title: Fixed flux boundary conditions in FiPy
body: <p>I have a follow-up question for this thread <a href="https://stackoverflow.com/questions/62640821/coupled-non-linear-equations-in-fypi">Coupled non-linear equations in FyPi</a>. I managed to set up the system and get the reasonable results when using the Neumann boundary conditions for all variables (i.e. keeping the constant concentrations of the electrons and holes and the potentials at the boundaries).</p>
  <p><a href="https://i.stack.imgur.com/C23uB.png" rel="nofollow noreferrer">The system of equations</a></p>
  <p>Now, I would like to compare it with the situation when the fluxes of electrons (n) and holes (p) are related by the relations:</p>
  <p><a href="https://i.stack.imgur.com/PQkZ3.png" rel="nofollow noreferrer">Boundary conditions</a></p>
  <p>where <em>n<sub>0</sub></em>(0), <em>p<sub>0</sub></em>(0) are the equilibrium concentrations at the left boundary, similar equations would be for the right-hand side bounary. <em>s</em> is the recombination velocity.</p>
  <p>I'm aware of this description in the documentation (<a href="https://www.ctcms.nist.gov/fipy/documentation/USAGE.html#applying-fixed-flux-boundary-conditions" rel="nofollow noreferrer">https://www.ctcms.nist.gov/fipy/documentation/USAGE.html#applying-fixed-flux-boundary-conditions</a>) but don't know how to apply it.</p>
  <p>Since the boundary conditions are considering the concentrations at the edges, should be the variables <em>n, p, phi</em> defined as FaceVariables? Could you please help me with defining these boundary conditions?</p>