Electrical force and diffusion are two important factors to consider in the study of neurons. Electrical force in a cell Figure 1 arises from the mutual attraction between particles with opposite electrical charges (positive and negative) and the mutual repulsion between particles with the same type of charge (both positive or both negative). Diffusion arises from the statistical tendency of particles to redistribute from regions where they are highly concentrated to regions where the concentration is low (due to thermal energy). The interplay of electric forces and diffusion establish the environment within and outside of a cell and impact the function of the cell greatly. An understanding of the units of measure of the electrical environment are necessary to understand the effects of movement of ions in and out of a cell.
Voltage, otherwise known as electrical potential difference (denoted ∆V and measured in volts, or joules per coulomb) is the potential difference between two points — or the difference in electric potential energy per unit charge between two points. Voltage is equal to the work that would have to be done, per unit charge, against a static electric field to move the charge between two points. A voltage may represent either a source of energy (electromotive force), or it may represent lost or stored energy (potential drop).
The same principle applies to voltage in cell biology. In electrically active tissue, the potential difference between any two points can be measured by inserting an electrode at each point, for example one inside and one outside the cell, and connecting both electrodes to the leads of what is in essence a specialized voltmeter. By convention, the zero potential value is assigned to the outside of the cell, and the sign of the potential difference between the outside and the inside is determined by the potential of the inside relative to the outside zero.