Diffusion does not continue until an equal number of cations are present on both sides of the membrane. Rather, an average of 8-9 cations ends up in the outside compartment, because we fixed (or more technically, "clamped") the membrane potential in this simulation at -60 mV.
If no voltage were applied to the membrane separating the two compartments, the cations would be free to diffuse from one compartment into the other. An equilibrium would soon be reached, in which equal concentrations of cations are found in both compartments. At equilibrium, a cation has an equal chance of diffusing across the membrane in either direction because the cations have equal electrochemical activities in both compartments, as we saw in an earlier simulation.
Voltage affects the movement of charged particles across the membrane. A voltage difference applied across the membrane makes positively charged cations more likely to cross the membrane when they are moving towards the negatively charged side (remember that opposite charges attract each other). Conversely, cations are less likely to leave the negative inside and cross to the outside.
Thus, when a membrane potential of -60 mV is applied, cations more likely stay in that compartment. If, occasionally, these cations do diffuse outward across the membrane, they are also much more likely to return.In this simulation, and in real cells, this amount of voltage causes a 10-fold concentration difference between the two sides! Voltage dramatically affects ion distributions and movements!
Alter the simulation by changing either the voltage or the side the particles are on. Based on these changes, estimate how the voltage and the differences in cation concentrations are related. We'll see next this relationship plays a major role in the generation of diffusion (or Nernst) potentials.