Concept:Overhead wires sag because they expand in length when heated by the sun. This is thermal linear expansion.
Explanation:When a solid is heated, its length increases. This effect is called linear expansion.
For a wire, the change in length is given by
ΔL=αL0ΔT, where
α is the coefficient of linear expansion and
ΔT is the temperature rise.
Overhead electrical wires are exposed to sunlight and become hot.
As the wires expand, they increase in length but the distance between the poles stays the same.
This extra length makes the wires hang loosely, causing them to sag.
Options A, B, and C do not explain the continuous sagging observed in heated overhead wires.
Answer:D. Linear expansion