Answer: It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero.
Step-by-step explanation:
The charge in plasma membrane occurs with diffusion and electric fields.
For example, if there are two solutions, let's assume solution X and Solution Y are separated by a porous barrier (to diffuse in equal proportions).
The fundamental principle of diffusion states that solution from a high concentration diffuses to a lower concentration.
For instance, if the X solution contains 40 sodium ions and 40 chloride ions, the Y solution only has 30 sodium and 30 chloride ions. Due to the diffusion, these two solutions will mix and reach a steady-state.
The like charges repel, and opposite charges attract.