Question

why the potato turns green when it is attached to the positive terminal of a circuit?

Answer 1

Suppose we want to pass current through potato. So, we connect ends of a battery by copper wire with potato by inserting in it. The electrons from negative potential of battery through copper wire reach in potato where they undergo in electrolysis of part of copper wire inside the potato in the presence of potato juice (starch). This process release electrons in potato and increase the negative particles. When these negative species reach the other end of copper it reacts with part of copper wire inside potato and make a complex of copper which is green in color. 

This is the reason of greenish spots in potato on passing current through it.
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Answer 2

Answer:

The potato turns green when connected to the positive terminal,the electrons from the negative terminal of the battery move inside the potato as the electrolysis occurs at the copper wire inside potato. This electrolysis conducted by the presence of starch in potato which produces more electrons.