Due to that positive charge build up, and so that's why the rate begins to decrease. And eventually, when the force due to that concentration gradient is equal to the force due to that electric repulsion, when these two forces are equal because they point in opposite directions, if we sum them up, the net force will be equal to zero. And we know by Newton's second law of motion, if the net force is zero, they will not actually move across that cell membrane. And this moment in time is known as the equilibrium point. So when these cations reach their equilibrium points, there will still be an unequal distribution of these molecules, these ions. And at that moment in time, we can calculate exactly what that voltage potential difference is.