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TOPICS ON MECHANICS & ELECTRICITY

Basic topics on mechanics, electricity and magnetism for students .

COMBUSTION

Combustion is the chemical union of a combustible substance with oxygen, resulting in the production of heat. The substances used to produce heat by their combustion in the air are called fuels. They are either carbon or compounds of carbon.

The process of combustion results in the transformation of fuel into carbon dioxide, water, and so on, by reactions with atmospheric oxygen, heat being evolved during the oxidation.

Fuels may be divided into solid, liquid and gaseous. Solid and liquid fuels are frequently transformed into gaseous state for convenience in use. Indeed, the modern tendency is to employ gaseous fuels whenever it is possible because of the cheaper cost of transport and greater flexibility of application.

SOLID FUELS

Solid fuels cover the different kinds of wood, peat, vegetable refuse, coal, charcoal and so on.

Wood has low calorific value. It is still in demand in those parts of the world where its cost is not great. Wood is the raw material for the production of charcoal.

Charcoal, which is not only used for fuel but also in some mechanic, artistic and chemical processes, is obtained by burning firewood.

It is very porous and can absorb big quantities of gases as well as solid and liquid bodies. It is used in refining sugar to remove the brown colouring matter and in filters to purify water.

Coal is a natural product consisting essentially of carbon. It is black or blackish, hard and opaque. It is formed by the decay of vegetable matter and solidified by great pressure. Peat is the first stage in this transformation; then brown coal or lignite which preserves its ligneous structure; then ordinary house coal or bituminous coal, and finally, anthracite which has a large proportion of carbon, but it is not easy to ignite.

Coke, which is a valuable fuel, is the solid left when the volatile parts are distilled from coal. It contains more than 50 % of carbon and it is largely used in the manufacture of iron and other metals, also for cooking and other purposes. It is hard, porous and gray and burns with an intense heat giving no smoke.-

LIQUID FUELS

Most of the liquid fuels are called hydrocarbon oils because they are distilled from petroleum which contains hydrogen and carbon.

Mineral oils are obtained by the distillation of coal or in a natural form in oil fields.

At first petroleum undergoes a general refining process called the separation into groups in which the more volatile parts of it, like gasoline and kerosene, are separated from the heavy oils, such as fuel oil, diesel oil, paraffin, etc.

Gasoline, a volatile distillate of crude petroleum, is a refined inflammable fuel that can be easily atomized to form an explosive mixture. When the heat in the refining retort gets to a temperature about 37° C to 100° C, the product obtained is termed kerosene, formerly called illuminating oil as it was used for lighting. Another name for this substance is paraffin oil.

From 100° C on the heavy oils obtained are known with different names according with their degree of density, viscosity and flashing point.

Gas oil and fuel oil are used in heavy oil engines, while a special oil called Diesel oil is employed in Diesel motors.

Alcohol is made by the distillation of vegetable matter such as corn, Indian corn, potatoes, beetroot, wood and by the fermentation of sugar.

Natalite and Discol are the trade names for two new fuels made of alcohol mixed with other substances. Who knows what scientists will invent or discover in the field of fuels...
Petroleum is a thick, greenish-black, unpleasant-smelling liquid occurring in large underground deposits in different parts of the world.

Petroleum is a complex mixture of hydrocarbons together with small amounts of nitrogen and sulphur compounds. It is extracted by drilling holes or wells through the soil until layers of oil are reached. Sometimes it is under great pressure and springs up from the well as a gusher, but after a time the flow stops and the gusher is changed into an ordinary well from which the fuel must be lifted by pumps. Crude petroleum is refined by distillation which can be done due to the fact that the different constituents of the oil have different boiling points.

The main substances into which natural petroleum is thus separated are: light naphtha, gasoline, heavy naphtha, paraffin oil, lubricating oil, vaseline and paraffin wax. The residue is a black, pitch-like substance known as petroleum pitch used in the making of roads.

GASEOUS FUELS

There are different kinds of gaseous fuels: natural gas, producer gas, coal gas, blast furnace gas, acetylene, etc.

Natural gas, used for domestic purposes and in internal combustion engines, consists largely of methane or marsh gas. It also contains some hydrocarbons such as ethylene.

Coal gas is a popular domestic and industrial fuel made in horizontal or vertical retorts in the absence of air. Its composition varies according to the nature of the coal and the temperature of decomposition. Before it leaves the holder it must be submitted to a process of cooling and cleaning.

Water gas is a mixture of carbon monoxide and hydrogen. It is obtained by passing an air blast through incandescent coal. When the coke is white-hot, a steam blast is sent to the coal by a nozzle. Then the gas is cooled and filtered. The calorific value of this fuel is considerably less than that of coal gas.

Producer gas, an inflammable mixture of carbon monoxide and nitrogen, is produced by passing steam over red-hot coke. It is not much used except in gas retorts, coke ovens, steelworks, etc.

Blast-furnace gas is a by-product of iron-smelting used mainly for steam-raising in factories.

Acetylene, a colourless gas with an unpleasant smell when impure, is obtained by the action of water on calcium carbide and it is used principally for cutting and welding steel. It produces a very hot, luminous, and smoky flame. In special burners which supply it with plenty of air, its flame is extremely bright and non-smoky.

THERMODYNAMICS

Thermodynamics is the study of the general laws governing processes which involve heat changes.

James Prescot Joule was the first who made quantitative experiments as regards heat.

The first law of Thermodynamics is stated as follows: When heat is converted into mechanical energy, or the reverse, the total quantity remains unchanged.

The second law states: It is impossible for any self acting machine unaided by an external agency, to convey heat from one body to another at a higher temperature.

One consequence of the second law is that although the total amount of energy is always the same, less and less of it becomes available to our use, since in all natural processes, energy of other kinds is being converted into heat energy. This heat energy only raises the temperature of the surroundings and cannot then be used again.

The third law states: The absolute zero of temperature can never be attained.

By absolute zero we understand the lowest temperature theoretically possible. It is zero on the absolute temperature scale. On this scale the temperature interval between the ice point and the steam point is defined to be 100 degrees, so that the magnitude of the degrees is the same as on the centigrade scale.

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