Before a neuron is stimulated to pass an electrical impulse down the potassium sodium ion pump, it is resting. In resting, the membrane of the neuron axon is polarized, having a slightly negative charge in relation to the fluid outside the axon. In order to pass an impulse down the axon, there must be depolarization, where the inside of the axon gradually becomes positive while the outside become negative. Two positive potassium ions enter while three negatively charged sodium ions exit.
The equilibrium potential is the state of the charge of resting ions on either side of the axon (inside and outside). One can calculate the potential and length for a successful impulse transport by knowing the charge and concentration of potassium ions in relation to sodium ions.