# boundary condition in COMSOL

I deal with PDE module in COMSOL and I would like to simulate Poisson's equation. Actually I need to simulate the distribution of the electric filed created by the spherical probe in the material, hence I need to consider at least two different domains: air and material. Air and material have different properties, so that I need to create a two seperate PDE modules for air and material respectively. I faced with a problem that I don't know how to specify continuity condition on the boundary between the air and material. Comsol allows me to specify only one boundary condition, however I need to specify two:$$\phi_{air}=\phi_{mat}$$ $$\epsilon_1\frac{\partial \phi_{air}}{\partial z}=\epsilon_2\frac{\partial \phi_{mat}}{\partial z}$$I know how to specify one condition but not two. Could anyone suggest me how I can specify both those conditions on the boundary in COMSOL?

I don't think you are restricted to only one boundary condition. You can specify boundary conditions through constraint, thereby you can set as many as you need.

On the other hand, your problem seems to fit AC/DC module in COMSOL. Why not try it?

• I do not quite understand, how exactly can I specify two boundary condition through the one constraint. Could you please provide some example or explain in more details. – Stepan Loginov Jul 9 '14 at 19:03
• It should be possible to add more than one constraint. – Pu Zhang Jul 9 '14 at 23:10
• I created 2 constraints and added one of them to the boundary. When I apply second constraint the first one becomes overridden. So it seems like it's not possible. – Stepan Loginov Jul 10 '14 at 20:34
• Your first condition doesn't seem necessary. The potential in two subdomains is the same unknown function to solve for, right? Then the mesh point on the boundary is shared by both subdomains, and the potential should be automatically continuous. – Pu Zhang Jul 11 '14 at 1:35