So essentially, from physics, we know that because there's a positive charge on the outside and a negative charge on the inside, there will be an electric field that exists across the cell membrane. And so this electric field that points this way from the outside to the inside will create, will exert an electric force that will try to pull on these sodium ions. Let's focus on this sodium ion here. 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.