen into water. Manganese dioxide, after the reaction with hydrogen, gets converted into manganese oxide. After this conversion, no electricity is produced by the cell.

          An alkaline dry cell battery is more powerful and lasts eight times more than a carbon-zinc cell. It also has a carbon electrode and a zinc casing electrode. The electrolyte is a strong alkali solution which has potassium hydroxide. Alkaline dry cells are used mainly for portable radios.

          In a mercury dry cell, the voltage remains constant till the end of the life of the battery. A mercuric oxide electrode is used. The other electrode is the zinc casing. The electrolyte is potassium hydroxide.

          Why these cells are called dry cells? It is simply because its electrolyte is not in a solution form but in a paste form. It would become useless the moment the electrolyte dries up. Dry cells are adversely affected by high temperatures. Hence they should be stored in a cool place, away from direct sunlight. It is better to take out the cells from the torchlight, camera or transistor if they are not being used because moisture leaks out from the cell and it swells up. 

An airconditioner is an electrically operated device used to keep houses, offices, and laboratories cool during summer and warm during winter. It not only controls temperature but also regulates humidity.

Today, airconditioners of all types and sizes are available. The big airconditioner plants are capable of cooling or heating an entire building.

          In general, an airconditioner keeps the temperature between 20°C and 25.5°C and relative humidity around 35-70%. An airconditioner plant consists of a compressor and a cooling liquid like the Freon gas. The cooling liquid evaporates in the cooling coil. This vapour is then carried to the electrically-operated compressor. It then goes to the condenser where it is cooled by air or water as it passes through the radiator. Here the vapour changes to a liquid giving off heat in the process. The compressor thus serves to transfer heat from one place to another. A fan sends fresh air into the room which keeps the temperature of the room to the desired level. The airconditioner has certain substances which remove the moisture from the room. It also has filters to remove the dust particles from the air. This is how an airconditioner controls the temperature and humidity and keeps the air clean. Some airconditioners have attachments so when turned on we get hot air in the winters.

          Many new buildings, factories and homes are now being designed to include air-conditioning. Ships, aeroplanes, cars, offices, restaurants, theatres, shops and space vehicles make use of this steady flow of comfortable, purified air.