Why do power lines sag in summer?
Show answer & explanation
Answer: Heat makes metal expand
Birds sitting on them more — Wrong. Birds don't affect sag noticeably. High temperatures cause metal thermal expansion, lengthening wires which sag more.
Poles shift in hot ground — Wrong. Poles are stable. Wire sag increases because heat causes thermal expansion—metal lengthens at higher temperatures.
Heat makes metal expand ✓ — Correct! Thermal expansion: materials expand when heated. Copper/aluminum power lines expand ~0.002% per °C. Summer heat (40°C vs winter -10°C = 50°C difference) makes wires significantly longer. Longer wire between poles = more sag. Engineers account for this—leave slack in summer to prevent winter contraction from snapping wires. Extreme heat can cause wires to touch trees/ground—fire risk. Physics: ΔL = αL₀ΔT!