As a most substance is heated, the the atoms and molecules of which they are made vibrate more violently. This forces them further apart, causing the substance to expand.
All substance expand to a different extent, but in general, solids expand the least, then liquids, and gases expand the most for degree Celsius increase in temperature. Strangely, water contracts as it is heated towards 4 degrees Celsius, expands as it is cooled towards zero degrees Celsius. As it freezes, it expands and becomes less dense, so that solid water - ice - floats. This is unique among liquids and solids.
Uses
o For moderate ranges of temperature many material displaly a linear relationship between volume or length and volume. This is a desirable property for thermometers and means that we can use a linear temperature scale. For a gas, volume is proportional to temperature and this can also be used to measure temperature.
o Bimetallic thermostats, made from strips of metals side by side that expand by different amounts when heated, bending the bar, are used to switch heating on and off, maintaining a steady temperature.
Often, thermal expansion is a nuisance, and engineers must take it into account.
Problems
o Overhead cables have to be slack so that on cold days the contraction does not snap the cable.
o Gaps have to be left in bridge and roads to allow for expansion
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