Overhead power lines are quite common in both urban and rural areas. Someone has raised a question: when installing electrical wires, the wires between two poles should sag slightly. What principle is this?

First of all, let me make a correction: the professional term for sagging is called sag.

Firstly, this sag is specially designed through calculation, rather than being unable to straighten due to gravity alone in the vertical direction. As long as one wants it to straighten, it can be achieved, but it is not possible for the wire to straighten due to other reasons. What's the reason for that? Let's listen to the interpretation of science students.

From a physical perspective: Electric wires are made of metal materials, and in winter, the metal will shrink when it is cold, and stretched wires will be pulled apart by the contraction force. Metal and wood are both significantly affected by thermal expansion and contraction, although metals have extensibility and can be stretched. But the elongation will decrease at low temperatures. So the wires will sag slightly to prevent them from breaking at low temperatures.

From a mechanical perspective, when analyzing the cables between two power towers, the cables can only be subjected to tension, and the direction of tension is along the tangent line. If the cables are completely horizontal, the tension at both ends of the cables will be horizontally to the left and to the right, resulting in an imbalance of vertical gravity. The tighter the jump, the smaller the angle between the tangent line at the point where the wire and pole come into contact with each other and the pole. The greater the pulling force along the direction of the wire=mg/sin θ that is generated to balance the gravity of the wire, the easier it is for the wire to break. If the ideal situation can be straightened, θ will approach 0 and the pulling force will approach infinity.

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