Telegraph & electrical lines
Telephone and electric wires are fixed between two poles in such a way asto allow for contraction in winter and expansion in summer. Buckled rail Gasoline drips from a freshly filled tank on a hot day because bothgasoline and the steel tank expand in summer, but gasoline expands muchmore than steel, and so it may overflow. The amount of gasoline left in the tank when the gauge hits "empty" is lessin the summer than in the winter.
Thermal expansion means "increase in size on heating". Various substancesexpand (increase in size) when their temperature is raised and contract(decrease in size) when their temperature is lowered. In general, thermalexpansion is increase in the volume of a material as its temperature isincreased. It is usually expressed as a fractional change in length or area or volume per unit temperature change.
For example, when a metal block is heated, it generally expands in length,breadth and height. This indicates that the metal block expands in volumeand is called as volume expansion or cubical expansion. However, if we heata solid, which is in the form of a sheet (its thickness can be neglected ascompared to its surface area), then the increase in area is calledsuperficial expansion. Similarly, if we heat a solid, which is in form of awire (its cross–section area is too small compared to its length and hencecan be neglected), then the increase in length is called linear expansion.
A linear expansion coefficient is usually employed in describing theexpansion of a solid, while a volume expansion coefficient is more usefulfor a liquid or a gas. If a crystalline solid is isometric (has the samestructural configuration throughout), the expansion will be uniform in alldimensions of the crystal. If it is not isometric, there may be differentexpansion coefficients for different crystallographic directions.
In a solid or liquid, there is a dynamic balance between the cohesiveforces holding the atoms or molecules together and the conditions createdby temperature; higher temperatures imply greater distance between atoms.Different materials have different bonding forces and therefore differentexpansion coefficients.
Materials generally change their size when subjected to a temperaturechange while the pressure is held constant. In the special case of solidmaterials, the pressure does not appreciably affect the size of an object,and so, for solids, it's usually not necessary to specify that the pressurebe held constant.