Quasi-Stationary Current

an alternating current that varies relatively slowly and whose instantaneous values obey the laws of direct currents with reasonable precision (direct proportionality between current and voltage—Ohm’s law, Kirchhoff s law).

Like direct current, a quasi-stationary current has the same current strength in all sections of a nonbranching circuit. However, when making calculations (in contrast to direct-current circuit calculations) it is necessary to take into account the electromotive force induced when the current varies. The inductance, capacitance, and resistance of the branches in a circuit with a quasi-stationary current can be treated as lumped parameters.

Certain conditions must be met for a given alternating current to be regarded as a quasi-stationary current. In the case of sinusoidal alternating current, this means that the geometric dimensions of the electric circuit must be small in comparison to the wavelength of the current under consideration. Commercial-frequency currents can generally be treated as quasi-stationary (a frequency of 50 hertz corresponds to a wavelength of ~ 6, 000 km). The currents in long-distance power-transmission lines, where the above condition is not satisfied, are an exception.