Question

Why must telephone or power line necessarily slag a little?(XI Physics)

Answer 1

because in summers they expand.
If the lines were fully tight,they wud have been broken due to thermal expansion.
Heat is also produced to flow of current.

Answer 2

Answer:

there is space for the wire to contract ,hence tighten up a bit. If the wires would have been kept stretched tightly between two poles ,the wires would surely break dur to contraction. Therefore,to prevent such a bizarresituation telephone or power lines are made to slag a little.

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