Is potential difference can be positive negative or zero?

Answer 1

That is correct. Since potential difference is a difference, Vb - Va ,then you can have all three possibilities. The potential at a point, say Va, is itself a difference. It is the potential difference between point "a" and a second point "0" where the potential is choosen to be zero. This zero point is completely arbitrary , provided it does not lead to mathematical difficulties. Sometimes it is convienient to choose a zero location for potential and sometimes its not necessary. For example; suppose the potential difference between charged parallel plates a & b is Vb - Va = 10 volts. This would require that Vb > Va , but the actual values of Vb & Va need not be known. Then Va - Vb = - 10 volts. If one wants to choose point "a" as the zero point, then Vb = 10 v. Or if one wants to choose Vb as the zero point then Va = - 10 v. Or if one wants to choose midway between the plates as the zero point then Vb = 5v & Va = - 5v.