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The Goldman-Hodgkin-Katz (GHK) equations have been widely applied to ion channel studies, simulations, and model developments. However, they are constructed under a constant electric field, causing them to have a low degree of approximation in the prediction of ionic fluxes, electric currents, and membrane potentials. In this paper, the equations are extended from the constant electric field to the nonlinear electric field induced by charges from an ionic solution and an ion channel protein. Furthermore, a novel numerical quadrature scheme is developed to estimate one major parameter, called the extension parameter, of the extended GHK equations in terms of a set of electrostatic potential values. To this end, the extended GHK equations become a bridge between the "macroscopic" ion channel kinetics and the "microscopic" electrostatic potential values across a cell membrane. To generate a set of required electrostatic potential values, a nonlinear finite element iterative scheme for solving a one-dimensional Poisson-Nernst-Planck ion channel model is developed and implemented as a Python software package. This package is then used to do numerical studies on the extended GHK equations, the numerical quadrature scheme, and the nonlinear iterative scheme. Numerical results confirm the importance of considering charge effects in the calculation of ionic fluxes. They also validate the high numerical accuracy of the numerical quadrature scheme, the fast convergence rate of the nonlinear iterative scheme, and the high performance of the software package.