1. The problem statement, all variables and given/known data This is from Prob. 2.25 Two point charges with separation d, P is a point at a distance z above the mid-point of the charges. The last sentence asked if one of the positive charges is changed to a negative one, what is the potential at P? What field does it suggest? Explain the discrepancy. 2. Relevant equations E = -grad V 3. The attempt at a solution V=0 obviously E = -grad V = 0 What is the reason behind that gives this result?