Resistive heating solver

The resistive heating solver is a simplified version of the eddy current model where only resistive and no inductive effects are computed. The vector potential A is equal to zero all over and only the scalar potential φ is kept. This way, the diffusion of the EM fields is not solved. The current density is proportional to the gradient of the scalar potential, which corresponds to a uniform current. There are no inductive effects since A=0, hence no coupling from a coil to the workpiece. This model is for very slow rising currents in a piece connected to a generator, where the diffusion and inductive effects can be considered as infinitely fast. The joule heating due to the current is still taken into account. Very large timesteps can be used and since A=0 and no BEM is needed, this makes this solver much faster than the full eddy current model.