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Методические указания и контрольные задания для студентов-заочников Салаватского индустриального колледжа по специальностям 140102 "Теплоснабжение и теплотехническое оборудование" - страница 14



9,8 SUNLIGHT INTO ELECTRICITY

Solar thermal electric generators have already been made. This is a semi-conductor thermal electric battery placed at the focal point of a mirror. The surface of the battery absorbs the reflected sunlight focused on it and warms up. Meanwhile the-other side of the battery is kept cool, for instance, by a stream of cold water. Owing to the difference in temperatures, electricity is generated. The capacity is equal to ten watts; although not very much, it is quite enough to work a desk fan or feed a radio set.

Construction has now begun on a one-kilowatt solar thermal generator. This sort of thing may be employed not only to supply electricity to radio sets, telephone lines, radio stations and like, but also to obtain mechanical energy in order to operate small water-pumping machines, and to provide lighting for buildings as well.

Another way of tackling the problem of the direct transformation of radiant, energy into electricity is to ; use a silicon photo - electric cell based on the principle of the so-called photo-effect. On impinging upon a silicon plate, light induces an electric current. Batteries of this king were installed in the Soviet sputniks and other space craft.

The cost of electricity generated by solar batteries is still comparatively high. One way of cutting down costs is to make the batteries more efficient, and in this respect, our scientists have made the noteworthy progress.


^ Для специальности 150411 «Монтаж и техническая эксплуатация промышленного оборудования»


0,1 METALS

Metals are materials most widely used in industry because of their properties. The study of the production and properties of metals is known as metallurgy.

The separation between the atoms in metals is small, so most metals are dense. The atoms are arranged regularly and can slide over each other. That is why metals are malleable (can be deformed and bent without fracture) and ductile (can be drawn into wire). Metals vary greatly in their properties. For example, lead is soft and can be bent by hand, while iron can only be worked by hammering at red heat.

The regular arrangement of atoms in metals gives them a crystalline structure. Irregular crystals are called grains. The properties of the metals depend on the size, shape, orientation, and composition of these grains. In general, a metal with small grains will be harder and stronger than one with coarse grains.

Heat treatment such as quenching, tempering, or annealing controls the nature of the grains and their size in the metal. Small amounts of other metals (less than 1 per cent) are often added to a pure metal. This is called alloying (легирование) and it changes the grain structure and properties of metals.

The ways of working a metal depend on its properties. Many metals can be melted and cast in moulds, but special conditions are required for metals that react with air.


2,5 STEEL

The most important metal in industry is iron and its alloy — steel. Steel is an alloy of iron and carbon. It is strong and stiff, but corrodes easily through rusting, although stainless and other special steels resist corrosion. The amount of carbon in a steel influences its properties considerably. Steels of low carbon content (mild steels) are quite ductile and are used in the manufacture of sheet iron, wire, and pipes. Medium-carbon steels containing from 0.2 to 0.4 per cent carbon are tougher and stronger and are used as structural steels. Both mild and medium-carbon steels are suitable for forging and welding. High-carbon steels contain from 0.4 to 1.5 per cent carbon, are hard and brittle and are used in cutting tools, surgical instruments, razor blades and springs. Tool steel, also called silver steel, contains about 1 per cent carbon and is strengthened and toughened by quenching and tempering.

The inclusion of other elements affects the properties of the steel. Manganese gives extra strength and toughness. Steel containing 4 per cent silicon is used for transformer cores or electromagnets because it has large grains acting like small magnets. The addition of chromium gives extra strength and corrosion resistance, so we can get rust-proof steels. Heating in the presence of carbon or nitrogen-rich materials is used to form a hard surface on steel (case-hardening). High-speed steels, which are extremely important in machine-tools, contain chromium and tungsten plus smaller amounts of vanadium, molybdenum and other metals.


3,6 METHODS OF STEEL HEAT TREATMENT

Quenching is a heat treatment when metal at a high temperature is rapidly cooled by immersion in water or oil. Quenching makes steel harder and more brittle, with small grains structure.

Tempering is a heat treatment applied to steel and certain alloys. Hardened steel after quenching from a high temperature is too hard and brittle for many applications and is also brittle. Tempering, that is re-heating to an intermediate temperature and cooling slowly, reduces this hardness and brittleness. Tempering temperatures depend on the composition of the steel but are frequently between 100 and 650 °C. Higher temperatures usually give a softer, tougher product. The colour of the oxide film produced on the surface of the heated metal often serves as the indicator of its temperature.

Annealing is a heat treatment in which a material at high temperature is cooled slowly. After cooling the metal again becomes malleable and ductile (capable of being bent many times without cracking).

All these methods of steel heat treatment are used to obtain steels with certain mechanical properties for certain needs.


4,7^ MECHANICAL SYSTEMS

Some kinematic concepts that apply to all mechanical systems are discussed in this paper. A mechanical system is defined as anything that is composed of matter. The first step in an analysis of a mechanical system should be a precise and definitive description of the system under, consideration. Since the modern theories of the constitution of matter will not be considered, the particles that compose a mechanical system are regarded as mathematical abstractions; they are more properly called "material points". The simultaneous positions of all the material points of a mechanical system are called the "configuration" of the system. For example, the displacement vector field of a deformable body defines a configuration of the body. To define the configuration of a mechanical system, we require a coordinate system that is attached to some rigid system, known as a "reference frame". In the theory of kinematics the reference frame is arbitrary.

A general problem of static is to determine the equilibrium configurations of mechanical systems under prescribed types of loadings and to ascertain which among them are stable. An important general problem of dynamics is to express the configuration of a given mechanical system as a function of time.

A mechanical system is said to experience a displacement if any of its material points are displaced. In other words, any change of the configuration of a mechanical system is a displacement.

Usually the material points of a mechanical system cannot be displaced independently. Geometrical restrictions on the displacements of the material points of a system are called "constraints". For example, the constraints in a rigid body are such that any two particles of the body remain at a constant distance from each other. The constraints in an incompressible fluid are such that the volume of any part of the fluid remains constant. The constraints of an ideal cantilever beam are such that the displacement vector vanishes at the clamped end.


9,8^ DEFINITION OF FORCE

The subject of mechanics is concerned with relationships between forces and motions which result from the application of forces. Later chapters in this book will consider the study of motion as such. The present chapter will be concerned only with forces and the analysis of combinations of forces. It is well known to anyone who has experimented with bodies subjected- to several forces that the effect of several forces cannot be obtained by simply adding numerically the magnitudes of the forces, since the directions in which the forces are applied are also significant.

In simple language, a force is a directed push or a pull on an object. We shall restrict our attention in this chapter to forces acting on particles, as previously defined. Occasionally finite sized objects will be considered as particles, but in general we shall restrict our attention to masses which occupy a very small region of space. The forces which we are considering and which we have defined as a push or a pull may result from a direct action by virtue of the contact of the particle in question with another particle or object. Forces may also result from the remote action of one mass on another, such as the attraction of the earth on the mass particle (gravity) or from the actions of electromagnetic forces.

Forces are vector quantities. A vector quantity has both magnitude and direction. In order to specify completely what a force vector is, we must give its numerical magnitude, e. g. the number of pounds, kilograms, or tons that measure the force, and we must also specify, relative to some convenient frame of reference, the direction in which the force is acting, such as 4.2 pounds east or northeast or vertically upward, etc. Since we shall consider the problem of combining the effect of several such forces, the rules of combination of vector quantities must be investigated next.


Для специальности 230106 «Техническое обслуживание средств вычислительной техники и компьютерных сетей»


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