Why does the rate of movement of these cations from this side to this side ultimately decrease to zero? Well as these cations are brought to this side, as they move to this side, that increases the total amount of positive charge on this side of the membrane, because as each one of these cations move this way, it brings with it a full positive charge. And so, as each one of these cations moves this way, as a result of that positive charge build up, they begin to feel an electrostatic repulsive force. 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.