The sagging of overhead electrical wires is caused by

Analyze the question: Overhead electrical wires (like the ones on NEPA/PHCN poles) are often seen hanging loosely between poles. We need to identify the physical cause of this sagging.

Explain the concept: Most solids, including metals such as copper and aluminium, undergo linear expansion when heated. The increase in length is given by:

ΔL = L₀ α ΔT

where

• L₀ = original length
• α = coefficient of linear expansion
• ΔT = change in temperature

Step-by-step reasoning:

1. During the day, especially in hot weather, the sun heats the wires. Current flowing through them also produces heat.
2. The wires get hotter, so their length increases (linear expansion).
3. The wires are fixed between two poles, so the distance between the supports does not change. The extra length has nowhere to go but to hang down, which produces the sag.
4. This is why engineers deliberately leave wires slightly slack when installing them. It allows room for expansion in hot weather and prevents the wires from contracting and snapping in cold weather (harmattan nights).

Checking the other options:

• A. Radiation of heat: Radiation is just a way heat is transferred. It is not what makes the wire sag.
• B. Rough handling: This is not a scientific cause, and the sag is a general, expected effect.
• C. Gravitational attraction: Gravity does pull the wire downward, and a wire has weight. However, the question asks what causes the sagging as a thermal effect. Gravity acts all the time, even on a tight wire. The change in length due to temperature is what allows the sag to develop or increase. The standard textbook answer for this question is linear expansion.
• D. Linear expansion: Correct, as explained above.

Answer: D. Linear expansion

Tip: When you see "overhead wires," "railway tracks," "bridges," or "gaps in concrete roads" in a question, think thermal expansion. Engineers leave gaps or slack in all of these for the same reason.