Let us consider a tank that is divided into two partitions by a biological membrane. On the right, we put in water, and on the left, we will put in a solution of potassium chloride (KCl). At this point, what happens depends entirely upon the properties of the biological membrane. For example, let us assume that the membrane acts as a barrier and is impermeable to all ions. Then apart from water movement due to osmosis, nothing would change and this picture would essentially remain the same.

But what happens if the membrane is permeable to both potassium and chloride ions? Both potassium and chloride ions diffuse across the membrane down the concentration gradient until the concentration of all connected chambers are even and equal. This is just like when you drop a droplet of ink in a glass of water, it eventually spreads evenly (assuming that the ink is not particulate and heavier than water, in which case it would settle the bottom).

So far you haven't achieved anything but to demonstrate that ions move down the concentration gradient until the concentration gradient disappears. How is this relevant to generation of electrical signals? Well, let's consider a situation in which the membrane is permeable to potassium ions but not to chloride ions.

This time, we will start with the same concentration of salt on the left, but we'll put a more dilute solution on the right. Let's say for this exercise, the concentration of the salt solution on the left is 10 times that on the right. It doesn't really change the principle, but for later calculations, it's inconvenient to have pure water. Being permeable, potassium ions flow down the concentration gradient across the membrane. However, chloride ions cannot follow because the membrane is impermeable to them.

As the potassium ions move, this creates an electrical imbalance (over-abundance of potassium ions on the right creating a net positive charge and a relative lack of potassium ions on the left creating a net negative charge). As you may remember from basic physics course, like charges repel each other and opposite charges attract each other. So, as the charge imbalance increases, the positively charged potassium ions will find it more and more difficult to enter the right half of the tank which has a net positive charge. At some point, the tendency to move down the concentration gradient will be cancelled by the tendency for positive charges to repel each other. There will then be a steady voltage between the two halves of the tank, defined as the Nernst potential. This value can be calculated from the log of the ratio of the concentrations. In our case (10:1 concentration ratio), then the potential will be 58 mV (at room temperature). In a typical neuron, the inside of the cell contains a much larger concentration of potassium ions than outside. The membrane is permeable to potassium at rest, and potassium leave the cell to create a potential that is negative on the inside compared to the outside. This is called the resting potential.