Question

Why do telephone wires sag in summers?

Answer 1

Telephone wires are made up of metals and as we know that metals tend to expand in hot temperatures and shrink in cold temperatures. The wires are usually made up of copper or aluminum. The wires expand due to thermal expansion, while in winters they contract and become taut.

Complete step by step solution:
Since metals expand with a rise in temperature, therefore telephone wires sag in summers. The wires are intentionally left a bit loose otherwise in winters they would contract and break. Thermal expansion is the tendency of materials to expand when exposed to heat. Some of the metals with high thermal expansion coefficient are aluminum and stainless steel. The telephone wires are made up of copper or aluminum which has a very high thermal expansion coefficient, therefore they expand in summers i.e. when they are subjected to heat.
Linear thermal expansion of a material is
${\alpha _L} = \dfrac{1}{L}\dfrac{{dL}}{{dT}}$,
where ${\alpha _L}$is the thermal expansion,
${\text{L}}$ is the length,
${\text{dL}}$ is the change in length and
${\text{dT}}$ is the change in temperature.

Additional Information:
When the temperature increases the frequency and the molecular motion inside the wires increase which causes more energetic collisions and therefore the molecules in the wire move further apart which causes the wire to expand. Heating of a substance causes three types of expansion, namely linear expansion, area expansion, and volume expansion.

Note: Usually metals have a higher thermal expansion coefficient. Different materials have different thermal expansion coefficients and they react differently when exposed to heat. Nowadays wires are coated with insulators so that they do not expand much in summers.