def METHOD_NAME(self, variables):
    param = self.param
    phi_s_cn = variables["Negative current collector potential [V]"]
    phi_s_cp = variables["Positive current collector potential [V]"]
    i_boundary_cc = variables["Current collector current density [A.m-2]"]
    self.algebraic = {
        phi_s_cn: (param.n.sigma_cc * param.n.L_cc) * pybamm.laplacian(phi_s_cn)
        - pybamm.source(i_boundary_cc, phi_s_cn),
        i_boundary_cc: (param.p.sigma_cc * param.p.L_cc)
        * pybamm.laplacian(phi_s_cp)
        + pybamm.source(i_boundary_cc, phi_s_cp),
    }