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Nothing prevents the positive sphere in the middle from moving towards the wall, unless the entire wall is positively charged.
Even then the sphere in the middle would fly towards the negative sphere embedded in the wall and find some balance point where the repuslive force of the outer wall is equivalent to the force of the negative to positive spheres attraction. Or the sphere would just smash into the negative embedded sphere.
That's assuming a structure like this is even possible to make which it isn't
