So let's this is our cell membrane in a neuron cell. Here's our outside, the cell and the inside of the cell. Now, assuming our cell reaches a rusting electrical potential, we're going to have many more of these sodium ions on the outside than on the inside. Now, what will entropy tell us about this situation? Well, entropy will tell us that our system is not even and we will want to move towards a more even system. In other words, some of these guys will want to move inside the cell so that we have the same number of ions on the inside as outside.