The property of a substance which changes uniformly with the uniform change in temperature is known as thermometric property. One of the most well known example is thermal expansion.Thermal expansion has many applications which we use in our daily life, few of them are listed below:
1) Thermostat which is used to maintain temperature.
2) Thermometer which is used to measure temperature.
First of all we must find what thermal expansion is all about and after that we will be able to understand the above list of applications.
Thermal Expansion:
Matter ( Solids, liquuds & gases) expands on heating, this is known as thermal expansion. Though solids, liquids and gases do not expand at the same rate but they all expand for sure. The rate of expansion in gases is greater than liquids and the rate of expansion in liquids is greater than solids.
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To understand this, consider the coloured liquid in a glass flask if we watch very carefully we will see the level of the water go down when we first start heating. This is because the glass gets hot first and so expands and the volume of the beaker therefore increases. This means that the liquid level will drop. Then the liquid starts getting hotter, it expands more than the glass because its rate of expansion is greater than glass, so the liquid level rises.
When a solid is heated, its molecules vibrate more energetically against the action of intermolecular forces, with greater amplitude. Since the average distance among the molecules increases hence the size of the body increases.
Solids have definite volume, so we can subdivide its expansion for our convinience and simplicity. Thermal expansion considered in only one dimension either length, width or height wise is said to be linear expansion. If a solid has length length L1 at any temperature T1, it is expanded due to rise in temperature so that its final length becomes L2 at temperature T2. Its change in length depends upon following factors:
1) Initial length (L1)
2) Change in temperature (ΔT)
3) Nature of material (α)
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The change in length can be formulized as follows:
ΔL = αL1ΔT
Where α is known as co-efficient of linear expansion, its value is different for different materials, for example for aluminium its value is 25 x 10-6K-1. What does this value tells us, basically co-efficient of linear expansion is define as, change in length per unit length per kelvin. This means that if an aluminium rod is 1m and its temperature is changed by 1K, it length will be changed by the value i wrote above i.e 25 x10-6 m or 0.000025 m. This change is nothing and cannot be observed by naked eye. This is the reason we cannot observe lengths expansion in our utensils , fans propellars etc. But in the case when lengths are large enough e.g materials used in buildings,railway tracks, bridges etc. In that case it create inconvience and we have to take care of it, otherwise structure can be d-shaped or even can be collapsed
Let us have some more discussion to understand completely by considering following cases:
Case#1:
Suppose we have two rods of aluminium A and B, rod A is of 1 meter and road B is of 2 meter. Their initial temperatures are same and both are heated upto same final temperature.
Guess which rod will expand more A or B. The answer is rod B, because the initial length of rod B is greater than rod A. So the winner is rod B.
Case#2:
Suppose we have two rods, rod A is made up of aluminium and rod B is made up of copper, both are of same length. The change of temperature in both cases also remain same.
Guess who will win this time, I mean which rod will expand more. The answer is rod A because its co-efficient of linear expansion is greater than that of rod B.
Case#3:
Suppose we have two rods, rod A and road B, both have same initial lengths and they are of same material and same initial temperatures. Rod A is heated more as compared to rod B.
Guess which rod will expand more, the answer is rod A, because the change in temperature in the case of rod Ais greater than rod's B change in temperature.
Similarly if whole volume of any solid, liquid or gas is considered. The change in volume ΔV also depends upon the factor as follows:
1)Initial volume ( V1)
2)Change in temperature (ΔT)
3)Nature of substance (β)
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The change in volume can be formulized as follows:
ΔV = βV1ΔT
Where β is known as co-efficient of volume expansion, it is the property of the material. It is define as change in volume per unit volume per unit change in temperature.In the case of solids β covers all three dimensions so it must be and is equal to three times of co-efficient of linear expansion.
β = 3α
Bi-mettalic thermostat:
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It works on the principle of thermal expansion. It acts as an electrical contact breaker in electrical heating systems.It is made by joining two strips of different metal together. On heating, one of the two strip bends more than the other due to different co-efficient of linear expansions. So the strip bends rightwards. The contact with the screw is broken, the current through the heating coil stops. When temperature falls, the strip contracts and electrical contact is restored. In this temperature can be maintained.
Bi-mettalic Thermometer:
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It is obvious from its name that it is used to measure temperature. In this type of thermometer a bi-mettalic coil shaped strip is used. Its one end is fixed and the other end is attached to a pointer which moves on the scale, when temperature rises, the coil expands and the pointer moves rightward. When temperature falls the coil contracts and the pointer moves leftward. In this way we can measure temperature.These types of thermometers work best at higher temperatures.These kind of thermometer is usually used for ordinary air thermometer, oven thermomete and in automobiles for automatic choke.
That is all for now. Upvotes , comment & Resteem would be appreciated. Have a nice day!