But what happens? Over time, the rate of movement begins to decrease until the rate of movement is equal to zero. The question is, why does that actually take place? 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.