Inductors

Core Sentence / 核心句

An inductor opposes changes in current by producing a self-induced EMF and stores energy in its magnetic field.

中文理解：电感通过 self-induction 反抗电流变化，并把能量储存在 magnetic field 中；它不反抗已经稳定的 DC current。

L = \frac{N Φ _{B}}{I}

E_{L} = - L \frac{d i}{d t}

unit：

1 H = 1 V s/A = 1 Ω s

i_{L} (0^{+}) = i_{L} (0^{-})

long air-core solenoid：

L = μ_{0} \frac{N ^{2} A}{ℓ}

U_{L} = \frac{1}{2} L I^{2}

resistor dissipates energy；ideal inductor stores and can return energy。

uncoupled inductors：

L_{series} = i \sum L_{i}

\frac{1}{L _{parallel}} = i \sum \frac{1}{L _{i}}

It resists the arrival of current and resists its departure.

来时拒绝变化，走时维持旧电流。

oppose current change, not current；
use actual voltage across the inductor in $d i / d t = V_{L} / L$ ；
ideal steady-state short circuit does not mean every real coil has zero resistance；
simple series/parallel rules assume negligible mutual coupling。