Electrostatic equilibrium is a state achieved by charged conductors in which an electrical charge between two conductors is at the required distance to optimally reduce the repulsive charges within and below the surfaces of the conductors. A conductor is a material that allows electrons to move freely within it from atom to atom.
When a conductor acquires an excess charge, the charge moves freely about the conductor in a way that reduces the conductor's repulsive forces. One of the main characteristics of a conductor that has achieved electrostatic equilibrium is that the electrical charge anywhere beneath the surface of the conductor or within it is zero.
This is best illustrated by considering the space between two concentric circles in which the inner circle and the outer circle are the charged conductors. For example, if the outer circle is positively charged and the inner circle is negatively charged, then the two conductors reach electrostatic equilibrium only when the space between the circles is just right to balance the negative and positive charges. Thus, if there are any electrical forces remaining within or below the surface of either conductor, they would be attracted to the surface until the space is proper for an equilibrium between the negative and positive charges. This principle was first demonstrated by the British physicist Michael Faraday in the 19th century.