Calculation of electrostatic forces

Dear all,

I have a querry regarding electrostatic force calculations in comsol 3.5(a). According to the manuals and few tutorials on calculation of electrostatic forces in MEMS module, it seems to me that we can only calculate the electrostatic forces on boundaries of the computational domain (i.e. they are boudary variables only). If in case I want to calculate the electrostatic forces inside a particular subdomain (as a body force N/m^3) what should I do???

If any one of u out there has ever tried to calculate this, I would really appreciate his/her cooperation.

Thanks in advance