Discussion Forum

Hi everyone,

I'm making a model using the Electrostatics Interface from within the limited AC/DC Module included with the base COMSOL Multiphysics package. Initially, I tried making my model using the Electric Currents (ec) Interface, but I was getting the 'wrong' answer (I will probably make a separate post for that problem, it would be interesting to see what you guys think about it). When I use the Electrostatics Interface, the answer I get is much better. The Electric Currents Interface includes the Current Density (J) variable from Ohm's Law, but the Electrostatics Interface does not.

Finding the Current Density (J) is relatively simple since the Electrostatics (es) Interface gives me the electric field (E). From Ohm's Law, I should easily be able to calculate J using: J = (sigma)(E). Where (sigma) is the electrical conductivity of the material (a vector/tensor quantity) and (E) is an electric field tensor.

How do I enter this into COMSOL? Is there a way to define this as a function somewhere? A variable? Or should I add this as another "physics" as some kind of PDE/ODE, even though it is only a matrix operation. Also, It may be hard to simply multiply an expression the sigma by E as sigma is a tensor/vector quantity and is unique to the material being modeled. The model is 2D and has two different materials in it, so the expression must take into account the E-field at that specific point and also the electrical conductivity associated with that point as well.

In case this post is too long for you to read or you don't understand the model after reading, I have attached it. It includes two different materials with assigned material electrical conductivities.

Thanks in advance for any help,

John

UPDATE: After some further investigation, I have found out that the Electrostatics Interface has an equation for x-direction current density as: es.Jx = es.Jdx. Where Jdx is the displacement current density, which happens to be zero for this modlue/model/interface. Investigating the Electric Currents Interface, the equation for equation for x-direction current density is: ec.Jx = ec.Jix + ec.Jdx. Where again, Jdx is displacement current density (again zero) and Jix is (ec.sigmaxx*ec.Ex+ec.sigmaxy*ec.Ey+ec.sigmaxz*ec.Ez) which is basically the Ohm's Law thing I was talking above where certain tensor quantities of of the electrical conductivity (sigma) is multiplied by the Electric Field (Ex, Ey, Ez). Why is there no induced current density in the Electostatics Interface? And when I try to add in the long expression for Jix in into the ex.Jx on the electrostatics I get an error saying that it doesn't know what es.sigmaxx, even though I have entered electrical conductivities for all materials I am studying.