electrostatic potential induced by charges

Hi,

I am fairly new to comsol, your help would be helpful,

I have an electrostatic model composed of two electrodes and medium( air, plastic), just like an parallel capacitor.

In my 2D model, I built a rectangle with a layer. The upper and lower boundary were assumed as electrodes, and surface charge density was set to the midterm boundary. What I would like to know is the potential difference between the two electrodes when the upper electrode moved toward or away from the midterm boundary . The action was dependent on parametric sweep.The question is I am not sure how to define the two electode boundary properties,

Any help would be appreciated,

Thank you,

Qize