'why does the color of copper change on heating but not in electric wires when they pass current?

This is a Chemistry question.

At high temperature, copper reacts with atmospheric oxygen to produce copper(II) oxide.
2Cu(s) + O₂(g) → 2CuO(g)
As copper(II) oxide is a black solid, the copper turns black in the reaction.

When copper conducts electricity, the temperature is not high enough for the above reaction to occur. Therefore, there is no color change when copper conducts electricity.

However, when there is bad contact in the circuit, the badly contacting point builds up would becomes very hot, and the high temperature would cause the above reaction to occur. Under such circumstances, the copper turns black at the badly contacting point.

The colour of copper only changes when copper is heated strongly (e.g. with a Bunsen burner). A chemical reaction happens - the copper gets oxidised, i.e. reacts with oxyggen in the air.

Cuprous oxide can be formed, this is orange, purple or red. If it's hot enough, cupric oxide can also be formed, this is black.

But when normal size currents are passed through copper, the copper does not get hot enough for oxidation to occur.

(A current *could* cause enough heating to produce oxidation, but this would be unusual because it requires thin copper wire and a very large current.)