What Is Expansivity In Physics?


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If we heat a 1 metre long iron rod through 1°C (or 1 K), its length increases by 0.000012m. We say that the linear expansivity of iron is 0.000012/°C. Linear expansivities of some solids in per degree Celsius are as follows:

Brass: 0.000019
Invar: 0.000001
Glass (ordinary): 0.000009
Glass (Pyrex): 0.000003

Since metals expand much more than glass, metal caps of glass bottles and jars can be loosened by heating them under hot water.

A thick glass tumbler is liable to crack when hot water is poured into it because glass is poor conductor of heat. When hot water is poured, the interior expands but the exterior remains unaffected and the tumbler cracks. Pyrex tumbler does not crack because Pyrex has low expansivity.

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