why is the blueish-green colour spot found only on the place were positive terminal?

In this case, electrons from the negative terminal of the battery move inside potato as the copper wire inserted in potato undergoes electrolysis. This electrolysis is conducted in presence of starch present in potato. This process produces more electrons. When these electrons move from the negative terminal to the positive terminal, they react with the  copper wire inserted in potato to form a bluish-green complex. Hence the colour at the positive terminal.

 

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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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