A neuron can be represented as an electrical equivalent circuit, which is a mathematical model that represents all of the major electrical properties of a given neuron. An equivalent circuit is made up of batteries, capacitors and resistors.
Basically, the model gives us a relatively intuitive idea of how currents are caused by ionic movement and how that generated nerve cell signaling. The model also gives us a quantitative way of modeling current and ionic movement, in relation to neuronal functioning.
To develop an equivalent circuit, you have to relate a membrane’s physical properties to its electrical properties. So, consider the lipid bilayer. It gives a membrane capacitance, which is the ability to store charge. Another way of understanding capacitance is that it is the ability of an insulator to separate electrical charge on either side of it.
So, the nonconducting bilayer separates the cytoplasm and the extracellular fluid. Both fluids are conductive, so the presence of a layer that separates them acts as a capacitors, which then gives rise to the electrical potential difference across the membrane.
The membrane, though, is a leaky capacitor, because it has ion channels, which endow the membrane with conductance and the ability to generate electromotive force.
(a source of electrical potential is called an electromotive force, and an electromotive force generated by a chemical potential difference is called a battery)