CFD Online Discussion Forums - electrostatic solver questions

Hello,

I recently started working with OpenFOAM and as I plan to do some electrostatic simulations. However I'm having some trouble with the electrostatic solver. The first question regards the code of the solver:

what do these two calculations do?

Code:

rhoFlux = -k*mesh.magSf()*fvc::snGrad(phi);



solve

(

    fvm::ddt(rho) + fvm::div(rhoFlux, rho)

);

What exactly is rhoFlux? I have no idea what it could be, especially as it has the strange unit [m^2/s]. k also is a mystery, as it has the unit [1/Tesla].

The second question is about the poisson equation

Code:

solve ( fvm::laplacian(phi) + rho/epsilon0 );

the charge density rho causes an electric potential phi, respectively the electric potential is caused by the charge.
I'm using the charged_wire tutorial and from a technican's view I only know the voltage (=potential) of the wire, but not its charge. However when I set the boundary conditions to "calculated" or to "zeroGradient" I either get an error from the solver or the charge is zero. How can I solve this problem without knowing or calculating the charge in advance?

Thanks for your help!
Lars