Question

why electric wires on poles kept loose

Answer 1

Electrical wires are made up of copper which is a metal, metals conduct heat and electricity. The main reason they conduct heat and electricity is because they have additional electrons , when heat is applied it provides energy to these electrons, this additional energy makes them move out of their original shell thus expanding the atom structure. This overall expansion of the shells makes the entire wire expand to a certain extent. Similarly the exact opposite is the case when metal is cooled ie at night, the atoms receive less or no energy from the sun this makes them go back to a more stable state and thus contracting to it's initial or even to smaller size depending on the surrounding temperature. Because of this phenomena the wire undergoes expansion and contraction every single day, thus if the wires were tightly held in the long run the wires might get snapped because of the contraction it faced. 
Similar is the case in railway tracks where they place small gap between two joints of the track to allow it to expand during daytime. 
Hope this clears it.

Answer 2

Answer:Electric wires are kept loose to prevent them from snapping in winter.

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