Problem with Shadow!

Ouss

I am currently working on a heat transfer problem. In fact, I am trying to model the heat transfer between a laptop and its environment (air, table, walls of the room.....) How do I define the intersections? I mean, since the laptop is put over a table, they share the same "wall". I have wall and wall.shadow . How can I know which part belongs to the Laptop and so define its material : aluminium or wood? I would really appreciate any help.