Any number of extra terms may be added, depending on the particular system being considered. Aside from mechanical work, a system may, in addition, perform numerous other types of work. For example, in the infinitesimal expression, the contractile work energy associated with a thermodynamic system that is a contractile fiber that shortens by an amount −d l under a force f would result in a term f d l being added. If a quantity of charge −d e is acquired by a system at an electrical potential Ψ, the electrical work associated with this is −Ψ d e, which would be included in the infinitesimal expression. Other work terms are added on per system requirements.