Electrostatics
The Electrostatics Interface solves for the electric potential V using the equation
For most MEMS devices the details of the charge distribution, ρ, inside material are unimportant and the charge usually resides only on surfaces held at fixed potentials. The Electric Potential, Ground, and Terminal boundary conditions are usually used to fix the potential on the surfaces of a MEMS device. At symmetry boundaries the Zero Charge boundary condition applies. To represent infinite regions adjacent to the model a common approach is to use infinite elements. Note that it is possible to model electric circuits associated with a terminal, as described in the Connecting to Electrical Circuits section.
Infinite Elements, Perfectly Matched Layers, and Absorbing Layers in the COMSOL Multiphysics Reference Manual
Electrostatic Forces
The Maxwell stress tensor is a general method for calculating electromagnetic forces and torques. Force and torque calculations using Maxwell’s stress tensor are available in the Electrostatics interface. In electrostatics the force is calculated by integrating
(2-12)
on the surface of the object that the force acts on where E is the electric field, D the electric displacement, and n1 the outward normal from the object.
In practice, electric fields are modeled not only in the air but also inside the solid materials. By entering the electromagnetic force variable name into the domain settings, the force calculations are activated, and the physics interface automatically generates new domain and boundary variables.
The form of the stress tensor in Equation 2-12 assumes no material electrostriction. Support for isotropic electrostriction is included in The Electromechanics Interface.
Capacitance Calculations
The capacitance of a system can be computed using the functionality built into the Terminal boundary condition. This is discussed in more detail in the section Lumped Parameters.