1 2 An energy domain pertains to a system or subsystem in which the energy and forces are all of a particular kind such as electrical, mechanical, acoustical, thermal, and so on. ↑ The five-element scheme can be extended to active devices such as transistors by the use of two-port networks containing dependent generators provided the transistor is operating in a substantially linear region. ↑ Pressure-momentum is a term coined by Busch-Vishniac. This variable is left unnamed by most authors.