Thermal Expansion
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