UDS for electric field
 

I want to simulate an electric field for an electric precipitator, therefore I need two equations:

1) the poisson equation I could solve with UDS (fluxfunction -> none)
2) charge conservation equation

How can I create an UDS for charge conservation?, it has this form:

E*gradient*a = -a²/b

uds flux
 

I will solve now charge conservation in the form:

gradient(rho*b*E)=0

E=gradient(phi) is the electric filed vector, which is calculated by other UDS as C_UDSI_G

so I need to create now an UDS flux for charge conservation, where I can include the C_UDSI_G from other UDS

is that possible?