Equipotentials surfaces are shown in figure a and b. The field is uniform in
In figure A as we move x0 distance v increase by V0 uniformly.
In figure B as we move r0 distance initially v increased from V0 to ΔV0 later from ΔV0 to qV0
∴ we know E=−dvdx
dvdx is constant in figure a
∴ E is uniform in A