什么是 The quasi-stationary approximations？

A system of size $L$ driven at frequency $f$ is quasi-stationary when $L \ll \lambda = c/f$: the time $L/c$ the fields need to cross it is negligible compared with the period, and all its points "see" the sources at the same instant. Two limits are used:

• the magnetic quasi-stationary regime (circuits with currents, coils, transformers, induction): the displacement current is dropped from Ampère’s law, $\operatorname{\vect{curl}}\vect B = \mu_0\vect j$ (so $\operatorname{div}\vect j = 0$ and currents flow in closed circuits), while Faraday’s law is kept in full;
• the electric quasi-stationary regime (a charging capacitor): Faraday’s term is dropped, $\vect E$ is the electrostatic field of the instantaneous charges, while the displacement current is kept to find $\vect B$.