So we have a force that is exerted on this ion. Let's call that force E because it's the force due to the presence of this electric field. Now, by the second law of motion, we know that if this is the only force acting on the sodium, then it will move in this direction. The problem is, it's not the only force. We have this barrier, a hydrophobic barrier that will not be able to or that the sodium ion will not be able to pass across. And this is analogous to the following situation.