How Does Changing Charge Affect Potential and Electric Field?

[E&M]

Homework Statement

Find the potential at a distance z above the center of the charge distributions q and -q, which are distance d apart. Also, compute E= -\nablaV and compare your answer. Suppose that we changed the right-hand charge to -q , what then is the potential at P? What field does that suggest?

Homework Equations

V = K \sum q_{}i/r_{}i

The Attempt at a Solution

I calculated V without changing the right side charge to -q and V came out to be 0 but how do I verify E = -\nablaV since, V is 0, but E won't be 0. I am confused =$.

 

[gabbagabbahey]

Someone else asked pretty much the same question here