Potential and Electric Field

1. Oct 13, 2008

E&M

1. The problem statement, all variables and given/known data
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= -$$\nabla$$V 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?

2. Relevant equations
V = K $$\sum q_{}i/r_{}i$$

3. 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 = -$$\nabla$$V since, V is 0, but E won't be 0. I am confused =\$.

2. Oct 13, 2008

gabbagabbahey

Someone else asked pretty much the same question here