Continuity Equation

A continuity equation, also called a transport equation, is an equation that describes the transport of some quantity, and is particularly simple and powerful when applied to a conserved quantity, though it can be generalized to any extensive quantity. Because mass, energy, momentum, electric charge and other natural quantities are conserved under the appropriate conditions, continuity equations of this same basic form describe a wide range of physical phenomena. The equation expresses, in local form, that whatever amount of the quantity leaves a region must equal the decrease of that quantity within the region, aside from any that is created or destroyed there.