Telegraph & electrical lines
Telephone and electric wires are fixed between two poles in such a way as
to allow for contraction in winter and expansion in summer.
Gasoline drips from a freshly filled tank on a hot day because both
gasoline and the steel tank expand in summer, but gasoline expands much
more than steel, and so it may overflow. The amount of gasoline left in the tank when the gauge hits "empty" is less
in the summer than in the winter.
Thermal expansion in solids
Thermal expansion means "increase in size on heating". Various substances
expand (increase in size) when their temperature is raised and contract
(decrease in size) when their temperature is lowered. In general, thermal
expansion is increase in the volume of a material as its temperature is
increased. 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 volume
and is called as volume expansion or cubical expansion. However, if we heat
a solid, which is in the form of a sheet (its thickness can be neglected as
compared to its surface area), then the increase in area is called
superficial expansion. Similarly, if we heat a solid, which is in form of a
wire (its cross–section area is too small compared to its length and hence
can be neglected), then the increase in length is called linear expansion.
A linear expansion coefficient is usually employed in describing the
expansion of a solid, while a volume expansion coefficient is more useful
for a liquid or a gas. If a crystalline solid is isometric (has the same
structural configuration throughout), the expansion will be uniform in all
dimensions of the crystal. If it is not isometric, there may be different
expansion coefficients for different crystallographic directions.
In a solid or liquid, there is a dynamic balance between the cohesive
forces holding the atoms or molecules together and the conditions created
by temperature; higher temperatures imply greater distance between atoms.
Different materials have different bonding forces and therefore different
Materials generally change their size when subjected to a temperature
change while the pressure is held constant. In the special case of solid
materials, the pressure does not appreciably affect the size of an object,
and so, for solids, it's usually not necessary to specify that the pressure
be held constant.