Electric field

To answer this question qualitatively, we can draw diagrams of the electric field lines both cases. And we know that

"Electric field strength" is proportional to "density of electric field lines"

For the case of two oppositely charged parallel plates, the density of electric field lines between the plates are the same everywhere. So the electric field is uniform

For the case of two oppositely charged spheres, the closer to the inner sphere, the greater is the density of electric field lines. So the electric field is not the same everywhere. It is not uniform.

2007-03-19 21:34:57 補充：
Although sometimes"The longer the line, the lower the electric field strength"may be true, it is generally not. We should consider the density of electric field lines.

How to calculate electric field strength?
E = V/d ?
d is uniform along parallel plates, p.d. applied to the two plates is the same along them, so E is uniform.
Although p.d. applied to oppositely charged sphere is the same along the surfaces of each sphere, the distance d between one position of a sphere to another is different from another position, then E is not uniform along the surfaces.

You can draw shortest lines joining from positively charged sphere to negatively charged, you will see lines of different lengths are formed. The longer the line, the lower the electric field strength.