Inductors

$v = L \frac{di}{dt}$

The current through an inductor cannot change instantly — interrupting it suddenly creates a large voltage spike (this is why relay coils need a flyback diode). Stored energy: $E = \tfrac{1}{2} L I^2$.

Inductance of a coil: $L = \dfrac{\mu N^2 A}{l}$ — it rises with the square of the number of turns.

Key specs: inductance, saturation current (above which the core stops working and $L$ collapses), DC resistance (DCR) and self-resonant frequency. Uses: switching power supplies, filters, chokes and tuned circuits.