In the previous picture, just look at the number of blue-dot particles (ignore the charges). There are more blue dots outside of the cell, so there is a chemical gradient for the blue dots to move inside of the cells. Insert a voltage rod to inject positive charges into the cell. The negative charge of the blue dots will want to enter the cell because they are attracted to the positive charges there. Thus, there are two reasons why these blue dots will quickly enter the cell. But when will the blue dots stop going into the cell They will be attracted to the positive charge, but if they are all inside, the chemical concentration gradient makes them want to diffuse out of the cell. When the electrical and chemical gradient is equally powerful (in opposite directions), that is the Nernst potential: No net gain or loss. Cells with resting membrane potential are at minus 70mV. They are not at their resting K potential. If you open more K channels than just the leak channels, there will be more movement of K out of the cell, and the potential will get closer to minus 94 mV (at which time, the cell will reach equilibrium, and the cell will die; but the body does not let it get that far).