It's a relativistic equation as well. The charge continuity equation is Lorentz invariant, and more, is covariant without connections on a curved spacetime.In a relativistic theory charge density and spatial current density form a four vector current density j_\mu. In the non relativistic limit they are related by the continuity equation.
You talked about a 'system'. Is it a circuit? Is it a component? What are the dimensions? Are there changes invloved that are fast enough that all of Maxwell's equations are needed, so that B is a factor? J is current density. Current density of what? You do need to be more specific.Well it is very simple. I want to compute capacitance C. Now from my simulation software i can only get the following outputs, which are
electric potential V
Total flux density B
Total Field Intensity H
Current Density J
So I just want an equation, which has these variables only to calculate Capacitance (and some constants)