Overhead lines are common in both urban and rural areas. Someone raised the question: When erecting wires, the wires between the two poles should sag slightly. What is the principle?
First correct it: the drooping terminology is called sag.
First of all, this sag is specially designed by calculation, instead of the vertical direction, it can't be straightened by gravity. If you want it to be straight, it can only be achieved because it is not for other reasons. . What is the reason? Come and listen to the interpretation of science students.
From a physical point of view: the wire is made of metal material, the metal will shrink when it is cold in winter, and the straight wire will be broken by the contraction force. Metal and wood are affected by thermal expansion and contraction, although the metal has elongation and can be elongated. However, the elongation is lowered at low temperatures. Therefore, the wire will sag slightly to prevent low temperature breakage.
From the point of view of mechanics: for the cable analysis between the two electric towers, the cable can only be pulled, and the pulling direction is along the tangent. If the cable is completely horizontal, the tension at both ends of the cable is horizontal to the left and one horizontal to the right, resulting in vertical The gravity of the direction is not balanced. The tighter the wire, the tighter the wire and the pole are in contact with the tangential line of the place and the smaller the angle between the poles. The tension in the direction of the wire in order to balance the gravity of the wire = mg/sin Î¸, the wire is more easily broken. If the ideal situation can be straightened, Î¸ approaches zero and the pulling force approaches positive infinity.
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