Unit 10 Text a. Heat.

Heat will flow whenever there is a difference in temperature between two bodies or between two parts of the same body. Today anyone knows that heat is a from of transmission of energy.

The transfer of heat from one body to another is known as heat exchange or heat transfer. Heat always flows from a more heated to a less heated body and therefore heat exchange is a natural process. It was our great scientist and poet Lomonosov who first stated that heat phenomena were due to the motion of molecules. The Russian Academician I. B. Rikhman first suggested to carry out the experiments in the field of heat transfer.

There are three principal modes of heat transfer: by conduction, convection and radiation.

Conduction is a process of transference of heat by which the heat is passed from particle to particle throughout the volume of body without any apparent movement occuring in the body. Convection is heat transfer performed by particles of gas, vapour or liquid moving in space. There are two kinds of convection: natural convection and forced convection, when heat transfer can be produced by the use of a fan or other mechanical device for stirring and mixing the warmer and cooler parts of the fluid. Radiation is heat transfer by thermal radiation.

The transfer of heat by the process of radiation does not require the use of material media. In practice heat may be transferred by two or even all three modes simultaneously. For example, in a steam boiler it is important that the heat be transferred from the glowing surface of the burning fuel to the heating surface of the boiler. This process is performed by means of radiation. At the same time this surface receives heat from flue gases by convection while the heat in the wall itself, which forms a heating surface, spreads by conduction.

Выучите слова.

conduction – проводимость

convection – конвекция

to depend on (upon) – зависеть от

flue gases – топочные газы

fuel – топливо

heat – тепло

to heat – нагревать

to transfer – передавать

heat transfer – теплопередача

heat exchange – теплообмен

heating surface – поверхность нагрева

liquid – жидкость

motion – движение

particle – частица

radiation – излучение

steam – boiler – паровой котел

thermal – термический, тепловой

vapour – пар, водяные пары

Text B. Heat Transfer

Heat is a form of energy transferred from one body to another because of a temperature difference. When two bodies at different temperature are brought into contact, the warmer body will be cooled and the colder body will be warmed. In this case heat flows from the hot body to the cold body by conduction. The molecules of the hot body being at a higher temperature have a higher level of kinetic energy than the molecules of the cold body.

This energy is transferred through the molecules of the cold body.

Heat may also be transferred from one body to another through space by means of radiation. The molecules of any substance of high temperature emit waves of energy which travel through the air with the speed of light. They differ from light waves only in their wave lengths. Heat may be transferred from one body to another be means of convection. For instance, heat is transferred by convection from the hot metal of a stove to a stream of moving air the temperature of which is increased. The warm air is then transported to a room in which the moving warm air transfers heat to the objects in the room by conduction.

Text C. Quality of water.

Impurities in water. Water is never found pure in nature. Rainwater falling to the earth contains dust, dissolved' oxygen, carbon dioxide and other gases taken from the air.

At the ground surface it takes up silt and other inorganic matter. The additional, impurities depend upon the composition of the ground over which and through which it flows after falling on earth.

Turbidity. A water is turbid when it contains material in suspension. It is generally caused by micro­scopic organisms, silt or clay. The turbidity of a stream may vary considerably during the year. Turbidity is the greatest during flood periods. Lake waters are clearer than stream waters, and streams in dry weather are clearer than streams in flood periods.

Ground waters are clear because the turbidity has filtered out by slow movement through the soil.

Determination of turbidity is important in the operation of a water treatment plant. Changes in turbidity require adjustment in the treatment process so that clear water may produced.

Colour. Colour is caused by material in solution or colloidal condition. It is generally caused by dyes de­rived from decomposing vegetation, but sometimes it is caused by iron and manganese that is combined with organic matter or by industrial processes.

Tastes and odours. Tastes and odours usually occur together. They are caused by dissolved gases and decomposing organic matter. A satisfactory

drinking water should be pratically tasteless and odourless.

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