Experiments to show that solids expand on heating.
A metal sphere passes through a ring when both are at room temperature. Heat the ball for about 5 minutes in a bunsen flame and you will find that it will no longer pass through the ring. Leave the ball on the ring. What happens when it cools? It may take quite a time before the ball contracts sufficiently on cooling to pass again through the ring.
A metal rod which just fits in a gauge when cold. The end of the rod also fits in the circular hole in the gauge. Allow the rod to cool and try again.
Now let us consider a further point. Can we prevent a solid expanding and contracting? This is an important point since metals are used so extensively and the difference in temperature between winter and summer will cause slight changes in their dimensions. In machinery, of course, even greater temperature variations may be experienced.
An iron bar is resting between the supports on a strong iron base. At the end Л of the bar is a hole through which a cast iron rod passes. The other end В is threaded and carries a stout nut.
Place the cast iron rod through the hole and bring it tightly up to the support by screwing the nut. Now heat the rod strongly. From time to time tighten the nut: expansion will have caused a slackening. After about 5 - 10 minutes' strong heating, finally screw the nut very tightly and leave the rod to cool. Suddenly the cast iron rod breaks with a loud crack showing that the forces are very great. A similar experiment can be used to show that equally great forces are produced if an attempt is made to stop a metal rod expanding.
Let us summarise what our experiments have shown us:
a) when heated a solid expands;
b) when cooled a solid contracts;
c) the forces set up when expansion or contraction is prevented are very great.
RELATIVE EXPANSION OF DIFFERENT SOLIDS
We shall consider two different metals of the same dimensions heated through the same temperature range. We shall not try to measure actual expansions but to see the effect of their different expansions.
Two metal strips of the same size are welded together. One strip is of brass and the other is of iron. Such an arrangement is known as a compound strip or bimetallic bar.
Heat the strip gently by passing it through a bunsen flame when it is seen to bend. What does this prove?
Of the commoner metals, for a given change of temperature, aluminium expands the most and brass and copper rather less. Iron expands about half as much as aluminium. However, metals expand much more than other solids although cement and concrete have expansions about equal of the expansion of iron. Ordinary glass has an expansion about two-thirds of the expansion of iron but Pyrex glass hardly expands at all when heated.
It is useful to have some metal the length of which hardly varied with changes of temperature. No such pure metal exists and the only possibility is the production of a suitable alloy. Such an alloy which has a very small expansion, can be made from 36 per cent nickel and 64 per cent iron and is known as invar since over normal temperature ranges its length is invariable. It is of great value in the construction of such things as measuring instruments and parts of clocks and watches.
Applications of Expansion of Solids
There are a number of useful applications of the expansion and contraction of metals...
It is obvious that if railway lines were laid as complete lines, the expansion on a hot summer's day would lead to distortion and buckling. Such distortions are avoided by leaving a small gap between successive rails.
about half as much почти вполовину меньше
buckling выгибание, изгиб
bunsen flame пламя бунзенской горелки
cast iron чугун
contract сокращаться, сжиматься
distortion искривление, перекашивание
gauge измерительный прибор
invar инвар, сплав железа и никеля
it is of great value имеет большое значение
nut гайка, муфта
Pyrex glass сорт стекла
temperature range диапазон температуры
tighten сжимать, натягивать
two-thirds две трети
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