Senin, 11 April 2011


Expansion is the general increase in the volume of a material as its temperature is increased. It is usually expressed as a fractional change in length or volume per unit temperature change; a linear expansion coefficient is usually employed in describing the expansion of a solid, while a volume expansion coefficient is more useful for a liquid or a gas. If a crystalline solid is isometric (has the same structural configuration throughout), the expansion will be uniform in all dimensions of the crystal. If it is not isometric, there may be different expansion coefficients for different crystallographic directions, and the crystal will change shape as the temperature changes.
In a solid or liquid, there is a dynamic balance between the cohesive forces holding the atoms or molecules together and the conditions created by temperature; higher temperatures imply greater distance between atoms. Different materials have different bonding forces and therefore different expansion coefficients.

Expansion In Solid
With few exceptions, substances expand when heated, and very large forces may be set up if there is an obstruction to the free movement of the expanding or contracting bodies. If concrete road surfaces were laid down in one continuous piece cracks would appear owing to the expansion and contraction brought about by the difference between summer and winter temperatures. To avoid this, surfaces are laid in small sections, each one being separated from the next by a small gap which is filled in with a compound of pitch. On a hot summer day this material will often be seen to have squeezed out of the joints on account of the expansion. In the older methods of laying railway tracks gaps have to be left between successive lengths of rail to allow for expansion. Even when such gaps have been left the rails may sometimes 'creep' and close up the gaps. If this happens a rise in temperature may lead to buckling of the track.
Free movement at the rail joints is allowed for by making the bolt holes of the plates joining the tracks, slotted. In modern practice, however, railway lines are welded together to form long, continuous lengths. With this method, it is only the last fifty to one hundred metres of any length which show expansion, usually of a few centimetres. This movement is taken up by planning the ends of the rails and overlapping them. The forces set up by expansion in the remainder of the rails are, so to speak, locked up in the metal. To keep these forces to a minimum, it is usual to lay the track at a time when the temperature is midway between the summer and winter averages. This technique has been made possible by the use of concrete sleepers and improved methods of fixing the rails so that the track may withstand the thermal stresses set up in it without buckling. Allowance also has to be made for the expansion of bridges and the roofs of buildings made of steel girders. Various methods are used to overcome the difficulty, a common one being to have one end only of the structure fixed while the other rests on rollers. Free movement is thus permitted in both directions. Over a very long period of years, expansion and contraction causes 'creeping' of the lead on the sloping roofs of buildings. When heated by the sun the lead expands and tends to move down the roof under its own weight. On cooling and contracting, the force of contraction is opposed by gravity and the friction of the lead against the roof. This sets up a strain in the lead gives it a slight permanent stretch. After many years the lead stretches so much it eventually forms folds and may break.
Expansion In Liquid and Gases
Liquids do not have a definite shape. They take the shape of the container. Thus, we can specify a liquid by its volume. Hence, we can speak of volume expansion only for liquids. Expansion of liquids is much greater than that of solids.
A liquid is heated in a container. Heat flows through the container to the liquid. Which means that the container expands first, due to which the level of the liquid falls. When the liquid gets heated, it expands more and beyond its original level. We cannot observe the intermediate state. We can only observe the initial and the final levels. This observed expansion of the liquid is known as the apparent expansion of the liquid.
If we consider the expansion of the container also and measure the total expansion in volume of the liquid, then the expansion is termed as the absolute expansion of the liquid.
The coefficient of apparent expansion is defined as the ratio of apparent increase in volume of the liquid to its original volume for every degree rise in temperature.
img82.gifCoefficient of apparent expansion


If we evaluate the increase in volume of the liquid taking into account the expansion of the vessel also, then we say it is absolute expansion of the liquid. We can show that,
The coefficient of absolute expansion of a liquid = coefficient of apparent expansion + coefficient of cubical expansion of the material of the container.

Gases are said to be perfectly elastic because they have no elastic limit and expand and contract alike under the action of heat. That is to say, every substance when in the gaseous state and not near its point of liquefaction has the same coefficient of expansion, this coefficient being 1/273 of its volume for each degree Centigrade or 1/459.4 of its volume for each degree Fahrenheit.
Since a gas contracts 1/273 part of its volume when its temperature is lowered 1° C, such a rate of contraction would theoretically reduce its volume to zero at a temperature of - 273° C ( - 459.4° F). Since all gases reach their liquefying point before this low temperature is attained, however, no such contraction exists. At the same time, it may be said that if heat is considered as a motion of the molecules of a substance, that motion is to be considered as having ceased when the temperature has reached - 273° C.
This temperature of -273° C (-459.4° F), therefore, is called the absolute zero, and from it all temperatures should properly be reckoned. Whenever a temperature is mentioned as being in degree absolute, either in the Centigrade or the Fahrenheit scale, it is understood to be counted from
The relation between the Centigrade and Fahrenheit thermometers is discussed in Chapter IX (Heat And Expansion. 100. Generation And Movement Of Heat), the absolute zero, and therefore is equal to the observed temperature plus 273 or 459.4 as the case may be.

Application Of Expansion
A)  Water Anomaly
The anomalous properties of water are those where the behavior of liquid water is quite different from what is found with other liquids  Frozen water (ice) also shows anomalies when compared with other solids. Although it is an apparently simple molecule (H2O), it has a highly complex and anomalous character due to its intra-molecular hydrogen bonding (see for example). As a gas, water is one of lightest known, as a liquid it is much denser than expected and as a solid it is much lighter than expected when compared with its liquid form. An interesting history of the study of the anomalies of water has been published

As liquid water is so common-place in our everyday lives, it is often regarded as a ‘typical’ liquid. In reality, water is most atypical as a liquid, behaving as a quite different material at low temperatures to that when it is hot. It has often been stated  that life depends on these anomalous properties of water. In particular, the high cohesion between molecules gives it a high freezing and melting point, such that us and our planet is bathed in liquid water. The large heat capacity, high thermal conductivity and high water content in organisms contribute to thermal regulation and prevent local temperature fluctuations, thus allowing us to more easily control our body temperature. The high latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling. Water is an excellent solvent due to its polarity, high dielectric constant and small size, particularly for polar and ionic compounds and salts. It has unique hydration properties towards biological macromolecules (particularly proteins and nucleic acids) that determine their three-dimensional structures, and hence their functions, in solution. This hydration forms gels that can reversibly undergo the gel-sol phase transitions that underlie many cellular mechanisms. Water ionizes and allows easy proton exchange between molecules, so contributing to the richness of the ionic interactions in biology.

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